At constant pressure, which of these systems do work on the surroundings? A (s) + B (s) ⟶ C (g) A (s) + B (s) ⟶C (g) 2 A (g) + 2 B (g) ⟶ 5 C (g) 2A (g) + 2B (g) ⟶5C (g) A (g) + B (g) ⟶ C (g) A (g) + B (g) ⟶C (g) 2 A (g) + 2 B (g) ⟶ 3 C (g)

Correct question:

At constant pressure, which of these systems do work on the surroundings?

(a) A (s) + B (s) ⟶ C (g)

(b) 2 A (g) + 2 B (g) ⟶ 5 C (g)

(c) A (g) + B (g) ⟶ C (g)

(d) 2 A (g) + 2 B (g) ⟶ 3 C (g)

Answer:

(a) A (s) + B (s) ⟶ C (g)

(b) 2 A (g) + 2 B (g) ⟶ 5 C (g)

Explanation:

Work done by a system on the surroundings at a constant pressure is given as;

W = - PΔV

Where;

ΔV is gas expansion, that is final volume of the gas minus initial volume of the gas must be greater than zero.

Part (a)

A (s) + B (s) ⟶ C (g)

ΔV = 1 - (0) = 1 (expansion)

Part (b)

2 A (g) + 2 B (g) ⟶ 5 C (g)

ΔV = 5 - (2 + 2) = 1 (expansion)

Part (c)

A (g) + B (g) ⟶ C (g)

ΔV = 1 - (1 + 1) = - 1 (compression)

Part (d)

2 A (g) + 2 B (g) ⟶ 3 C (g)

ΔV = 3 - (4) = - 1 (compression)

Thus, systems where there is gas expansion are in part (a) and part (b). The correct answers are:

(a) A (s) + B (s) ⟶ C (g)

(b) 2 A (g) + 2 B (g) ⟶ 5 C (g)