984 00 SAC SP Prelims - Pearson...

2Salters Advanced Chemistry, Pearson Education Ltd 2008. © University of York.

This document may have been altered from the original.

IntroductionNo one yet has been able to look inside atoms to see what they are really like. The typical picture of an atom we have in our minds is neither ‘the truth’ nor ‘the right answer’ – it is a good working model which helps to explain many phenomena. Much evidence has been gathered to support the current model of an atom. The model may change as more evidence comes to light, and it is very likely to become more detailed. We can sometimes explain things using only a simplified model of the atom. Thinking of atoms as tiny spheres is sufficient to explain the states of matter (the properties of solids, liquids and gases) – but this model is not detailed enough to explain why metals tend to react with non-metals. Models can be simple or elaborate, depending on the job they need to do. Keep this in mind as your ideas and understanding of chemistry develop.

What you doHow has the current model of the atom developed? Many scientists contributed to the sequence of gathering knowledge about the atom, but some made particularly important discoveries – they were:

• JosephJ.Thomson(keydiscovery1897–1899)• HansGeiger,ErnestMarsdenandErnestRutherford(keydiscovery1909)• HenryMoseley(keydiscovery1913)• JamesChadwick(keydiscovery1932).

You will need to work in a group of three for this activity.

1 Eachgroupisgoingtotakethepartofoneofthesescientists–choosewhoisgoingtocoverwho.(NotethatGeiger,MarsdenandRutherfordrepresentone choice.)

2 Prepare a series of PowerPoint® slides on the scientist you have chosen. Your presentation should cover the following points:

•whoyouare •whenyoudidtheworkyouwilldescribe •whatyoualreadyknewabouttheatom •whatyoudid •whatyoufoundout •whatconclusionsyoudrewfromyourresults. Use suitable textbooks, magazine articles or the Internet to help you to find

the information you need. You could start by searching the Salters’ Advanced Chemistrywebsite.Youwillneedtodiscusswhatinformationandimagestoinclude and what to leave out as a group.

3 The members of your group should now deliver their presentations to the rest of the class – make sure the reports are presented in chronological order.

4 At the end of the activity everyone in the class will need notes on your presentations – prepare handouts of your group’s set of presentations.

In this activity you will learn how some of our ideas about atomic structure have developed.

How do we know about atoms?

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3Salters Advanced Chemistry, Pearson Education Ltd 2008. © University of York. This document may have been altered from the original.

IntroductionYou are going to carry out a quantitative investigation – trying to answer, as accurately as possible, a question which begins ‘How much …?’ A titration is a method of quantitative analysis that can be used when two solutions react together. One solution of a known concentration is placed in a burette – the second solution is placed in a conical flask. The solution in the burette is run into the flask until just enough has been added for the reaction to be complete. An indicator is often added to show when the reaction has finished, but this is not necessary if the reaction is accompanied by a very obvious colour change. An analysis involving a titration is sometimes called a volumetric analysis.

How it works

In this investigation, you are going to find out how much iron there is in a solution of an iron(II) salt by titrating the solution with potassium manganate(VII) solution. The salt is called hydrated iron(II) ammonium sulfate, which contains Fe2+ ions, as its name suggests. These react with the MnO4

− ions in the potassium manganate(VII), as shown in the equation below:

5Fe2+(aq) + MnO4−(aq) + 8H+(aq) → 5Fe3+(aq) + Mn2+(aq) + 4H2O(l)

pale deep light colourlessgreen purple brown

This looks rather complicated, but it tells you that the colour of the potassium manganate(VII) disappears as it reacts with the Fe2+(aq) ions. This provides a way of deciding when the titration is complete because when all the Fe2+(aq) ions are gone just one drop more of potassium manganate(VII) solution will make the titration mixture turn pale purple.

•accesstoabalance•weighingbottle•spatula•glassrod•250cm3 conical flask•250cm3 volumetric flask•burette•100cm3beakers(2)•25cm3 pipette•pipettefiller•smallfilterfunnel•plasticdropperpipette•washbottleanddistilled/deionisedwater•asampleofhydratediron(II)ammoniumsulfate(5g)•sulfuricacid,1moldm−3(250cm3)•potassiummanganate(VII)solution,0.010moldm−3(100cm3)

CaRe Take care when pouring potassium manganate(VII) solution as it stains the hands. Wear protective gloves if necessary.

Requirements

IRRITANT

dilute sulfuric acid

HARMFUL

iron(II) ammonium sulfate solid

WEAR EYE PROTECTION

CaRe Eye protection must be worn.

This activity introduces you to the technique of titration – one method of quantitative analysis. You will learn how to perform a titration and make use of accurately calibrated apparatus. Titration will be used in later modules.

How muCH IRon Is In a sampLe of an IRon Compound?

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What you do 1 Weigh a clean, dry weighing bottle accurately. Add about 5 g of iron(II)

ammonium sulfate to the bottle and record the mass of the bottle plus crystalsaccurately.Tipthecrystalsintoaclean100cm3beaker.Carefullyrinseoutthebottletwoorthreetimeswith1moldm−3 sulfuric acid, transferring the washings to the beaker each time. It is important that all the solid goes into the beaker.

2 Pouraboutafurther25cm3 of sulfuric acid into the beaker – but do not fill the beaker more than half full. Stir the acid and the solid together with a glass rod until you are sure that all the solid has dissolved.

3 Transferthecontentsofthebeakerthroughasmallfunnelintoa250cm3 volumetricflask.Rinsethebeakerandglassrodtwicewithsmallquantitiesofthe dilute sulfuric acid and transfer the washings to the volumetric flask. Then rinse the funnel with a small amount of the acid. This technique ensures that all the iron compound from the beaker is transferred to the volumetric flask.

4 Finally,adddilutesulfuricacidtothevolumetricflaskuntilitisabout1cmbelow the graduation mark. Now add more acid slowly from a clean dropping pipette until the bottom of the meniscus is just touching the graduation mark. Stopper the flask and invert it several times to mix the solution.

5 Useapipetteandpipettefillertowithdraw25.0cm3 of the solution from the volumetric flask and transfer it to a conical flask.

6 Useaclean,dry100cm3 beaker to fill a burette with the potassium manganate(VII)solution.Runalittleofthesolutionoutoftheburetteintothe beaker to make sure the jet is full of solution – ask your teacher for help if an air bubble stays in the jet. Be careful how you turn the burette tap – some burettes have tapered keys which leak if they are used wrongly. If you are not sure, ask your teacher for advice on how to use a burette correctly.

7 Recordthevolumereadingontheburettebeforestartingthetitration–readtheburettetothenearest0.05cm3.

8 Add small volumes of potassium manganate(VII) solution from the burette to the solution in the conical flask, swirling the flask after each addition. The purple colour of the MnO4

−(aq) ions will disappear as they react with the Fe2+(aq) ions. The end point of the titration is when you first get a permanent faint purple colour from excess manganate(VII) ions.

9 Recordthefinalburettereadingandcalculatethevolumeofsolutionyouhave run out into the flask. This first attempt will be a rough titration but it will give you a general idea of where the end point comes.

10 Do several further accurate titrations, in which you approach the end point adding the manganate(VII) solution drop by drop, until you have three volumeswhichagreetowithin0.1cm3.

Recording your results

Recordyourresultsasfollows:

mass of weighing bottle and solid = _______ gmass of weighing bottle = _______ gmass of solid = _______ g

titration Rough 1 2 3 4 5

final burette reading/cm3

initial burette reading/cm3

titre/cm3

Average titre = _______ cm3

eL1.2 How much iron is in a sample of an iron compound?

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5

Using your results

11 Work out the average of your three closest figures for the volume of potassiummanganate(VII)usedinatitration.Correctthistoanappropriatenumber of decimal places.

