tolong bantu yang ngerti:)

a) H2(g) + 1/2O2(g) -> H2O (l) ΔHf =-187.8
b) H2 (g) + S(s) -> H2S (g) ΔHf = -20.2
c) Ca (s) +C (s)+3/2O2(g) -> CaCO3 ΔHf =-207.8
d) H2(g)+S(s)+2O2(g)->H2SO4 ΔHf=-843.99
e) C(s)+3/2H2(g)+1/2Cl2(g)->CH3Cl ΔHf =+74.81