1. Reaction kinetics can be investigated using the iodine clock reaction. The equations for two reactions that occur are given below.

Reaction B is much faster than reaction A, so the iodine, I2, formed in reaction A immediately reacts with thiosulfate ions, S2O32–, in reaction B, before it can react with starch to form the familiar blue-black, starch-iodine complex.

In one experiment the reaction mixture contained:

5.0 ± 0.1 cm3 of 2.00 mol dm–3 hydrogen peroxide (H2O2)

5.0 ± 0.1 cm3 of 1 % aqueous starch

20.0 ± 0.1 cm3 of 1.00 mol dm–3 sulfuric acid (H2SO4)

20.0 ± 0.1 cm3 of 0.0100 mol dm–3 sodium thiosulfate (Na2S2O3)

50.0 ± 0.1 cm3 of water with 0.0200 ± 0.0001 g of potassium iodide (KI) dissolved in it.

After 45 seconds this mixture suddenly changed from colourless to blue-black.

(a) Calculate the amount, in mol, of KI in the reaction mixture.

(b) Calculate the amount, in mol, of H2O2 in the reaction mixture.

(c) The concentration of iodide ions, I, is assumed to be constant. Outline why this is a valid assumption.

(d) For this mixture the concentration of hydrogen peroxide, H2O2, can also be assumed to be constant. Explain why this is a valid assumption.

(e) Explain why the solution suddenly changes colour.

(f) Apart from the precision uncertainties given, state one source of error that could affect this investigation and identify whether this is a random error or a systematic error.

(g) Calculate the total uncertainty, in cm3, of the volume of the reaction mixture. 

(h) The colour change occurs when 1.00 × 10–4 mol of iodine has been formed. Use the total volume of the solution and the time taken, to calculate the rate of the reaction, including appropriate units.

(i) In a second experiment, the concentration of the hydrogen peroxide was decreased to 1.00 mol dm–3 while all other concentrations and volumes remained unchanged.The colour change now occurred after 100 seconds. Explain why the reaction in this experiment is slower than in the original experiment.

(j) In a third experiment, 0.100 g of a black powder was also added while all other concentrations and volumes remained unchanged. The time taken for the solution to change colour was now 20 seconds. Outline why you think the colour change occurred more rapidly and how you could confirm your hypothesis. 

(k) Explain why increasing the temperature also decreases the time required for the colour to change.




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