12 You will learn how to do calculations following titrations in the Elements from the Sea module. For the moment, simply multiply the volume of potassiummanganate(VII)youusedinthetitrationby2.8.Thiswillgiveyouthe mass (mg) of iron(II) ions pipetted into the conical flask each time.

13 Calculatehowmuchironwasinthe250cm3 of solution in the volumetric flask.

14 Calculatehowmuchironmusthavebeeninthemassofhydratediron(II)ammonium sulfate that you used.

15 Calculatethepercentageofironinthehydratediron(II)ammoniumsulfatecrystals.

16 The percentage of iron in a pure sample of hydrated iron(II) ammonium sulfate crystals, Fe(NH4)2(SO4)2·6H2O,is14.3%.Comparethisvaluewithyourresult for the percentage of iron in the compound.

Evaluating your results and procedures

In any analysis involving a titration, there are errors or uncertainties related to the precision of the equipment used. The glassware has been designed so that, if it is used appropriately, the precision errors are:• Volumetric or standard flask (class B)–whena250cm3 volumetric flask is

filled correctly (i.e. the bottom of the meniscus rests on the calibration line) theerroris0.2cm3or0.08%

• Burette (class B) – one drop from a burette has a volume of approximately 0.05cm3.Allburettereadingsshouldinclude2decimalplacesinwhichthesecondfigureiseither0or5.Anerrorofonedropinavolumeof25.00cm3 givesapercentageerrorof0.2%foreachreading.

• Pipette (class B)–whena25cm3 pipette is used correctly (i.e. it is allowed to drainandretainthelastdrop)theerroris0.06cm3or0.24%.

Procedural errors can arise if your practical technique is not good – a good technique would include the following:

• thesolutioninthevolumetricflaskneedsthoroughmixing• theburetteandpipetteshouldbewashedoutwiththesolutionsbeingused• theconicalflaskshouldbethoroughlywashedoutwithpurifiedwater

between titrations• theendpointofatitrationcanonlybedeterminedaccuratelyifthesolution

from the burette is added drop by drop, with swirling, as the end point is reached

• whenanindicatorisusedinatitration(notnecessarywhenusingpotassiummanganate(VII)) only the minimum number of drops is added each time.

How much iron is in a sample of an iron compound? eL1.2

Questions

1 Fill in the following table for your experiment.

Quantity measured % error

Mass of iron compound weighed on balance

250 cm3 solution made up in volumetric flask

25 cm3 solution delivered by pipette

Your average titre delivered by burette

2 Which of the stages in your procedure do you think could have led to errors? In each case, say whether it would make the result higher or lower.

3 Which of all the sources of error that you have identified is likely to have most impact on your overall result?

percentage error = error _______

reading × 100

It is important to repeat a titration several times to check that your results are reliable. After calculating the average titre, you should correct the value to an appropriate number of decimal places.

Salters Advanced Chemistry, Pearson Education Ltd 2008. © University of York. This document may have been altered from the original.

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IntroductionWhen metal compounds are placed in a Bunsen flame, the electrons in the metal atoms absorb energy and are promoted to higher (excited) energy levels. The electrons then emit energy as they fall back to lower energy levels – this energy is emitted in the form of radiation, some of which is in the visible part of the spectrum. The radiation is emitted at specific frequencies and if the emission spectrum of a metal is examined closely, it is found to be made up of a series of lines. Using only our eyes, we see the predominant colour resulting from the main frequencies at which each type of metal atom emits the radiation.

What you doYou are going to look at the light emitted when metal compounds are put into a Bunsen burner flame. You will capture an image of the colours with a mobile phone camera or digital camera. You will use these images to create a PowerPoint presentation in which you describe and explain the visible emission spectra of some metals.

1 Light your Bunsen burner and adjust the flame until no yellow colour appears in it.

2 Select one of the splints that has been soaked in a metal compound solution overnight.

3 Hold the splint in the Bunsen flame long enough to observe the colour that it imparts to the flame, but do not allow the splint to burn.

4 Repeatwithanothersplint,ifnecessary,toensurethatyoucanseethecolour accurately.

5 Repeattheprocedurewiththehelpofanotherstudent,sothatoneofyouholds the splint in the flame while the other takes a photograph of the colour.

6 Repeatwithsplintsthathavebeensoakedinothermetalcompounds.

Preparing your PowerPoint presentation

7 Download the images from your camera onto a computer.8 Use these images, and the ideas you have met during your study of this

topic, to produce a PowerPoint presentation which will: • illustratethedifferentcoloursemittedbydifferentmetalcompounds

when they are heated in a Bunsen burner flame •explainthebackgroundtheorywhichaccountsfortheemittedlightand

the formation of atomic emission spectra.

In this activity you will have an opportunity to view the visible light emitted when some metal compounds are heated. This visible radiation is part of the emission spectra of the metals.

InvestIgatIng vIsIbLe emIssIon

speCtRa

eL1.3

•Bunsenburnerandheat-resistantmat•mobilephonewithcamera,oradigitalcamera•accesstoanICTpresentationpackage(e.g.PowerPoint®)•woodensplintsthathavebeenpre-soakedovernightinthefollowing

solutions: – lithiumchloride0.1moldm−3

– sodiumchloride0.1moldm−3

– potassiumchloride0.1moldm−3

– bariumchloride0.1moldm−3

– calciumchloride0.1moldm−3

Requirements

HARMFUL

barium chloride

WEAR EYE PROTECTION


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IntroductionRadioactivedecayisarandomprocess–eachnucleusinasampleofaradioactive isotope decays in a random manner regardless of what other nuclei are doing. We can’t predict when a particular nucleus will decay but we can predict that half the radioactive nuclei in the sample will decay in a fixed time – the radioactive half-life. In this simulation of radioactive decay, you will drop a cardboard tray containing pieces of pasta onto a table. This causes some of the pasta to change from lying on their sides to standing on one of their flat ends – this is a random process, in a similar way that radioactive decay is a random process. We are going to take the pieces of pasta lying on their sides to represent radioactive nuclei, and the pieces which stand on a flat end to represent a radioactive nucleus which has decayed.

What you do1 Weighoutabout86gofditalinipasta(or162gofditaliliscipasta)intoa

cardboard tray.2 Swirl the tray to get the pasta into a single layer – make sure that all the

piecesofpastaarelyingontheirsidesandcountthem.Recordthestartingnumber of pieces of pasta in a table like the one opposite – this is most convenientlydoneusingcomputerspreadsheetsoftwaresuchasExcel®.

3 Dropthetrayontoatablefromaheightof5to10cm.4 Countandremovethepiecesofpastawhicharenowstandingonflatends,

and enter this number in the table.5 Repeatthisprocessafurtherninetimes.6 Plot a graph, using the computer package if possible, of unchanged pasta

(represents the number of undecayed radioactive nuclei) in the sample against the ‘drop number’ (represents time).

7 Share your results with other groups of students so that you can find a class average for the number of ‘undecayed nuclei’ at each stage – draw another graph using the average figures.

In this activity you will use pasta to simulate radioactive decay – this will enable you to practise working out radioactive half-lives.

sImuLatIng RadIoaCtIve deCay

eL1.4

•ditalinipasta(approximately86g)orditaliliscipasta(162g)•tweezers•cardboardtraysuchasthetopofaboxcontainingpacketsofpaper

Requirements

drop number

unchanged pasta

pasta removed

0 0

1

2

3

4

5

6

7

8

9

10

Questions

1 Work out three values of ‘radioactive half-life’ based on the graph drawn using your own data.

2 What do you notice when you compare the half-life values?

3 Use the graph drawn using the class average values to work out three values of the half-life. How do these values compare with each other and with the values found from your own graph?

