Label each of the following reactions as exothermic or endothermic ("exo" or "endo"), and according to whether work is done on or by the system ("on" or "by")? Note that no "en-on" cases appear here, as these are always thermodynamically unfavourable. There is no need to do any calculation, use chemical arguments to predict the sign of the enthalpy change.

Question

3 NaN3(s) --> Na3N(s) + 4 N2(g)
(Simplified version of the chemical reaction in air bags. NaN3 is sodium azide.)

2 Cl(g) --> Cl2(g) H2O(l) --> H2O(g)

2 Li(s) + H2O(l) --> 2 LiOH(aq) + H2(g)

H2(g) + O2(g) --> 2 H2O(l)

i would say that all of them are exothermic except for the last one...is that right?

but i don't know how to find out whether work is done on or by the system.

Answer 1

Your answer is correct. If a reaction forms a gas, that gas expands into the surroundings and work is done.