Year 12 Chemistry Chapter 7

The diagram shown to the left represents the Maxwell-Boltzmann distribution of an uncatalysed reaction.

The effect of adding a catalyst could be:

1. the curve flattens to reflect the lower activation energy barrier

2. the curve shifts to the right

3. the Ea shifts to the left.

Which of the statements above are correct?

• 1 only

• 1 and 2

• 2 and 3

• 3 only

For the reaction N₂(g) + 3H₂(g) ⇌ 2NH₃(g) ΔH = −92.3 kJ mol⁻¹

• a catalyst increases the number of collisions between the reactants.

• the rate of the forward reaction increases when the temperature increases.

• a catalyst reduces the activation energy of the forward and backward reactions by the same proportion.

• the activation energy of the forward reaction is greater than the activation energy of the reverse reaction.

An energy profile for a particular uncatalysed reaction is shown to the left.

If a catalyst was used in the reaction, which values would change?

• W, X and Z only

• W, Y and Z only

• W and Z only

• X and Y only

The effectiveness of two catalysts, MnO₂ and CuO, in catalysing the decomposition of hydrogen peroxide was tested by measuring the volume of oxygen gas released over time. The results of the experiment are

shown to the left.

Some variables of the decomposition reaction which can be measured in the laboratory include:

I enthalpy change

II rate of reaction

III activation energy

IV equilibrium constant

Which of the above variables are changed by using CuO as a catalyst rather than MnO₂?

• I and II only

• II and III only

• I, III and IV only

• I, II, III and IV

• decreasing the size of the solid copper particles

• increasing the temperature of HNO₃ by 20 °C

• increasing the concentration of HNO₃

• allowing NO₂ gas to escape

In the above reaction, the number of successful collisions per second is a small fraction of the total number of collisions.

The major reason for this is that

• the nitric acid is ionised in solution.

• some reactant particles have too much kinetic energy.

• the kinetic energy of the particles is reduced when they collide with the container’s walls.

• not all reactant particles have the minimum kinetic energy required to initiate the reaction.

Shown below in Figure 6 are Maxwell-Boltzmann curves for four different reactions, all occurring at the same temperature.

The activation energies of the four reactions are represented by the symbols E_a1, E_a2, E_a3 and E_a4 respectively.

The order of reaction rates from slowest to fastest is

• Reaction 1 < Reaction 2 < Reaction 3 < Reaction 4.

• Reaction 3 < Reaction 1 < Reaction 4 < Reaction 2.

• Reaction 4 < Reaction 2 < Reaction 1 < Reaction 3.

• Reaction 3 < Reaction 1 < Reaction 2 < Reaction 4.

Two experiments are conducted to investigate the rate of reaction between CaCO₃ (molar mass = 100.1 g mol–1) and HCl.

Experiment 1 – 1 g of powdered CaCO3 is added to 20 mL of 1.0 M HCl.

Experiment 2 – 2 g of powdered CaCO3 is added to 20 mL of 1.0 M HCl.

In experiment 2

• gas is evolved at the same rate but twice the volume of gas is evolved.

• gas is evolved at the same rate and the same volume of gas is evolved.

• gas is evolved faster but the same volume of gas is evolved.

• gas is evolved faster and twice the volume of gas is evolved.

The kinetic energy of the particles involved in the reaction can be represented by the diagram shown to the left.

If a catalyst is added:

• the whole graph will shift to the right meaning that more molecules will have enough energy to react

• the activation energy shifts to the right meaning that more molecules will have enough energy to react

• the activation energy shifts to the left meaning that more molecules will have enough energy to react

• the activation energy line stays where it is but the height of the graph at the activation energy point will increase meaning that more molecules will have enough energy to react

A catalyst

• slows the rate of reaction.

• ensures that a reaction is exothermic.

• moves the chemical equilibrium of a reaction in the forward direction.

• provides an alternative pathway for the reaction with a lower activation energy.