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What you doThe statements in the table below refer to the formation of ions, using sodium and chlorine as examples.

1 Readthrougheachstatementcarefullyandputatickinoneoftheboxestoshowwhetheryouthinkthestatementistrueorfalse.YoumayfindtheinformationinTable1useful.

statement true false

a A sodium atom spontaneously loses an electron to get a full shell of electrons

b An Na7− ion is more stable than a sodium atom because it has a full shell of electrons

c A Cl7+ ion is just as stable as a Cl− ion because they both have a full shell of electrons

d Each proton in the nucleus of an atom attracts one specific electron

e Energy is required to remove an electron from an atom

f When an atom is ionised, it requires even more energy to remove a second electron

g If you remove an electron from a sodium atom you can never put it back

h Once you have removed one electron from a sodium atom you can’t remove another because that would mean it no longer had a full electron shell

i Solid sodium chloride contains pairs of sodium and chloride ions which are kept together by their opposite charges

j When sodium chloride dissolves, the solution contains molecules of sodium chloride

▼ table 1

particle electron arrangement

Na 2.8.1

Na+ 2.8

Na7− 2.8.8

Cl 2.8.7

Cl− 2.8.8

Cl7+ 2.8

2 Joinwithtwootherstudentsandcompareyouranswerstothe10statements.Whereyouhavedifferentanswers,explaintoeach other why you have chosen your particular answer and agree between you what your group thinks is the best answer for each statement.

3 Your teacher will help you to compare your group’s answers with the answers chosen by other groups. Be prepared to explain why your group has chosen answers, and also be ready to challenge other groups if you think that your answer is more appropriate than theirs.

4 Write down how you have modified your original ideas about why atoms form ions after talking and listening to other students.

In this activity you can check and clarify your ideas about why some atoms form ions.

wHy do atoms foRm Ions?

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What you do1 Blow up the balloons and tie the necks – do not blow them up too hard or

you will not be able to twist them.2 Twist one of the balloons in the middle. It now represents two electron pairs

aroundacentralatom(Figure1).Thespacetakenupbyalobeoftheballoon resembles the space occupied by a pair of electrons, and the balloon lobes push each other out of the way – just as electron pairs repel one another. You can see that the ‘balloon molecule’ takes the linear shape you wouldexpect(likeBeCl2).

3 Twist a second balloon in the middle, and then twist the middle of this balloon several times around the middle of the first balloon to represent fourelectronpairs(Figure2).Itmayhelptoputafewdropsofglycerolonthe twist to lubricate the join.

a What shape does the ‘molecule’ made in step 3adopt?Giveanexampleof a molecule with such a shape.

4 Now, get a third balloon – twist it in the middle and twist it around the middle of the ‘molecule’ you made in step 3. Again, make sure the balloons are twisted round each other several times. Now you have a representation of a molecule with six electron pairs round a central atom.

b What shape does the ‘molecule’ made in step 4 adopt?Giveanexampleof a molecule with such a shape.

5 Now comes the exciting bit – get a pin and pop one of the balloon lobes. If you did your twisting well, the air won’t escape from the other half of the popped balloon, and you will have five ‘electron pairs’.

c What shape does the ‘molecule’ made in step 5 adopt?Giveanexampleof a molecule with such a shape.

6 Pop two more lobes to get three ‘electron pairs’. d What shape does the ‘molecule’ made in step 6 adopt?Giveanexample

of a molecule with such a shape.7 Summarise your findings in a table like the one below. When you draw the

diagrams, represent the electron pairs with lines or wedges, as shown in Chemical Ideas 3.2.

•sausage-shapedballoons(3)•apin•glycerol

Requirements

The shapes adopted by twisted balloons can closely resemble the shapes of molecules. You have to be careful if this modelling process is to work well.

sHapes of moLeCuLes:

baLLoon moLeCuLes

eL2.2 (1)

figure 1 Two electron pairs round a central atom.

figure 2 Four electron pairs around a central atom.

number of electron pairs round central atom

shape bond angles

diagram example

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What you do1 Working in pairs, draw the dot–cross diagram for each of the following

molecules/ionsonseparatepiecesofblankcard:

BCl3 BeCl2 CH4 H2O NH3 NH4+ PCl5 SCl2 SCl6 SiH4

2 Write the following headings across the top of a piece of A4 paper (landscape format):

Formulaofmolecule/ion Dot–crossdiagram Shape Bondangle

3 Place your dot–cross cards under that heading on the piece of paper. Now match the other cards with your dot–cross diagrams to show the formula, shapeandbondangle(s)foreachmolecule/ion.

4 Discuss the placing of the cards with your partner, then compare your card arrangements with those of other pairs of students. Are there any ideas you are not sure about?

5 Finally, construct a table to summarise the information shown by your cards.

•10blankpiecesofcard,eachmeasuring5cmby4cm•setofcardsshowingmoleculeorionnames,molecularshapesandbondangles

Requirements

In this part of the activity you will draw dot–cross diagrams for simple molecules and ions and use them to predict molecular shapes and bond angles.

sHapes of moLeCuLes: sHapes and bond angLes

eL2.2 (2)

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Shapes of molecules: shapes and bond angles eL2.2

Linear Triangular planar Tetrahedral

Tetrahedral Tetrahedral Triangular pyramid

V-shaped V-shaped Triangular bipyramid

Octahedral 90° 90° and 120°

109° 109° 109°

109° 109° 109°

120° 180° BCI3

BeCI2 CH4 H2O

NH3 PCl5

SCI2 SCI6 SiH4

NH4+

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What you do1 For each of the substances named in the table, describe its structure and

properties by choosing one of the responses from each of Sections A–D and putting ticks, 3, in the appropriate boxes.

2 Compareyourgridwithoneproducedbyanotherstudent.Discussanydifferences between them and decide on any changes you think you should make to your original grid.

3 Compareyouragreedgridwiththoseofotherpairsofstudents.Again,makeany changes that are necessary and make a note of any features that you were less certain about. This will serve as a reminder for you to focus on these points when you revise this topic in the future.

methane Iron diamond sodium chloride

a: structure

Giant lattice (metallic)Giant structure (ionic)Giant structure (covalent network)Simple molecular

b: melting temperature

HighLow

C: solubility in water

SolubleInsoluble

d: Conduction of electricity

Conducts as a solid and when moltenConducts in solution and when moltenDoes not conduct electricity

In this activity you will summarise the physical properties of different types of structures.

wHat type of pRopeRtIes

do dIffeRent stRuCtuRes Have?

eL2.3

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IntroductionThe checklist below covers the key points in Chemical Storylines EL1 and EL2. The statements listed correspond to learning outcomes in the specification for the AS examinations. They are listed in the order in which they occur in this module.Rememberthatyouwillbecomingbacktomanyoftheideasinlatermodules. You will probably have made summary notes of the main ideas that you have met. Now is a good time to make sure that your notes cover all the points you need. If you feel that you are not yet able to meet the requirements of all of the statements in the list, you should look again at the areas concerned, seek help from your teacher if necessary and develop your notes accordingly. Most of the points are covered in Chemical Ideas, with supporting information in Chemical Storylines or the activitites. However, if the main source of information is in a storyline or an activity this is indicated.

