Question

At constant pressure, which of these systems do work on the surroundings? Check all that apply.

A(s) + 2B(g) = C(g)
A(s) + B(s) = C(g)
A(g) + B(g) = C(g)
A(g) + B(g) = 3C(g)

Answer 1

A(g) + B(g) → 3C(g)

This is because the volume of gas after reaction is greater than that before the reaction (because there are more moles in the product than in the reactants) so the system's boundary will have to expand in order to maintain constant pressure; this means that the system will do work on its surroundings.