Question

Differentiate between:
(x) Order of a reaction and molecularity
(y) Average and instantaneous velocity

Hint:

Order is related to the rate law and can be fractional, while molecularity is a whole number and applies only to elementary reactions. Instantaneous velocity requires calculus to determine the rate at a specific point in time.

Answer 1

(x) Order of a reaction and molecularity:
- Order of a reaction refers to the sum of the exponents of the concentration terms in the rate law. It indicates how the rate of reaction is affected by the concentration of the reactants. For example, in a reaction with the rate law \( \text{Rate} = k[A]^m[B]^n \), the order of the reaction is \( m + n \), where \(m\) and \(n\) are the orders with respect to reactants A and B, respectively.
- Molecularity refers to the number of reacting species (atoms, molecules, or ions) that collide in an elementary step of the reaction. Molecularity is an integer and can only be 1 (unimolecular), 2 (bimolecular), or 3 (termolecular) for most elementary reactions. Unlike order, molecularity is determined from the balanced equation of the elementary reaction, not from the experimental data.
Key Difference: The order of a reaction is determined experimentally, while molecularity is determined from the balanced equation of an elementary reaction.
(y) Average and instantaneous velocity:
- Average velocity is defined as the total displacement (change in position) divided by the total time taken. In terms of reaction rate, it can be defined as the total change in concentration of reactants or products divided by the total time interval.
- Instantaneous velocity is the velocity of an object at a specific instant in time. In terms of reaction rate, it refers to the rate of change of concentration at a particular moment, and it can be calculated by taking the derivative of the concentration with respect to time.
Key Difference: Average velocity is the total change over a time interval, while instantaneous velocity refers to the rate of change at a specific point in time.