Question

at constant pressure, which of these systems do work on the surroundings? check all that apply!!!

A(s) +B(g) -------------->2C(g)

2A(g)+3B(g)------------->4C(g)

2A(g)+B(g)--------------->C(g)

A(g)+B(g)----------------->3C(g)

Answer 1

When a gas is evolved during a chemical reaction, the gas can be imagined as displacing the atmosphere - pushing it back against the atmospheric pressure. The work done is therefore V*P where V is the volume of gas evolved, and P is the atmospheric pressure.

A(g) + B(g) ---> C(g) In this reaction 2 volumes of reagent combine to form 1 vol of product. The total volume falls, so the work done is positive (the atmosphere does work on the gaseous system).

2A(g) +2B(g) ---> 5C(g) 4 vols of reagent combine to form 5 vols of product. The total volume increases and negative work is done.

A(s) + B(s) ---> C(g) we can assume that the volume of gas produced exceeds the volume of solid consumed, so again negative work is done.

2A(g) + B(g) ---> C(g) In this reaction 3 volumes of reagent combine to form 1 vol of product. The total volume falls, so the work done on the atmosphere is positive.