Activation energy = (Threshold energy) - (Average energy of the reactants)= ET - ER -------------(18)
The concept of activation can be easily visualized from the below figure.
fig 6.11 - Illustration of energy barrierand activation energy involved in a reaction
Each reaction has a characteristic value of energy of activation. At a given temperature, a reaction with low activation energy will proceed faster than a reaction with high energy of activation. This can be easily determined from the Arrhenius equation. The exponential value is larger for a low Ea as compared to the value of the exponential with high Ea. In other words, if low Ea is Ea1 and high Ea is Ea2, then k1greater than k2 where the indices 1 and 2 represent reaction 1 and reaction 2 respectively.
Activation energy is low for fast reactions.
Activation energy is high for slow reactions.
Example 11:The rate constants of reaction at 700 K and 760 K are 0.011M-1s-1 and 0.105 M-1s-1 respectively. What are the values of 'A' and 'Ea'?
Arrhenius equation is ln k= ln A-Ea / RTT1 = 700K, T2 = 760K, k1 = 0.011M-1s-1, k2 = 0.105 M-1s-1
Substitution of these values in the equation gives
= 166.311 KJ / mol
ln A = 24.06A = 2.8 x 1010 M-1 s-1
Example12:What is the activation energy of a reaction whose rate quadruples when the temperature is raised from 293 K to 313 K.
or, Ea = 52.85 KJ / molExample 13
For a reaction with activation energy of 55 KJ/mol, by what factor will the rate constant go up with a rise in temperature from 300 K to 310 K.
where k2 is the rate constant at 310k and k1 is the rate constant at 300k.
This example shows that for a 10o rise in temperature near the room temperature results in doubling of the rate of the reaction.
In the manufacture of ammonia, iron is used as a catalyst and it enhances the rate of this reaction.
Catalysts are substances, which alter the rate of chemical reactions without undergoing any overall chemical change themselves.
as a catalyst.
Sometimes catalysts are used to retard or slow down the rates of reaction. For example., glycerol is used to slow down the decomposition of hydrogen peroxide. Such catalysts are known as negative catalysts.Many industrial processes like the manufacture of polythene and polystyrene require catalysts.
In the living bodies, a large number of complex chemical reactions occur which are catalysed by complex organic molecules called enzymes. For example., enzyme amylase present in saliva catalyses the decomposition of starch into maltose.