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, or no work is done at all ("on", "by" or "none")? Note that no "endo-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.
Pick from these > endo-by ,exo-none ,exo-by ,exo-on
Zn(s) + 2HCl(aq) → ZnCl2(aq) + H2(g)
Cl2(aq) + 2 KBr(aq) → 2 KCl(aq) + Br2(aq)
N2(l) → N2(g)
NH3(g) + HCl(g) → NH4Cl(s)
S(s) + 3/2 O2(g) → SO3(g)