9.1    Factors Affecting the Rate of Reaction

How to increase the rate of  reaction?

• According to the collision theory, the rate of reaction will increase if:
  1. the frequency of collision and effective collision increases
  2. the proportion of particles with energy greater than activation energy increases.

 

Effect of surface area

• The larger the surface area of the reactant particles, the higher the rate of reaction.
• This is because the surface area exposed for collision is larger, frequency of collision between the reactant particles increases and hence the frequency of effective collision also increases. More products are formed per unit time and hence the rate of reaction increases

• For example, the reaction between hydrochloric acid and marble chips(calcium carbonate) is as follow:

HCl(aq) + CaCO3(s) → CaCl2(aq) + CO2(g) + H2O(l)

The rate of reaction can be found by measuring the volume of CO2 gas given out per unit time. The volume of CO2 is determined at regular intervals. The set up of apparatus is as follow:

It is found that powdered marble chips react faster than big lumps of marble chips

• This is because powdered marble chips are smaller in size and hence have a larger total surface area

 

Effect of concentration

• The higher the concentration of the reactant particles, the higher the rate of reaction.
• This is because in a more concentrated solution, there are more reactant particles per unit volume. The frequency of collision between the reactant particles increases and hence the frequency of effective collision also increases. More products are formed per unit time and hence the rate of reaction is higher
• For example, the reaction between magnesium ribbon and hydrochloric acid is as follow:

Zn(s) + HCl(aq) → ZnCl2(aq) + H2(g)

The rate of reaction can be found by measuring the time taken for the magnesium ribbon to disappear from  sight.

 

It is found that when HCl has a higher concentration, the time taken for the magnesium ribbon to disappear is shorter.

 

• Since rate of reaction is inversely proportional to time, the shorter the time, the higher the rate of reaction

• There are also times where increasing the concentration does not help in increasing the rate of reaction:

i.   Reactions involving only a single reactant particle splitting

• The rate of reaction is not governed by the rate of  collision between them
• The rate is governed by the amount of energy possessed by the reactant particles.
• If a large proportion of the particles have energy greater than activation energy, more particles will split up per unit time and hence the rate of reaction is higher

ii.   Where a catalyst is working as fast as it can

• If a high enough concentration of reactant particles is mixed with a small amount of catalyst, the reaction proceeds
• The rate of reaction will not increase if the concentration of the reactant particles is increased because the catalyst is saturated.
• The rate of reaction can be increased by adding more catalysts

 

Effect of pressure

• The higher the pressure of the system, the higher the rate of reaction
• The explanation is the same as the one in the effect of concentration. However, increasing the pressure will only increase the rate of a reaction involving gases. Changing the pressure of a reaction which involves only solid or liquid has no effect on it

 

Effect of temperature

• The higher the temperature of the reacting system, the higher the rate of reaction.
• This is because at higher temperature, the average kinetic energy of the particles increases, so:
  1. more reacting particles have energy equal to or greater than the activation (This is the major reason)
  2. the reacting particles travel at higher speed, the frequency of collision between the reacting particles increases. (This is the minor reason)

This causes the frequency of effective collision to increase. More products are formed per unit time and hence, the rate of  reaction is  higher.

• In fact, the rate of reaction doubles for every increase in 10 °C.
• The effect of temperature on rate of reaction can be shown in a Maxwell- Boltzmann distribution A typical curve looks like this:

 

• The area under the curve represents the total number of molecules, it is constant if no additional molecules are added
• An increase in temperature increases the number of particles with higher energy, the curve shifts to the right. So that there are more particles with energy greater than the activation energy, as illustrated below:
• When drawing the curve at a higher temperature, the height of the curve should reduce so that the area under the curve remains constant