What you doReadandthinkabouteachofthestatementsinthechecklist.Putatickinthecolumn that best represents your current ability to do what is described:A – I am confident that I can do thisB – I need help to clarify my ideas on thisC – I am not yet able to do this.You will be sharing this information with your teacher so that you can work together to improve your understanding.

at the end of Chemical storylines eL1 and eL2 you should be able to: a b C

• describeprotons,neutronsandelectronsintermsoftheirmassandrelativecharge

• describethestructureofatomsintermsofelectronsandacentralnucleuscontainingprotonsandneutrons

• explaintheoccurrenceofabsorptionandemissionatomicspectraintermsofchangesinelectronicenergylevels; compare and contrast the features of these spectra:

– similarities – both line spectra, lines in same position for a given element, lines become closer at higher frequencies, sets of lines representing transitions to or from a particular level

– differences – bright/coloured lines on a black background or black lines on coloured/bright background• understandtherelationshipbetweentheenergyemittedorabsorbedandthefrequencyofthelineproduced

in the spectra, ΔE = hυ• describetheelectronstructureofatomsintermsofmainenergylevels(electronshells)uptoZ = 36

• recallthatthenucleiofsomeatomsareunstableandthattheseatomsareradioactive• recallandexplainthedifferentpropertiesofα, β and γ radiations• recallthatthetermhalf-lifereferstothetimetakenforhalftheradioactivenucleiinasampletodecay,and

that the half-life is fixed for any given isotope• carryouthalf-lifecalculations activity eL1.4

• usenuclearsymbolstowriteequationsfornuclearprocesses,includingfusionandradioactivedecay

• recallthatinfusionreactionslighteratomsjointogiveheavieratoms(underconditionsofhightemperatureand pressure) and understand that this is how certain elements are formed

• understandhowradioactiveisotopescanbeusedas‘tracers’inthebodyand(giveninformation)forotheruses

• explainthatthehalf-lifeof‘tracers’mustbeofanappropriatelengthtoallowdetectionbutnotcauseunduedamage

• understandtheuseofradioisotopesinthedatingofarchaeologicalandgeologicalmaterial

continued

This activity helps you check your knowledge and understanding of the topics that you have covered in Chemical Storylines EL1 and EL2.

CHeCk youR knowLedge and undeRstandIng

(paRt 1)

eL2.4

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eL2.4 Check your knowledge and understanding (part 1)

• understandthatknowledgeofthestructureoftheatomdevelopedintermsofasuccessionofgraduallymoresophisticated models

• giveninformation,interprettheseandotherexamplesofsuchdevelopingmodels activity eL1.1

• explainandusethetermsatomicnumber,massnumber,isotope,Avogadroconstant,relativeisotopicmass,relative atomic mass, relative formula mass and relative molecular mass

• drawandinterpretsimpleelectron‘dot–cross’diagramstoshowhowatomsbondthroughionic,covalentanddative covalent bonds, and be able to describe a simple model of metallic bonding

• describesomelimitationsofthesemodels

• recallthetypicalphysicalproperties(meltingtemperature,solubilityinwater,abilitytoconductelectricity)characteristic of giant lattice (metallic, ionic, covalent network) and simple molecular structure types activity eL2.3

• usetheelectronpairrepulsionprincipletopredictandexplaintheshapesofsimplemolecules(suchasCH4, NH3, H2O and SF6) and ions (such as NH4

+) with up to six outer pairs of electrons (any combination of bonding pairs and lone pairs) activity eL2.2

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IntroductionEpsomsaltsoccurnaturallyandareahydratedformofmagnesiumsulfate.Magnesium sulfate can be made in the laboratory in the reaction between magnesium carbonate and dilute sulfuric acid:

MgCO3(s)+H2SO4(aq) → MgSO4(aq)+CO2(g)+H2O(l)

What you doMaking hydrated magnesium sulfate crystals (Epsom salts)

1 Usingameasuringcylinder,pour40cm3of1moldm−3 sulfuric acid into a 250cm3 beaker.

2 Weigh approximately 6 g of magnesium carbonate – this is more than is needed to react with all of the acid so you don’t need to weigh it accurately.

3 Add spatula measures of the solid magnesium carbonate to the acid and stir until all visible signs of reaction have stopped.

4 Support a filter funnel in a clamp and place an evaporating basin underneath it.

5 Prepare a filter paper, put it into the filter funnel and filter the mixture from the beaker.

6 Heat the filtrate in the evaporating basin to reduce it to about one-third of its original volume – do NOT heat to dryness. (CARE There may be considerable ‘spitting’.) Wear goggles and heat gently.

7 Put the evaporating basin in a safe place, covered by a dry filter paper, to allow the solution to crystallise – this may take a day or two.

8 Removethecrystalsfromtheremainingfiltrate,blotthemdrywithabsorbent tissues and then allow the crystals to air dry.

Analysing magnesium sulfate crystals

The magnesium sulfate crystals you have made have the formula MgSO4·xH2O. When they are heated, water of crystallisation is driven off leaving anhydrous magnesium sulfate, MgSO4.

MgSO4·xH2O(s) → MgSO4(s)+xH2O(g)

By weighing the hydrated and anhydrous magnesium sulfate before and after

IRRITANT

sulfuric acid

•250cm3 beaker•100cm3measuring cylinder•weighingbottleorweighingboat•spatula•evaporatingbasin•filterfunnel•filterpaper•tripod,gauzeandpipeclaytriangle•Bunsenburner•crucible,lidandcrucibletongs•accesstobalance(2or3d.p.)•magnesiumcarbonatepowder(6g)•sulfuricacid,1moldm−3(40cm3)

CaRe When heating the hydrated magnesium sulfate, there can be considerable ‘spitting’. Goggles should be worn and the Bunsen burner flame should be turned down as low as possible.

Requirements

WEAR EYE PROTECTION


Epsom salts are widely sold as a mild laxative – they are hydrated magnesium sulfate. In this activity you will make magnesium sulfate crystals and then analyse them to find out their exact formula.

makIng and anaLysIng epsom

saLts

eL3

984_01_SAC SP_EL.indd 15 16/4/08 15:52:39



heatingyoucancalculatethenumberofmolesofwaterassociatedwith1moleof the magnesium sulfate. This will allow you to determine the exact formula for the crystals. As you carry out steps 9–12, record your results like this:

mass of crucible = _______ gmassofcrucible+hydratedmagnesiumsulfate =_______gmassofcrucible+anhydrousmagnesiumsulfate =_______gmass of magnesium sulfate in the magnesium sulfate crystals = _______ gmass of water in the magnesium sulfate crystals = _______ g

9 Weigh a clean, dry crucible and record its mass.10 Put some of the dry crystals of hydrated magnesium sulfate you have made

intothecrucible.Reweighthecrucibleandrecordthemass.11 Heat the crucible gently for about a minute, and then more strongly for a

further 5 minutes (although you should try and keep the heat as low as possible). You may see the crystals appear to ‘melt’. The liquid is likely to ‘spit’ as the water of crystallisation is driven off (CARE Wear goggles). You will need to use the crucible lid to prevent loss of solid. Allow the crucible to cool and then reweigh it, recording your mass.

12Heatthecrucibleagainforabout2minutesandreweighitagain.Ifthelasttwomassesdifferbymorethan0.05g,repeattheheatingandweighingagain– heating a substance until its mass remains the same is called ‘heating to constant mass’.

Questions

1 What is meant by the terms: a hydrated b anhydrous c water of crystallisation?

2 Why is it necessary to heat the hydrated magnesium sulfate to constant mass?

3 Calculate the relative formula masses of: a magnesium sulfate (MgSO4) b water.

4 a Calculate the number of moles of magnesium sulfate in the crystals you weighed out.

b Calculate the number of moles of water in the crystals.

5 a Calculate the number of moles of water which are combined with 1 mole of magnesium sulfate in the crystals.

b What is the exact formula for the magnesium sulfate crystals?

eL3 Making and analysing Epsom salts

984_01_SAC SP_EL.indd 16 16/4/08 15:52:40


What you doReaction of the elements with water

1 Halffilla100cm3 beaker with water. Use a pair of tweezers to select a small piece of calcium metal (CARE Avoid skin contact) and add it to the water.

