Measuring initial rates of reaction

In some reactions, it is not easy to measure the rate of reaction directly, and easier to mention the time taken for a particular stage in the reaction to be reached.

Since rate is the change in concentration per unit time, it follows that the rate is inversely proportional to time taken. A graph of 1/t against initial concentration will give curves like those shown above.

Examples of such measurements could be:

- time taken for fixed amount of gas to be produced

- time taken for absorbance to change by a certain amount

- use of a clock reaction: the appearance of a certain coloured product is delayed by adding a fixed amount of another species.

Eg S₂O₈^2-(aq) + 2I^-(aq) à 2SO₄^2-(aq) + I₂(aq)

Iodine is produced in this reaction. If starch was added to the original mixture, a blue-black colour would appear immediately. However if a fixed amount (ie 0.02 moles) of sodium thiosulphate is also added to the mixture, it reacts with the iodine and a blue-black colour is only seen when all the thiosulphate has been used up.

It is possible to measure the time taken for the blue-black colour to appear.

EXPLAINING ORDERS OF REACTION

The orders of reaction for a chemical equation are not always the same as the reaction coefficients:

Eg the reaction NO₂ + H₂ à NO + H₂O has the following rate equation:

rate = k[NO₂]²

It is therefore not possible to predict the rate equation of a reaction simply by looking at the reaction coefficients.

Many reactions consist of a series of different steps, some of which are slow and some of which are very fast.

It is the slowest step in a chemical reaction which determines how fast a reaction is. For this reason the slowest step in a chemical reaction is called the rate-determining step. Changing the rate of this step will affect the overall rate of reaction; changing the rate of fast steps won’t.

Eg consider the reaction NO₂ + H₂ à NO + H₂O

This reaction happens in two steps:

Step 1: NO₂ + NO₂ à NO₃ + NO this step is slow

Step 2: NO₃ + H₂ à NO₂ + H₂O this step is fast

Step 1 is the slowest step and is therefore the rate-determining step. This involves two molecules of NO₂, and so doubling the concentration of NO₂ will make collisions in this step four times more likely. So the reaction is second order with respect to NO₂. H₂ is not involved in this step; it is only involved in the second, fast step. Changing the concentration of H₂ therefore has no effect on the rate of reaction, and the reaction is zero order with respect to H₂.

The rate equation of a chemical reaction is determined by the number of each species involved in the rate-determining step of that reaction.