Reaction Rates, Catalysts and Collision Theory

Rates of reaction

The rate of a reaction measures how fast it happens. Increased reaction rate means that the reactants are used up faster and products are formed quicker.

Rates of reaction depend on• Particle size• Temperature• Concentration• The presence or absence of a Catalyst– A catalyst is something that speeds up the

reaction without being used up in the reaction

Collision theory

Collision theory says that a reaction will only take place between chemicals when their particles collide with sufficient energy to break the bonds within the reacting molecules

Bonds can only be broken and remade when the reactants touch each other and with sufficient energy to break old bonds.

Activation Energy (EA)The activation energy is the name for the

amount of energy required that starts the reaction. Collisions at lower energies do not react. Once a reaction has started the energy released by the reaction can be enough to continue the reaction.

Particle size

Smaller particles of a substance have much greater surface area than large particles relative to their volume.

The greater the surface area of a substance the more collisions it can take part in.

The more collisions that occur the faster the reaction rate.

Concentration

The higher the concentration of a solution the more ‘active’ particles there are. In a dilute solution of acid reacting with a marble chip, most of the particles in the solution are water. Increasing the number of acid particles increases the likelihood of a collision between the acid particle and the marble chip. More collisions mean a faster reaction rate.

Temperature

Increasing the temperature increases the movement of the particles in the reaction. Each particle will move faster and collide more often. As each particle has more energy then more of the collisions will be equal to, or greater than, the activation energy.

Increasing the temperature increases the number of sufficiently forceful collisions.

CatalystsCatalysts change the rate of reaction without

being used up themselves. Catalysts reduce the activation energy so more of the collisions are powerful enough for the reaction to occur.

Catalysts provide an alternative pathway for the reaction to occur.