Quick Notes

Quick Notes

Chemistry Chapter 7: Rate of Reaction
7.4 7.4 Collision Theory
7.4 Collision Theory
CHEMISTRY • Form 4 • Chapter 7: Rate of Reaction

7.4Collision Theory

Matter consists of tiny and discrete particles that are constantly moving and colliding with one another. During collisions, energy is transferred between particles, resulting in particles possessing different amounts of kinetic energy. The collision theory explains how reactant particles interact to cause chemical reactions and form products. For a reaction to happen, particles must collide effectively by possessing energy equal to or greater than the activation energy and colliding in the correct orientation.

Learning Objectives

  • Describe the collision theory.
  • Explain activation energy using examples and analogies.
  • Interpret energy profile diagrams for exothermic reactions and endothermic reactions.
  • Apply collision theory to explain the effects of concentration, pressure, size of reactant, temperature, and catalyst on the rate of reaction.

Energy Profile Diagrams with and without a Catalyst

Compare activation energy, heat of reaction and the lower-energy pathway provided by a catalyst.

Exothermic Reaction
Exothermic energy profile diagram Energy profile showing reactants higher than products, with a higher activation energy hump without catalyst and a lower dotted hump with catalyst. Energy Reaction pathway ΔH Eₐ' Eₐ Reactants Products Without catalyst With catalyst
Endothermic Reaction
Endothermic energy profile diagram Energy profile showing products higher than reactants, with a higher activation energy hump without catalyst and a lower dotted hump with catalyst. Energy Reaction pathway ΔH Eₐ' Eₐ Reactants Products Without catalyst With catalyst

Brief Explanation

Understand the key ideas behind collision theory.

Collision theory explains that chemical reactions can only happen when reactant particles collide with sufficient energy and the right orientation. The minimum energy barrier is called the activation energy (\(E_a\)). Like a horse needing enough energy to clear a barrier, reactant particles must acquire this minimum energy before a reaction can begin.

1. Effective Collision

A collision between reactant particles that has energy equal to or greater than the activation energy and occurs in the correct orientation, successfully forming products.

2. Ineffective Collision

A collision with energy less than the activation energy or in the wrong orientation, resulting in particles bouncing back without reacting.

Factor Effect on collision theory
Concentration Number of particles per unit volume increases → frequency of collisions increases → frequency of effective collisions increases → rate of reaction increases.
Gas pressure Number of gas particles per unit volume increases → frequency of collisions increases → frequency of effective collisions increases → rate of reaction increases.
Size of reactant Total surface area exposed to collision increases → frequency of collisions increases → frequency of effective collisions increases → rate of reaction increases.
Temperature Kinetic energy of particles increases → more particles have energy to overcome the activation energy → frequency of effective collisions increases → rate of reaction increases.
Catalyst Provides an alternative reaction pathway with a lower activation energy (\(E_a'\)) → more particles can achieve this lower activation energy → frequency of effective collisions increases → rate of reaction increases.

Try to Answer First

Answer mentally, then check the explanation.

1

What are the two essential conditions required for a collision between reactant particles to be effective?

Answer: The reactant particles must possess kinetic energy equal to or more than the activation energy (\(E_a\)), and they must collide in the correct orientation.
2

How does a catalyst increase the rate of reaction based on the collision theory?

Answer: A catalyst provides an alternative reaction pathway with a lower activation energy (\(E_a'\)), allowing more reactant particles to achieve the activation energy and increasing the frequency of effective collisions.
3

In an exothermic reaction, how does the total energy content of reactants compare to products, and what is the sign of the heat of reaction (\(\Delta H\))?

Answer: The total energy content of reactants is higher than that of products, and the heat of reaction (\(\Delta H\)) has a negative sign.

Common Mistakes

Avoid these errors when explaining collision theory.

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Stating that all collisions between reactant particles produce chemical reactions, forgetting that only effective collisions produce products.
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Stating that increasing temperature only increases the collision frequency, while missing the critical factor: higher temperature increases particle kinetic energy so that significantly more particles have energy equal to or greater than the activation energy.
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Confusing how a catalyst works by thinking it gives energy to the particles, rather than providing an alternative reaction pathway with a lower activation energy.
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Forgetting that the activation energy arrow on an energy profile diagram must always be drawn starting from the reactant energy level to the peak of the curve, not from the base/zero line.

Concept Misunderstandings

Compare each misconception with the correct fact.

Misconception

Adding a catalyst increases the kinetic energy of reactant particles.

Correct Fact

A catalyst does not increase the kinetic energy of particles. Instead, it provides an alternative pathway that lowers the required activation energy barrier (\(E_a'\)).

Misconception

Breaking a solid into smaller pieces changes its total volume.

Correct Fact

Breaking a solid into smaller pieces keeps the total volume constant, but significantly increases the total surface area exposed to collisions.

Misconception

Collisions between particles occur at a uniform velocity because temperature is constant.

Correct Fact

At any given temperature, energy transfers continuously occur during random collisions; thus, particles possess a range of different velocities and kinetic energies.

Summary

Key facts to remember.

  • Collision theory states that for a reaction to occur, reactant particles must collide with energy \(\ge E_a\) and in the correct orientation.
  • Rate of reaction is directly proportional to the frequency of effective collisions.
  • Activation energy (\(E_a\)) is the minimum energy required to break bonds in reactants to initiate a reaction.
  • In energy profile diagrams, \(E_a\) is the difference between reactant energy and the curve peak. \(\Delta H\) is negative for exothermic reactions (reactants higher than products) and positive for endothermic reactions (products higher than reactants).
  • Increasing concentration, gas pressure, or surface area increases collision frequency, leading to higher effective collision frequency.
  • Increasing temperature increases kinetic energy so more particles exceed \(E_a\), increasing effective collision frequency.
  • Adding a catalyst provides an alternative pathway with lower activation energy (\(E_a'\)), enabling more particles to exceed \(E_a\) and increasing effective collision frequency.

Short Activity

Complete all parts before checking your score.

A. Objective Quiz

1

Which statement correctly explains why increasing the temperature increases the rate of reaction?

2

What happens when reactant particles collide with energy lower than the activation energy?

B. Fill in the Blanks

3

The minimum energy required by reactant particles to initiate a chemical reaction is called the ______ energy.

4

A catalyst increases the rate of reaction by providing an alternative pathway with a ______ activation energy.

C. Match the Factor with Its Explanation

Drag each factor on desktop. On mobile, tap a factor and then tap the matching box.

Factors Affecting Rate of Reaction

Increase in concentration Smaller size of solid reactant Increase in temperature Presence of a catalyst

Explanation Based on Collision Theory

1Number of particles per unit volume increases, leading to higher collision frequency.
2Total surface area exposed increases, allowing more collisions per unit time.
3Kinetic energy increases so more particles possess energy to overcome activation energy.
4Provides an alternative pathway with lower activation energy.
Complete every answer before checking.
 

Keywords

Tap or hover over a term to read its meaning.

Collision theory Activation energy Effective collision Ineffective collision Collision orientation Catalyst Energy profile diagram Heat of reaction Exothermic reaction Endothermic reaction