AQA A-Level Chemistry Paper 3, 2023: Question 10

1 mark · Medium difficulty · Multiple Choice

Reaction with Standard Enthalpy Change Equal to Formation of Barium Chloride This question requires selecting the reaction equation representing the formation of BaCl₂(s) directly from its elements in their standard states under standard conditions.

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Question

AQA A-Level Chemistry Paper 3, 2023: Question 10
Question text

10 Which reaction has a standard enthalpy change equal to the

standard enthalpy of formation for barium chloride?

[1 mark]

A Ba(g) + Cl2(g) → BaCl2(s)

B Ba2+(g) + 2 Cl–(g) → BaCl (s)

C Ba(s) + Cl2(g) → BaCl2(s)

D Ba2+(s) + 2 Cl–(g) → BaCl (s)

Mark scheme

Show the mark scheme Mark scheme for AQA A-Level Chemistry Paper 3, 2023: Question 10

10 C 1 (AO1) Ba(s) + Cl2(g) → BaCl2(s)

How to answer it

Understanding Standard Enthalpy of Formation

Question

Which reaction has a standard enthalpy change equal to the standard enthalpy of formation for barium chloride?

  • A: Ba(g) + Cl2(g) → BaCl2(s)
  • B: Ba2+(g) + 2 Cl-(g) → BaCl2(s)
  • C: Ba(s) + Cl2(g) → BaCl2(s)
  • D: Ba2+(s) + 2 Cl-(g) → BaCl2(s)

Correct Answer: C

The standard enthalpy of formation is defined as the enthalpy change when one mole of a compound is formed from its elements in their standard states under standard conditions (298 K and 1 atm).

For barium chloride (BaCl2), the elements in their standard states are:

  • Barium (Ba) in its solid state
  • Chlorine (Cl2) as a diatomic gas

The reaction in option C satisfies this definition:

Ba(s) + Cl2(g) → BaCl2(s)

Teacher Tip: Always check that the reactants are in their standard states and that the product is a single mole of the compound being formed.

Explanation of Other Options

  • A: Barium (Ba) in the gas phase is not in its standard state.
  • B: Both Ba2+(g) and Cl-(g) are ions, not elements in their standard states.
  • D: Ba2+(s) is not the standard state of barium, as it exists as a neutral solid atom.
Teacher Tip: Remember that standard states refer to the most stable physical form of an element at 298 K and 1 atm.

Final Answer

The reaction with the correct standard enthalpy change of formation is:

C: Ba(s) + Cl2(g) → BaCl2(s)

Topics

Physical Chemistry · 3.1.4 Energetics

Question and mark scheme from the AQA A-Level Chemistry examination, Paper 3, 2023. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.