Chemistry of the Oceans Workbook

7/23/2019 Chemistry of the Oceans Workbook

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All at sea?

The chemistryof the oceans

The chemistry of the biosphere is based onthat of long chain polymers.

This article will concentrate of the chemistryof the hydrosphere but it will also look athow this interacts with the other spheres.

Q1.

a) Explain what is meant by the termsgiant ionic structure, smallmolecule and long chain polymer.You will probably need to drawdiagrams to help your explanation.For each category, give an example

of a substance with that structure.b) In a substance made up of small

molecules, what type of bonding occurs

(i) between the atoms within themolecule?

(ii) between the moleculesthemselves?

c) Explain why substances with giantstructures and substances with longchain polymeric structures are solidswhile those made up of smallmolecules are liquids or gases.

Those few people lucky enough to have seenthe planet Earth from space often call it theblue planet because of the large area of surfacecovered by oceans (Figure 1). In fact the Earths

oceans hold about 1.5 x 1018 tonnes of water,

which in turn contains 0.05 x 1018 tonnes ofdissolved salts. So a lot of chemistry can take

place in the oceans. The oceans also interactchemically with the Earths rocks, with theatmosphere and with living things.

For many purposes, the Earth can be dividedinto four sections or spheres. These are:

the lithosphere this consists of the solidrock and soil component of the crust andupper mantle;

the hydrosphere the water on, in andaround the Earth;

the atmosphere the gases surroundingthe Earth; and

the biosphere the living things on theEarth.See Figure 2

Although the detail of the chemistries ofeach of these areas is complex, we can makesome very broad generalisations.

The chemistry of the lithosphere isessentially that of giant ionic structures.

The hydrosphere consists of small moleculeswith dissolved ions.

The atmosphere is made up of smallmolecules.

Figure 1The PacificOcean viewedfrom space bythe solid state

imager on theGalileo spaceprobe

lithosphere

biosphere

hydrosphere

atmosphere

biosphereatmospherehydrospherelithosphere

Figure 2The Earths

four spheres


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The hydrosphere facts, figures and estimates

The total mass of water on the Earth is about

1.5 x 1018 tonnes. (1018 is a shorthand wayof writing 1000 000 000 000 000 000 one

followed by 18 zeros, so 1.5 x 1018 could bewritten as 1500 000 000 000 000 000).Many of the quantities we will talk aboutwhen dealing with the hydrosphere can

conveniently be expressed in units of 1018

tonnes. The prefix exa is used to signify

1018 in the same way that kilo means 103

and mega 106, so the mass of water on theEarth could be referred to as 1.5 exatonnes.

For comparison, the mass of the Earth is

about 6 x 1024 tonnes, or 6000 000exatonnes.

95.7% of this water is found in the oceans,4.1% in rocks and soil, 0.2% as ice, lakesand rivers and 0.001% in the atmosphere.

The likely source of all this water is from thegases that were released by volcanoes in thefirst 500 million years after the formation of theEarth. To put this in context, the Earth is about

4.5 billion (4.5 thousand million, ie 4.5 x 109)years old. This source of water seems likelybecause 70% of gases ejected from present dayvolcanoes consist of water vapour.

Perhaps surprisingly, no one knows for

certain where this water came fromoriginally. Possibly the answer will be foundas we explore other planets, by unmannedprobes as we are doing now, and later bymanned expeditions.

Q2. Use the percentages above to workout the actual masses of water in:

a) the oceans,

b) rocks and soil,

c) ice, lakes and rivers,

d) the atmosphere.

Use units of exatonnes.

The water cycle

Although the proportions of water in theoceans, soil, ice, rivers and atmosphereremain more or less constant, the water isnot static. It is estimated that every year,

0.000 42 x 1018 tonnes of water circulatearound the water cycle, Figure 3. That is, itevaporates from the oceans to form watervapour in the atmosphere and thencondenses to fall as rain, snow or hail,

collectively called precipitation. It has beencalculated that the average time that amolecule of water remains as vapour in theatmosphere between evaporation andprecipitation is a mere 12 days.