2 Observe what happens to the contents of the beaker. When the reaction seems to be over, test the pH of the mixture using universal indicator solution. Make a note of your observations.

3 Repeattheexperimentwithapieceofmagnesium;andthenwithbarium.(CARE Barium and its compounds are harmful. Do not swallow any. Barium is kept under oil or liquid paraffin. You will need to dry your piece of metal on filter paper before using it.)

a Use a reference book to find out what the products are in these reactions. Then summarise your results in a table like the one below.

b Make a note of any common properties of these metals and the compounds produced from them.

c Make a note of any patterns you can spot in your results.

Solubilities of the hydroxides and carbonates

In this part of the experiment you will be looking to see whether precipitates form when you add drops of solutions of sodium carbonate and sodium hydroxidetodropsofGroup2metalionsolutions.ThiswillenableyoutomakejudgementsabouttherelativesolubilitiesofGroup2carbonatesandhydroxides.4 You will be given a worksheet with boxes on which to place drops of

solution.Coverthisworksheetwithaclearplasticsheet(orplasticpocket)ifit is not already laminated.

5 Put2dropsofthemetalionsolutionsineachboxoftheappropriaterow.6 Nowadd2dropsofsodiumcarbonateorsodiumhydroxidesolutiontothe

appropriate column. d Recordyourresults.Canyouidentifyanypatternsinthesolubilitiesof

Group2carbonatesandhydroxides?

•testtubes•100cm3beakers(3)•tweezers•filterpaper•smallpiecesof: – magnesium ribbon – calcium – barium•magnesiumnitratesolution,0.1moldm−3(1cm3)•calciumnitratesolution,0.1moldm−3(1cm3)•strontiumnitratesolution,0.1moldm−3(1cm3)•bariumnitratesolution,0.1moldm−3(1cm3)•sodiumhydroxidesolution,1moldm−3(2cm3)•sodiumcarbonatesolution,1moldm−3(2cm3)•universalindicatorsolution•worksheetresultstable

Requirements

HIGHLY FLAMMABLE

calcium

WEAR EYE PROTECTION


This activity introduces you to some of the chemistry of these elements and gives you practice at spotting patterns and looking for generalisations in your results.

InvestIgatIng tHe CHemIstRy of gRoup 2 eLements

eL4.1

CORROSIVE

sodium hydroxide solution

universal indicator solutionHIGHLY

FLAMMABLE

bariumHIGHLY

FLAMMABLEHARMFUL

metal observation when metal added to water

Chemical equation

pH of mixture produced

magnesium

984_01_SAC SP_EL.indd 17 16/4/08 15:52:41



table 1 Reactions of Group 2 metal ions in solution

Solu

tion

ofhy

drox

ide

ions

Solu

tion

ofca

rbon

ate

ions

Solu

tion

ofm

agne

sium

ions

Solu

tion

ofca

lciu

m io

ns

Solu

tion

ofst

ront

ium

ions

Solu

tion

ofba

rium

ions

eL4.1 Investigating the chemistry of Group 2 elements

984_01_SAC SP_EL.indd 18 16/4/08 15:52:41


IntroductionThe naturally occurring form of most elements is made up of a mixture of isotopes of the element. For example, natural chlorine is made up of the isotopes 35Cland37Clintheratioof75%to25%.Thismeansthatinevery100atoms of chlorine

75havemassnumber35and25havemassnumber37.

So the relative atomic mass of chlorine = (75×35)+(25×37)

___________________ 100

=35.5

The relative atomic mass (Ar) of a naturally occurring element (the form of the element that normally takes part in chemical reactions) is the weighted mean of the mass numbers of the stable isotopes of the element. The mass numbers and relative abundances of the stable isotopes can be found from the mass spectrum of the element.

What you doThemassspectrumofnaturalkrypton,seeFigure1,showsthatithasfivestableisotopesatmassnumbers80,82,83,84and86.

This activity shows how a mass spectrometer can be used to give information about isotopes. You will use mass spectra data to determine the relative abundances of the isotopes of an element and then calculate its relative atomic mass.

IsotopIC abundanCe and ReLatIve atomIC

mass

eL4.2

The peak heights of a mass spectrum are often adjusted so that the most abundantionisgivenarelativeintensityof100%inordertoachievemaximumdifferentiation between the peak intensities.

1 Measure the peak height for each isotope of krypton and calculate its relative abundance in natural krypton. Use these data and the mass numbers of the isotopes to calculate the relative atomic mass (Ar) of krypton.

2 Draw the mass spectrum you would expect to obtain for naturally occurring chlorine.

figure 1 The mass spectrum of krypton.

100

80

60

40

20

040 80 9050 60 70

m/z

Inte

nsity

/ %

984_01_SAC SP_EL.indd 19 16/4/08 15:52:42



What you doYou need to download and print, or draw, graphs of the melting temperatures and boiling temperatures of the elements from hydrogen to argon. To download a graph, find an Internet site that provides information about the Periodic Table. A good example is www.webelements.org – a site designed and developed at the University of Sheffield. Navigate through the site until you reach a graph showing the information you are looking for. It may contain information about more elements than those you need but that is fine. Print copies of the graphs. Anothergooddataandgraph-drawingresourcecanbefoundattheRoyalSocietyofChemistry’swebsite(www.chemsoc.org/networks/learnnet/ptdata/welcome.htm). This interactive Periodic Table allows you to select the data you need and plot your own graph, which you can then print. If you cannot download and print off a graph from the Internet, you will need to find the required information from a chemistry data book. In this case, use spreadsheet software to allow you to show the data graphically and to print a hard copy.

This activity helps you to identify how the melting and boiling temperatures of elements change across a row in the Periodic Table.

patteRns In tHe pHysICaL pRopeRtIes of

eLements

eL4.3

Questions

1 On your graphs, indicate which elements are from Period 1, which are from Period 2 and which are from Period 3.

2 For each graph, describe the patterns that you observe across each period.

3 How do the patterns across Period 2 compare to those across Period 3?

4 Explain how your graphs show ‘periodicity’.

5 For each graph, note which group of elements appear at:

a the peaks b the troughs.

6 Part of the boiling temperature graph is shown below. Label the point you think represents the element sodium. Explain how you arrived at your answer.

Successive elements in the Periodic Table

Boili

ng te

mpe

ratu

re

984_01_SAC SP_EL.indd 20 16/4/08 15:52:42


IntroductionThe checklist below covers the key points in Chemical Storylines EL3 to EL5. The statements listed correspond to learning outcomes in the specification for the AS examinations. They are listed in the order in which they occur in this module.Rememberthatyouwillbecomingbacktomanyoftheideasinlatermodules. You will probably have made summary notes of the main ideas that you have met. Now is a good time to make sure that your notes cover all the points you need. If you feel that you are not yet able to meet the requirements of all of the statements in the list, you should look again at the areas concerned, seek help from your teacher if necessary and develop your notes accordingly. Most of the points are covered in Chemical Ideas, with supporting information in Chemical Storylines or the activities. However, if the main source of information is in a storyline or an activity this is indicated.

