Reactants and products can be solids, liquids or gases. Equations always include state symbols because there will be additional energy needed to change the state of a reactant or product.Hydrogen burns in oxygen to form water which could be liquid or steam. The amount of energy needed to turn water into steam is 44kJmol-1Hence the two possible equations are:H2(g) + ½O2(g) → H2O(l)ΔH = -285.8kJmol-1.
H2(g) + ½O2(g) → H2O(g)ΔH = -241.8kJmol-1.
H2(g) + ½O2(g) → H2O(g)ΔH = -241.8kJmol-1.