FACTORS AFFECTING THE RATE OF CHEMICAL REACTION

From previous classes we know that: Reactants------------------> Products, or a number of reactants combine to form a product / products. We can also say that the particles in the products collide in order for a reactant to be formed.

Depending on the types of reactant the time taken for a reaction to occur can vary considerably.

For example, the chemical reactions that occur in the formation of crude oil occur over huge periods of time whereas the chemical reactions betwen Sodium and Acid occur very quickly. So how do we determine / measure the rate of a reaction? We can say:

Rate of Reaction = amount of reactant(s) used or product(s) formed / Time Taken

We can look at either how much reactant is used up in the chemical reaction or the amount of product formed. Below is a simplified graph to show this relationship:

It can be seen that with higher temperatures and higher concentration and smaller pieces the line of the graph showing the relationship between the total amount of products used and the time taken is steeper, thus indicating the reaction is happening more quickly.

Equally if this graph was showing the total amount of reactants being used against time then the steeper line would, again, indicate, a faster rate of reaction.


What this graph tells us then is that there are a number of factors that determine the rate of the reaction. These are listed below:

• The temperature at which the reaction takes place
• The concentration of reactants being used
• The pressure at which the reaction is occuring
• The size of the pieces of reactants being used in the reaction
• The addition of a catalyst

In simple terms:

•  the higher the temperature the faster the rate of reaction
• the greater the concentration of dissolved reactants the faster the rate of reaction
• The higher the pressure of a reactsnt gas the faster the rate of reaction
• the smaller the pieces of a solid reactant the faster the rate of reaction
• if a catalyst is added the faster the rate of reaction

Again, keeping it simple:

• with higher temperatures the greater the energy of the particles in the products and the more likely they are to react when they collide with other particles
• with greater concentrations of particles or pressure in gases the greater number of particles there are thereby increasing the chances of collisions at high enough energy levels to create a reaction
• with smaller pieces of solids both the surface area and the number of pieces are increased, thereby increasing the number of collisions which will speed up the chemical reaction
• if a catalyst is added the activation energy needed to result in a reaction when particles collide is lowered, thereby increasing the rate of the reaction. Catalysts are added to a reaction but are not used up

The slide show below gives a greater depth of detail if you are interested:

Reaction Rates

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