The table lists the standard enthalpy changes of formation, ΔH, for some compounds and aqueous ions.

| species | ΔHᶠ/kJ mol⁻¹ |
|------------|-------------|
| Ba²⁺(aq) | -538 |
| OH⁻(aq) | -230 |
| CO₂(g) | -394 |
| BaCO₃(s) | -1216 |
| H₂O(l) | -286 |

If CO₂(g) is bubbled through an aqueous solution of Ba(OH)₂ for a long time, the precipitated BaCO₃(s) dissolves, as shown in reaction 2.
BaCO₃(s) + CO₂(g) + H₂O(l) → Ba(HCO₃)₂(aq)
reaction 2
The standard enthalpy change for reaction 2, ΔHᵣ₂ = −26 kJ mol⁻¹.

Q: What topic does this question test?

This question tests Group 2 in A-Level Chemistry (syllabus code 9701). It is worth 13 marks.

Q: Which exam paper is this question from?

This question appeared in the Cambridge A-Level Chemistry May/June 2017 examination, Paper 4 Variant 3 (9701_s17_qp_43.pdf).

Question

The table lists the standard enthalpy changes of formation, ΔH, for some compounds and aqueous ions.

| species    | ΔHᶠ/kJ mol⁻¹ |
|------------|-------------|
| Ba²⁺(aq)   | -538        |
| OH⁻(aq)    | -230        |
| CO₂(g)     | -394        |
| BaCO₃(s)   | -1216       |
| H₂O(l)     | -286        |

If CO₂(g) is bubbled through an aqueous solution of Ba(OH)₂ for a long time, the precipitated BaCO₃(s) dissolves, as shown in reaction 2.
BaCO₃(s) + CO₂(g) + H₂O(l) → Ba(HCO₃)₂(aq)
reaction 2
The standard enthalpy change for reaction 2, ΔHᵣ₂ = −26 kJ mol⁻¹.

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