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 )