After falling as rain or snow, water returns tothe oceans via streams and rivers. In doing

this it carries weathered and eroded materialfrom the Earths crust into the oceans. Each

year, 8 x 1012 tonnes of material istransported from the Earths crust into theoceans. About two-thirds of this is insoluble(rocks, pebbles, sand, silt etc) and the rest isin solution.

The ions in seawater

Everyone knows that the sea is salty.Common salt is sodium chloride. It exists insolution as separate hydrated sodium ions

(Na+(aq)) and hydrated chloride ions

(Cl(aq)). Dissolved ions are independent,and we cannot say that a particular ion ispart of a particular compound. In fact thesodium and chloride ions that make the seasalty almost all came from different sources virtually none came from solid sodiumchloride that dissolved. So it is better to thinkabout the ions dissolved in the sea, ratherthan different compounds.

Tables 1 and 2 show the ions dissolved inthe sea.

Q3. To break up a giant ionic structureinto separate ions, ions of oppositecharge have to be pulled apart aprocess which requires an input ofenergy. Explain how it is that manyionic compounds, such as sodiumchloride, dissolve in water withlittle energy change. You may needto include in your answer anenergy level diagram and a diagramto show the part played by watermolecules.

In fact, since sodium (Na+

) is by far the mostcommon positive ion and chloride (Cl) byfar the most common negative ion, it is notunrealistic to say that the sea containssodium chloride.

evaporation

erosion

precipitation

Figure 3The water

cycle


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Q4. Look at Tables 1 and 2

a) From which two groups in thePeriodic Table do all the metal ionscome? Suggest why this is so.

b) Use the charges of the ions in thetable to work out the formulae ofthe following compounds:

(i) potassium bromide(ii) strontium fluoride

(iii) sodium silicate(iv) strontium carbonate

c) The relative atomic mass ofchlorine is 35.5. Do a calculationto confirm that a concentration by

mass of 19 000 mg dm3 of chlorideions is the same as a concentration

of 0.535 mol dm3.1 mg is 1/1000 g.

d) Which ion, Na+, or Cl, is in excessin seawater? Hence what is theeffective concentration of sodium

chloride in seawater?

e) What is the maximum mass ofsodium chloride that could be

extracted from 1 dm3 of seawater?

Where do these ions come from?

The positive ions in seawater are all metalions, and all come from the weathering ofminerals in the Earths crust. The commonnegative ions (except for silicate) arederived from simple molecules, and most ofthem probably came originally from theatmosphere. For example, sulfate(VI) ions arederived from sulfur dioxide by reaction withoxygen and water in the atmosphere:

2SO2(g) + O2(g) 2SO3(g)

SO3(g) + H2O(l) H2SO4(l)

The H2SO4 exists in solution as the ions H+

and SO42.

Weathering mainly affects the continentalcrust of the Earth (iedry land). This is madeup mostly of silicates and aluminosilicates ofmetals in Groups 1 and 2 of the PeriodicTable and of silicates and aluminosilicates ofiron. The crust also contains carbonates ofthese metals. These are present in smaller

amounts but weather more rapidly.

Table 1 The twelve most common ions in the sea

Ion Concentration % by mass of the Concentration

/ mg dm3 total dissolved solids / mol dm3

Chloride, Cl 19 000 55.04 0.535

Sodium, Na

+

10 500 30.42 0.457Sulfate(VI), SO4

2 2 655 7.69 0.028

Magnesium, Mg2+ 1 350 3.91 0.056

Calcium, Ca2+ 400 1.16 0.010

Potassium, K+ 380 1.10 0.009 7

Carbonate, CO32 140 0.41 0.002 3

Bromide, Br 65 0.19 0.000 81

Borate, BO33 20 0.06 0.000 34

Silicate, SiO32 8 0.02 0.000 11

Strontium, Sr2+ 8 0.02 0.000 09

Fluoride, F 1 0.003 0.000 05

Table 2 The most common ions in the seaarranged as positive and negative ions

Positive ions Concentration Negative ions Concentration

/ mol dm3 / mol dm3

Sodium 0.457 Chloride 0.535

Magnesium 0.056 Sulfate(VI) 0.028

Calcium 0.010 Carbonate 0.002 3

Potassium 0.009 7 Bromide 0.000 81

Strontium 0.000 09 Borate 0.000 34

Silicate 0.000 11

Fluoride 0.000 05


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Q5. a) What elements are present in:

(i) the silicate ion?(ii) the aluminosilicate ion?

b) Explain how the names tell you

what elements are present.

