 # Rates of Reaction

## Collision Theory

• The collision theory can be used to explain the factors affecting rates of reaction.
• The basic theory behind the collision theory is that chemicals react when their particles successfully collide.
• Take for instance the reaction between hydrogen and nitrogen to produce ammonia.
• The collision theory states that the reaction occurs when the hydrogen particles and nitrogen particles successfully collide.
• The more frequently they collide, the faster the rate of reaction.

## Effect of Pressure

• The higher the pressure, the more particles there are per volume. If there are more particles in a space, then there will be more collisions within a given time period. This means that the rate of reaction will be higher.

## Effect of Temperature

• When the temperature increases, the particles gain kinetic energy and move around faster. This results in more collisions.
• The speed of the molecules is proportional to the square root of the temperature (K).
• However, it’s not only how frequently the particles collide that matters, but also how much energy they collide with.
• The minimum energy needed for the reaction to occur is called the activation energy.
• Unless the particles collide with the energy equal to the activation energy, they just bounce off each other, and remain un-reacted.
• This is what happens most of the time during reactions, for example when N2 and H2 react at 300 K, only 1 in 1011 collisions results in a successful reaction.
• The collision theory states that reactions occur when molecules collide with a certain amount of energy. The more frequent these reactions, the faster the reaction.

## Distribution of Energies

• At a temperature, the kinetic energy of the molecules varies; the different molecules move at different speeds.
• Some molecules have high kinetic energies, some have medium, and some have low kinetic energies.
• The Maxwell-Boltzmann distribution can be used to represent the distribution of kinetic energies. • As the temperature increases, the molecules kinetic energy increases; they move around faster.
• There is still a distribution of energies at higher temperatures, however a greater proportion of molecules have higher energies. • These may look irrelevant to reaction rates; however they do have a purpose.
• If we add the activation energy to the distribution, then we can see how much of the distribution will result in successful collisions (below). • The molecules to the right of the activation energy line can react, whereas the molecules to the left of the line are unable to.
• Notice that the distributions with higher temperatures have more of the graph to the right of the line; therefore more of the molecules react.
• This can be summarized by saying that reactions go faster at higher temperatures because a larger proportion of molecules have the required activation energy needed to react.
• Increasing the temperature may not make a difference to the energy of each individual molecule, but it makes a big difference to the proportion of the molecules.

## Useful books for revision:

Revise AS Chemistry for Salters (Written by experienced examiners and teachers of Salter's chemistry) Revise AS Chemistry for Salters (OCR) (Salters Advanced Chemistry) Home