Rates of Reaction - Collision Theory

This document provides an essential overview of reaction rates and collision theory, highlighting the factors that affect chemical reactions.

Explanation of Reaction Rates

• Reaction Rate: The rate at which a chemical reaction proceeds.
• It indicates the speed at which reactants are converted to products.
• Understanding reaction rates is vital for optimising production, managing pollutants, and comprehending biochemical processes in both industrial and biochemical contexts.

Definition of Key Terms

Term	Definition
Reaction Rate	The rate at which reactants are converted into products.
Reactants	The initial substances involved in a chemical reaction.
Products	The substances produced as a result of a chemical reaction.

Factors Affecting Reaction Rates

Key Influencing Factors:

• Concentration:

  • Higher concentration increases the likelihood of collisions, thus accelerating the reaction rate.

• Temperature:

  • Elevated temperatures increase particle energy, promoting more collisions and enhancing the reaction rate.

• Catalysts:

  • Catalysts facilitate faster reactions by reducing the required activation energy without being consumed.

• Particle Size:

  • Smaller particles offer a greater surface area, which enhances the frequency of collisions.

• An understanding of these factors is linked to collision theory.

Introduction to Collision Theory

Collision Theory: Reactions occur when particles collide with adequate energy and the correct orientation.

• Effective collisions are imperative for accelerating reactions.

Collision Theory Basics

• Collision Occurrence: Only colliding particles have the potential to react.
• Sufficient Energy: Particles must have sufficient energy to surpass the activation energy barrier.
• Proper Orientation: Accurate alignment of particles during collisions is crucial.

Molecular Collisions and Orientation

• Molecular Orientation: The arrangement of molecules plays a crucial role in determining the success of collisions, akin to the alignment of puzzle pieces.
• Correct alignment, combined with sufficient energy, is required for effective reactions.

Impact of Molecular Geometry

• Molecular Geometry: The shape of molecules affects collision pathways.
• CO₂ (linear): Permits direct collisions.
• H₂O (bent): Necessitates angle navigation for effective collisions.

Visual Representation:

Activation Energy

Understanding Activation Energy

• Activation Energy: The minimum amount of energy required for a chemical reaction. It represents the energy barrier that must be overcome.

Activation Energy as an Energy Barrier

• Energy Barrier: Determines reaction rates.
• Higher activation energy results in slower reactions.

• Effects of Catalysts and Temperature:
  • Catalysts decrease the activation energy.
  • Temperature increases the energy available to reactants.

Key Points to Remember

• Collision Theory Criteria: Mastery of these criteria is essential:

  • Sufficient energy: Particles must possess adequate energy for reactions.
  • Proper orientation: Correct alignment is crucial.
  • Physical collision: A tangible collision is necessary.

• Activation Energy:

  • Functions as an energy barrier.
  • Catalysts lower this barrier, accelerating reactions.

Common Pitfalls to Avoid

• Molecular Orientation Misunderstanding:
  • Correct orientation is vital.
• Energy Profile Interpretation:
  • Identify sections representing activation energy.

Practical Applications

• Real-world examples, such as enzyme catalysts, provide valuable insights.
• Use annotated diagrams for clarity.
• Conduct simple experiments to reinforce the effects of temperature and catalysts.

Questions and Solutions

Multiple Choice Question

Question: What does the collision theory state about reactions?

Options:

• A. Reactions occur if reactants have proper orientation and energy.
• B. All reactions are spontaneous at room temperature.
• C. Temperature does not affect rates.
• D. Catalysts are required for all reactions.

Solution: A. According to collision theory, chemical reactions occur when particles collide with both the proper orientation and sufficient energy to overcome the activation energy barrier.

Calculation Problem

Question: Calculate activation energy $E_a$

Given:

• Frequency factor $A = 2.0 \times 10^{13}$ s⁻¹
• Rate constant $k = 4.0 \times 10^{-3}$ s⁻¹
• Temperature $T = 300$ K
• Gas constant $R = 8.314$ J mol⁻¹ K⁻¹

Formula: $E_a = -RT \ln\left(\frac{k}{A}\right)$

Solution:

1. Substitute the values into the formula: $E_a = -(8.314 \times 300) \ln\left(\frac{4.0 \times 10^{-3}}{2.0 \times 10^{13}}\right)$

2. Calculate the natural logarithm: $\ln\left(\frac{4.0 \times 10^{-3}}{2.0 \times 10^{13}}\right) = \ln(2.0 \times 10^{-16}) = -36.24$

3. Complete the calculation: $E_a = -(8.314 \times 300) \times (-36.24) = 2494.2 \times 36.24 = 90,370$ J mol⁻¹

4. Convert to kJ mol⁻¹: $E_a = 90.4$ kJ mol⁻¹

Therefore, the activation energy is 90.4 kJ mol⁻¹.