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2206-008 Chemistry HL

Candidate Name Candidate Number Date of Practical

: Eileen Cham Yee Lin : 2206-008 : 4th May 2009

Practical Assessment 22 – Iodine clock reaction

Research Question How does the concentration of a reactant (manipulated by using different concentrations of potassium iodide solution) affect its rate of reaction with another reactant (hydrogen peroxide solution, of a fixed concentration and volume), which is calculated as the inverse of the time measured for the triiodide ion formation from the reaction to completely obscure the “X” mark at the bottom of a flask?

Introduction There are a few factors that affect the rate of reaction: temperature, reactant concentration, reactant surface area, pressure, the presence of catalyst and the physical states of reactants. This experiment will investigate the effect of reactant concentration on the rate of reaction in the reaction between potassium iodide and hydrogen peroxide. The hydrogen peroxide concentration is fixed while the concentration of potassium iodide is manipulated; the formation of the product – triiodide ions can be detected with the blue-black starch complex. Because both of these reactants react quickly with each other, sodium thiosulphate solution is used as a delaying mechanism to delay the formation triiodide ions by reacting with the triiodide ions to reform more iodide ions. When the sodium thiosulphate is exhausted, remaining triiodide ions form a blue-black starch complex with the starch present in the solution. The chemical equations involved in this reaction are: H2O2 (aq) + 3 I- (aq) + 2 H+ (aq) → I3- (aq) + 2 H2O (l)

I3- (aq) + 2 S2O32- (aq) → 3 I- (aq) + S4O62- (aq)

A stopwatch is used to measure the time taken for the triiodide ions to completely obscure a mark “X” at the bottom of the flask; the times recorded are then inversed and the rate of reactions for individual runs can be found. This experiment is also known as the Iodine Clock Experiment.

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2206-008 Chemistry HL

Hypothesis The collision theory states effective collisions between reactant molecules must occur in order for the reaction to occur. The more reactant particles present, the more frequently they collide effectively, and the more often the reaction occurs to form products. Therefore, the increase in reactant concentration will increase the rate of reaction. Rate of reaction  frequency of effective collision  concentration of reactants

In this experiment, the concentration of one of the reactant, which is potassium iodide, is increased; this means that there will be more potassium iodide available to effectively collide with hydrogen peroxide as the concentration increases. As such, the hypothesis for this experiment is that the rate of reaction will increase with the concentration of potassium iodide.

Rate of reaction

The expected graph from this experiment is as follows:

Concentration of potassium iodide Figure 1: Expected graph of rate of reaction against concentration of potassium iodide

Variables Dependent variable

: Rate of reaction –

Independent variable

This can be determined by measuring the time taken for a mark “X” to be completely obscured by the blue-black of triiodide – starch complex. The inverse of the time taken is the rate of reaction.

: Concentration of potassium iodide –

0.1 mol dm-3 potassium iodide solution is prepared. The solution is then diluted to form potassium iodide solutions of different concentrations using dilution technique. 2

2206-008 Chemistry HL

Controlled variables

: 1. Amount of hydrogen peroxide solution –

To ensure that the same amount of hydrogen peroxide is used in the reaction, the same volume of the same concentration of hydrogen peroxide solution is reacted with potassium iodide solution.

However, hydrogen peroxide solution has to be in excess; therefore, a solution with high concentration (6 %) is used.

2. Amount of starch –

Starch is the indicator of the presence of triiodide, the final product of the iodine clock reaction.

The same volume of homogenous starch should be used in the reaction as to ensure that the formation of black-blue colour is standard for all reactions.

3. pH or H+ concentration –

Low pH or high H+ concentration causes the reactants to react more quickly.

The same volume of the same concentration of dilute hydrochloric acid should be used to acidify the hydrogen peroxide solution.

4. Temperature –

Reactants react more quickly at high temperatures.

All iodine clock runs are carried out at room temperature.

Apparatus and materials          

6 % hydrogen peroxide, H2O2 solution 0.025 mol dm-3 potassium iodide, KI powder Sodium thiosulphate, NaS2O3 solution Starch solution Dilute hydrochloric acid, HCl solution Digital stopwatch (10.00 ± 0.02) cm3 pipette 250 cm3 conical flask (100.0 ± 0.1) cm3 volumetric flask. Pipette filler 3

2206-008 Chemistry HL

      

Dropper White tile Beakers Micropipette Wash bottle Measuring cylinder Electronic balance, correct to 0.001 g

Procedure 1. 100 cm3 of 0.1 mol dm-3 potassium iodide solution is prepared by dissolving 1.66 g of potassium iodide powder into 100 cm3 of distilled water in a volumetric flask. 2. 30 cm3 stocks of potassium iodide solutions of different concentration are prepared using the dilution technique in measuring cylinder. A micropipette is used when necessary. Volume of 0.1 mol dm-3 potassium iodide solution measured / cm3 (± 0.4 cm3)

Volume of distilled water added / cm3 (± 0.4 cm3)

Concentration of potassium iodide solution, Mpotassium iodide / cm3

30

0

0.10

15.75

14.25

0.075

10.5

19.5

0.050

5.25

14.75

0.025

2.1

27.9

0.010

Table 1: Volumes required for the preparation of potassium iodide solutions of different concentrations

