Q.The number of collisions per second per unit volume of the reaction mixture is known as \rule{2cm}{0.4pt}.

Proposed by Trautz (1916) and Lewis (1918): reactions occur because molecules collide, so $\text{Collision rate}=Z[A][B]$ for reactants A and B, with Z from the kinetic theory of gases. At room temperature molecules collide roughly $10^9$ times per second, yet most reactions take far longer than a nanosecond to complete -- so most collisions must FAIL to react. The missing requirement is a minimum energy, the activation energy $E_a$: only the fraction $f=e^{-E_a/RT}$ of collisions carry enough energy to react (for $E_a=100\ \text{kJ mol}^{-1}$ at 300 K, $f\approx4\times10^{-18}$ -- just 4 in every $10^{18}$ collisions). …