S-Cool Revision Summary

S-Cool Revision Summary

Chemical reactions occur at a definite rate which is determined by the reaction conditions.

Example of calculation:

Rate = change of volume/time

Consider the reaction:

A + BC + D

The rate equation can be expressed as:

To find out how [A] or [B] affect the rate we have to perform a series of experiments in which one concentration is varied whilst the other remains constant.

Usually it is found that:

is proportional to [A]x and [B]y

Or

= k [A]x [B]y

k = Rate constant

x and y are orders with respect to A and B.

The order of a reaction with respect to a given reactant is defined as 'power of its concentration in the rate equation'.

The overall order is the sum of the powers of the concentrations of the reactants that appear in the rate equation.

To find the order of a reaction with respect to one of the reactants A,

  1. Plot [A] against time
  2. Calculate the rate at 5 or 6 different times by drawing tangents to the curve at these times and finding the gradients.
  3. Plot the rate against [A]. If this is a straight line then the reaction is first order in A. If not a straight line then, plot rate against [A]2. A straight line shows the reaction is second order in A.

Examples of these graphs are shown below:

Effect of Concentration on Rate

The value of k, the rate constant is found by taking the gradient of the graph.

The half life of a reactant is the time taken for the initial concentration to fall by half.

The effect of temperature on rate of reaction is summarised by two theories:

  1. The collision theory states that molecules must collide with sufficient energy (activation energy) if a reaction is to take place. As temperature increases more molecules gain this activation energy, hence more collisions occur per second, rate increases.
  2. The transition state theory explains the nature of an 'energy barrier' by the existence of an intermediate 'activated complex' or transition state formed during the reaction. This is a high energy species in which old bonds are partially broken and new bonds partially made.

Catalysts provide an alternative pathway that has a lower activation energy than the original one. The catalyst is involved in the reaction but is reformed at the end.