Chemistry class 12th Rate_Expression_and_Rate_Constant

Rate Expression and Rate Constant


The rate of a chemical reaction describes how the concentration of reactants or products changes
with time.
The rate expression (rate law) and the rate constant (k) are fundamental concepts in chemical
kinetics.


1. Rate Expression (Rate Law)
The rate expression (or rate law) gives the mathematical relationship between the rate of a reaction
and the concentration of reactants. It is typically written as:


Rate = k [A]^m [B]^n


where:
- k = rate constant
- [A], [B] = molar concentrations of reactants
- m, n = reaction orders (determined experimentally)


The overall reaction order is the sum of m and n.


2. Rate Constant (k)
The rate constant (k) is a proportionality factor in the rate law. Its value depends on:
- Temperature (determined using Arrhenius equation: k = A e^(-Ea/RT))
- Presence of catalysts
- Nature of reactants


The units of k depend on the overall order of the reaction. For example:
- First-order: k (s^-1)
- Second-order: k (M^-1 s^-1)


3. Example
Consider the reaction: 2A + B -> C


Experimental data shows that Rate = k [A]^2 [B]. This means: