This lesson explains how fast chemical reactions occur, the factors affecting reaction rates, collision theory, activation energy, and how rate equations are determined.

1. Core Concepts (Short Notes)

15.1 What Is Reaction Rate?

Reaction rate is the change in concentration of reactants or products per unit time.

• Measured in mol dm⁻³ s⁻¹.

• Fast reaction → high rate.

• Slow reaction → low rate.

15.2 Factors Affecting Reaction Rate

1. Concentration: More particles → more collisions.

2. Temperature: Higher temperature → faster movement → more effective collisions.

3. Surface Area: Finely divided solids react faster.

4. Catalyst: Lowers activation energy (Ea) → increases rate.

5. Pressure (gases): Higher pressure → more collisions.

15.3 Collision Theory

For a reaction to occur:

• Particles must collide.

• Collisions must have energy ≥ activation energy.

• Particles must collide with correct orientation.

Only a small fraction of collisions are successful.

15.4 Activation Energy (Ea)

The minimum energy required for a reaction to occur.

• Lower Ea → faster reaction.

• Catalysts lower the Ea without being consumed.

15.5 Energy Profile Diagrams

Show energy changes during reactions.

• Exothermic: Products lower in energy than reactants.

• Endothermic: Products higher in energy.

• Catalyzed reaction → lower peak (Ea).

15.6 Rate Laws

Rate = k[A]ᵐ[B]ⁿ Where:

• k = rate constant.

• m, n = reaction orders (found experimentally).

15.7 Half-Life (t½)

Time for concentration to drop to half its initial value.

• First-order reactions have constant half-life.

15.8 Arrhenius Equation

Shows how temperature affects rate constant:

k = A e^(−Ea/RT)

Higher temperature → higher k.

2. Key Formulas to Memorize

Rate Law

Rate = k[A]ᵐ[B]ⁿ

Arrhenius Equation

k = A e^(−Ea/RT)

Half-Life (1st Order)

t½ = 0.693 / k

3. Tips & Tricks for Exams

• A straight line in ln[A] vs. time means first-order.

• If rate doubles when [A] doubles → first-order in A.

• Catalysts change rate, not ΔH or equilibrium position.

• Small temperature changes cause large increases in rate.

• Activation energy diagrams are frequently tested.

4. Important Points to Remember

• Rate depends on collision frequency and energy.

• Only effective collisions form products.

• Rate laws are determined experimentally.

• Catalysts provide an alternative low‑energy pathway.

• First-order reactions show exponential decay.