Core practical 13b - Use a clock reaction to determine a rate equation

Identify the main sources of uncertainty in the procedure used and the measurements recorded in this experiment. Calculate the percentage uncertainty for any measurements taken.

One procedural error is misjudging the appearance of the blue colour in the solution. Another arises from the addition of the starch, which increases the total volume of the mixture slightly. Measurement uncertainties can occur in measuring volumes of solutions.

For a 2cm3 volume, the uncertainty is: ± 0.1cm3

% uncertainty = 0.2/2 × 100 = 10%

For a 10cm3 volume, the uncertainty is ± 0.1cm3

% uncertainty = 0.2/10 × 100 = 2%

Suggest ways of minimising these uncertainties.

The procedural errors are difficult to overcome; one change is to have two students timing simultaneously and using the average value. Measurement uncertainties can be minimised by using a graduated pipette or a burette.

What is the overall rate equation for this reaction? The equation for the reaction is: S₂O₈²⁻ + 2I⁻ → 2SO₄²⁻ + I₂

Rate = k[S₂O₈²⁻][I⁻]

A suggested mechanism for the reaction is:

step 1

I⁻ + S₂O₈²⁻ → (S₂O₈I)³⁻

step 2

(S₂O₈I)³⁻ + I⁻ → 2SO₄²⁻ + I₂

Which of these steps is the rate-determining step? Use the rate equation to justify your answer.

Step 1 is the rate-determining step. The rate of the reaction is second order and involves one peroxodisulfate ion and one iodide ion.