Chemical weathering is the action of waterthat contains acids on metal silicates andcarbonates. Natural water is usually acidicbecause of its reaction with carbon dioxide,in the atmosphere and in soil, to formcarbonic acid, H2CO3, which dissociates to

produce H+ ions in aqueous solution.

Q6. Write equations for the reaction ofwater with carbon dioxide to form

carbonic acid and for thedissociation of carbonic acid.

The atmosphere also contains smalleramounts of sulfuric(VI) acid and nitric(V) acidproduced when oxides of sulfur and nitrogenrespectively react with water in theatmosphere. The concentrations of theseacids in rainwater are increasing at presentbecause of the activities of humans.

Q7. Explain how human activities areresponsible for producing sulfuroxides and nitrogen oxides in theatmosphere.

Some compounds dissolve as acidic rainwaterpercolates through the soil and underlyingrocks, and the dissolved ions are eventuallywashed into the sea. This is where most of thedissolved metal ions in the sea have come from.

However, the Earths crust does not containmany compounds that contain fluoride,chloride, bromide, sulfate(VI) and borateions. Since these are found in seawater, they

must have come from another source. It isprobable that they have come from gasesproduced by volcanoes. These gases thendissolved in rainwater.

So the salt (sodium chloride) dissolved inseawater has come from two sources thesodium ions directly from the Earths crustand the chloride ions from volcanic gases.

Is the sea getting saltier?

The simple answer to this is no, we believethat the oceans reached their present degreeof saltiness (on average 3.5% of dissolved

ions) quite rapidly (compared with the age ofthe oceans) and that they have maintainedthe same degree of saltiness for a very longtime.

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VARIATIONS IN THE SALTINESSOF THE SEA

Although on average the sea contains 3.5%of dissolved solids, there are considerablevariations from place to place, see Table 3

and Figure 4.

Table 3The saltiness of different seas

Location Dissolved solids / %

Open ocean 3.5

Mediterranean Sea 3.9

Red Sea (northern end) 4.1

Dead Sea 27.0

Figure 4World map showing location of the sea areas shownin the table.

Figure 5The Dead Sea is so salty that floating is easy.

Q 8. What sort of climates do theMediterranean Sea, the Red Seaand the Dead Sea have? What sortof geographical surroundings dothey have? You could use an atlas,CD-ROM or an internet search to

help you find this information.Suggest why these seas (especiallythe Dead Sea) are saltier than theopen ocean.

Open ocean 3.5%

Mediterranean Sea 3.9%

Red Sea 4.1%

Dead Sea27.0%


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Maintaining the composition ofseawater

Since rivers are transporting around

2.5 x 1012 tonnes of dissolved material intothe sea each year, this means that dissolved

ions must be being removed at the same rateto keep the composition of seawater thesame. The ways in which ions are removedfrom seawater include:

EvaporationThis takes place especially in hot, dryclimates where the sea is shallow andenclosed. Solid deposits of salts such as rocksalt (sodium chloride, NaCl) and gypsum(calcium sulfate(VI), CaSO4) are formed.

Q9. What ions are removed from

seawater when a deposit of

a) rock saltb) gypsumis formed?

PrecipitationIf the concentrations of the ions that makeup a particular compound become too great,that compound will come out of solutionand form a solid precipitate. For examplecalcium ions and carbonate ions combinetogether to form insoluble calcium carbonate(limestone):

Ca2+(aq) + CO32(aq) CaCO3(s)

Q10. Describe a simple test to show thata fossil sea shell contains acarbonate.