What you doReadandthinkabouteachofthestatementsinthechecklist.Putatickinthecolumn that best represents your current ability to do what is described:A – I am confident that I can do thisB – I need help to clarify my ideas on thisC – I am not yet able to do this.You will be sharing this information with your teacher so that you can work together to improve your understanding.

at the end of Chemical storylines eL3 to eL5 you should be able to: a b C

• explainandusethetermsatomicnumber,massnumber,isotope,Avogadroconstant,relativeisotopicmass,relative atomic mass, relative formula mass and relative molecular mass

• usetheconceptofamountofsubstancetoperformcalculationsinvolvingmassesofsubstances,empiricalandmolecular formulae and percentage composition

• writeandinterpretbalancedchemicalequations,includingstatesymbols

• describeandexplainthemainstagesintheoperationofatime-of-flightmassspectrometer

• usedatafromamassspectrometerto: – calculate relative atomic mass and the relative abundance of isotopes – work out the relative molecular mass of molecules• understandthatotherpeaksarecausedbyfragmentsofmolecules

• recallthatthePeriodicTablelistselementsinorderofatomic(proton)numberandgroupselementstogetheraccording to their common properties

• usegiveninformationtodescribetrendsinagroupofthePeriodicTableandtomakepredictionsconcerningthe properties of an element in this group

• describeperiodictrendsinthepropertiesofelements,intermsofmeltingtemperatureandboilingtemperature activity 4.3

• recallthatthepositionofanelementinthePeriodicTableisrelatedtoitselectronstructure(mainenergylevels or electron shells) and vice versa

• describeandcomparethefollowingpropertiesoftheelementsandcompoundsofMg,Ca,SrandBainGroup 2:

– reactions of the elements with water – acid–base character of the oxides and hydroxides – thermal stability of the carbonates – solubilities of hydroxides and carbonates activity 4.1

• understandhowMendeleevdevelopedthePeriodicTablebyleavinggapsandrearrangingsomeelementsfrom their atomic mass order and how subsequent research validated this knowledge

• givenrelevantinformation,discussotherexamplesofhowscientificresearchcanbeusedtoassessthevalidity of a discovery Chemical storylines eL4

This activity helps you check your knowledge and understanding of the topics that you have covered in Chemical Storylines EL3 to EL5.

CHeCk youR knowLedge and undeRstandIng

(paRt 2)

eL5

984_01_SAC SP_EL.indd 21 16/4/08 15:52:43



Elements of Life end of module test 60 marks (1 hour)

A copy of the Periodic Table is required for this test.

1 The element helium was discovered in the Sun’s atmosphere by Lockyer and Frankland around 1870. They saw lines in the emission spectrum of the Sun which corresponded to no element known at the time.

a Copy and complete the table to show the relative mass and charge of protons, neutrons and electrons.

proton neutron electron

mass (on relative atomic mass scale) 1 v. small

charge +1 0 [2]

b Helium is made in the Sun by fusion reactions such as

21H + 31H → 42He + ……. equation 1.1

i What is meant by the term fusion reaction? [2] ii Explain why 2

1H and 31H are called isotopes of hydrogen. [2] iii Complete equation 1.1 for the fusion reaction. [2]

c Helium was identified by its absorption spectrum. Describe the appearance of an absorption spectrum. [2]

d Draw some of the electronic energy levels of an atom such as helium and use your diagram to explain: • The way in which absorption and emission occur. • Why only certain frequencies are involved in the emission spectrum of a particular element. • Why there is more than one frequency in an emission spectrum. [4]

e Helium nuclei occur as α-particles in radioactive decay. For example, polonium-216 decays by loss of an α-particle. i What feature of the 216Po nucleus causes it to decay? [1] ii Write a nuclear equation for the decay of 216Po. [2] iii The half-life of the decay of 216Po is 0.145 s. Explain what this means. [1] iv Imagine you have a sample of 1 million atoms of 216Po. How many atoms would be left after 0.435 s? [1]

f Suggest why β-particle emitters are more useful than α-particle emitters as radioactive tracers. [2]

g Geiger and Marsden fired α-particles at a thin sheet of gold foil. Most passed through undeflected while just a very few bounced back.

What feature of atomic structure did this experiment reveal? Explain your answer. [2] [totaL: 23 maRks] (OCR Chemistry B (Salters) question, adapted for the 2008 specification)

2 Mendeleev is usually regarded as the founder of the modern Periodic Table.

a Mendeleev listed the elements in order of ascending atomic mass, but he reversed the positions of tellurium and iodine. (Ar: Te, 127.6; I, 126.9)

i Explain why he thought it necessary to do this. [1] ii Which feature of atoms is used today to determine their position in the Periodic Table? [1]

b Mendeleev grouped zinc with calcium because they have some similar properties. i Suggest an equation for the thermal decomposition of zinc carbonate. [2] ii Draw a dot–cross diagram for the ionic substance calcium oxide, showing outer electron shells only. [2] iii Zinc oxide is an ionic solid that is insoluble in water.

Suggest two physical properties of zinc oxide that can be deduced from its ionic structure. [2]

c Give the symbol for an element from Period 3 (Na to Cl) which: i has an outer shell of five electrons; [1] ii forms a basic oxide XO; [1] iii has the highest melting point; [1] iv forms a compound with fluorine which has an octahedral shape. [1]

eL End of module test

984_01_SAC SP_EL.indd 22 16/4/08 15:52:43


End of module test eL

d Sulfur is in Group 6 and forms a hydride (hydrogen sulfide). The electron arrangement and shape of this molecule are similar to those of water.

i Draw a dot–cross diagram for hydrogen sulfide, showing the outer electron shells only. [2] ii Draw a diagram to predict the shape of the hydrogen sulfide molecule, indicating the bond angle. [2] iii Explain your answer to ii. [3]

e An oxide of sulfur contains 40% sulfur by mass, the rest being oxygen. Calculate the empirical formula of the oxide. [3] [totaL: 22 maRks] (OCR Chemistry B (Salters) question, adapted for the 2008 specification)

3 In 1848, a party led by Sir John Franklin set out on an ill-fated expedition to find the Northwest Passage between the Atlantic and Pacific Oceans. Scientists have now been able to show how the members of the party died. Their method involved analysing the lead in human remains from the site of their last camp, and comparing it with lead from the solder used to seal their food containers. This showed that members of the party had absorbed large (and presumably fatal) quantities of lead from their contaminated food.

The analysis was carried out using mass spectrometry, and showed that the ratio of 208Pb to 206Pb was very similar in the human remains and in the solder seals.

a i Draw a diagram of a time of flight mass spectrometer. On your diagram, write the following labels: sample inlet; ionisation area; acceleration area; ion detector. [4] ii Which of 208Pb+ ions or 206Pb+ ions will pass through the spectrometer faster? Explain why this happens. [2]

b The mass spectrum of a sample of lead is shown below. Calculate the relative atomic mass (Ar) for this sample of lead by reading values off the mass spectrum. Give your answer to four significant figures.

Inte

nsity

/%

m/z

100

80

60

40

20

0200 201 202 203 204 205 206 0207 208 209 210

25 22

53

[3]

c Solder is an alloy of tin and lead. A sample is found to contain one mole of lead for every two moles of tin. Use the following steps to calculate the percentage by mass of lead in the solder sample. i What is the mass of two moles of tin atoms? [1] ii What is the mass of one mole of lead atoms? [1] iii Calculate the percentage of lead in the solder. [2]

d Draw a labelled diagram to illustrate the metallic bonding in a sample of lead. [2] [totaL: 15 maRks] (OCR Chemistry B (Salters) question, adapted for the 2008 specification)

984_01_SAC SP_EL.indd 23 16/4/08 15:52:43



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984_07_SAC SP_TT EL.indd 150 17/4/08 08:00:33


Concept mapThe concept map shows how the major chemical ideas in this teaching module develop throughout the AS course.

Concept First introduced in module

Developed in module(s)

Assumed in module(s)

Relative atomic mass and relative formula mass EL – all

Amount of substance EL DF all

Chemical formulae and inorganic nomenclature EL ES all

Balanced chemical equations EL DF all

Atomic structure EL – –

Atomic orbitals and electron configuration EL ES –

Nuclear processes EL –

Ionic bonding EL – ES

Ionisation enthalpy EL ES –

Size of ions EL – –

Covalent bonding EL – DF, ES, PR

Giant covalent (network) structures EL A ES

Shapes of molecules EL DF ES

Metallic bonding EL – ES

Relationship between properties, and bonding and structure EL – DF, ES, PR

Electromagnetic spectrum EL A –

Quantisation of energy EL A –

Interaction of radiation with matter EL A –

Atomic absorption and emission spectra EL – –

Mass spectrometry EL – –

The Periodic Table EL – ES

Groups 1 and 2 EL – –

Advance warningThe following items needed for activities in this module may not be in your school currently, and might take a little time to obtain.

