Reaction Rates

25min Part 2 / Ch4 / Lesson 1

Prerequisites: 1-2-3

Objectives

Define reaction rate and its expression
Explain factors that affect reaction rates
Understand activation energy and the role of catalysts

What Is Reaction Rate?

Reaction rate = change in concentration per unit time

$v = -\frac{\Delta[\text{reactant}]}{\Delta t} = \frac{\Delta[\text{product}]}{\Delta t}$

Unit: $\mathrm{mol/(L \cdot s)}$

Factors Affecting Reaction Rate

Factor	Effect	Reason
Increase concentration	Rate ↑	More collisions
Increase temperature	Rate ↑	More molecules exceed the activation energy
Add a catalyst	Rate ↑	Lowers activation energy
Decrease particle size	Rate ↑	Greater surface area

Activation Energy and Catalysts

Activation energy and the effect of a catalyst

Activation energy ( $E_a$ ): the minimum energy required for a reaction to occur.

Catalyst: a substance that lowers $E_a$ , increasing the reaction rate without being consumed.

Check Your Understanding

Q1 Why does increasing temperature increase reaction rate?

Q2 What does a catalyst change?

Q3 The correct unit for reaction rate is:

Exercises

Q1. Explain why adding $\mathrm{MnO_2}$ speeds up the decomposition of hydrogen peroxide ( $\mathrm{H_2O_2}$ ). What happens to $\mathrm{MnO_2}$ after the reaction?

Solution

$\mathrm{MnO_2}$ acts as a catalyst, lowering the activation energy of the decomposition reaction, which increases the reaction rate.

As a catalyst, $\mathrm{MnO_2}$ is not consumed — its amount and properties remain unchanged after the reaction.

$2\mathrm{H_2O_2} \xrightarrow{\mathrm{MnO_2}} 2\mathrm{H_2O} + \mathrm{O_2}$