3. 10 ml of 6 % hydrogen peroxide, H2O2 solution is pipette into one conical flask. 6 drops of dilute hydrochloric acid is added to acidify it. The conical flask is labeled “A”. 4. In another conical flask, 7 ml of 0.10 mol dm-3 of potassium iodide solution is measured with a measuring cylinder, and poured into it. 10 ml of starch, and 1 ml of 0.025 mol dm-3 of sodium thiosulphate solution are also poured into the conical flask. The conical flask is labeled “B” and placed on a white tile with a mark “x”. 5. The contents of conical flask “A” are emptied into conical flask “B”, and the stopwatch is immediately started. The “x” is viewed through the top of the conical flask “B”. The stopwatch is immediately stopped when the “x” is obscured by the dark-blue formation in the solution in conical flask. The time taken is recorded. 6. Steps 3 – 5 are repeated 2 times to obtain 2 more replicates. 7. Steps 3 – 6 are repeated with different concentrations: 0.075 mol dm-3, 0.05 mol dm-3, 0.0025 mol dm-3 and 0.001 mol dm-3. 4

2206-008 Chemistry HL

Data collection – quantitative data

Concentration of potassium iodide solution, Mpotassium iodide / cm3

Time taken for “xâ€? to be obscured, t’ /s (Âą 0.01) Run 1

Run 2

Run 3

0.10

5.28

4.75

4.47

0.075

6.25

6.62

8.75

0.050

17.75

15.13

14.94

0.025

36.00

28.12

24.75

0.010

45.63

42.81

52.00

Table 2: Collected data

Data collection – qualitative data The colourless solution in conical flask “B� turns dark blue at the end of each run.

Data processing The average time taken for each potassium iodide solution concentration is calculated: Concentration of potassium iodide solution, Mpotassium iodide / cm3

Average time taken, t /s (Âą 0.01)

1

Rate of reaction, đ?‘Ą / s-1

0.10

5.28+4.75+4.47 3

= 4.83

1 4.83

= 0.207

0.075

5.28+4.75+4.47 3

= 7.21

1 7.21

= 0.139

= 15.94

1 15.94

= 0.063

0.050

6.25+6.62+8.75 3

0.025

36.00+28.12+24.75 3

= 29.61

1 29.61

= 0.034

0.010

45.63+42.81+52.00 3

= 46.81

1 46.81

= 0.021

Table 3: Calculation of the average time taken and rate of reaction

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2206-008 Chemistry HL

Data presentation

Concentration of potassium iodide solution, Mpotassium iodide / mol dm-3

Average time taken, t' / s against concentration of potassium iodide solution, Mpotassium iodide / mol dm-3 0.120

0.100

4.83, 0.1

0.080 7.21, 0.075

0.060 15.94, 0.05 0.040

29.61, 0.025 0.020 46.81, 0.01 0.000 0.00

5.00

10.00

15.00

20.00

25.00

30.00

35.00

40.00

45.00

50.00

Average time taken, t' / s

Figure 2: Average time taken, t' / s against concentration of potassium iodide solution, Mpotassium iodide / mol dm-3

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2206-008 Chemistry HL

concentration of potassium iodide solution, Mpotassium iodide / mol dm-3

Rate of reaction, 1/t / s-1 against concentration of potassium iodide solution, Mpotassium iodide / mol dm-3 0.12

0.10

0.207, 0.1

0.08 0.139, 0.075

0.06 0.063, 0.05 0.04

0.034, 0.025 0.02 0.021, 0.01 0.00

0.000

0.050

0.100

0.150

0.200

0.250

Rate of reaction, 1/t / s-1 Figure 3: Rate of reaction, 1/t / s-1 against concentration of potassium iodide solution, Mpotassium iodide / mol dm-3

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2206-008 Chemistry HL

Uncertainties Uncertainty in time taken due to stopwatch = 0.01 s Uncertainty in time taken due to human reaction time = 0.09 s Total absolute uncertainty in time taken = 0.1 s

Percentage uncertainty in 0.10 mol dm-3 potassium iodide solution prepared in Step 1 of the Procedures = percentage uncertainty due to electronic balance + percentage uncertainty due to volumetric flask =(

0.001 1.66

0.1

× 100% ) + ( 100 × 100%)

= 0.16 %

Uncertainty in the different concentrations of potassium iodide solution, Mpotassium iodide: Concentration of potassium iodide solution,

Mpotassium iodide

Percentage uncertainty in volume of potassium iodide solution measured / %

Percentage uncertainty in volume of distilled water measured / %

/ mol dm-3

0.10

0.4 30

0.075

0.4 15.75

× 100 % = 2.540

0.4 14.25

0.050

0.4 10.5

× 100 % = 3.810

0.4 19.5

0.025

0.4 5.25

× 100 % = 7.619

0.4 14.75

0.010

0.4 2.1

× 100 % = 1.333

× 100 % = 19.048

0.4 0

0.4 27.9

× 100 % = 0.000

Percentage uncertainty in 0.1 mol dm-3 potassium iodide solution / %

0.16

× 100 % = 2.807

0.16

× 100 % = 2.05

0.16

× 100 % = 2.712

0.16

× 100 % = 14.434

0.16

Table 4: Calculation of the percentage uncertainties in the preparation of solutions of different concentrations

