(a) Explain the following terms :
(i) Order of a reaction
(ii) Molecularity of a reaction
(b) The rate of a reaction increases four times when the temperature changes from 300 K to 320 K. Calculate the energy of activation of the reaction, assuming that it does not change with temperature. `(R = 8.314 J K^(-1) mol^(-1))`

Explanation

(a)(i) Order of a reaction : The sum of powers of the concentration of the reactant in the rate law expression is called the order of that chemical reaction.
(ii) Molecularity of a reaction : The number of reacting species (atoms, ions or molecules) taking part in an elementary reaction which must colliode simultaneously in order to bring about a chemical reaction is called molecularity of a reaction
(iii) Here ` k_(2) = 4 k_(1), T_(1) = 300k, T_(2) = 320 k, E_(a)` = ?
`:. log. k_(2)/k_(1) = (Ea)/(2.303 R) [(T_(2)-T_(1))/(T_(1) T_(2))]`
` log.(4k_(1))/k_(1) = (Ea)/(2.303 R) [(320-300)/(300 xx 320)]`
` log 4 = (Ea)/(2.303 R) = [20/96000]`
` 2 log 2 = (Ea)/(2.303 R) [1/4800]`
` 2 xx 0.3010 = (Ea)/(2.303 xx 8.314) xx 1/4800`
`Ea = 0.6020 xx 2.303 xx 8.314 xx 4800 = 55327.581" J "mol^(-1)`