L3 Factors Affecting Rate

Core Objectives:

Describe the effect of concentration, particle size, catalysts and temperature on the rate of reaction

Describe how concentration, temperature and surface area create a danger of explosive combustion with fine powders (e.g. flour mills) and gases (e.g. methane in mines)

Extended Objectives:

Describe and explain the effect of changing concentration in terms of frequency of collisions between reacting particles

Describe and explain the effect of changing temperature in terms of the frequency of collisions between reacting particles and more colliding particles possessing the minimum energy (activation energy) to react

Temperature

The average speed at which particles travel is measured as Temperature.

Particles at a higher temperature will travel faster than those with lower energy.

In the above diagram you can see that particles at a higher temperature will travel faster and speed is represented by the size and thickness of the arrows.

As particles will travel faster they will collide more often thus - At higher Temperatures the frequency of successful collisions will increase

Concentration

Concentration is defined as the number of particles of a substance in a given area. In Chemistry we use Litres as the standard area.

In a chemical reaction - if you have more particles of reactant then you are more likely to have more collisions.

Thus Increasing Concentration will increase the frequency of successful collisions.

Pressure

As Gases are the only phase of matter that can be compressed changing Pressure only effects gaseous reactions.

When the Pressure of a reaction is increased Volume is reduced. This means that the space between the particles is reduced at higher pressure and thus they have a smaller distance to travel to collide.

Therefore the frequency of successful collisions is higher at higher pressures for gaseous reactions.

Surface Area

The Surface Area of Reactants only relates to Solids.

When particles are clumped together in solids the inner particles are unavailable to react as other particles are found in between them and the particles on the surface.

Thus if you split these particles up so they are all exposed to react you would increase the rate of reaction.

We say that particles are more available to react this would increase the frequency of successful collisions

Catalysts

Catalysts are termed as 'A Species that Speeds up a Chemical Reaction without being used up (or chemically altered) in the process'

They are often (but not always) metals found in the Transition Metals area of the Periodic Table.

They work by either offering an alternate reaction pathway of lower activation energy

OR

Providing a surface for Reaction

Dangerous Applications of Rates of Reaction

So you know that variables affecting the rate of a reaction are temperature, concentration, and surface area. These are applicable to real life situations.

In a flour mill, flour is produced. Flour particles are very small, so they have a large surface area. If there is a lot of flour in the air, a small spark can cause an explosion between flour and oxygen.

In a coal mine, the air is filled with flammable gases. If the gases reach a certain concentration they can form an explosion with air.