7.4 - Collision Theory

Particle Motion and Collisions

Matter consists of tiny, discrete particles in constant motion: particles vibrate about fixed positions in solids and move freely in liquids and gases.
Particles collide and transfer energy. Their kinetic energies are not all equal and change continually.
Collision theory explains how reactant particles interact to form products.
Reactant particles must collide for a reaction to occur, but not every collision produces a reaction.

Effective Collisions

An effective collision produces a reaction. It requires both conditions:

particles collide with energy equal to or greater than the activation energy; and
particles collide in the correct orientation so old bonds can break and new bonds can form.

A collision with insufficient energy or incorrect orientation is ineffective; the particles separate without reacting.

higher frequency of effective collisions ⟹ higher rate of reaction

Two collision sequences compare an orientation forming new molecules with an orientation leaving the original molecules unchanged — Correct and incorrect collision orientations

Activation Energy and Energy Profiles

Activation energy, $E_{a}$ , is the minimum energy that reactant particles must acquire to initiate a reaction by breaking existing bonds and enabling new bonds to form.

Energy-barrier analogy: A horse needs enough energy to clear a hurdle; similarly, reactant particles need enough energy to overcome the activation-energy barrier.
Different reactions have different activation energies.
Heat of reaction, $Δ H$ , is the change in energy content when reactants become products.
On an energy profile, $E_{a}$ is the difference between the reactant energy level and the peak of the curve.
In an exothermic reaction, reactants have more energy than products; heat is released and $Δ H$ is negative.
In an endothermic reaction, products have more energy than reactants; heat is absorbed and $Δ H$ is positive.
Every exothermic and endothermic reaction has an activation-energy barrier.

Effect of Concentration and Pressure

When concentration increases, the number of reactant particles per unit volume increases, collision frequency rises, effective-collision frequency rises, and the rate increases. For gases, increasing pressure at constant temperature has the same particle-level effect as increasing concentration.

Effect of Reactant Size

Breaking a fixed amount of solid into smaller pieces keeps its total volume constant but increases its total exposed surface area. More particles are exposed, so collision frequency, effective-collision frequency and rate all increase.

Effect of Temperature

Increasing temperature raises the average kinetic energy of reactant particles. A larger fraction has energy equal to or greater than $E_{a}$ , so the frequency of effective collisions and the reaction rate increase.

Effect of a Catalyst

A catalyst provides an alternative reaction pathway with lower activation energy, $E_{a}^{'}$ , than the uncatalysed pathway, $E_{a}$ ; therefore, $E_{a}^{'}$ is lower than $E_{a}$ .

More reactant particles can overcome the lower barrier, so effective-collision frequency and rate increase. The catalyst remains chemically unchanged and does not change the energy levels of reactants or products; therefore, it does not change $Δ H$ .

Catalyst energy profile for an exothermic reaction

Catalyst energy profile for an endothermic reaction