Introduction  (Page 2/3)

Factors affecting reaction rates

Several factors affect the rate of a reaction. It is important to know these factors so that reaction rates can be controlled. This is particularly important when it comes to industrial reactions, so that productivity can be maximised. The following are some of the factors that affect the rate of a reaction.

1. Nature of reactants Substances have different chemical properties and therefore react differently and at different rates.
2. Concentration (or pressure in the case of gases) As the concentration of the reactants increases, so does the reaction rate.
3. Temperature If the temperature of the reaction increases, so does the rate of the reaction.
4. Catalyst Adding a catalyst increases the reaction rate.
5. Surface area of solid reactants Increasing the surface area of the reactants (e.g. if a solid reactant is broken or ground up into smaller pieces) will increase the reaction rate.

Experiment : the nature of reactants.

Aim:

To determine the effect of the nature of reactants on the rate of a reaction.

Apparatus:

Oxalic acid ((COOH) ${}_2$ ), iron(II) sulphate (FeSO ${}_4$ ), potassium permanganate (KMnO ${}_4$ ), concentrated sulfuric acid (H ${}_2$ SO ${}_4$ ), spatula, test tubes, medicine dropper, glass beaker and glass rod.

Method:

1. In the first test tube, prepare an iron (II) sulphate solution by dissolving about two spatula points of iron (II) sulphate in 10 cm ${}^3$ of water.
2. In the second test tube, prepare a solution of oxalic acid in the same way.
3. Prepare a solution of sulfuric acid by adding 1 cm ${}^3$ of the concentrated acid to about 4 cm ${}^3$ of water. Remember always to add the acid to the water , and never the other way around.
4. Add 2 cm ${}^3$ of the sulfuric acid solution to the iron(II) and oxalic acid solutions respectively.
5. Using the medicine dropper, add a few drops of potassium permanganate to the two test tubes. Once you have done this, observe how quickly each solution discolours the potassium permanganate solution.

Results:

• You should have seen that the oxalic acid solution discolours the potassium permanganate much more slowly than the iron(II) sulphate.
• It is the oxalate ions (C ${}_2$ O ${}_4^{2-}$ ) and the Fe ${}^{2+}$ ions that cause the discolouration. It is clear that the Fe ${}^{2+}$ ions act much more quickly than the C ${}_2$ O ${}_4^{2-}$ ions. The reason for this is that there are no covalent bonds to be broken in the ions before the reaction can take place. In the case of the oxalate ions, covalent bonds between carbon and oxygen atoms must be broken first.

Conclusions:

The nature of the reactants can affect the rate of a reaction.

Experiment : surface area and reaction rates.

Marble ( $CaC{O}_3$ ) reacts with hydrochloric acid (HCl) to form calcium chloride, water and carbon dioxide gas according to the following equation: