Enthalpy changes and standard conditionsAQA A-Level Chemistry: Subtopic test

1. 1
   An instant cold pack used for sports injuries contains solid ammonium nitrate and a separate bag of water. When the bag is broken and the solid dissolves, the temperature of the mixture falls. An instant hand warmer works in the same way using calcium chloride, and the temperature rises. Both are measured at constant pressure.

   (a)

   Which statement correctly describes the dissolving of ammonium nitrate in the cold pack?

   [1 mark]

   • AIt is exothermic and ΔH is positive
   • BIt is endothermic and ΔH is positive
   • CIt is exothermic and ΔH is negative
   • DIt is endothermic and ΔH is negative

   (b)

   Which of these describes standard conditions for an enthalpy change?

   [1 mark]

   • A10 kPa and 298 K
   • BAny pressure and 298 K
   • C100 kPa and a stated temperature, usually 298 K
   • DZero pressure and 298 K

   (c)

   Explain why the temperature of the cold pack falls when the ammonium nitrate dissolves.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Ethanol, C₂H₅OH, is used as a biofuel. Its standard enthalpy of combustion is −1367 kJ mol⁻¹. Standard enthalpy changes refer to 100 kPa and 298 K.

   (a)

   Which equation represents the standard enthalpy of combustion of ethanol?

   [1 mark]

   • AC₂H₅OH(l) + 3O₂(g) → 2CO₂(g) + 3H₂O(g)
   • B2C(s) + 3H₂(g) + ½O₂(g) → C₂H₅OH(l)
   • C2C₂H₅OH(l) + 6O₂(g) → 4CO₂(g) + 6H₂O(l)
   • DC₂H₅OH(l) + 3O₂(g) → 2CO₂(g) + 3H₂O(l)

   (b)

   Which equation represents the standard enthalpy of formation of ethanol?

   [1 mark]

   • A2C(s) + 3H₂(g) + ½O₂(g) → C₂H₅OH(l)
   • B2C(g) + 6H(g) + O(g) → C₂H₅OH(l)
   • CC₂H₅OH(l) → 2C(s) + 3H₂(g) + ½O₂(g)
   • D2CO₂(g) + 3H₂O(l) → C₂H₅OH(l) + 3O₂(g)

   (c)

   Calculate the enthalpy change when 2.00 mol of ethanol burn completely under standard conditions, and state what the sign of the value shows.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Hydrogen is a possible clean fuel for vehicles. Under standard conditions, H₂(g) + ½O₂(g) → H₂O(l) has ΔH = −286 kJ mol⁻¹, and the standard enthalpy of combustion of methane, the main component of natural gas, is −890 kJ mol⁻¹. The relative molecular masses are 2.0 for hydrogen and 16.0 for methane.

   (a)

   Define standard enthalpy of combustion, and explain why the value for hydrogen is also the standard enthalpy of formation of liquid water.

   [3 marks]

   (b)

   Calculate the energy released, in kJ, when 5.0 g of hydrogen and when 5.0 g of methane burn completely, and state which fuel releases more energy per gram.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Nitrogen and oxygen react at the high temperature in a car engine to form nitrogen monoxide: ½N₂(g) + ½O₂(g) → NO(g), ΔH = +90 kJ mol⁻¹ under standard conditions. Three other reactions of carbon are: (1) C(s) + O₂(g) → CO₂(g), ΔH = −394 kJ mol⁻¹; (2) C(s) + ½O₂(g) → CO(g), ΔH = −111 kJ mol⁻¹; (3) CO(g) + ½O₂(g) → CO₂(g), ΔH = −283 kJ mol⁻¹. In each, carbon is graphite.

   (a)

   Explain what is meant by an enthalpy change, why standard conditions are specified, and what the value for nitrogen monoxide shows about the reaction and about why it forms in engines.

   [6 marks]

   (b)

   Identify which of reactions (1), (2) and (3) represent a standard enthalpy of formation and which represent a standard enthalpy of combustion, explaining each decision.

   [6 marks]

   Total for question 4: 12 marks

End of questions