1. 1
   Butan-1-ol, C₄H₉OH, can be made by fermenting plant material and is being trialled as a fuel for portable heaters. In a well-ventilated room it burns with a clean blue flame; in a small, closed tent the flame turns yellow and a black deposit forms on the heater.

   (a)

   Which equation represents the complete combustion of butan-1-ol?

   [1 mark]

   • AC₄H₉OH + 6½O₂ → 4CO₂ + 5H₂O
   • BC₄H₉OH + 6O₂ → 4CO₂ + 5H₂O
   • CC₄H₉OH + 4O₂ → 4CO + 5H₂O
   • DC₄H₉OH + 6O₂ → 4CO₂ + 4H₂O

   (b)

   Which equation represents incomplete combustion of butan-1-ol in which the only products are carbon and water?

   [1 mark]

   • AC₄H₉OH + 4O₂ → 4CO + 5H₂O
   • BC₄H₉OH + 2½O₂ → 4C + 5H₂O
   • CC₄H₉OH + 3O₂ → 4C + 5H₂O
   • DC₄H₉OH + 2O₂ → 4C + 5H₂O

   (c)

   Explain why the yellow flame and black deposit appear in the closed tent, and state one hazard of using the heater there.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A direct methanol fuel cell powers a laptop. Methanol and water are supplied to one electrode and air to the other; the two electrodes are separated by a membrane that conducts hydrogen ions, and the only products are carbon dioxide and water. The cathode half-equation is O₂ + 4H⁺ + 4e⁻ → 2H₂O.

   (a)

   Which half-equation represents the reaction at the anode?

   [1 mark]

   • ACH₃OH + H₂O + 6e⁻ → CO₂ + 6H⁺
   • BCH₃OH → CO₂ + 4H⁺ + 4e⁻
   • CCH₃OH + H₂O → CO₂ + 6H⁺ + 6e⁻
   • D2H₂O → O₂ + 4H⁺ + 4e⁻

   (b)

   Which statement about the fuel cell is correct?

   [1 mark]

   • AIt converts the chemical energy of the fuel directly into electrical energy.
   • BIt stores electrical energy supplied by charging and releases it later.
   • CIt produces no carbon dioxide because methanol is not burned.
   • DIt needs an external electrical supply to make the reaction happen.

   (c)

   Deduce the overall equation for the reaction in the fuel cell.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   An energy analyst compares four fuels by the energy released when one mole burns completely, in kJ mol⁻¹: methane (natural gas), CH₄, 890; octane (petrol), C₈H₁₈, 5470; carbon (a model for coal), C, 394; ethanol, C₂H₅OH, 1367. The molar mass of carbon dioxide is 44.01 g mol⁻¹. The ethanol is bioethanol made by fermenting sugar cane.

   (a)

   Calculate the mass of carbon dioxide produced when octane burns completely to release 1.00 MJ of energy. Give your answer to three significant figures.

   [3 marks]

   (b)

   Evaluate the claim that switching power stations from coal to natural gas, and cars from petrol to bioethanol, would both reduce the carbon dioxide added to the atmosphere per unit of energy.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Measurements at the Mauna Loa observatory show atmospheric carbon dioxide rising from 316 ppm in 1959 to 421 ppm in 2023. Over the same period, the burning of coal, crude oil and natural gas has increased several-fold. Energy released on complete combustion: octane (petrol) 47.9 kJ g⁻¹; ethanol 29.7 kJ g⁻¹. Growing sugar cane for bioethanol in some countries has involved clearing forest land.

   (a)

   Explain how the burning of fossil fuels contributes to the greenhouse effect, using the data to support your answer.

   [6 marks]

   (b)

   Discuss the advantages and disadvantages of bioethanol compared with petrol as a fuel. Include the equation for photosynthesis in your answer.

   [6 marks]

   Total for question 4: 12 marks

End of questions