Question

A.Define collision frequency.
B.Give an example of a Pseudo first order reaction.

Answer 1

A.Number of collisions increases with increase in temperature. Collisions happen due to increase in kinetic energy. Kinetic energy is important for the motion of gaseous particles. These leads produce elastic collisions.
B.Bimolecular reaction behaves like a first order reaction that means the rate of reaction depending on only one reactant and other reactant concentration is negligible.

Complete step by step answer:
A.The gas molecules are taken into a closed container. The gas molecules are of low density and show diffusion processes. So the gas molecules show random motion. At the time movement of molecules hits one and each other and meanwhile hits the walls of the container and shows elastic collisions. Elastic collisions are defined as the collisions between two bodies’ results in no change in kinetic energy. So in between gas molecules so many collisions will occur that only effective collisions turn into products. So the number of collisions taking place in given volume in unit time is called Collision frequency. Collision frequency is denoted as Z.
B.Order of reaction can be defined as the sum of power of concentration of reactants is expressed as order of reaction Pseudo first order reaction is defined as suppose the reaction is having two reactants A and B. In the given reaction the concentration of Reactant B is very high. So change in concentration of B observed is negligible and becomes constant. Even Though it is a bimolecular reaction the order is 2nd Order, but can be approximated as a 1st order reaction with respect to A. So such types of reactions are called pseudo first order reactions.
Example for pseudo first order reaction
Hydrolysis of Cane sugar
${C_{12}}{H_{22}}{O_{11}} + {H_2}O \to {C_6}{H_{12}}{O_6} + {C_6}{H_{12}}{O_6} \\\ \\\$
Sucrose Glucose Fructose
In this reaction water is excess, so concentration of water is treated as constant
Rate of reaction can be written as $R = K[{C_{12}}{H_{22}}{O_{11}}]$
So the hydrolysis of cane sugar is pseudo first order reaction

Note:
All bimolecular reactions are not second order reactions, this means due to the presence of excess concentration of one reactant, and the change of reactant considered as negligible. Collisions take place between molecules which are travelling through space in straight lines.