In order to start a reaction, molecule requires energy. This can be understood by a simple example – molecules need some kinetic energy or velocity to collide with other molecules to start a reaction. No reaction will take place, if collision don’t happen, or molecules don’t have enough kinetic energy. The energy needs to initiate the reaction is known as Activation energy.
“Activation Energy is the minimum amount of energy that is needed to start a chemical reaction.”
Activation Energy Formula
k = rate constant
A = frequency factor
Ea = activation energy
R = gas constant
T = absolute temperature
If we know the rate constant k1 and k2 at T1 and T2 the activation energy formula is
Question 1: The rate constant for the reaction
2N2O5(g) →→ 4NO2(g) + O2(g) is 5.0 ×× 10-4S-1. Frequency factor is 2.812 ×× 1013S-1. Find the activation energy of the reaction at 45oC.
T = 273 + 45oC
Therefore,replace all the values and rearrange the equation to get Ea value.
log (5.0 ×× 10-4) = log (2.812 ×× 1013S-1) + Ea2.303 ×8.314 ×318Ea2.303 ×8.314 ×318
Ea = 102000J
Ea = 102KJ
Question 2: The rate of a reaction increases when the temperature changes from 293 to 313K. Find the energy of activation of the reaction assuming that it does not change with temperature.
The T1and T2 are 293K and 313K
Rearrange the following equation to get Ea.
So that Ea = 2.303R [T1T2T2−T1T1T2T2−T1 ×× logK2K1K2K1]
T1T2T2−T1T1T2T2−T1 = 4585.45K
logK2K1K2K1 = log4141 = log4 = 0.6021
Ea = 2.303 ×× 8.314JK-1mol-1 ×× 4585.45K ×× 0.6021
Ea = 52848 Jmol-1
Ea = 52.8 KJ mol-1