Unit of second order reaction is L mol–1 s–1
It the time required for the concentration of a reactant to decrease to half of its initial value.
Activation energy of a reaction is defined as the extra energy over and above the average potential energy of the reactants, which must be supplied to the reactants to enable them to cross-over the energy barrier between the reactants and products. It is denoted by EA.
First half reaction gets completed within 2 hours and next half reaction gets completed within next 2 hours. That means, t1/2 is indepedent of concentration. Hence, the order of reaction is first order reaction.