A2-Level Rates of Reaction

  • Changing the concentration of reactants in a reaction can affect the rate of the reaction.

  • The orders of a reaction describe how much the rate of a reaction changes when the concentrations of reactants are changed.

  • The order of a reaction with respect to a reactant describes how much the rate of a reaction change when the concentration of that reactant is changed by a given factor

    • Zero order means changing the concentration of the reactant has no change on the rate of reaction.

    • First order means changing the concentration of the reactant by a given factor will change the rate of the reaction by the same factor.

    • Second order means changing the concentration of the reactant by a given factor will change the rate of the reaction by the same factor squared.

QUICK NOTES

Orders of Reaction

 

The rate of a reaction is a measure of how fast a reaction is occurring. Changing the concentrations of reactants in a reaction can alter how fast the reaction happens.

 

Boltzmann-Distribution curves from AS-level chemistry show us that when you increase the concentration of reactants there are more reacting particles with the required activation energy to react, meaning there is a greater frequency of successful collisions. This should lead to a faster rate of reaction. Common sense would tell you that by doubling the concentration of reactants, you would double the rate of reaction. Experimental results show that this does not always happen.

 

Changing the concentration of reactants can, and often does, change the rate of a reaction but not always by the factor we have changed the concentration by.

 

How much the rate of a reaction is determined by the concentrations of reactants is described by the order of that reaction.

 

Orders of a reaction just link the concentrations of reactants to the rate of a reaction. The order of a reaction ‘with respect to…’ just means how changing the concentration of a particular reactant affects the rate of the reaction. There are three types of orders you need to know for A-level chemistry – zero order, first order and second order.

 

For example, in the following reaction:

 

 

Let’s say we are measuring the rate of this reaction and we want to see how changing the concentrations of each reactant affect the overall rate.

 

If we double the concentration of A and the rate does not change, then the order of the reaction with respect to A is zero order.

 

It does not matter what we change the concentration of A to, the rate will always be the same.

 

 

If we double the concentration of B and the rate also doubles, then the order of the reaction with respect to B is first order. Whatever factor we change the concentration of B by, the rate will change by the same factor (i.e. doubling concentration of B results in a doubling of the rate, halving the concentration of B results in a halving of the rate, and so on).

 

 

If we double the concentration of C, and the rate quadruples, then the order of the reaction with respect to C is second order. Here, the rate of a reaction is changed by a factor that is the change in concentration squared. If we double the concentration of C, then the rate would change by 22 = 4. If we halved the concentration of C, then the rate would change by 0.52 = 0.25 (the rate would be quartered).