Reaction of sodium Reaction of sodium thiosulphate and thiosulphate and hydrochloric acidhydrochloric acid

GCSE InvestigationGCSE Investigation

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The reaction…The reaction…

• Sodium thiosulphate, NaSodium thiosulphate, Na22SS22OO33 reacts reacts

with HCl to give a reaction mixture that with HCl to give a reaction mixture that gets more cloudy as yellow sulphur is gets more cloudy as yellow sulphur is formed.formed.

• NaNa22SS22OO3(aq)3(aq) + 2HCl + 2HCl(aq)(aq) 2NaCl 2NaCl(aq)(aq) + S + S(s)(s) + +

SOSO2(g)2(g) + H + H22OO(l)(l)

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What will we measure?What will we measure?

• The time for a cross beneath the The time for a cross beneath the reaction flask to disappear as the reaction flask to disappear as the reaction gets cloudy.reaction gets cloudy.

• We will do the reaction at several We will do the reaction at several temperatures and try to work out the temperatures and try to work out the rate of reaction.rate of reaction.

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The InvestigationThe Investigation

• The investigation will involve looking at how The investigation will involve looking at how concentration can affect the rate of this reaction.concentration can affect the rate of this reaction.

• We can control concentration by diluting solutions of We can control concentration by diluting solutions of known strenghts known strenghts

• We want to investigate 5 or 6 concentrations – what We want to investigate 5 or 6 concentrations – what would be a suitable range for investigation of would be a suitable range for investigation of reactions reactions in solutionin solution??

• How many times would you attempt to repeat the How many times would you attempt to repeat the reaction to improve reliability of results?reaction to improve reliability of results?

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AAnswer Q’s

ErrorsErrors

• There are several errors in the experiment.There are several errors in the experiment.• The devices for measuring volume are at best The devices for measuring volume are at best

accurate to 0.2 mL. accurate to 0.2 mL. • What would the result be of having too much or too What would the result be of having too much or too

little of each reagent?little of each reagent?• If the total volume of the reaction is 50mL what is the If the total volume of the reaction is 50mL what is the

percentage error from measuring volume?percentage error from measuring volume?• Error % = error/range x 100Error % = error/range x 100• Especially at lower temperature the end of the Especially at lower temperature the end of the

reaction is difficult to judge.reaction is difficult to judge.• Why is it inappropriate to use reaction times to Why is it inappropriate to use reaction times to

fractions of 1/100 seconds?fractions of 1/100 seconds?

AAnswer Q’s

BackgroundBackground

• Using your background knowledge Using your background knowledge write a short write a short account of how and why you would expect account of how and why you would expect concentration to affect react rateconcentration to affect react rate. .

• Then use the reference materials to Then use the reference materials to make a make a prediction of how rate will change with concentration prediction of how rate will change with concentration – – for higher (Level 8) you need to make a prediction for higher (Level 8) you need to make a prediction that has numbers in itthat has numbers in it!. In your write up !. In your write up quote the quote the source of the informationsource of the information (including web addresses if (including web addresses if the internet is used).the internet is used).

DDo this

When hydrochloric acid is added to the sodium thiosulphate a reaction occurs which produces a cloudy suspension of solid sulphur.

The time it takes for the cross to disappear can be used to investigate rate of reaction.

The ReactionThe ReactionNa2S2O3 + 2HCl 2NaCl + S + SO2 +H2O

The Plan (1)The Plan (1)

• Describe what you will be investigating Describe what you will be investigating and what you will be measuring.and what you will be measuring.

• Give a list of the apparatus you plan to Give a list of the apparatus you plan to use and a method which could be use and a method which could be followed by a student who has not done followed by a student who has not done the experiment. Your method should the experiment. Your method should include quantities of chemicals.include quantities of chemicals.

Plan (2)Plan (2)

• Describe how you will make the experiment more Describe how you will make the experiment more reliable and what things you will have to look out for reliable and what things you will have to look out for that could go wrong.that could go wrong.

• Include some details on safety of the chemicals Include some details on safety of the chemicals and the lab.and the lab.

• Give background research into things that will Give background research into things that will affect reaction rate and try to make a quantitative affect reaction rate and try to make a quantitative prediction – how much faster will the experiment go prediction – how much faster will the experiment go when you change concentration?when you change concentration?

• Do a Do a trial experimenttrial experiment that can be used to help that can be used to help decide how exactly to do the reaction.decide how exactly to do the reaction.

Changing ConcentrationChanging Concentration• It is important that the total volume of the It is important that the total volume of the

reaction stays the same.reaction stays the same.• If you add more acid you need to take away If you add more acid you need to take away

the same amount of water. For example…the same amount of water. For example…

Amount of Amount of thiosulphate (cmthiosulphate (cm33))

Amount of acid (cmAmount of acid (cm33)) Amount of water (cmAmount of water (cm33))

4040 1010 00

4040 88 22

4040 66 44

4040 44 66

Analysis of results (1)Analysis of results (1)

• Describe what the basic pattern of results Describe what the basic pattern of results shows – what is the effect of the changes shows – what is the effect of the changes you make?you make?

• Describe the pattern in more detail using the Describe the pattern in more detail using the numbers you have obtained in your results.numbers you have obtained in your results.

• Produce a graph of the results with a line of Produce a graph of the results with a line of best fit. Make sure the axes are labelled best fit. Make sure the axes are labelled with the correct units.with the correct units.

Analysis of results (2)Analysis of results (2)

• Using your results calculate the reaction rate Using your results calculate the reaction rate for each concentration using…for each concentration using…

• Rate = 1000/time to react (units = 1000/s)Rate = 1000/time to react (units = 1000/s)

• Produce another graph with Rate on the y-Produce another graph with Rate on the y-axis and concentration on the x-axis.axis and concentration on the x-axis.

• Does the rate double if you double the Does the rate double if you double the concentration? If it does not then how much concentration? If it does not then how much faster does it get? How does the increase in faster does it get? How does the increase in rate relate to collision theory?rate relate to collision theory?

EvaluationEvaluation

• Did the experiment work effectively to answer the Did the experiment work effectively to answer the aim of the investigation?aim of the investigation?

• Did any of the results you obtained not fit the Did any of the results you obtained not fit the expected pattern? If they did say which ones and expected pattern? If they did say which ones and think of why this result might be off.think of why this result might be off.

• What could you do to improve the experiment or What could you do to improve the experiment or make it more accurate?make it more accurate?

• Describe in detail an experiment that you could do to Describe in detail an experiment that you could do to test more ideas about collision theory.test more ideas about collision theory.

I’ve checked results from the experiments and while some were not too bad most were pants!! So I have put together the best results so we can use them as a class

1 2 3 4 5 6 7 8 9 avg1000/avg

10 34 36 36 37 34 35 37 33 35 35.1 28

8 37 38 38 39 38 38 39 38 39 38.2 26

6 43 41 44 44 45 44 45 42 44 43.6 23

4 50 49 52 51 50 48 50 50 51 50.2 20

2 61 63 66 62 65 66 62 65 65 63.8 16

Choose 3 sets of results from sets 1-9 – do not use the same set as the person sitting next to you.Then work out values for your average and then 1000/average.Remember to put the correct units into the table for volume and time…Rewrite your own table using the results you have Making sure you have the correct units in the table….

thiosulphate acid water 1 2 3 average 1000/average

40 10 0

40 8 2

40 6 4

40 4 6

40 2 8