Assertion (A) The enthalpy of reaction remains constant in the presence of a catalyst.
Reason (R) A catalyst participating in the reaction forms different activated complex and lowers down the activation energy but the difference in energy of reactant and product remains the same.
Assertion (A) All collision of reactant molecules lead to product formation.
Reason (R) Only those collisions in which molecules have correct orientation and sufficient kinetic energy lead to compound formation.
Assertion (A) Rate constant determined from Arrhenius equation are fairly accurate for simple as well as complex molecules.
Reason (R) Reactant molecules undergo chemical change irrespective of their orientation during collision.
All energetically effective collisions do not result in a chemical change. Explain with the help of an example.
Only effective collision lead to the formation of products. It means that collisions in which molecules collide with sufficient kinetic energy (called threshold energy = activation energy + energy possessed by reacting species).
And proper orientation lead to a chemical change because it facilitates the breaking of old bonds between (reactant) molecules and formation of the new ones i.e., in products. e.g., formation of methanol from bromomethane depends upon the orientation of the reactant molecules.
$$\mathrm{CH}_3 \mathrm{Br}+\mathrm{OH}^{-} \longrightarrow \mathrm{CH}_3 \mathrm{OH}+\mathrm{Br}^{-}$$
The proper orientation of reactant molecules leads to bond formation whereas improper orientation makes them simply back and no products are formed. To account for effective collisions, another factor $P$ (probability or steric factor) is introduced $K=P z_{A B} e^{-E a / R T}$.
What happens to most probable kinetic energy and the energy of activation with increase in temperature?
Kinetic energy is directly proportional to the absolute temperature and the number of molecules possessing higher energies increases with increase in temperature, i.e., most probable kinetic energy increases with increase in temperature.
Energy of activation is related to temperature by the following Arrhenius equation
$$k=A e^{-E_a / R T}$$
Thus, it also shows an increase with rise in temperature.
