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