Advanced Physical Chemistry Core Practicals

Imagine you want to know how a reaction's speed depends on the concentration of one particular reactant — iodine. The problem is, you can't just "watch" a concentration change directly. What you can do is repeatedly grab small samples of the reaction mixture at known times, instantly stop ("quench") each sample from reacting further, and then titrate it to work out exactly how much iodine was left at that moment. Plot those values against time, and the shape of the graph tells you the order of reaction.

The reaction studied here is the acid-catalysed iodination of propanone:

Both propanone and the H⁺ catalyst are used in large excess. This is the clever trick of the whole experiment: because there's so much of them compared to the iodine, their concentrations barely change over the course of the reaction, even though iodine's concentration is dropping steadily. That means any change in rate we observe can only be blamed on the changing iodine concentration — every other variable is effectively held constant. This is sometimes called the "isolation method."

1. Mix 25 cm³ of 1.0 mol dm⁻³ propanone with 25 cm³ of 1.0 mol dm⁻³ sulfuric acid in a beaker.
2. Add 50 cm³ of 0.02 mol dm⁻³ iodine solution — start the timer at this exact moment.
3. At regular intervals, pipette out a 10 cm³ portion of the mixture into a conical flask.
4. Quench the sample immediately by adding a spatula of sodium hydrogencarbonate. This neutralises the sulfuric acid catalyst, so the reaction in that particular sample stops dead — it's now a frozen snapshot of the reaction mixture at that exact time.
5. Titrate the quenched sample against 0.01 mol dm⁻³ sodium thiosulfate(VI) solution, using starch as the indicator, to find out how much iodine remains.
6. Record time and titre in a table, and repeat for several time points.

titre vs time