Compare — 4 marks
A chemical engineer is designing a new industrial process to manufacture ammonia. The process can operate at different temperatures and with or without a catalyst. The engineer has obtained two reaction profile diagrams: one showing the uncatalysed reaction pathway and one showing the same reaction with an iron catalyst present.
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(a) Compare the activation energies shown on the two reaction profile diagrams for the catalysed and uncatalysed ammonia synthesis reactions.
[1 mark]
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(b) Explain why the catalyst lowers the activation energy without being consumed in the reaction.
[2 marks]
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(c) The engineer notes that both reaction profiles show the same overall energy change (ΔH) between reactants and products. Compare and explain why the overall energy change remains the same whether or not a catalyst is used.
[1 mark]
Show mark scheme
- (a) The catalysed reaction has a lower activation energy than the uncatalysed reaction (accept: the catalyst reduces/decreases the activation energy)
- (b) The catalyst provides an alternative reaction pathway with lower activation energy (accept: alternative mechanism/route)
- (b) The catalyst is regenerated at the end of the reaction so it is not consumed/used up (accept: it is reformed/recovered)
- (c) The overall energy change depends only on the energy of reactants and products, not the pathway taken / the catalyst does not change the relative stability of reactants and products (accept: catalysts do not affect the starting or ending energy levels)
Explain — 2 marks
A student is investigating how a catalyst affects the rate of a chemical reaction between hydrochloric acid and marble chips (calcium carbonate). They measure the time taken for the reaction to complete with and without a catalyst present.
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(a) Explain what is meant by activation energy.
[1 mark]
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(b) Explain how a catalyst increases the rate of the reaction between hydrochloric acid and marble chips.
[1 mark]
Show mark scheme
- (a) The minimum/least energy required for reactant particles to collide and react successfully / to break bonds and form products
- (a) Accept: the energy barrier that must be overcome for a reaction to occur
- (b) A catalyst lowers/reduces the activation energy (of the reaction)
- (b) This means more particles have sufficient energy to react when they collide / more successful collisions occur per unit time
- (b) Accept: the reaction pathway requires less energy, so the reaction proceeds faster
Calculate — 2 marks
A student investigates the decomposition of hydrogen peroxide (H₂O₂) into water and oxygen. This reaction is very slow at room temperature. When manganese dioxide is added as a catalyst, the reaction speeds up significantly. The student draws a reaction profile showing the energy changes for the uncatalysed reaction. The energy of the reactants is 150 kJ/mol, the energy of the products is 50 kJ/mol, and the energy of the transition state (peak) is 350 kJ/mol.
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(a) Calculate the activation energy for the uncatalysed reaction.
[1 mark]
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(b) A catalyst provides an alternative pathway with an activation energy of 100 kJ/mol. Calculate the energy change when the catalyst is used.
[1 mark]
Show mark scheme
- (a) 200 (kJ/mol)
- (b) 100 (kJ/mol)
Explain — 3 marks
Hydrogen peroxide is a chemical used for bleaching hair and disinfecting wounds. It decomposes to form water and oxygen gas. The reaction is very slow at room temperature but speeds up when manganese(IV) oxide is added.
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(a) State what is meant by the term activation energy.
[1 mark]
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(b) Explain, using the idea of activation energy, how manganese(IV) oxide speeds up the decomposition of hydrogen peroxide.
[2 marks]
Show mark scheme
- (a) The minimum energy needed for a reaction to occur
- (b) The catalyst provides an alternative reaction pathway
- (b) Which has a lower activation energy (than the uncatalysed reaction)
Evaluate — 3 marks
A chemical company is choosing a catalyst for the Haber process to produce ammonia. Two catalysts were tested: an iron catalyst and a ruthenium catalyst. The reaction profile diagrams for the uncatalysed reaction and each catalysed reaction were recorded. The iron catalyst reduces the activation energy to 62 kJ/mol, while the ruthenium catalyst reduces it to 45 kJ/mol. However, iron costs £2 per kg while ruthenium costs £15,000 per kg.
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(a) State what is represented by the highest point on a reaction profile diagram.
[1 mark]
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(b) Evaluate which catalyst the company should choose for the Haber process.
[2 marks]
Show mark scheme
- (a) transition state / activated complex
- (b) ruthenium gives faster reaction rate (due to lower activation energy)
- (b) iron is much cheaper / more cost-effective
- (b) iron is sustainable choice for large-scale production
- (b) allow comparison of cost vs rate benefit
GCSE Perfect