An increase in the concentration of reactants in any reaction system will increase the rate of the reaction; while a decrease in the concentration of the reactants will decrease the rate.

This is because increasing the concentration brings the reactant particles closer to each other, decreasing the distance between reacting particles, making it possible for many effective collisions to take place per unit time and thus increasing the rate of the chemical reaction.

On the other hand, decreasing the concentration of the reactants entails that fewer reactant particles are present, increasing the distance between reacting particles hence fewer effective collisions and as a consequence decrease in the rate of a chemical reaction.

• Experimental investigation on the effect of concentration on rate of reaction.

Example:

Reaction between magnesium and hydrochloric acid.

Mg (s) + 2HCI (aq)                       MgCl₂ (aq) + H₂ (g)

Requirements

Magnesium ribbon Hydrochloric acid of different concentrations E.g. 0. 5M 1. 0M, 2. 0M, three 100cm3 beakers, a stop-watch.

Procedure

Place 50Cm³ of 0.5M hydrochloric acid into a beaker and add 3cm of magnesium ribbon. Start a stop-watch immediately. Make a note of how long it takes for the magnesium to react completely. Repeat the experiment using 50Cm³ each of 1.0M and 2.0M hydrochloric acid respectively. Plot the rate curves for the different concentrations of the acid on the same graph axes.

Observation

The rate of reaction is faster, especially at the beginning, if the acid is more concentrated. The reaction also goes to completion in the case of a more concentrated Acid.

Conclusion: the more concentrated the acid solution, the steeper the curve and the greater is the rate of reaction

• Pressure (gases)

Pressure is important only in gaseous systems.

This is because increasing pressure will compress the gaseous molecules, bringing them closer to one another. In effect, increasing the pressure is synonymous to increase in concentration. Me overall effect is that there will be more effective collisions and hence an increase in the reaction rate. A decrease in pressure will lead to the reactant molecules being further apart and thus fewer collisions. As a consequence the reaction rate will decrease.

For example in the Haber process for the manufacture of ammonia, a high pressure of about 200 atmospheres is used to ensure that the process does not become too slow and become economically non profitable.