BiochemicalLiving things in the sea may remove ionsfrom seawater for a variety of purposes. Theshells of sea creatures are made fromcalcium carbonate, as is coral. A number of

organisms concentrate ions from seawater,sometimes by a factor of 105 or more. Someexamples include:

sea squirts which concentrate vanadium; other tunicates (creatures that feed by

filtering seawater) which concentrateniobium;

oysters which concentrate zinc (somepeople believe that zinc is the reason thatoysters have their supposed aphrodisiaceffect);

lobsters which concentrate copper; and shellfish which concentrate mercury.The last example was the cause of a tragedyat Minamata in Japan. Here, industrial wastecontaining mercury was discharged into thesea. Once diluted by the seawater, the

mercury concentration was quite low, but itgot into the following food chain:

algae shellfish fish humans

eventually causing a number of deaths ofhumans as well as birth defects.

TectonicIn some places below the sea bed, themargins of tectonic plates are movingtowards each other and one plate slidesbelow the other a process calledsubduction, see Figure 6. During thisprocess, seawater is dragged into the Earthscrust.

Trace elements in seawater

Small amounts, called trace amounts, of allthe other elements are present in seawatermany of them in tiny concentrations lessthan one millionth of that of sodium. Theseconcentrations have been measured bylooking at the light given out when they areheated in a high temperature flame. This is avariation on the flame test method that youmight have used to identify metals from thecolours they produce in flames orange-yellow for sodium, green for copper etcand is called Flame EmissionSpectrophotometry see Box.

Gold in seawaterEven gold compounds are present dissolvedin seawater. The concentration of gold is

0.000 004 mg dm3. This may not soundmuch, but this means that there are fourmillionths of a gram of gold in a tonne ofseawater and 5.6 million tonnes of gold inthe whole of the Earths oceans!

Not surprisingly, chemists have tried toextract gold from seawater. Perhaps the mostfamous attempt was that by the Germanchemist Fritz Haber, (who also developed

the process for making ammonia that isnamed after him). Haber intended to use themoney he hoped to make to help pay offGermanys debts resulting from the FirstWorld War. However, the scheme failed.

plate movement

Figure 6Subduction


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FLAME EMISSION SPECTROPHOTOMETRY

One way of measuring small concentrations is by atechnique called Flame EmissionSpectrophotometry. The electrons in atoms exist in

particular energy levels called orbitals. The spacingof these levels is different in every element.Normally the electrons in an atom are found in thelowest possible levels this is called the groundstate of the atom. Heating the atom to a hightemperature will supply the energy to move one ormore of the electrons into orbitals of higher energythan normal - this is called an excited state of theatom. The electrons in an excited atom then dropback to the lower levels of the ground state. Aseach one does so, it gives out an amount of energy,E, equal to the difference in energy between thetwo levels, see Figure 8 below. This energy is givenout in the form of electromagnetic radiation whosefrequency, , is given by the equation

E = h

where h is Plancks constant (6.6 x 10-34 Js).

This radiation is normally in the visible or ultraviolet regions of the electromagnetic spectrum.

Figure 8 The energy levels in the sodium atom

Because each element has a unique set of electronic energy levels, every element has its own pattern offrequencies that it gives out after being excited. This can be used to identify the element. By comparing the

intensity of the radiation with that from a sample of known concentration, the concentration of a particularelement in, say, seawater can be measured. With this technique, concentrations of the order of 10 8 mol dm3

(depending on the element) can be measured.

Q11. Look at Figure 8 above. The yellow light given out when the electron in level 3p of the excited

state falls back into level 3s has a frequency of 5.08 x 1014 s1.

a) Use the equation E = h to calculate the energy gap between levels 3p and 3s in sodium.

b) Your answer to (a) will be in J / atom. What does this correspond to in kJ mol1? (Take the

Avogadro constant to be 6 x 1023mol1.)

c) Compare your answer with the value of a typical bond energy. Does your answer seemreasonable?

4p3d4s

3p

3s

2p

2s

1s

4p3d4s

3p

3s

2p

2s

1s

Lightgivenout

(a) The ground state of sodium (b) An excited state of sodium. The lightgiven out when the electron falls backfrom 3p to 3s is in the yellow region ofthe spectrum and is the reason thatsodium gives a yellow flame.

