1. 1
   A student dissolves 8.00 g of solid ammonium nitrate, NH₄NO₃ (M = 80.0 g mol⁻¹), in 100 g of water in an insulated polystyrene cup. The temperature of the water falls from 22.0 °C to 15.2 °C. (specific heat capacity of the solution, c = 4.18 J g⁻¹ K⁻¹; assume the mass of the solution equals the mass of water plus the mass of solid dissolved)

   (a)

   Is this dissolution process exothermic or endothermic, and how can you tell?

   [1 mark]

   • AEndothermic — the temperature of the solution decreases, showing that the dissolving process absorbs thermal energy from the solution
   • BExothermic — the temperature decrease shows that energy is being released to the surroundings
   • CEndothermic — the temperature decrease shows energy is released by the solid as it dissolves
   • DExothermic — dissolving processes are always exothermic regardless of temperature change

   (b)

   What is the total mass of solution used in the calculation?

   [1 mark]

   • A100 g
   • B108 g
   • C92 g
   • D116 g

   (c)

   Calculate the heat absorbed by the dissolution process, in J.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Methane burns in oxygen: CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(g). Average bond enthalpies, in kJ mol⁻¹: C–H 414, O=O 498, C=O 804, O–H 463.

   (a)

   How many C–H bonds must be broken per mole of methane that reacts?

   [1 mark]

   • A1
   • B2
   • C4
   • D3

   (b)

   How many O=O bonds must be broken per mole of methane that reacts, according to the equation?

   [1 mark]

   • A1
   • B3
   • C4
   • D2

   (c)

   Using the bond enthalpies given, calculate the enthalpy change, ΔH, for the combustion of one mole of methane.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Bioethanol is produced by fermenting sugars from crops such as sugarcane, then used as a fuel blended with petrol. The complete combustion of ethanol is: C₂H₅OH(l) + 3O₂(g) → 2CO₂(g) + 3H₂O(l). During growth, the sugarcane plants absorb CO₂ from the atmosphere by photosynthesis: 6CO₂(g) + 6H₂O(l) → C₆H₁₂O₆(aq) + 6O₂(g) (using light energy).

   (a)

   Explain why bioethanol is often described as 'carbon neutral', using the two equations given.

   [3 marks]

   (b)

   Evaluate the claim that bioethanol is completely carbon neutral, considering the whole production process rather than just growth and combustion.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Data for the Born–Haber cycle of sodium bromide, NaBr, in kJ mol⁻¹: standard enthalpy of formation, ΔHf⦵(NaBr,s), −361; enthalpy of atomisation of sodium, +107; first ionization energy of sodium, +496; enthalpy of atomisation of bromine (Br₂(l) → 2Br(g), per mole Br), +112; electron affinity of bromine, −325. The standard entropy change for Na(s) + ½Br₂(l) → NaBr(s) is ΔS⦵ = −190 J K⁻¹ mol⁻¹.

   (a)

   Use the Born–Haber cycle data given to calculate the lattice enthalpy of sodium bromide, ΔH_lattice(NaBr).

   [6 marks]

   (b)

   Given the standard entropy change ΔS⦵ = −190 J K⁻¹ mol⁻¹ for Na(s) + ½Br₂(l) → NaBr(s), calculate ΔG⦵ for this reaction at 298 K, and use your answer to justify whether the formation of NaBr is spontaneous at this temperature.

   [6 marks]

   Total for question 4: 12 marks

5. 5
   A student reacts 2.10 g of solid sodium hydrogencarbonate (M = 84.0 g mol⁻¹) with excess dilute citric acid solution in an insulated cup containing 50.0 cm³ of the acid. The temperature falls from 21.5 °C to 16.3 °C. (Assume the density of the acid solution is 1.00 g cm⁻³, its specific heat capacity is 4.18 J g⁻¹ K⁻¹, and the total mass of the final solution equals the mass of acid solution plus the mass of solid dissolved.)

   (a)

   Is this reaction exothermic or endothermic, and how can you tell?

   [1 mark]

   • AExothermic — the temperature decrease shows that energy is being released to the surroundings
   • BEndothermic — the temperature of the solution decreases, showing that the reaction absorbs thermal energy from the solution
   • CExothermic — reactions that produce a gas are always exothermic
   • DEndothermic — the temperature decrease shows energy is released by the products

   (b)

   What is the total mass of the final solution used in the calculation?

   [1 mark]

   • A52.1 g
   • B50.0 g
   • C44.5 g
   • D84.0 g

   (c)

   Calculate the heat absorbed by the reaction, in J.

   [2 marks]

   Total for question 5: 4 marks

6. 6
   Ethyne (acetylene), C₂H₂, is formed directly from its elements: 2C(graphite) + H₂(g) → C₂H₂(g). Standard enthalpies of combustion, ΔHc⦵, in kJ mol⁻¹: C(graphite) −393.5; H₂(g) −285.8; C₂H₂(g) −1300.0.

   (a)

   Which expression correctly uses Hess's law to find ΔHf⦵(C₂H₂) from these combustion data?

   [1 mark]

   • AΔHc(product) − ΣΔHc(reactants)
   • BΣΔHc(reactants) + ΔHc(product)
   • CΔHc(product) alone
   • DΣΔHc(reactants) − ΔHc(product)

   (b)

   What is the sum of the enthalpies of combustion of the reactants, 2C(graphite) + H₂(g)?

   [1 mark]

   • A−679.3
   • B−1300.0
   • C−1072.8
   • D−393.5

   (c)

   Calculate ΔHf⦵(C₂H₂), in kJ mol⁻¹, using the data given.

   [2 marks]

   Total for question 6: 4 marks

7. 7
   A butane gas heater, C₄H₁₀, is used in a small, poorly ventilated room. With a good supply of air the butane burns completely, but with limited air it burns incompletely, producing a mixture of carbon monoxide and carbon dioxide as well as water vapour.

   (a)

   Deduce balanced equations for the complete combustion of butane and for its incomplete combustion to carbon monoxide only.

   [3 marks]

   (b)

   Explain why incomplete combustion producing carbon monoxide is dangerous, and suggest, with a reason, how the risk could be reduced.

   [4 marks]

   Total for question 7: 7 marks

8. 8
   Hydrogen peroxide decomposes according to the equation 2H₂O₂(l) → 2H₂O(l) + O₂(g). Standard enthalpies of formation, ΔHf⦵, in kJ mol⁻¹: H₂O₂(l) −188; H₂O(l) −286; O₂(g) 0. Standard entropies, S⦵, in J K⁻¹ mol⁻¹: H₂O₂(l) 110; H₂O(l) 70; O₂(g) 205.

   (a)

   Calculate the standard enthalpy change, ΔH⦵, for the decomposition of hydrogen peroxide, using the enthalpy of formation data given.

   [6 marks]

   (b)

   Calculate the standard entropy change, ΔS⦵, and the standard Gibbs energy change, ΔG⦵, for this reaction at 298 K, using your enthalpy value from part (a). Hence deduce whether the decomposition of hydrogen peroxide is spontaneous at 298 K.

   [6 marks]

   Total for question 8: 12 marks

End of questions