What is Activation Energy $(E_a)$? Explain the reaction profile graph for activation energy and discuss the probability of effective collisions.

(N/A) The additional energy that must be supplied to the reactant molecules to reach the threshold energy level is known as activation energy $(E_a)$.
It is the energy required to form the intermediate, called the activated complex $(C)$.
Explanation using the reaction profile graph:
Consider the reaction: $H_{2(g)} + I_{2(g)} \rightarrow 2HI_{(g)}$.
According to collision theory, this reaction occurs when a molecule of hydrogen and a molecule of iodine collide with sufficient energy and proper orientation to form an unstable intermediate called the activated complex $(C)$.
This complex exists for a very short time and then decomposes to form two molecules of hydrogen iodide.
Activation Energy $(E_a) = (\text{Potential energy of activated complex } (C)) - (\text{Potential energy of reactants})$.
Probability of effective collisions: Not all collisions between reactant molecules lead to product formation. Only those collisions where the molecules possess kinetic energy greater than or equal to the activation energy and collide with the correct orientation are effective, leading to a reaction.