碰撞理论

The theory of how reactions happen

COLLISION THEORY: Reactions can only happen when the reactant particles collide, but most collisions are NOT successfulin forming product molecules despite the high rate of collisions. about 109per second!) The reason is that particles have a wide range of kinetic energy BUT only a small fraction of particles have enough kinetic energyto break bonds and bring about chemical change. The minimum kinetic energy required for reaction is known as the activation energy. The minority high kinetic energy collisions between particles which do produce a chemical change are called 'fruitful collisions'. Here the reactant molecules collide with enough kinetic energy to break the original bonds and form new bonds in the product molecules.

Nearly all the rate-controlling factors described below are to do with the collision frequency (chance of collision) OR the energy of reactant particle collision (>= activation energy)which can be summed up as the 'chance of a fruitful collision' leading to product formation. In the case of temperature, the energy of the collision is even more important than the frequency effect (see later). The particle theory of gases and liquidsand the particle diagrams and the explanations below, will all help you understand or describe in your coursework what is going on.

The Factors affecting the Rate of Chemical Reactions

a The effect of Concentration

If the concentration of any reactant in a solution is

increased, the rate of reaction is increased.

Increasing the concentration, increases the probability

of a collision between reactant particles because there

are more of them in the same volume and so increases

the chance of a fruitful collision forming products.

e.g. Increasing the concentration of acid molecules

increases the frequency or chance at which they hit the

surface of marble chips to

dissolve them.

b The effect of Pressure

If one or more of the reactants is a gasthen increasing

pressure will effectively increase the concentration of the

reactant molecules and speed up the reaction.

The A and B particle diagrams right could represent

lower/higher pressure , resulting in lesser or greater

concentrationand so slower or faster reaction all because of the increased chance of a 'fruitful' collision.

c The effect of Surface Area -particle size of a soli

d reactant

If a solid reactant or a solid catalyst is broken down into smaller pieces the rate of reaction increases. The speed increase happens because smaller pieces of the same mass of solid have a greater surface areacompared to larger pieces of the solid. Therefore, there is more chance that

a reactant particle will hit the solid

surfaceand react.

The diagrams right illustrate the

acid –marble chip reaction

d Th

e effect o

f Temperature

When gases or liquids are heated the particles gain kinetic energy and move faster.The increased speed increases the chance (frequency) of collisionbetween reactant molecules and the rate increases. BUTthis is NOTthe main reason for the increased reaction speed.Most molecular collisions do not result in chemical change.Before any change takes place on collision, the colliding molecules must have a minimum kinetic energy called the Activation Energyshown on the energy level diagrams below.

e. The effect of a Catalyst

It does not matter whether the reaction is an exothermic or an endothermic in terms of energy change, its the activation energy which is the most important factor in terms of temperature and reaction

speed. Now heated molecules have a greater average

kinetic energy, and so at higher temperatures, a greater proportion of them have the required activation energy to react.This means that the increased chance of 'fruitful' higher energy collisiongreatly increases the speed of the reaction, depending on the fraction of molecules with enough energy to react. Catalysts increase the rate of a reaction by helping

break chemical bonds in reactant molecules and

provide a 'different pathway' for the reaction. This

effectively means the Activation Energy is reduced,

irrespective of whether its an exothermic or

endothermic reaction (see diagrams left). Therefore

at the same temperature, more reactant molecules

have enough kinetic energy to reactcompared to the

uncatalysed situation. The catalyst does

NOTincrease the energy of the reactant molecules!