Collision theory

Collision theory models chemical reaction rates by treating reactions as outcomes of particle collisions. A collision produces reaction only when the particles have enough energy and a suitable orientation.

Activation energy: The minimum energy barrier that reacting particles must overcome for a chemical reaction to proceed. Only collisions with enough energy to overcome this barrier can produce reaction.
Chemical kinetics: The study of the rates of chemical reactions and the factors that influence them. Collision theory is a particle-level model within this broader study of reaction rates.
Temperature dependence of reaction rates: The change in chemical reaction rate as temperature changes, commonly described through the rate constant. Higher temperature raises the fraction of collisions energetic enough to react.
Unimolecular reaction: A reaction step in which one molecular species transforms without colliding with another reactant molecule. Such steps cannot be explained as direct encounters between two reactant particles.
Maxwell–Boltzmann distribution: A statistical distribution describing particle speeds or energies in a classical ideal gas at thermal equilibrium. It determines what fraction of collisions exceed the activation energy at a given temperature.
Reaction rate: The change in concentration of a reactant or product per unit time during a chemical reaction. Collision theory seeks to explain why this rate changes with conditions.
Concentration: The amount of a substance present per unit volume of a mixture or solution. Greater reactant concentration generally increases collision frequency.
Diffusion-controlled reaction: A reaction whose rate is limited mainly by how quickly reactants diffuse together. Here, encounter frequency rather than crossing an energy barrier limits the rate.
Steric factor: A factor that accounts for the fraction of molecular collisions with orientations suitable for reaction. It captures collision theory’s requirement that molecules meet in a productive orientation.
Rate constant: The proportionality constant in a chemical rate law at a specified temperature. Collision theory estimates how often collisions lead to reaction, contributing to the rate constant.

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