Collision Theory

Introduction to Collision Theory

Collision theory explains how and why chemical reactions occur at the molecular level. It is fundamental to understanding reaction rates in organic chemistry. For a chemical reaction to proceed, reactant molecules must collide under specific conditions.

• Successful Collision: A reaction only occurs if molecules collide with sufficient energy and proper orientation.

• Conditions for Reaction:

  1. Molecules must collide with sufficient energy to break existing bonds.

  2. Molecules must collide in the correct orientation to allow bond breaking and formation.

• If these conditions are not met, no reaction occurs.

Orientation of Collisions

Importance of Molecular Orientation

Molecules are three-dimensional, and the way they collide affects whether a reaction will occur. Only collisions with the correct orientation lead to successful bond breaking and formation.

• Correct Orientation: Atoms must align properly during collision for bonds to break and new bonds to form.

• Example: In the reaction of ethene with hydrochloric acid, HCl must collide with the double bond of ethene for the reaction to occur:

Energy of Collision

Activation Energy and Reaction Requirements

For a reaction to occur, molecules must possess a minimum amount of energy, known as the activation energy. This energy is required to break the bonds in the reactants.

• Activation Energy (): The minimum energy required for a reaction to proceed.

• Energy Requirement: Molecules must have energy equal to or greater than the activation energy before the reaction can occur.

• Units: Activation energy is usually measured in kilojoules per mole (kJ/mol).

Activation Energy and Energy Profiles

Energy Diagrams for Chemical Reactions

Activation energy can be visualized using energy profile diagrams, which show the energy changes during a reaction.

• Exothermic Reaction: Energy is released; products have lower energy than reactants.

• Endothermic Reaction: Energy is absorbed; products have higher energy than reactants.

• Activation Energy: The energy difference between reactants and the transition state (peak of the curve).

Maxwell–Boltzmann Distribution

Kinetic Energy Distribution of Molecules

The Maxwell–Boltzmann distribution describes the spread of kinetic energies among molecules in a sample at a given temperature.

• Distribution: Most molecules have moderate kinetic energy, but some have much higher or lower energies.

• Graph: The area under the curve represents the total number of molecules; only those with energy equal to or greater than the activation energy can react.

• Temperature Effect: Increasing temperature shifts the distribution, increasing the number of molecules with sufficient energy to react.

Activation Energy and Particle Distribution

Fraction of Molecules Able to React

Not all molecules in a sample have enough energy to react. The fraction of molecules with energy greater than or equal to the activation energy determines the reaction rate.

• Key Point: Only molecules in the high-energy tail of the Maxwell–Boltzmann distribution can participate in the reaction.

• Implication: Most particles do not have enough energy to react at any given moment, but increasing temperature or lowering activation energy increases the fraction that can react.

Transition State and Activated Complex

Nature of the Transition State

The transition state (or activated complex) is a high-energy, unstable arrangement of atoms that exists momentarily as bonds are breaking and forming during a reaction.

• Transition State: Occurs at the peak of the energy profile diagram; has higher energy than both reactants and products.

• Activated Complex: The specific molecular arrangement at the transition state.

• Role: The transition state quickly forms products if the correct orientation and energy are present.

Factors Affecting Reaction Rate

Influence of Activation Energy and Collision Frequency

The rate of a chemical reaction depends on both the activation energy and the frequency of successful collisions.

• Low Activation Energy: Leads to a high rate of reaction.

• High Activation Energy: Leads to a low rate of reaction.

• Changing Conditions: Reaction rate can be increased by:

  • Increasing the frequency of collisions (e.g., raising temperature, concentration).

  • Increasing the probability of successful collisions (e.g., using a catalyst, proper orientation).

Summary Table: Key Concepts in Collision Theory

Additional info: These notes expand on the provided slides and handwritten content, adding academic context and examples relevant to organic chemistry, such as the ethene and HCl reaction. All equations and diagrams referenced are standard in college-level organic chemistry.