Activity Item(s) Essential/optional Typical quantity per activity

EL1.4 Ditalini pasta (or ditali lisci pasta) Essential 86 g ditalini (or 162 g ditali lisci) per pair of students (reusable)

EL2.2 Balloons (sausage-shaped) and balloon pump Essential 3 per pair of students

EL4.1 Small pieces of barium metal (do not use strontium)Strontium and barium compounds – solutions of the nitrates

EssentialEssential

small pieces1 cm3 of each per pair

ELELEMENTS OF LIFE

984_07_SAC SP_TT EL.indd 151 17/4/08 08:00:34



Storyline: answers to assignments 1 a 4 2 He b 15

8 O c 1 1 H2 Sodium, magnesium, iron, helium and hydrogen lines are

all visible.3 a The radioactivity of blood samples would be taken

before, during and after receiving the tracer. b Detectable quantities of radiation have to be given off

during the short time the patient is being observed. There is also a health risk involved in using long-lived isotopes which may become built into the body, e.g. radioactive calcium in bones.

c Ionising radiation is being produced inside the patient in sensitive organs and may cause damage. Isotopes must be rushed from the production lab to the hospital before too much decay has occurred.

4 Communication and presentation of data are important aspects of this course. Students should be able to see that the pie charts show up the differences between mass and percentage of atoms more clearly than the table. This may be an appropriate point at which to discuss other ways of presenting data.

Activities: notes and answers to questions EL1.1 How do we know about atoms?

CommentsThis activity is an opportunity for students to practise and develop their communication and IT skills by extracting information from several sources, including the Internet, and preparing a PowerPoint presentation.

EL1.2 How much iron is in a sample of an iron compound?

Safety note Information about hazardous chemicals is given on the activity sheet.

CommentsThis activity extends students’ experience of quantifiable errors by considering the specialist glassware used in volumetric analysis. Calculations involving reacting amounts and concentrations are dealt with in modules DF and ES, respectively. Students who have already covered these ideas could perform a standard calculation from their results. Students who have not previously encountered mole calculations should be able to do the activity by making use of the conversion factor provided in the notes. You may wish to spend some time showing students how to make up a solution and perform a titration before starting this activity.

Answers1 The actual percentage errors depend on the size of the

values measured. The answers for the volumetric flask, burette and pipette are given in the activity.

2 Students should be able to identify the stages in the procedure that could lead to errors. They should also be encouraged to think through how the error would affect the result.

3 Students should identify the stages in their procedure and the measurements that are likely to have the greatest impact on the closeness of the final answer to the ‘true’ value. Since the titration is repeated, they should comment on the reliability of their average titre as indicated by the closeness of successive values.

EL1.3 Investigating visible emission spectra

Safety note Information about hazardous chemicals is given on the activity sheet.

CommentsThe wooden splints need to be pre-soaked in the salt solutions overnight. Alternatively, the splints can be soaked overnight in water. A splint can then be removed, excess water drained off and the end dipped into a small sample of the salt. The activity is best organised as a ‘circus’ with each station being dedicated to a specific salt. Pairs of students can then move from station to station observing the different flame colours. This activity aims to help students to see and explain the link between flame colours and emission spectra. Teachers may want students to use handheld spectroscopes to view the flame colours. However, this is not essential because students are asked to explain the background theory that accounts for the emitted light and the formation of atomic emission spectra in their PowerPoint presentation. Demonstration of a spectroscope (which might be found in a physics department) or the use of a digital video clip showing particular atomic spectra would complement this activity. The colours seen are:

• Li–brightred• Na–yellow• K–lilac• Ca–brickred• Ba–applegreen.

EL1.4 Simulating radioactive decay

CommentsDitalini (‘little thimble’) pasta is available from a number of suppliers (an Internet search will provide a list). Each manufacturer produces different sizes of ditalini pasta so the mass required will vary. Another pasta (which may be more readily available) that works quite well is ditali lisci. This is a bigger form of pasta which is easier to count and to pick out with tweezers. 162 g of ditali lisci pasta is needed per pair of students. The pasta can be reused. Pre-weighed bags of pasta are easy to use and store. The tops of boxes used to supply photocopy paper make good trays. The first drop of the tray should produce between 70 and 90 pieces of pasta standing on end. If this is not the case then try dropping the pasta from a slightly greater height. Teachers could set up a spreadsheet in which pairs of students enter their results in order to get ‘class average’ results. Use of computer spreadsheet software will enable the graphs to be drawn quickly and accurately.

EL Storyline: answers to assignments

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Activities: notes and answers to questions EL


Answers1 The ‘radioactive’ half-life values will depend on students’

data.2 Students should notice that the three values for the half-

lives are similar.3 The class average ‘radioactive’ half-lives should be very

similar to each other, and similar to the data from students’ own graphs. Emphasise how averaging a larger data sample reduces anomalies which may be apparent in any one set of data.

EL2.1 Why do atoms form ions?

CommentsThis activity is based on ideas in Chemical Misconceptions – prevention, diagnosis and cure; Volume II: Classroom Resources, London: Royal Society of Chemistry, 2002. It aims to help students to identify misunderstandings they may have about the formation of ions. The statements they are given should promote discussion between students which help reveal any misconceptions.

Answers1 a False: energy needs to be supplied to remove an

electron from an atom (the ionisation enthalpy) b False: the Na7− ion would be highly unstable c False: Cl− is much more stable than Cl7+ since a great

deal of energy would be needed to remove 7 electrons from an atom

d False: all the protons attract all the electrons (and vice versa)

e True: this is the ionisation enthalpy f True: in the case of both sodium and chlorine, more

energy is needed to remove a second electron, but the difference is greater with sodium because the second electron to be removed is in a lower energy level

g False: an electron can be added to a sodium ion to reform the atom

h False: a second electron can be removed from a sodium atom but it would need more energy to do so –thisisthesecondionisationenthalpy

i False: solid sodium chloride is made up of an ionic lattice containing many ions with each ion attracting several oppositely charged ions

j False: sodium chloride solution contains hydrated sodium and chloride ions

EL2.2 Shapes of molecules

Part 1The activity is fun to do, but students must also realise the significance and importance of the summary table they obtain.

Answersa Tetrahedral; e.g methane, CH4

b Octahedral; e.g. sulfur(VI) fluoride, SF6

c Trigonal bipyramid; e.g phosphorus(V) chloride, PCl5d Planar triangular; e.g boron(III) fluoride, BF3

Part 2It is helpful if students work in pairs so that they can talk with eachotheraboutdrawingappropriatedot–crossdiagramsandabout assigning shapes and bond angles to them. The summary table will be useful for revision later.

Teachers may choose to prepare laminated cards with molecule names, shapes and bond angles before the lesson to avoid using class time for cutting out the cards.

EL2.3 What type of properties do different structures have?

This is a quick assessment for learning activity to check the students’ understanding about structure and properties. It is important that students identify any features explicitly that they are less certain about so that they can focus on them in the future.

