In this chapter of AS/A-Level Chemistry, we will learn about rate of reaction and activation energy.

Activation Energy

We can calculate the activation energy using the Arrhenius equation:

*k = Ae ^{_Ea/RT}*

Where;

k = rate constant E_{A} = activation energy (J)

T = temperature (K) R = gas constant (8.31 JK^{-1}mol^{-1})

A = another constant

Some relationships to note:

- As E
_{A}increases, k will get smaller. Therefore, large activation energy, means a slow rate – this makes sense! - As T increases, k increases. Therefore at high temperatures, rate will be quicker – this makes sense too!

If we “ln” both sides of Arrhenius’ equation, we get;

ln k = – E_{A}/RT + ln A

This looks a bit like:

y = mx + c

If we plot ln k (y) against 1/T (x), the gradient we produce will be –E_{A}/R (m). Then R is just a number that we know (8.31 JK^{-1}mol^{-1}) we can rearrange and find the activation energy in AS/A-Level Chemistry.

__Iodine clock reaction__

e can use iodine clock reaction as an example to demonstrate the calculation under the AS/A-Level Chemistry.

S_{2}O_{8}^{2-} (aq) + 2I^{-} (aq) —> 2SO_{4}^{2-}_{ (aq) }+ I_{2} (aq)

Rate of reaction is inversely proportional to the time taken for the solution to change colour

i.e. increased rate = decreased time taken

k α 1/t

We can say that 1/t is the same as k (rate constant) and we can substitute 1/t instead of k in Arrhenius’ equation and find the gradient again to find a value for E_{A}

Congratulations! Now you understand the calculation of activation energy in AS/A-Level Chemistry.