Chemical Kinetics – Complete JEE Notes
1. Introduction to Chemical Kinetics
Chemical kinetics deals with the **speed (rate)** of chemical reactions and the
factors affecting this rate.
It explains **how fast** a reaction occurs, not whether it will occur.
2. Rate of Reaction
Rate of reaction is defined as the **change in concentration of reactant or product
per unit time**.
For reaction: $aA \rightarrow bB$
Rate $= -\frac{1}{a}\frac{d[A]}{dt} = \frac{1}{b}\frac{d[B]}{dt}$
Rate $= -\frac{1}{a}\frac{d[A]}{dt} = \frac{1}{b}\frac{d[B]}{dt}$
3. Average Rate and Instantaneous Rate
- Average rate: Change in concentration over a finite time interval
- Instantaneous rate: Rate at a particular instant (slope of tangent)
4. Units of Rate of Reaction
Unit of rate = $\text{mol L}^{-1}\text{s}^{-1}$
5. Rate Law (Rate Equation)
Rate law expresses the relationship between rate and concentration of reactants.
Rate $= k[A]^m[B]^n$
$m$ and $n$ are determined experimentally, not from stoichiometry.
6. Order of Reaction
Order of reaction is the **sum of powers of concentration terms** in rate law.
Order $= m + n$
7. Molecularity of Reaction
Molecularity is the number of reacting species involved in an elementary step.
| Molecularity | Description |
|---|---|
| Unimolecular | One molecule involved |
| Bimolecular | Two molecules involved |
| Termolecular | Three molecules involved (rare) |
Order can be zero or fractional, but molecularity is always a whole number.
8. Integrated Rate Equations
Zero Order Reaction
$[A] = [A]_0 - kt$
First Order Reaction
$\ln\frac{[A]_0}{[A]} = kt$
Second Order Reaction
$\frac{1}{[A]} = \frac{1}{[A]_0} + kt$
9. Half-Life of a Reaction
Zero Order
$t_{1/2} = \frac{[A]_0}{2k}$
First Order
$t_{1/2} = \frac{0.693}{k}$
Second Order
$t_{1/2} = \frac{1}{k[A]_0}$
10. Temperature Dependence of Reaction Rate
Increase in temperature increases kinetic energy of molecules, increasing rate.
11. Arrhenius Equation
$k = A e^{-E_a/RT}$
Where:
- $k$ = rate constant
- $A$ = frequency factor
- $E_a$ = activation energy
- $R$ = gas constant
- $T$ = temperature
12. Logarithmic Form of Arrhenius Equation
$\ln k = \ln A - \frac{E_a}{RT}$
Slope of $\ln k$ vs $\frac{1}{T}$ graph = $-\frac{E_a}{R}$
13. Effect of Catalyst
Catalyst lowers activation energy by providing an alternate reaction pathway.
Catalyst does not change equilibrium constant.
14. Collision Theory
Reaction occurs only when:
- Molecules collide
- Collision energy ≥ activation energy
- Proper orientation
15. Pseudo First Order Reaction
Reaction that appears first order due to excess of one reactant.
Example: Hydrolysis of ester in excess water
16. Graphical Representation
- $[A]$ vs $t$ → Zero order → Straight line
- $\ln[A]$ vs $t$ → First order → Straight line
- $1/[A]$ vs $t$ → Second order → Straight line
17. Factors Affecting Rate of Reaction
- Concentration
- Temperature
- Nature of reactants
- Presence of catalyst
- Surface area
18. Common JEE Traps
- Confusing order with molecularity
- Using stoichiometry to find order
- Wrong half-life formula
- Incorrect Arrhenius slope sign
19. Important JEE Observations
- First order reactions depend only on $k$
- Half-life independent of concentration for first order
- Units of $k$ depend on order
20. Final Revision Checklist
You have mastered Chemical Kinetics if you can:
- Identify order from data
- Use correct integrated rate equation
- Solve Arrhenius equation numerically
- Interpret rate graphs
- Apply catalyst and temperature concepts