Factors that Affect the Rate of a Reaction

There are 4 basic factors that can affect the rate of a chemical reaction:

1. Temperature

Temperature and rate at AS level is connected very closely with the Maxwell-Boltzmann Distribution.

An increase or decrease in temperature will change the shape of the curve which means fewer or more particles will have the required activation energy.

As the number of molecules is constant so the areas under the 2 curves are the same.

BUT, the average energy of the molecules in curve T2 is greater, so there are more particles with enough energy to react. So there are basically 2 factors to consider:

  • Increase in energy of particles due to increase in average energy of particles.

  • Increase in speed of particles do there are more successful collisions with particles having the required activation energy.

These two factors result in the rate of the reaction increasing.

2. Concentration and pressure

If the concentration or pressure of a chemical increases, there will be more particles within a given space. 

  • Particles with therefore collide more often

  • As concentration or pressure increases, the rate of reaction also increases.

3. Physical state

If particles are in the same phase (liquid/liquid) or (gas/gas), then it is very easy for them to mix with each other. This gives particles the maximum opportunity to collide.

BUT, if one of the reactants is a solid, then the reaction can only take place on the surface of the solid.

  • The smaller the size of the solid particles, the greater the area that the reaction can take place in.

So finely divided powder reacts more quickly than the same stuff in a great big lump!

A situation that can arise with a 2 liquids or 2 gases is if they are immiscible (can't mix), this will mean that the reaction can only occur at the interface between the two fluids.

4. Catalyst

What is a catalyst?

A catalyst is a substance that alters the rate of a chemical reaction without being used up or permanently changed chemically.

How does it work?

A catalyst works by changing the energy pathway for a chemical reaction. It provides an alternative route (mechanism) that lowers the Activation Energy meaning more particles now have the required energy needed to undergo a successful collision. 

The above graph demonstrates what a catalyst does to the reaction profile. You will be expected to produce one of these!