Which of the following reactions would be expected to not only transfer thermal energy (heat) to the surroundings, but would also perform pressure-volume work on the surroundings?

Question

Which of the following reactions would be expected to not only transfer thermal energy (heat) to the surroundings, but would also perform pressure-volume work on the surroundings?

1) CO₂(g) + 2H₂O(l) --> CH₄(g) + 2O₂(g) ΔH = + 890 kJ/mol

2) 2Na(s) + 2H₂O(l) --> 2NaOH(aq) + H₂(g) ΔH = -368 kJ/mol

3) C(graphite) + O₂(g) --> CO₂(g) ΔH = -368 kJ/mol

4) 2H₂(g) + O₂(g) --> 2H₂O(l) ΔH = -572 kJ/mol

5) CaCO₃(s) --> CaO(s) + CO₂(g) ΔH = +178 kJ/mol

Please explain the reasoning behind this. The correct answer is 2, but I would like to understand why that is and why it is not any of the other choices.

Answer 1

To transfer thermal energy to the surrounding, ∆H must be negative

To perform pressure-volume work, the pressure or volume must change.

1) and 5) have positive values of ∆H, so they are out

2 has negative ∆H and since a gas is formed (no gas on reactant side), the pressure or volume would increase. So, this reaction is a "yes".

3) has negative ∆H but no change in the moles of gas from reactant to products (1 mole on each side), so this would be a "no".

4) negative ∆H but a decrease in moles of gas from reactant to product side, so PV work is in the wrong direction.