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

Below are the choices:

Check all that apply

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

() 2A (g) + 2B (g) - - - > 5C (g)

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

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

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 negative (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 positive 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 positive 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 negative