Figure 7

Flame emission spectrophotometer(photograph by courtesy of Pekin Elmer Ltd)


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GETTING A FEEL FOR THEFIGURES

1 cm3 of water has a mass of 1 g, so 1 m3 ofwater has a mass of 100 x 100 x 100 =1000 000 g, which is 1000 kg or 1 tonne.The water in an average bath has a mass ofabout one-fifth of a tonne.

Q12. The price of gold is about

5000 kg1. (The price of gold varies,you could look up the current pricein a newspaper, Ceefax or on the

internet.)a) What is all the gold in the oceansworth?

b) Use the figures above to work outwhat the gold in a bath of seawaterwould be worth.

Q13. Suggest what problems would befound when trying to extract goldfrom seawater.

Q14. Entropy, S, is a measure of thedisorder of chemical systems.

a) What can be said about theentropy change, S, of processes

which occur naturally?b) Which is larger, the entropy of a

block of metallic gold or that of thesame amount of gold present as adilute solution of gold ionsdissolved in water? What does thissuggest about the feasibility ofextracting gold from seawater?

Q15. Gold is present in seawater in the

form of AuCl4 ions.

a) What is the oxidation numberof gold in this ion?

b) What is the oxidation numberof metallic gold?

c) What type of reaction is

required to convert AuCl4

ions into metallic gold?

Useful materials from the sea

The dissolved ions in the sea form anessentially free source of materials to anyonewith access to the sea.

Evaporation of seawater produces sodiumand potassium chlorides. This is carried outmostly in hot countries where the heat of theSun is used to evaporate the water (and thereis little rain to re-dissolve the solid salt thathas been produced). In some countries, suchas Saudi Arabia, seawater is separated toprovide pure water (for drinking etc) while

the salts form a by-product.

Q16.

a) Explain why using the Suns heat toevaporate seawater is commerciallyattractive.

b) Find out the main source of sodiumchloride in the UK. What is theconnection with seawater?

Treating seawater (which containsmagnesium chloride) with an alkali such ascalcium hydroxide forms a precipitate ofmagnesium hydroxide. This is used in themanufacture of magnesium metal.

Q17. Write the word and balancedsymbol equations for the reactionof magnesium chloride withcalcium hydroxide.

Bromine is made by reacting seawater,which contains bromide ions, with chlorine.

Q18.

a) Write an equation for the reactionof chlorine with bromide ions.

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Figure 9Salt lagoons inFrance


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b) Work out the oxidation numbers ofall the species involved before andafter the reaction. Which species isthe oxidising agent?

c) Given the following values of Eo

:Br2(aq) + 2e

2Br(aq) E o = +1.07 V

Cl2(aq) + 2e

2Cl(aq) E o = +1.36 V

Work out E o for the reaction ofchlorine with bromide ions.

d) Using the sign and value of Eo,state which of the followingphrases best describes theequilibrium position for the

reaction between chlorine andbromide ions:

(i) goes to completion;

(ii) does not occur;

(iii) an equilibrium well over tothe right; or

(iv) an equilibrium well over tothe left?

Acknowledgements

Written by Ted Lister and Alastair Fleming.

Designed by Imogen Bertinwww.cork-teleworking.com and TrevorBounford www.bounford.com.

Figure 1 courtesy of NASA.

Figure 5 courtesy of Eric Zellerwww.ewz.com..

Figure 7 courtesy of Perkin Elmer Ltd.

Figure 9 Spectrum Photo Library.

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Answers

1. a) Giant ionic structure. A three-dimensional array of positive andnegative ions in which eachpositive ion is surrounded bynegative ions and vice versa. Thestructure can extend without limit.See Figure 10.

Figure 10 Part of a giant ionic structure

Small molecule. A group of a fewatoms held together with covalentbonds.

Long chain polymer. A largemolecule held together withcovalent bonds and consisting ofmany smaller molecules(monomers) linked together.

b) (i) Covalent

(ii) Intermolecular forcessuch as van der Waals,dipole-dipole orhydrogen bonding,depending on the actualmolecule.

c) Giant ionic and covalentstructures are held together byionic and covalent bondingrespectively. These forces arestronger than the intermolecular

forces that act between smallmolecules. In a long chain polymer,the sum total of the intermolecularforces between the molecules isgreater than those between smallmolecules.

