Chemical Kinetics is the branch of chemistry that deals with the rate of chemical reactions and the factors affecting it.
๐ Core Idea: It explains how fast a reaction occurs and why.
1. Rate of Reaction
Definition
The rate of reaction is the change in concentration of reactants or products per unit time.
Mathematical Expression
Rate = โ d[R]/dt (for reactants)
Rate = + d[P]/dt (for products)
Units
mol Lโปยน sโปยน
Average vs Instantaneous Rate
- Average rate โ over a time interval
- Instantaneous rate โ at a specific moment
2. Rate Law
Definition
The rate of reaction depends on concentration of reactants.
General Form
Rate = k [A]แต [B]โฟ
Where:
- k = rate constant
- m, n = orders of reaction
Order of Reaction
Order = m + n
Types
- Zero order
- First order
- Second order
3. Molecularity
Definition
Number of molecules involved in an elementary step.
Difference Between Order and Molecularity
| Order | Molecularity |
|---|---|
| Experimental | Theoretical |
| Can be zero/fraction | Always whole number |
4. Integrated Rate Laws
(A) Zero Order Reaction
Rate Law
Rate = k
Integrated Form
[A] = [Aโ] โ kt
Graph
[A]
|
|\
| \
| \
|___\_____ t
Half-life
tโ/โ = [Aโ] / 2k
(B) First Order Reaction (Very Important)
Rate Law
Rate = k[A]
Integrated Form
ln[A] = ln[Aโ] โ kt
Graph
ln[A]
|
|\
| \
| \
|___\_____ t
Half-life
tโ/โ = 0.693 / k
๐ Independent of initial concentration
(C) Second Order Reaction
Rate Law
Rate = k[A]ยฒ
Integrated Form
1/[A] = 1/[Aโ] + kt
Graph
1/[A]
|
| /
| /
|/
|_____ t
Half-life
tโ/โ = 1 / (k[Aโ])
5. Factors Affecting Rate of Reaction
(A) Concentration
Higher concentration โ higher rate
(B) Temperature
Increase in temperature โ increases rate
Arrhenius Equation
k = A e^(โEa/RT)
(C) Catalyst
Increases rate without being consumed
(D) Surface Area
More surface area โ faster reaction
6. Activation Energy
Definition
Minimum energy required for reaction to occur
Concept Clarity
๐ Lower activation energy โ faster reaction
7. Arrhenius Equation (Very Important)
Equation
k = A e^(โEa/RT)
Log Form
log k = log A โ Ea / (2.303RT)
Graph
log k
|
|\
| \
| \
|___\_____ 1/T
8. Collision Theory
Idea
Reactions occur due to collisions between molecules
Conditions for Effective Collision
- Proper orientation
- Sufficient energy
9. Pseudo First Order Reaction
Definition
Reaction that appears first order due to excess concentration of one reactant
Example
Hydrolysis of ester
10. Important Numericals
Numerical 1
Find rate if concentration changes from 2 to 1 mol in 5 sec
Numerical 2
Find half-life for first order reaction if k = 0.693 sโปยน
tโ/โ = 1 s
Numerical 3
Find order if rate doubles when concentration doubles
Answer: First order
Numerical 4
Use Arrhenius equation to find Ea
11. Important Formula Sheet
- Rate = k[A]แต[B]โฟ
- tโ/โ (first order) = 0.693/k
- Arrhenius equation
- Integrated equations
12. Concept Clarity (Very Important)
๐ WHY first order half-life constant?
Because it does not depend on concentration.
๐ WHY temperature increases rate?
Because more molecules cross activation energy.
๐ WHY catalyst works?
Because it lowers activation energy.
13. Common Mistakes
- Confusing order and molecularity
- Wrong formula usage
- Ignoring units of k
Conclusion
Chemical Kinetics helps understand how reactions occur and how fast they proceed. It is a highly scoring chapter if concepts and formulas are clear.
๐ Focus on:
- Rate laws
- Integrated equations
- Arrhenius equation