1. 1
   A reusable hand warmer contains a supersaturated solution of sodium acetate sealed inside a plastic pouch, along with a small metal disc. When the disc is clicked, sodium acetate crystallises out of solution rapidly and the pouch becomes noticeably warm to touch.

   (a)

   What type of change is occurring inside the hand warmer when the disc is clicked?

   [1 mark]

   • AAn exothermic process, because energy is transferred to the surroundings
   • BAn endothermic process, because energy is taken in from the surroundings
   • CAn exothermic process, because energy is taken in from the surroundings
   • DAn endothermic process, because energy is transferred to the surroundings

   (b)

   Which statement about the sign of ΔH for this change is correct?

   [1 mark]

   • AΔH is positive, since energy is released
   • BΔH is negative, since energy is released
   • CΔH is positive, since energy is absorbed
   • DΔH is zero, since no bonds change

   (c)

   Explain, in terms of energy transfer, why the pouch feels warm when the disc is clicked.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A student wants to estimate the energy content of a small sample of a snack food. She skewers a 1.20 g sample on a mounted needle, ignites it, and holds it beneath a boiling tube containing 25 cm3 of water, keeping the flame beneath the tube until the food sample burns out completely. She records the temperature of the water before and after.

   (a)

   Which equation should the student use to calculate the energy transferred to the water?

   [1 mark]

   • AQ = m + cΔT
   • BQ = mcΔT
   • CQ = mΔT / c
   • DQ = m / (cΔT)

   (b)

   Which value in this equation represents the specific heat capacity of water?

   [1 mark]

   • Athe 1.20 g mass of the food sample
   • Bthe volume of water used, 25 cm3
   • Cthe temperature change, ΔT
   • Dc, a fixed constant for water

   (c)

   State two variables the student should keep constant if she repeats this experiment with a different snack food, in order to make a fair comparison of energy content.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A car's catalytic converter contains a platinum-based catalyst that speeds up the reaction between carbon monoxide and oxygen, converting them into carbon dioxide, without the catalyst itself being used up. Engineers compare how quickly this reaction proceeds with and without the catalyst present.

   (a)

   Describe, without drawing a diagram, how the activation energy of this reaction compares with and without the catalyst present, and explain what this means for the energy needed to start the reaction.

   [3 marks]

   (b)

   Explain why the platinum catalyst is not used up during this reaction, and describe one advantage of using a catalyst in this process rather than simply increasing the temperature of the exhaust gases to speed up the reaction.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A chemical engineer is studying the Haber process reaction, in which nitrogen gas reacts with hydrogen gas to form ammonia gas: N2(g) + 3H2(g) → 2NH3(g). The average bond energies are: N≡N = 945 kJ/mol, H–H = 436 kJ/mol, N–H = 391 kJ/mol.

   (a)

   Use the bond energy data given to calculate the overall energy change for this reaction, and use your answer to explain whether the reaction is exothermic or endothermic.

   [6 marks]

   (b)

   A separate part of the plant tests a related reaction — the reaction of ammonia gas with water to form aqueous ammonia solution — using a simple solution calorimetry method in an insulated container, similar to testing a neutralisation reaction. Describe how this experimental method could be used to measure the energy change of this reaction, and discuss one limitation of this method compared with the bond energy calculation method used in part (a).

   [6 marks]

   Total for question 4: 12 marks

5. 5
   Members of a school science club build a simple electrical cell by pushing a strip of zinc and a strip of copper into a lemon, without letting the two metal strips touch each other. They connect the strips to a voltmeter using wires and observe a small potential difference reading.

   (a)

   Which feature of this set-up allows the cell to produce a potential difference?

   [1 mark]

   • AThe lemon provides sugar for the reaction to use as fuel
   • BThe two different metals and the acidic juice (electrolyte) allow charge to be transferred between them
   • CThe yellow colour of the lemon skin reacts with light to generate charge
   • DThe zinc and copper strips touching each other directly completes the circuit

   (b)

   The students record a decreasing voltmeter reading over several hours as the club continues using the cell. What does this decrease suggest?

   [1 mark]

   • AThe reactants at the electrodes are gradually being used up
   • BThe lemon is absorbing electricity from the wires
   • CThe zinc strip is dissolving into the copper strip
   • DThe voltmeter is broken and needs replacing

   (c)

   Suggest two changes the students could make to the cell that might increase the potential difference it produces, other than replacing the lemon.

   [2 marks]

   Total for question 5: 4 marks

6. 6
   A logistics warehouse replaces its diesel-powered forklift trucks with forklifts powered by hydrogen fuel cells, which react hydrogen gas from an onboard tank with oxygen from the air to generate electricity for the motor.

   (a)

   What is the only waste product released by the hydrogen fuel cell as the forklift operates?

   [1 mark]

   • Acarbon dioxide
   • Bcarbon monoxide
   • Csoot (particulates)
   • Dwater

   (b)

   Which of these is a practical disadvantage of using hydrogen fuel cells in the warehouse, compared with diesel forklifts?

   [1 mark]

   • AHydrogen fuel cells release toxic exhaust gases into the warehouse
   • BHydrogen fuel cells cannot produce electricity continuously
   • CHydrogen gas is difficult to store safely and requires specialised refuelling infrastructure
   • DHydrogen fuel cells produce more carbon dioxide per journey than diesel engines

   (c)

   Give two advantages of using hydrogen fuel cell forklifts rather than diesel forklifts inside an enclosed warehouse.

   [2 marks]

   Total for question 6: 4 marks

7. 7
   A school science department compares two ways of investigating energy changes. In the first, a technician dissolves a sample of anhydrous copper sulfate in 50 cm3 of water in an insulated cup and records the temperature change as the solution turns blue. In the second, a different technician burns a small candle made of paraffin wax beneath a metal can of water, recording the temperature change of the water as the candle burns for a fixed time.

   (a)

   Describe how the technician using the insulated cup method could calculate the energy released when the copper sulfate dissolves.

   [3 marks]

   (b)

   Explain one reason why the energy value calculated for the candle-and-can method is likely to be less accurate than the value calculated for the insulated-cup method, and suggest one improvement that could be made to the candle-and-can method.

   [4 marks]

   Total for question 7: 7 marks

8. 8
   A rail transport company is deciding between two possible power technologies for a new fleet of short-distance passenger trains on a route that is not electrified: hydrogen fuel cell trains, which generate electricity on board from a hydrogen tank, or trains powered by large rechargeable chemical cells (batteries) that are charged at stations and then discharge their stored chemical energy as they travel.

   (a)

   Evaluate the advantages and disadvantages of choosing hydrogen fuel cell trains for this route, compared with using diesel trains.

   [6 marks]

   (b)

   Evaluate whether large rechargeable chemical cells (batteries) would be a more suitable choice than hydrogen fuel cells for this train route, referring to how a chemical cell produces a potential difference.

   [6 marks]

   Total for question 8: 12 marks

End of questions