Edexcel International A Level (IAL) Chemistry (YCH11) - Unit 4 - 11.12 Core Practical: iodine-propanone & clock reaction-Study Notes - New Syllabus

11.12 Core Practicals 9a and 9b: Rate Studies using Titration and Clock Reactions

Reaction rates can be studied experimentally using methods that track changes in concentration over time. Two important core practicals involve following the iodine–propanone reaction using titration and investigating a clock reaction, where a visible change occurs after a fixed time. These methods allow the rate of reaction and its dependence on concentration to be determined.

Practical 9a: Iodine–Propanone Reaction (Titrimetric Method)

This reaction involves iodine reacting with propanone in acidic conditions. The rate is followed by measuring the decrease in iodine concentration over time using titration.

\( \mathrm{CH_3COCH_3 + I_2 \rightarrow CH_3COCH_2I + HI} \)

Method

• A known mixture of iodine, propanone and acid is prepared.
• At regular time intervals, samples are withdrawn from the reaction mixture.
• Each sample is quenched (reaction stopped) and titrated with sodium thiosulfate solution.
• The volume of thiosulfate used determines the concentration of iodine remaining.

Key Principle

• The concentration of iodine decreases as the reaction proceeds.
• By measuring iodine concentration at different times, the rate of reaction can be determined.
• A graph of concentration vs time can be used to find the rate.

Important Observations

• The brown colour of iodine fades as the reaction proceeds.
• Sodium thiosulfate reacts with iodine:

\( \mathrm{I_2 + 2S_2O_3^{2-} \rightarrow 2I^- + S_4O_6^{2-}} \)

Practical 9b: Clock Reaction (Harcourt–Esson / Iodine Clock)

A clock reaction is designed so that a visible change (usually a colour change) occurs after a measurable time interval. This allows the rate to be determined by timing how long it takes for the change to occur.

Key Idea

The time taken for a fixed amount of product to form is measured, and rate is inversely proportional to time.

Typical Reaction System

• Iodine is produced slowly in a reaction.
• Sodium thiosulfate is present and immediately reacts with iodine formed.
• When all thiosulfate is used up, iodine accumulates and reacts with starch to give a blue-black colour.

Key Reactions

\( \mathrm{I_2 + 2S_2O_3^{2-} \rightarrow 2I^- + S_4O_6^{2-}} \)

Interpretation

• The time taken for the blue-black colour to appear corresponds to the time taken to produce a fixed amount of iodine.
• Rate \( \propto \frac{1}{\text{time}} \).
• Changing concentration allows investigation of reaction order.

Key Features of These Methods

• Titration provides quantitative measurement of concentration over time.
• Clock reactions provide a simple visual method to measure rate.
• Both methods allow determination of rate and investigation of factors affecting it.