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

1. 1
   Hydrogen reacts with oxygen to form water, both in fuel cells and in combustion engines. The standard enthalpy change of formation of liquid water is −286 kJ mol⁻¹.

   (a)

   Which of the following are standard conditions for an enthalpy change?

   [1 mark]

   • A101 kPa and 273 K
   • B10 kPa and 298 K
   • C100 kPa and a stated temperature, usually 298 K
   • D100 kPa and 373 K

   (b)

   Which equation represents the standard enthalpy change of formation of water?

   [1 mark]

   • A2H₂(g) + O₂(g) → 2H₂O(l)
   • BH₂(g) + ½O₂(g) → H₂O(l)
   • CH₂(g) + ½O₂(g) → H₂O(g)
   • DH₂O(l) → H₂(g) + ½O₂(g)

   (c)

   Define the term standard enthalpy change of formation.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A company makes two types of instant pack. The hot pack contains calcium oxide, which reacts with water in an exothermic reaction. The cold pack contains ammonium nitrate, which dissolves in water in an endothermic process with ΔH = +25.7 kJ per mole of ammonium nitrate.

   (a)

   Which statement correctly describes the reaction in the hot pack?

   [1 mark]

   • AΔH is positive and the products have lower enthalpy than the reactants
   • BΔH is positive and the products have higher enthalpy than the reactants
   • CΔH is negative and the products have higher enthalpy than the reactants
   • DΔH is negative and the products have lower enthalpy than the reactants

   (b)

   Which feature is shown on a reaction profile but not on an enthalpy level diagram?

   [1 mark]

   • AActivation energy
   • BEnthalpy change of reaction
   • CEnthalpy level of the reactants
   • DThe sign of ΔH

   (c)

   A cold pack contains 32.0 g of ammonium nitrate, NH₄NO₃ (Mr = 80.0). Calculate the energy absorbed, in kJ, when all of it dissolves.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   The standard enthalpy change of combustion of ethanol is −1367 kJ mol⁻¹, and the standard enthalpy change of neutralisation for a strong acid reacting with a strong alkali is −57 kJ mol⁻¹.

   (a)

   Define the term standard enthalpy change of combustion and write the equation, including state symbols, for the standard enthalpy change of combustion of ethanol.

   [3 marks]

   (b)

   Calculate the energy released when 4.60 g of ethanol (Mr = 46.0) is burned completely under standard conditions. Then calculate the mass of ethanol that would release the same energy as the neutralisation of 0.500 mol of hydrochloric acid by excess sodium hydroxide solution.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A teacher introduces enthalpy level diagrams and standard enthalpy changes using the burning of methane in a Bunsen burner, and uses data from two different data tables to make a point about conditions and state symbols.

   (a)

   The standard enthalpy change of combustion of methane is −890 kJ mol⁻¹. Describe how to draw and label an enthalpy level diagram for this reaction and explain what it shows. Explain also why activation energy does not appear on it.

   [6 marks]

   (b)

   One data table gives the standard enthalpy change of formation of water as −286 kJ mol⁻¹ and another gives −242 kJ mol⁻¹, and both tables are correct. Explain how this can be, and explain why a data table must state the standard conditions and the state symbols.

   [6 marks]

   Total for question 4: 12 marks

End of questions