1.3 Collision Theory

The collision theory
■ Collision theory

	►	A chemical reaction may occur as a result of the collisions between particles.
	►	For a reaction to occur, an effective collision must take place.

■ Effective collision

	►	The particles of the reactants must collide with each other to produce energy that greater than or equal to the activation energy.
	►	The collisions must happen in the correct orientation.
	►	The animation below shows the collisions of particles.

■ This video contains information on the collision theory in the rate of reaction.

■ Activation energy (Ea)

	►	Minimum amount of energy required to initiate a chemical reaction.
	►	Energy profile diagram can be used to shows the energy change that takes place in a chemical reaction. Energy profile diagram for exothermic reaction Energy profile diagram for endothermic reaction
	►	This video contains information on the effective collision and activation energy.
	►	Example: Reaction of hydrogen with iodine to produce hydrogen iodide. H2(g) + I2(g) → 2HI(g)

■ Use of collisions theory to explain the rate of reaction:

	►	Relationship of frequency of effective collisions and rate of reaction ○ Reaction with higher frequency of effective collisions means the rate of reaction is higher.
	►	Relationship of activation energy and rate of reaction ○ Reaction with high activation energy occurs slowly while the reaction with a low activation energy occur quickly.

Collision theory and the effect of surface area (particle size)
■ The effect of the size of reactant

	►	If the size of the solid material that reacts is small, the amount of surface area exposed to reaction increased.
	►	The chance for collisions increased, the collision frequency is also increased, resulting in the increased of effective collisions.
	►	The rate of reaction also increased.
	►	The animation below shows the effect of size of reactant on the rate of reaction.

Collision theory and the effect of concentration
■ The effect of concentration

	►	When the concentration increases, the number of particles per unit volume increases.
	►	Therefore, the total number of collisions and the effective collision frequency between particles of materials response will increase.
	►	Thus, the reaction rate increases.
	►	The animation below shows the effect of concentration on the rate of reaction.

Collision theory and the effect of temperature
■ The effect of temperature

	►	When the reaction temperature increased, the particles of the reactants have more kinetic energy.
	►	Particles reactants move more quickly.
	►	This increases the chances of a collision, the collision frequency is increased.
	►	Then the effective collision frequency increases and the rate of reaction increases.
	►	The animation below shows the effect of temperature on the rate of reaction.

Collision theory and the effect of catalyst
■ The effect of catalyst

►

Catalysts change the rate of a reaction by varying the activation energy of the reaction.

■ Positive catalysts

	►	Provide a reaction path requires a lower activation energy.
	►	More particles will have enough energy to overcome the lower activation energy.
	►	The effective collision frequency is increased.
	►	Thus, it accelerate the rate of a reaction.

■ Negative catalyst

►

Delay the rate of reaction by increasing the activation energy of the reaction so that the effective collision frequency is reduced.

Pressure and rate of reaction
■ The effect of pressure

	►	When pressure increase, the particles are close together.
	►	The number of particles per unit volume increases.
	►	Frequency of collisions between particles increase, higher frequency of effective collisions.
	►	More particles have activation energy, thus the rate of reaction increase.
	►	The animation below shows the effect of pressure on the rate of reaction.

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