5.3.2 Factors Affecting Rate of Reactions

Introduction

Several factors influence how fast a reaction happens by changing the number of successful collisions between particles. Some of the important factors are:

• Concentration of Reactants
• Surface Area
• Temperature
• Pressure of Gases
• Catalyst

This section explains each factor and how it affects the rate of a reaction.

Definition

Factors Affecting Rate of Reaction: Factors that influence the speed of a chemical reaction by changing the number of successful collisions between particles.

In simple words: These are the things that can make a reaction faster or slower.

Main Idea

The rate of a reaction depends on the number of effective collisions between particles. Factors that increase the number of effective collisions make the reaction faster. Factors that decrease the number of effective collisions make the reaction slower.

Core Concept

Key point: All these factors work by changing the number of effective collisions between particles.

Detailed Explanation

1. Concentration of Reactants

What happens:

• The more reactant particles there are in a given space, the more likely they are to collide.
• This means that increasing the concentration of reactants makes the reaction happen faster.

Examples from the source:

• Acid rain damages marble more quickly when the acid is stronger.
• Two antacid tablets neutralize stomach acid faster than one tablet because there are more reacting particles.
• Hydrogen and chlorine gases react twice as fast if their concentrations are doubled.

Key point: Higher concentration → more collisions → faster reaction.

2. Surface Area

What happens:

• When solid reactants have more surface area exposed, the reaction happens faster.
• This is because there are more opportunities for particles to collide.

Examples from the source:

• Powdered zinc reacts with dilute HCl faster than a solid chunk of zinc because the powder has more exposed surface area.
• Similarly, powdered aluminium reacts quickly with NaOH, while aluminium foil reacts more slowly.

Key point: More surface area → more collisions → faster reaction.

3. Temperature

What happens:

- Raising the temperature increases the speed of particles, leading to more frequent and energetic collisions.

However, for a collision to cause a reaction, the particles must:

• Have enough energy (called activation energy).
• Be oriented correctly when they collide.

What the source says:

• At normal temperatures, only a small number of particles have enough energy to react.
• A graph called the Maxwell-Boltzmann curve shows how energy is distributed among particles.
• Most particles have medium energy, while only a few have the activation energy needed for a reaction.
• When the temperature increases, more particles gain enough energy to overcome the activation energy barrier, leading to faster reactions.

The Maxwell-Boltzmann curve:

• The curve shows the distribution of available energy between the molecules of a gas at constant temperature.
• It clearly shows that most of the molecules do not have the necessary activation energy.
• When temperature increases (say from T₁ to T₂), the energy of the molecules also increases.
• Thus, the proportion of molecules having required activation energy increases.

Key point: Higher temperature → more particles have enough energy → faster reaction.

4. Pressure of Gases

What happens:

• Pressure increases the concentration of gases for a given volume.
• There will be more collisions, which will increase the reaction rate.

Example from the source:

- A mixture of H₂ molecules and Cl₂ molecules will react twice as quickly when the partial pressure of one of the H₂ molecules is doubled compared to the partial pressure of the other.

Key point: Higher pressure → more collisions → faster reaction.

5. Catalyst

What happens:

• A catalyst speeds up a chemical reaction without being used up itself.
• It works by providing an easier pathway for the reaction to happen, which has a lower activation energy.
• With a lower activation energy, more particles can collide effectively, and the reaction happens faster.

Key point: A catalyst lowers the activation energy → faster reaction.

Note: A detailed account of the influence of catalyst on reaction rates is given in section 5.3.

Summary Table

Intermediate Level

How each factor affects the rate:

Key point: All factors work by changing the number of effective collisions.

Maxwell-Boltzmann curve:

• Shows the distribution of energy among particles.
• At higher temperatures, more particles have enough energy to overcome the activation energy barrier.

Solved Example

Concept Assessment Exercise 5.2

1. In what way does increasing the surface area of a solid reactant affect the rate of reaction?

Answer: Increasing the surface area of a solid reactant increases the rate of reaction. This is because more surface area means more particles are exposed to the other reactant, leading to more frequent collisions and a faster reaction.

