16.2 Activation Energy

     The Arrhenius equation can be represented as k = A in its exponential form.

 State the effect on k of an increase in; 

i)     The constant, A, (frequency factor) 

ii)     Activation energy, E_a 

iii)    Temperature, T

The table below shows how temperature affects the rate of reaction.

Rate constant k/s-1	ln k	Temperature / K
2.0 x 10-5	-10.8	278	0.00360
4.7 x 10-4	-7.7	298	0.00336
1.7 x 10-3	-6.4	308	0.00325
5.2 x 10-3	-5.3	318	0.00314

 Use the results to plot a labelled graph of ln k against .

            2NO₂ (g) + O₃(g) → N₂O₅ (g) + O₂ (g) 

Experimental data shows the reaction is first order with respect to NO₂ and first order with respect to O₃. 

State the rate expression for the reaction.

Calculate a value for the rate constant k at this temperature and state its units.

For this reaction ln A = 15.8 mol⁻¹ dm³.

d)       The relationship between the rate constant and temperature is given by the Arrhenius equation, k = A 

State how temperature affects activation energy.

 b)       An Arrhenius plot of ln k against  for the reaction between A (g) and B (g) at different temperatures is shown in Figure 1 below.

            The equation of the line of best fit was found to be: 

            ln k = -6154 - 8.2 

Calculate the activation energy, E_a, for the reaction between A (g) and B (g).

A graph of ln k against  for another general reaction is shown.

             Sketch the expected line for the same reaction with an added catalyst.

H₂ (g) + I₂ (g) → 2HI (g) 

Table 1

Using Sections 1 and 2 of the Data Booklet, calculate the activation energy, in kJ mol^-1, for the reaction.