10.05.2 Changing the Rate of Reaction

Syllabus

- Factors which affect the rates of chemical reactions include: the concentrations of reactants in solution, the pressure of reacting gases, the surface area of solid reactants, the temperature and the presence of catalysts.
- Students should recall how changing these factors affects the rate of chemical reactions.
- Collision theory explains how various factors affect rates of reactions.
- According to this theory, chemical reactions can occur only when reacting particles collide with each other and with sufficient energy.
- The minimum energy that particles must have to react is called the activation energy.
- Increasing concentration of reactants in solution, pressure of reacting gases, and surface area of solid reactants increases collision frequency & so increases rate of reaction.
- Increasing the temperature increases the frequency of collisions and makes the collisions more energetic, and so increases the rate of reaction.

Students should be able to :

1. predict and explain using collision theory the effects of changing conditions of concentration, pressure and temperature on the rate of a reaction
2. predict and explain the effects of changes in the size of pieces of a reacting solid in terms of surface area to volume ratio
3. use simple ideas about proportionality when using collision theory to explain the effect of a factor on the rate of a reaction.

Required practical 5: investigate how changes in concentration affect the rates of reactions by a method involving measuring the volume of a gas produced and a method involving a change in colour or turbidity. This should be an investigation involving developing a hypothesis.

What does this mean?

What is collision theory?

Reactant particles cannot react unless they are touching.

Particles of gases and particles dissolved in water are always moving.

They only touch when they collide.

And even when they do collide there is no guarantee they will react unless they collide with enough energy.

We call the minimum energy needed the Activation Energy.

If we can increase the number of collisions each second (collision frequency) we can speed up the reaction.

Speeding up reactions

#1 Increase Temperature.

If we speed up the number of collisions per second (collision frequency) we speed up the rate of reaction.

We can make gas particles and dissolved particles collide more frequently if we help them to move around faster.

We can help them more faster by giving them more energy - by raising the temperature.

#2 Increased Pressure

We can make gas particles collide more frequently by pushing them closer together

We can do this by raising the pressure.

It doesn't give the particles any more energy.

So they don't move any faster.

THey're just close together and so can't avoid hitting each other frequently.

#3 Increased Concentration.

In a dilute solution of acid most particles are water.

This means the collision frequency between the acid particles and any solid dropped in it is low and so is the rate of reaction.

In a concentrated solution of acid far fewer particles are water.

This means the collision frequency between the acid particles and any solid dropped in it is higher and so is the rate of reaction.

#4 Increased Surface Area.

Particles in a solid can't move.

Only the surface of the solid can be collided with and react.

The bigger its surface area the higher the collision frequency between the solid and the solution it is reacting with.

We can increase surface area by breaking a solid into smaller pieces, exposing more of it to the solution.

#5 Add a Catalyst

Catalysts allow a reaction to happen in a different way.

This new way requires less energy - a lower Activation Energy.

A greater proportion of particles will have enough energy to react when they collide - there are fewer unsuccessful collisions.

So the rate of reaction increases even though there is no extra energy available and the collisions may not occur more frequently.