EquilibriumAcids and BasesRedoxOrganic

 Kinetics

  • Introduction
  • Collision Theory
  • Factors affecting rates
  • Graphs
Content 1

 

Reactions can only happen when the reactant particles collide, but most collisions are NOT successful in forming product molecules despite the high rate of collisions.

The reason is that only a small fraction of particles have enough kinetic energy to 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 'successful collisions'.

The reactant molecules collide with enough kinetic energy to break the original bonds and form new bonds in the product molecules.

As the number of particles having enough energy to react increases, the frequency of successful collisions will increase.

In the case of temperature, the energy of the collision is even more important than the frequency effect.

 

 

Effect of concentration on the rate

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 successful collision forming products.

rate concentration

Effect of temperature on the rate

For particles to collide successfully they need to have a minimum amount of energy to overcome the activation energy of a reaction.

As the temperature increases, more and more particles will have energy greater than or equal to the activation energy resulting in more successful collisions.

Increasing the temperature has the biggest effect on the rate of a reaction compared to the other factors, like concentration, surface area or a catalyst.

rate temperature particles

Effect of the surface area on the rate

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 area compared to larger pieces of the solid.

Therefore, there is more chance that a reactant particle will hit the solid surface and react.

effect of surface area on rate

Effect of a catalyst on the rate

A catalyst is substance that speeds up a chemical reaction without being used up.

A catalyst provides an alternative route for the reaction by lowering its activation energy so more particles will have enough energy to react.

catalysts could be either homogeneous ( same phase as the reactants) or heterogeneous ( different phase)

effect of catalyst on rates

rates graph

The shape of the graph is quite characteristic.
The reaction is fastest at the start when the reactants are at a maximum (steepest gradient).
The gradient becomes progressively less as reactants are used up and the reaction slows down.
Finally the graph levels out when one of the reactants is used up and the reaction stops.
The amount of product depends on the amount of reactants used.
The initial rate of reaction is obtained by measuring the gradient at the start of the reaction. A tangent line is drawn through the first part of the graph and passes through the origin 0,0.