Factors Affecting Reaction Rates

There are FOUR factors that affect the rate of a reaction:

1. Temperature

2. Pressure/Concentration

3. Surface Area

4. Using a Catalyst

 

Collision Theory

To understand the factors affecting the RATE of a reaction, you can think of all the reactant and products as PARTICLES.

Let's consider the following reaction:

 

At the start of the reaction, only REACTANT particles are present. These particles are constantly moving around the container and COLLIDE with each other.

Most of these collisions are UNSUCCESSFUL and do NOT result in a REACTION.

If two particles COLLIDE with an energy GREATER than the ACTIVATION ENERGY, they REACT and form the product particles. This is known as a SUCCESSFUL COLLISION.

 

 

ACTIVATION ENERGY: The minimum amount of energy particles need to collide with in order to react.

The rate of reaction INCREASES when:

• There are MORE FREQUENT COLLISIONS.
• There are MORE ENERGETIC COLLISIONS.

 

 Temperature

• As TEMPERATURE increases, the KINETIC ENERGY of particles INCREASES. This means particles move with GREATER SPEEDS, leading to MORE FREQUENT COLLISIONS.
• Furthermore, the collisions are MORE ENERGETIC, meaning a greater proportion of the collisions have an energy greater than the ACTIVATION ENERGY.
• This means the rate of reaction is FASTER.

 

Concentration and Pressure

• The CONCENTRATION of reactants in a solution or the PRESSURE of reacting gases affect the number of particles in a given volume.

• A higher CONCENTRATION or PRESSURE results in particles being CLOSER together, meaning there are MORE FREQUENT COLLISIONS, which INCREASES the rate of reaction.

 

Surface Area

• You can increase the surface area to volume ratio of a reactant by breaking it down into SMALLER PARTICLES (such as powder).

• By increasing the SURFACE AREA, we expose more reactant particles to possible collisions.
• Smaller particle sizes or finely divided materials present a larger SURFACE AREA to volume ratio, allowing for MORE FREQUENT COLLISIONS and a faster reaction rate as compared to larger chunks of the same substance.

 

Catalysts

• CATALYSTS are substances that INCREASE the reaction RATE without being USED UP in the reaction.
• This means they will NOT appear in the chemical equations.
• Catalysts are SPECIFIC, meaning different reactions need different catalysts.
• ENZYMES are examples of CATALYSTS that increase the rate of biological reactions.
• Catalysts work by providing an ALTERNATIVE REACTION PATHWAY with a LOWER ACTIVATION ENERGY.
• You can see activation energies on REACTION PROFILES:

 

The ACTIVATION ENERGY is represented by the height from the REACTANTS to the PEAK of the graph.

You can see that the activation energy for the reactions WITH a catalyst is LOWER.

This means a greater proportion of the particles will have an energy GREATER than the ACTIVATION ENERGY, meaning there will be MORE FREQUENT COLLISIONS, which will INCREASE the rate of the reaction.