标况或者常温下，氢气和氧气是气态，H2(g)+1/2O2(g)=H2O(g);△H=-241.8KJ/mol
H2(g)+1/2O2(g)=H2O(g);△H=-241.8KJ/mol
H2+1/2O2=H2O H=-241.8KJ
