Question

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

Answer 1

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