Enthalpy changes and enthalpy level diagramsEdexcel International A Level Chemistry: Subtopic test

1. 1
   A food company sells self-heating drink cans. The base of each can holds solid calcium oxide separated from water by a seal. Pulling a tab mixes them: CaO(s) + H₂O(l) → Ca(OH)₂(s), for which ΔH = –65 kJ mol⁻¹ at constant pressure.

   (a)

   Which statement about this reaction is correct?

   [1 mark]

   • AThe reaction is endothermic and the temperature of the can falls.
   • BThe reaction is exothermic: the products are at a lower enthalpy than the reactants and heat is given out.
   • CThe reaction is exothermic because the products are at a higher enthalpy than the reactants.
   • DThe reaction is endothermic because ΔH is measured at constant pressure.

   (b)

   Which equation represents the standard enthalpy change of formation of calcium hydroxide?

   [1 mark]

   • ACa(s) + O₂(g) + H₂(g) → Ca(OH)₂(s)
   • BCaO(s) + H₂O(l) → Ca(OH)₂(s)
   • CCa²⁺(aq) + 2OH⁻(aq) → Ca(OH)₂(s)
   • D2Ca(s) + 2O₂(g) + 2H₂(g) → 2Ca(OH)₂(s)

   (c)

   Describe the enthalpy level diagram for this reaction, stating what is on the vertical axis and what the labels and arrow show.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A teacher demonstrates two energy changes. First, ethanol is burned in a spirit burner; the standard enthalpy change of combustion of ethanol is –1367 kJ mol⁻¹. Second, dilute hydrochloric acid is mixed with aqueous sodium hydroxide in an insulated cup; the standard enthalpy change of neutralisation of a strong acid by a strong alkali is –57 kJ mol⁻¹.

   (a)

   Which is the correct definition of the standard enthalpy change of combustion of a substance?

   [1 mark]

   • AThe energy released when 1 g of the substance burns in air.
   • BThe enthalpy change when 1 mol of the substance is formed from its elements in their standard states.
   • CThe energy needed to start the substance burning.
   • DThe enthalpy change when 1 mol of the substance is completely burned in oxygen under standard conditions.

   (b)

   Which equation represents the standard enthalpy change of combustion of ethanol?

   [1 mark]

   • AC₂H₅OH(l) + 3O₂(g) → 2CO₂(g) + 3H₂O(g)
   • B2C₂H₅OH(l) + 6O₂(g) → 4CO₂(g) + 6H₂O(l)
   • CC₂H₅OH(l) + 3O₂(g) → 2CO₂(g) + 3H₂O(l)
   • DC₂H₅OH(l) + 2O₂(g) → 2CO(g) + 3H₂O(l)

   (c)

   Write the ionic equation, including state symbols, for the neutralisation reaction, and use it to explain why the enthalpy change of neutralisation is almost the same for any strong acid and strong alkali.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A chemist is studying the enthalpy changes of chlorine and its compounds. The standard enthalpy change of atomisation of chlorine is +122 kJ mol⁻¹ and the standard enthalpy change of formation of hydrogen chloride gas is –92 kJ mol⁻¹.

   (a)

   Define the term standard enthalpy change of atomisation and write the equation for the standard enthalpy change of atomisation of chlorine.

   [3 marks]

   (b)

   Write the equation for the standard enthalpy change of formation of hydrogen chloride. Calculate the enthalpy change when 3.65 g of hydrogen chloride is formed from its elements under standard conditions, and state whether the products are at a higher or lower enthalpy than the reactants. (Ar: H = 1.0, Cl = 35.5.)

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A company is choosing a fuel for a small power plant and compares hydrogen with methane. The standard enthalpy changes of combustion are –286 kJ mol⁻¹ for hydrogen, with liquid water as the product, and –890 kJ mol⁻¹ for methane. (Ar: H = 1.0, C = 12.0.) The enthalpy change for burning hydrogen to form steam, H₂O(g), is –242 kJ mol⁻¹.

   (a)

   Calculate the energy released per gram of hydrogen and per gram of methane burned, and use your values to evaluate which is the better fuel for a plant where the mass of fuel carried must be as small as possible.

   [6 marks]

   (b)

   Explain why the enthalpy change for burning hydrogen to form steam (–242 kJ mol⁻¹) differs from the standard enthalpy change of combustion, and describe how the enthalpy level diagrams for the two reactions differ. Explain why enthalpy changes must be quoted under standard conditions, and state what happens to the energy released if the steam from the burner is condensed.

   [6 marks]

   Total for question 4: 12 marks

End of questions