A LevelChemistryCambridge (CIE)Exam Questions8. Reaction KineticsHomogeneous & Heterogeneous Catalysts

Homogeneous & Heterogeneous Catalysts (Cambridge (CIE) A Level Chemistry): Exam Questions

Exam code: 9701

48 mins13 questions

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Homogeneous & Heterogeneous Catalysts

Medium
Hard
1a
2 marks

The first step in the production of nitric acid involves the reversible reaction of gaseous ammonia in air to form nitrogen(II) oxide gas and water vapour.

Construct an equation for this reaction. Include state symbols.

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1b
2 marks

For this reaction, a fine mesh is powder-coated with a mixture of platinum and rhodium catalysts.

State whether the catalyst acts as a homogeneous or heterogeneous catalyst in this reaction. Explain your answer.

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1c
1 mark

Explain why the catalyst does not affect the yield of the products in the reaction described in (a).

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2a
2 marks

The decomposition of 3% hydrogen peroxide solution into water and oxygen is a very slow chemical reaction. 

Construct an equation for this reaction. Include state symbols.

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2b
2 marks

The decomposition of 3% hydrogen peroxide solution can be catalysed by different chemicals including solid manganese dioxide and catalase solution.

State whether each catalyst acts as a homogeneous or heterogeneous catalyst. Explain your answer.

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2c
3 marks

For the exothermic decomposition of hydrogen peroxide, manganese dioxide gives a greater increase in the rate of reaction than catalase solution.

On Fig. 2.1:

  • Sketch a reaction pathway diagram for the reaction using each catalyst.

  • Label the diagram to show the enthalpy change, ΔH, and the activation energy, Ea, for each reaction.

blank-reaction-pathway-diagram

Fig. 2.1

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3a
3 marks

Gaseous A and B were added together to produce C as shown.

A (g) + 2B (g) ⇌ C (g)

Fig. 3.1 shows the production of C over time.

Concentration-time graph showing the concentration of C increasing and levelling off at equilibrium

Fig. 3.1

i) On Fig. 3.1, sketch a line to show what happens to the concentrations of A and B during the progress of the reaction.

[2]

ii) On Fig. 3.1, label the point at which an equilibrium is first established.

[1]

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3b
3 marks

State and explain the effect of a catalyst on the rate of reaction and the equilibrium yield of the reaction in (a).

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3c
6 marks

A catalyst was added to speed up the rate of reaction in part (a).

i) Sketch a Boltzmann distribution on the axes in Fig. 3.2 to show the distribution of molecular energies at a constant temperature with and without a catalyst.

Use Ea to label the activation energy without a catalyst. Use Ea,cat to label the activation energy with a catalyst.

[3]

Blank axes for sketching a Boltzmann distribution, with number of molecules on the y-axis and energy on the x-axis

Fig. 3.2

ii) Use the Boltzmann distribution to explain how the addition of a catalyst increases the rate of reaction.

[3]

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1a
3 marks

Reaction rates can be affected by a range of factors including changes in pressure and temperature.

Fig 1.1

Blank axes for sketching Boltzmann distribution curves, with number of molecules on the y-axis and energy on the x-axis

On Fig 1.1:

i) Sketch one Boltzmann distribution labelled T1 and a second Boltzmann distribution at a higher temperature labelled T2.

[2]

ii) Use Fig 1.1 to state how the proportion of molecules with energy greater than or equal to the activation energy, Ea, changes when temperature increases from T1 to T2.

[1]

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1b
6 marks

Hydrogen iodide can be used in the manufacturing of pharmaceuticals and can be broken down back into its elements in standard form, iodine and hydrogen.

2HI (g) → H2(g) + I2(g) ΔH = −52 kJ mol-1

The activation energy when uncatalysed is +183 kJ mol-1 and when catalysed with gold it is +105 kJ mol-1.

i) Sketch a reaction profile for the reaction, including the curves for the activation energies for both the catalysed and uncatalysed reactions.

[2]

ii) Calculate the activation energy for the reverse reaction in both the uncatalysed and catalysed reactions. Show your working.

[2]

iii) Explain why increasing the concentration of hydrogen iodide gas results in a faster reaction rate.

[2]

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1c
3 marks

Catalysts are often used in industrial processes and can be used in a variety of forms.

i) Explain why it is likely that the solid gold catalyst was used in powder form to catalyse the reaction mentioned in part (b).

[1]

ii) Gold is a heterogeneous catalyst used in the formation of hydrogen iodide. State the difference between a homogeneous and heterogeneous catalyst.

[1]

iii) State how, if at all, the area under the curve of a Boltzmann distribution changes as a catalyst is introduced, without changing the temperature or the total number of molecules.

[1]

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1d
3 marks

The Contact process is an important industrial process, contributing to the production of sulfuric acid. In the Contact process, solid vanadium(V) oxide, a heterogeneous catalyst, is used to make sulfur trioxide from sulfur dioxide and oxygen. This process is reversible.

i) Construct a balanced symbol equation for this reaction. Include state symbols in your answer.

[1]

ii) Explain the mode of action of the vanadium(V) oxide catalyst in the Contact process.

[2]

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