2.

a) 1.44 exatonnes

b) 0.062 exatonnes

c) 3 x 103 exatonnes

d) 1.5 x 105 exatonnes

3. The process of dissolving an ioniccompound involves putting energyin to separate the ions (the latticeenergy) but also involves hydration

of the ions, a process in whichenergy is given out. In many cases,the two energy changesapproximately cancel one anotherout as is the case with sodiumchloride, see Figure 11.

Figure 11An enthalpy diagram for the dissolution of sodiumchloride

Figure 12Hydration of cations, M+, and anions, X,by water molecules

4.

a) Groups 1 and 2, because most

simple compounds containing theseions are soluble in water.

b) (i) KBr (ii) SrF2 (iii) Na2SiO3(iv) SrCO3.

c) 19 000 mg is 19 g This is 19/35.5 =0.535 mol

d) The chloride ion is in excess, so theeffective concentration of sodiumchloride in seawater is determinedby the sodium ion concentration,

0.457 mol dm3

e) 0.457 x 58.5 = 26.7 g

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EnthalpyLE (NaCl)+771 kJ mol

1

Na+

(g) + Cl

(g) + (aq)

NaCl (s) + (aq) +1 kJ mol1

Ho

solution(NaCl)

770 kJ mol1

Ho

hydration(Na

++ Cl

)

Na+(aq) + Cl

(aq)

O

H H

O

H

M+ O

H

H HO

HH

+ +

+

+

++

+

+

H

O

H

+

+

H

O

H

+

+

X

H

O

HH

O

H

++

+

+


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5.

a) (i) Silicon and oxygen

(ii) Aluminium, silicon andoxygen.

c) The ending ate indicates thepresence of oxygen.

6. H2O(l) + CO2(g) H2CO3(aq)

H2CO3(aq) H+(aq) + HCO3

(aq)

7. When wood and fossil fuels areburned, small amounts of sulfurthat they contain are converted tosulfur dioxide. In high temperatureburning processes, some of theoxygen and nitrogen in the aircombine to form a mixture ofnitrogen oxides.

8. They have hot, dry climates. They

are largely surrounded by land.Water evaporates readily fromthese seas leaving behind greaterthan normal salt concentrations. Asthe seas are landlocked, or nearlyso, little or no mixing with theoceans occurs.

9.

a) Sodium and chloride

b) Calcium and sulfate(VI).

10. Add some of the shell to a little

dilute hydrochloric acid. Test anygas that comes off with limewater(calcium hydroxide solution). If agas is produced that turns thelimewater cloudy, then the shellcontains carbonate.

11.

a) E = h

= 6.6 x 1034 x 5.08 x 1014

= 3.35 x 1019 J per atom

b) E = 201 kJmol1

c) This is comparable with a typicalbond energy and is thereforereasonable.

12.

a) 2.8 x 1013

b) A typical bath contains 0.2 tonne of

water, which is 200 dm3.

This contains 200 x 0.000 004

= 8 x 104 mg gold.

This is 8 x 1010 kg of gold.

This is worth 4 x 106

. This isabout 1/2000 p.

13. Vast quantities of water must beremoved. The small quantities ofgold compounds must be separatedfrom much larger quantities ofother compounds. The goldcompounds must be reduced tometallic gold.

14.

a) The entropy increases.

b) The entropy of the block of gold ismuch smaller.Energy will be needed to drive theprocess.

15.

a) +III

b) 0

c) Reduction

16.

a) The energy is essentially free.

b) Underground deposits in Cheshireand the North East of England.These were formed by theevaporation of ancient seas.

17. magnesium chloride +calcium hydroxide magnesium hydroxide +calcium chloride

MgCl2(aq) + Ca(OH)2(aq)

Mg(OH)2(s) + CaCl2(aq)

18.

a) Cl2(aq) + 2Br(aq)

Br2(aq) + 2Cl(aq)

b) Cl2 0; Br -I; Br2 0; Cl

-I;

c) + 0.29 V

d) (iii) an equilibrium well over to theright.

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Chemistry of the Oceans Workbook

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