1.5.3

# Increasing Rates

Test yourself

## Changing Reaction Rates

There are many chemical reactions that we would like to go faster (e.g. the Haber process), and many that we would like to slow down (e.g. explosions). The main methods for changing reaction rates are:

### Changing temperature

• Earlier, we saw the effect of heating on the Maxwell-Boltzmann distribution of energies.
• It raises the average energy but lowers the peak of the graph.
• This gives more molecules energy greater than or equal to the activation energy.
• This means more collisions will have the energy to react.
• So the rate increases.
• Heating the mixture also increases the number of collisions.
• So heating increases rate through two pathways.

### Changing concentration

• In solution, the majority of collisions a solute particle experiences will be with water.
• We want more of these collisions to be with the other reactant, as then the rate will increase.
• By increasing the concentration of the solution, we increase the number of collisions that happen between reactants.
• There will be more successful collisions.
• The rate will increase.

### Changing pressure

• In a gas, the molecules are far apart.
• Consider the ideal gas law: pV=nRT.
• Increasing pressure reduces the volume of the gas.
• This means the molecules are closer together.
• If the molecules are closer together, they will collide more often
• Think of this as like being on a tube platform in rush hour; the closer everyone is, the more you bump into people.
• This means that there are more successful collisions
• The rate will increase.

### Catalysis

• A catalyst helps to increase the reaction rate by offering an alternative reaction pathway.
• The rate is higher because the new pathway has a lower activation energy.
• So a greater proportion of molecules will have energy greater than the activation energy.
• So there will be more successful collisions.

## Diagrams to Explain Catalysis

Enthalpy profile diagrams and the Maxwell-Boltzmann Distribution can be used to explain how catalysts work.

### Enthalpy profile

• Here you can see both reaction pathways.
• The catalytic pathway has a lower hump.
• The hump represents the activation energy.
• So the catalytic pathway has a lower activation energy.

### Maxwell-Boltzmann graph

• Here you can see the different activation energies for each pathway.
• Some molecules can use the old pathway.
• Some molecules can use the new pathway but not the old pathway.
• Some molecules can't use any.
• Overall, the catalytic pathway can be used by more molecules.
• More molecules have enough energy to react via the catalysed pathway.