Which of the following reactions releases thermal energy to the surroundings but does no significant pressure-volume work on the surroundings?

Question

Which of the following reactions releases thermal energy to the surroundings but does no significant pressure-volume work on the surroundings?

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

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

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

Reaction 4: CaCO₃(s) --> CaO(s) + CO₂(g) ΔH = +178 kJ/mol

The correct answer is Reaction 3 but can someone please explain why that is and why it is not any of the other answers?

Answer 1

The reactions that release thermal energy to the surroundings are the ones showing a negative Δh (#2 and #3)

Looking at #2, you'll notice that you start with a solid and liquid and end with a liquid and a gas. That gas will expand because it takes up more space than the liquid and solid. That means it WILL do pressure-volume work on the surroundings.

Looking at #3, you'll notice that you start with a gas (O₂) and for every mole of O₂ you have, you produce one mole of CO₂ so basically, the CO₂ just replaces an equivalent amount of O₂. That means no appreciable pressure-volume work is done on the surroundings