Example: Powdered zinc reacts with dilute HCl faster than a solid chunk of zinc because the powder has more exposed surface area.

2. How does temperature affect the rate of reaction?

Answer: Increasing the temperature increases the rate of reaction. This is because higher temperature gives particles more kinetic energy, causing them to move faster and collide more frequently and with more energy. More particles have enough energy to overcome the activation energy barrier, leading to a faster reaction.

3. Discuss the effect of changing the concentration of a reactant on the rate of reaction.

Answer: Increasing the concentration of a reactant increases the rate of reaction. This is because a higher concentration means there are more reactant particles in a given space, leading to more frequent collisions and a faster reaction.

Example: Two antacid tablets neutralize stomach acid faster than one tablet because there are more reacting particles.

Important Points

• - Concentration: Higher concentration → faster reaction.
• - Surface Area: More surface area → faster reaction.
• - Temperature: Higher temperature → faster reaction.
• - Pressure: Higher pressure → faster reaction (for gases).
• - Catalyst: Presence of catalyst → faster reaction.
• - All factors work by changing the number of effective collisions.
• - Maxwell-Boltzmann curve shows the distribution of energy among particles.
• - At higher temperatures, more particles have enough energy to overcome the activation energy barrier.
• - A catalyst lowers the activation energy.

Remember

• - Concentration ↑ → rate ↑
• - Surface area ↑ → rate ↑
• - Temperature ↑ → rate ↑
• - Pressure ↑ → rate ↑
• - Catalyst → rate ↑ (lowers activation energy)
• - All factors affect the number of effective collisions.
• - Maxwell-Boltzmann curve shows energy distribution.
• - More particles with enough energy → faster reaction.

Common Mistakes

Quick Revision

Factors Affecting Rate of Reactions:

Several factors influence how fast a reaction happens by changing the number of successful collisions between particles. Some of the important factors are:

1. Concentration of Reactants: The more reactant particles there are in a given space, the more likely they are to collide. This means that increasing the concentration of reactants makes the reaction happen faster. Examples: Acid rain damages marble more quickly when the acid is stronger. Two antacid tablets neutralize stomach acid faster than one tablet because there are more reacting particles. Hydrogen and chlorine gases react twice as fast if their concentrations are doubled.

2. Surface Area: When solid reactants have more surface area exposed, the reaction happens faster. This is because there are more opportunities for particles to collide. Examples: Powdered zinc reacts with dilute HCl faster than a solid chunk of zinc because the powder has more exposed surface area. Similarly, powdered aluminium reacts quickly with NaOH, while aluminium foil reacts more slowly.

• Have enough energy (called activation energy).
• Be oriented correctly when they collide.

4. Pressure of Gases: Pressure increases the concentration of gases for a given volume. There will be more collisions, which will increase in reaction rate. For instance, a mixture of H₂ molecules and Cl₂ molecules will react twice as quickly when the partial pressure of one of the H₂ molecules is doubled compared to the partial pressure of the other.

• 5. Catalyst:
• A detailed account of the influence of catalyst on reaction rates is given in section 5.3.

Concept Assessment Exercise 5.2:

• In what way does increasing the surface area of a solid reactant affect the rate of reaction?
• How does temperature affect the rate of reaction?
• Discuss the effect of changing the concentration of a reactant on the rate of reaction.

60-Second Revision

• Concentration ↑ → rate ↑
• Surface area ↑ → rate ↑
• Temperature ↑ → rate ↑
• Pressure ↑ → rate ↑
• Catalyst → rate ↑ (lowers activation energy)
• All factors affect the number of effective collisions.
• Maxwell-Boltzmann curve shows energy distribution.
• More particles with enough energy → faster reaction.
• Powdered zinc reacts faster than solid zinc.
• Two antacid tablets work faster than one.
• ⚠️ Warning: all factors work by changing the number of effective collisions — not just the number of collisions.