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2206-008 Chemistry HL

Concentration of potassium iodide solution,

Total percentage uncertainty / %

Mpotassium iodide / mol dm-3

0.10

1.333 + 0.16 = 1.493

0.075

2.540 + 2.807 + 0.16 = 5.507

0.050

3.810 + 2.05 + 0.16 = 6.02

0.025

7.619 + 2.712 + 0.16 = 10.491

0.010

19.648 + 14.434 + 0.16 = 33.684

Table 5: Calculation of the total percentage uncertainties in the preparation of solutions of different concentrations

Concentration of potassium iodide solution, Mpotassium

Total percentage uncertainty /%

Absolute uncertainty / mol dm-3

0.10

1.493

0.10 × 1.493 % = 0.001

0.075

5.507

0.075 × 5.507 % =0.004

0.050

6.02

0.050 × 6.02 % =0.003

0.025

10.491

0.025 × 10.491 % =0.003

0.010

33.684

0.010 × 33.684 % =0.003

iodide

/ mol dm-3

Table 6: Calculation of the absolute uncertainties in the preparation of solutions of different concentrations

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2206-008 Chemistry HL 1

Uncertainty in the different rates of reaction, đ?‘Ą : Concentration of potassium iodide solution, Mpotassium

Percentage uncertainty time taken / %

Absolute uncertainty in rate of reaction / mol dm-3

iodide

/ mol dm-3

0.10

0.1 4.83

Ă— 100 % = 2.070

0.207 Ă— 2.07 % = 0.004

0.075

0.1 7.21

0.050

0.1 15.94

Ă— 100 % = 0.627

0.063 Ă— 0.627 % = 0.0004

0.025

0.1 29.61

Ă— 100 % = 0.338

0.034 Ă— 0.338 % = 0.0001

0.010

0.1 46.81

Ă— 100 % = 0.214

0.021 Ă— 0.214 % = 0.00004

Ă— 100 % = 1.387

0.139 Ă— 1.387 % = 0.002

Table 7: Calculation of the percentage and absolute uncertainties in rate of reactions for different potassium iodide solution concentrations

Conclusion The rate of reaction increases as the concentration of potassium iodide increases; the graph of the relationship between the rate of reaction and the concentration of potassium iodide shows a positive linear regression curve. This result is in agreement with the hypothesis that the rate of reaction will increase with the increase in concentration of potassium iodide solution.

Evaluation Two reactions occur when contents in conical flask “A� is poured into conical flask “B�: 1. Between hydrogen peroxide solution and the iodide ions from the potassium iodide solution to form triiodide ions: H2O2 (aq) + 3 I- (aq) + 2 H+ (aq) → I3- (aq) + 2 H2O (l) 2. Between triiodide ions and thiosulphate ions to re-form iodide ions. I3- (aq) + 2 S2O32- (aq) → 3 I- (aq) + S4O62- (aq) Triiodide ions and starch molecules combine to form triiodide-starch complexes which give the solution its dark blue colour.

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2206-008 Chemistry HL

When the thiosulphate ions in the 2nd reaction is fully consumed, and thus hinder the reformation of iodide ions, the triiodide ions will form complexes with starch and present the black-blue colour in the solution. The stopwatch is stopped when the dark blue triiodide-starch complexes completely obscures the “X” mark at the bottom of the flask. If the time taken for the triiodide-starch complexes to obscure the “X” is to be prolonged, the concentration of the sodium thiosulphate solution should be increased; and vice versa. This is because as the more the amount of sodium thiosulphate solution present, the later the sodium thiosulphate solution will be used up and thus delaying the formation of blue black triiodide-starch complexes. The experiment can be carried out with potassium iodide concentrations of more than 0.200 mol dm-3 to extend the linear regression curve in Figure 3. However the rate of reaction will stop increasing with the increase of potassium iodide concentration at a point, where the hydrogen peroxide concentration becomes the limiting factor. The rate of reaction remains constant; unless the concentration of hydrogen peroxide solution is increased. If the conical flasks are left on their own for some time, the dark-blue colour becomes more intense as more triiodide – starch complexes are formed. The colour will stop darkening when the hydrogen peroxide solution or potassium iodide solution is completely used up; the reactant that is exhausted is the limiting factor of the reaction. There is a “human reaction time” that is included as part of the uncertainty due to stopwatch. This is because there may be a delay between the first detection of an obscured “X” with when the thumb presses the stop button.

Ways to improve experiment The starch solution added into the beaker must be clear and does not cloud up the solution and decrease the visibility of the “X” mark before the potassium iodide solution is added. The preparation process of starch solution used in this experiment included boiling the starch solution to ensure that it became homogenous; hence the starch was warm when added into the conical flask with hydrogen peroxide solution. The starch should be cooled down to room temperature first, as high temperature causes rate of reactions to be faster.

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Rate of reaction for iodine clock reaction