A2-Level Rates of Reaction
Measuring the initial rate of a reaction is difficult because the concentrations of the reactants are constantly changing, meaning the rate of reaction is also changing.
Clock reactions are used to find the initial rate of a reaction by measuring the length of time taken to form a small amount of product.
The iodine clock experiment is a common example of a clock reaction at A-level chemistry.
In the main reaction, hydrogen peroxide reacts with iodide ions to produce iodine molecules.
In a second reaction, thiosulfate ions react with the iodine produced in the main reaction to form iodide ions again.
Starch indicator is added to the reaction mixture, which turns dark blue in the presence of iodine. As long as there are thiosulfate ions in the mixture the starch will not cause a color change, as the thiosulfate ions are instantly converting any iodine molecules formed into iodide ions.
When all the thiosulfate ions are used up the starch will cause a color change, as iodine molecules will no longer be reacted back to iodide ions by the thiosulfate ions and they can react with the starch.
In the clock reaction, the amount of thiosulfate ions in the mixture is known and the length of time taken for the mixture to change color is recorded. This tells you how long it took to form the amount of iodine required to completely react with all the thiosulfate ions present.
The time taken can be used to find the rate of reaction.
Clock Reactions (Iodine Clock Reaction)
The initial rate of a reaction refers to how fast a reaction is happening at the start of the reaction. The rate of a reaction changes as the reaction happens, due to changes in the concentrations of the reactants.
To find the initial rate of a reaction, the change in concentration of a reactant (or product) needs to be measured against time. Finding changes in concentration during the initial stages of a reaction can be very difficult because a concentration is changing rapidly as the reaction proceeds.
One effective method is to time how long it takes to produce a small amount of product (compared to the starting concentrations of reactants). In order to determine when this amount of product has been made, we need a visual indication of some kind – usually an indicator (although sometimes precipitate formation can be used). This process is called a clock reaction. Clock reactions can be confusing to A-level students, but their idea is actually very simple.
A common example of a clock reaction at A-level chemistry is the iodine clock reaction.
The basic reaction involves hydrogen peroxide and potassium iodide (in the presence of an acid catalyst).
The hydrogen peroxide oxides the iodide ions to iodine and iodine (I ) is formed.
Using a simple starch indicator enables the iodine formed to be observed (starch turns dark blue in the presence of iodine). We want to time how long it takes to form a specific amount of iodine.
By adding a second reaction, this can be easily achieved.
Sodium thiosulfate is added. Thiosulfate ions react with iodine to form iodide ions.
This means that every time a molecule of iodine is made in the first reaction, it is instantly converted back into iodide ions by the thiosulfate ions. As long as the thiosulfate ions are reacting with the iodine formed, there will be no colour change to the mixture (as there is no iodine to turn the starch indicator dark blue).
As soon as the thiosulfate in the mixture is used up, however, the iodine formed by the hydrogen peroxide and potassium iodide stays as iodine. This means the solution turns dark blue.
The time it takes the solution to turn blue is controlled by the amount of thiosulfate ions there are at the start. The higher the concentration of thiosulfate ions, the longer it will take for the solution to change colour.
If, for example, 1 mole of thiosulfate ions is placed in the mixture and it takes 30 seconds for the solution to change colour, this means it has taken 30 seconds to produce 0.5 moles of iodine (the ratio of thiosulfate ions to iodine is 2:1).
This means we now have a ‘rate of reaction’, as we know how long it took to produce a set amount (0.5 moles) of iodine.
By changing the concentration of hydrogen peroxide or potassium iodide, but keeping the amount of thiosulfate ions the same, we can see how the time it takes to produce 0.5 moles of iodine changes as we change the concentrations of reactants. This is now just the same as determining a rate equation from reaction data.