AnswersMethane Iron Diamond Sodium

chloride

A: Structure

Giant lattice (metallic) 3

Giant structure (ionic) 3

Giant structure (covalent network)

3

Simple molecular 3

B: Melting temperature

High 3 3 3

Low 3

C: Solubility in water

Soluble 3

Insoluble 3 3 3

D: Conduction of electricity

Conducts as a solid and when molten

3

Conducts in solution and when molten

3

Does not conduct electricity

3 3

EL2.4 Check your knowledge and understanding (Part 1)

CommentsThis activity ensures that students are aware of the learning outcomes (specification statements) that their assessment will be based on, and provides an opportunity for them to reflect on how well they understand the ideas they have covered. Crucially, it enables teachers to identify areas where individual students are less confident, and to provide appropriate additional support to improve their understanding.

EL3 Making and analysing Epsom salts

Safety note Information about hazardous chemicals is given on the activity sheet. Note that care must be taken when heating the salt.

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EL Activities: notes and answers to questions



CommentsStudents have the opportunity to make crystals of hydrated magnesium sulfate, and then to analyse the resulting salt to find its exact formula. Some teachers may prefer to provide crystals for analysis to avoid waiting for students’ own samples to crystallise.

Answers1 a Crystals which contain molecules of water associated

with the cations and anions are said to be hydrated. b Crystals which do not contain any molecules of water

are said to be anhydrous. c The water molecules associated with ions in a crystal

are called water of cystallisation.2 Heating to constant mass ensures that all the water of

crystallisation has been driven off to leave only the anhydrous salt.

3 a 120.4 b 18.05 a 7 b MgSO4·7H2O

EL4.1 Investigating the chemistry of Group 2 elements

Safety note Information about hazardous chemicals is given on the activity sheet. Barium metal is difficult to cut into smaller pieces. Barium rods can be held in a vice and a junior hacksaw used, with care, to cut off small pieces of the metal. It is better to buy granules if possible. Do not use strontium. When barium reacts with water, the resulting alkaline solution will be an irritant. The solution will absorb carbon dioxide and turn cloudy (barium carbonate).

CommentsThis activity needs careful management. Students often have difficulty making observations from test tube experiments at this stage of the course. Also, it may be better to divide up the work so that different groups are responsible for different parts of the activity or for different elements. The second part of the activity introduces student to microscale techniques in chemistry. The student sheets can be covered by a plastic sheet such as those used with OHPs, or put into plastic file pockets. Some teachers may prefer to laminate the sheets instead.

AnswersReactions of the elements with water:3 • Magnesium does not appear to react. Ca and Ba react

to produce an alkaline solution of the metal hydroxide and hydrogen.

• The metals are reactive and are denser than water. The hydroxides are white, slightly soluble compounds which form alkaline solutions. The metals become more reactive from Mg to Ba.

6 The hydroxides become more soluble from Mg(OH)2 to Ba(OH)2. The carbonates are insoluble.

EL4.2 Isotopic abundance and relative atomic mass

Answers1 Students may find the intensity of each peak from the

scale, or measure the peak heights with a ruler. The results here use the relative intensity of each peak.

Mass Relative intensity Relative abundance

80 4.0 2.3

82 20.5 11.7

83 20.5 11.7

84 100 56.9

86 30. 5 17.4

175.5 100.0

Ar of krypton = [(80 × 2.3) + (82 × 11.7) + (83 × 11.7) + (84 × 56.9)

+ (86 × 17.4)] / 100 = 83.9 (the Data Book value is 83.8)2 The mass spectrum of chlorine consists of two peaks at

mass 35 and 37 with relative intensities in the ratio of 3 : 1 respectively.

EL4.3 Patterns in the physical properties of elements

CommentsStudents are expected to make use of the Internet to find information about the melting temperatures and boiling temperaturesofelementsfromhydrogentoargon– Period 1: H, He; Period 2: Li to Ne; Period 3 Na to Ar.

Answers2 Melting and boiling temperatures rise to elements in

Group 4 (C, Si) then fall to low values.3 The pattern across Period 2 is similar to that in Period 3.4 Periodicity describes how a pattern in a property across

one period is repeated across another period5 a Peaks occur at Group 4. b TroughsoccuratGroup0–thenoblegases–although

the melting and boiling temperatures of elements in Groups 5, 6 and 7 are also low.

6 Sodium is represented on the graph by the fifth point from the left. The point at the peak must represent an element in Group 4; so sodium in Group 1 is three places to the left. Alternatively, the sequence of four low points must represent elements in Groups 5, 6, 7 and 0; so sodium must be represented by the next point.

EL5 Check your knowledge and understanding (Part 2)

CommentsThis activity ensures that students are aware of the learning outcomes (specification statements) that their assessment will be based on, and provides an opportunity for them to reflect on how well they understand the ideas they have covered. Crucially, it enables teachers to identify areas where individual students are less confident, and to provide appropriate additional support to improve their understanding. This activity could be used as part of the preparation for an end of module test.

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EL Answers to end of module tests



Answers to Elements of Life end of module test

Q Answer Maximum mark

1 (a) Neutron: mass = 1 (1); electron: charge = –1 (1) 2

1 (b) (i) Two nuclei join together (1); to form a heavier nucleus (1) 2

1 (b) (ii) Same number of protons (1); different number of neutrons/mass number (1) 2

1 (b) (iii) 10n (1) for numbers; (1) for ‘n’ or ‘neutron’ 2

1 (c) Dark lines (1); on a coloured or bright background (1) 2

1 (d) At least three horizontal lines, spacing smaller at the top (1); arrows between levels – up labelled ‘absorption’, down ‘emission’ (1); transitions only occur between energy levels (1); different gaps give rise to different lines (1)

4

1 (e) (i) It is unstable 1

1 (e) (ii) 21864Po → 4

2He (1) + 21822Pb (1) 2

1 (e) (iii) This is the time taken for half the sample to decay 1

1 (e) (iv) 125 000 1

1 (f) β-particles penetrate more (1); therefore more easily detected (1) 2

1 (g) Central nucleus (which is very small and very heavy) (1); the few particles that hit it bounced back (1)

2


2 (a) (i) Their properties/reactions/behaviour matched the reversed groups (1) 1

2 (a) (ii) Atomic number/number of protons 1

2 (b) (i) ZnCO3 (1) → ZnO + CO2 (1) 2

2 (b) (ii)

Ca2� 2�

O

(1) for calcium (can have eight electrons); (1) for oxygen; [(1) if no charges but otherwise correct]

2

2 (b) (iii) High melting point (1); conducts electricity when molten (1) 2

2 (c) (i) P 1

2 (c) (ii) Mg 1

2 (c) (iii) Si 1

2 (c) (iv) S (allow SF6) 1

2 (d) (i) H S H Lone pairs (1); bonding pairs (1)

2

2 (d) (ii)

109°

S

HH

Shape (1); bond angle 105–110° (1)

2

2 (d) (iii) Four pairs of electrons (1); repel (1); and get as far away from each other as possible (1) 3

2 (e) 40/32.1 moles S (= 1.25) (1); 60/16.0 moles O (= 3.75) (1); formula SO3 (1)

3

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3 (a) (i)

Sampleinlet

Ionisationarea

Accelerationarea

Ion detector

(1) for each correctly labelled area; only award ion detector mark if flight path area is shown (not necessarily labelled)

4

3 (a) (ii) 206Pb2+ faster because lighter ions faster (1); all have same kinetic energy initially, lower mass ⇒ higher velocity (1)

2

3 (b) ((206 × 25) + (207 × 22) + (208 × 53))/100 = 207.3 (1) for top of fraction; (1) for dividing by 100; (1) for answer to 4 sf

3

3 (c) (i) 237.4 g (238 g) 1

3 (c) (ii) 207.2 g (207 g) 1

3 (c) (iii) 207.2 × 100/444.6 (1) = 46.6% (46.5%) (1) (credit correct working and answers from previous incorrect steps)

2

3 (d) Diagram showing positive ions (1); in a ‘sea’ of electrons (1) 2

Answers to end of module tests EL

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