PAG 10 Initial rates 1 - Iodine Clock

What is the initial rate?

The instantaneous rate at the start of a reaction when the time t=0

How does a clock reaction work?

• We can obtain the initial rate of a reaction by taking a single measurement

• The time t from the start of an experiment can be measured for a visual change to be observed (colour or precpitate)

What is the assumption for the experiment?

There is no significant change in rate during this experiment, so the average rate of reaction will be the same as the initial rate

How is initial rate calculated?

Initial rate = 1 / t

Show the ionic equation for the reaction of hydrogen peroxide (H₂O₂) and iodide ions in acid solution

H₂O_2(aq) + 2H⁺_(aq) + 2I^-_(aq) → I_2(aq) + 2H₂O_(l)

Explain how the iodine clock experiment works

• By varying the concentration of iodide ion, you can determine the order of this reaction with respect to [I^-]

  • H₂O_2(aq) + 2H⁺_(aq) + 2I^-_(aq) → I_2(aq) + 2H₂O_(l)

• The liberated iodine reacts with thiosulfate ions

  • I₂ + 2S₂O₃^2- → 2I^- + S₄O₆^2-

• When all the thiosulfate ions have been used up, free iodine remains in solution and this is detected by the formation of a blue-black colour with starch indicator. The appearance of the blue-black colour represents the same extent of reaction in each case

  • Therefore initial rate - 1 / t

How is the order of reaction found?

By plotting a graph of rate (1/t) against concentration of iodide ions and observing the shape

Give the procedure

1. Mix set volumes of sulfuric acid (H+), starch (colour), potassium iodide (I^-), sodium thiosulfate (thiosulfate S₂O₃^2- ions) in a conical flask.

2. Add a set volume of a certain concentration of hydrogen peroxide into the conical flask and time immediately

3. Record the time taken for the blue colour of the starch-iodine complex to appear

4. Repeat for each concentration

2S₂O₃^2-_(aq) + I_2(aq) → S₄O₆^2-_(aq) + 2I^-_(aq)

1. Use the equation to explain why thiosulfate ions delay the colour

2. Why is it important to use the same volume of thiosulfate ions each time?

1. The reaction results in I^- ions which are not coloured. As soon as all the thiosulfate has reacted, iodine will start to build up in the solution to produce the colour

2. The amount of iodine removed by the thiosulfate in each experiment is constant. This means rate is proportional to 1/t.