Chemical Kinetics MCQs for NEET — Chemistry Questions with Answers

Refer NCERT, Heading 9.12.1, page-154,155

According to the NCERT text, 'The reason could be that all collisions do not lead to the formation of products.' For a collision to be effective, molecules must collide with sufficient kinetic energy (threshold energy) AND proper orientation. Option o3 incorrectly states that all collisions with energy greater than activation energy lead to product formation, ignoring the orientation factor.

As per the NCERT text, 'The number of collisions per second per unit volume of the reaction mixture is known as collision frequency (Z).'

The NCERT text states, 'where $Z_{AB}$ represents the collision frequency of reactants, A and B and $e^{-E_a/RT}$ represents the fraction of molecules with energies equal to or greater than $E_a$.'

The NCERT text clearly defines, 'The collisions in which molecules collide with sufficient kinetic energy (called threshold energy) and proper orientation, so as to facilitate breaking of bonds between reacting species and formation of new bonds to form products are called as effective collisions.'

According to the NCERT text, 'To account for effective collisions, another factor P, called the probability or steric factor is introduced. It takes into account the fact that in a collision, molecules must be properly oriented.'

The NCERT text states, 'According to collision theory, another factor P called steric factor which refers to the orientation of molecules which collide, is important and contributes to effective collisions, thus, modifying the Arrhenius equation to $k = P Z_{AB}e^{-E_a/RT}$'.

The NCERT text mentions, 'Collision theory also has certain drawbacks as it considers atoms/molecules to be hard spheres and ignores their structural aspect.'

The NCERT text provides an example: 'For example, formation of methanol from bromoethane depends upon the orientation of reactant molecules... The proper orientation of reactant molecules lead to bond formation whereas improper orientation makes them simply bounce back and no products are formed.' This directly illustrates the significance of proper orientation.

The NCERT text states, 'Comparing (3.23) with Arrhenius equation, we can say that A is related to collision frequency.' Also, 'A (Arrhenius factor or pre-exponential factor) corresponds to the collision frequency.'