## 1.4 Short Notes: Collision Theory Part 3

1. Effect of concentration on rate of reaction

Example: 1g of Magnesium ribbon in 10cm3 of H2SO4 aqueous solution with concentration of 0.2 mol dm-3 and 0.5 mol dm-3

• The higher the concentration, the greater the number of particles per unit volume of solution.
• The frequency of collisions between particles increases.
• The frequency of effective collisions also increases.
• Hence, the rate of reaction is higher at higher concentrations.
• To explain the concentration on rate of reaction, it can only be applied on aqueous solutions

2. Effect of pressure on rate of reaction

Example: Mixture of Hydrogen and Iodine gas in different pressure

• At higher pressure, the gas molecules have been squashed closer to each other.
• Pressure are only applicable to reactants made by gas. Only gasses can be affected by the change of pressure
• The higher the pressure, the greater the number of particles per unit volume
• The frequency of collisions between particles increases
• The frequency of effective collisions also increases
• Hence, the rate of reaction is higher at higher pressure

3. Effect of catalyst on rate of reaction.

Example: 20cm3 of 0.1mol dm-3 of Hydrogen Peroxide and 20cm3 of 0.1 mol dm-3 of Hydrogen Peroxide with 1g of Manganese Dioxide, MnO2 (catalyst)

• 2H2O2 → 2H2O + O2
• The experiment with the presence of catalyst only requires 10 seconds for the reaction to be completed, whereas the one without catalyst took up to 60 seconds for the reaction to be completed.
• Before the reactant particles can collide effectively to form products, they have to overcome an energy barrier known as the activation energy.
• When the catalyst is added, the activation energy is lower, the catalyst acts by lowering the activation energy, thus resulting in the increase of the frequency of effective collisions.
• Hence, a catalyst increases the rate of reaction.

Summary

1. When the particle size is small, there is a large surface area.
2. When the temperature is high, there are more kinetic energy
3. When the concentration is high, there are more particles per unit volume.
4. When the pressure is high, there are more particles per unit.
5. The existence of catalyst will lower the activation enrgy.
6. These will all lead to increase of collision energy (effective collision frequency) and thus increasing the rate of reaction.

Questions

1. What is the catalyst used for the decomposition of hydrogen peroxide?
2. How does a catalyst increase the rate of reaction?