Can someone please explain this to me? I get when we are using Hess law to work out the enthalpy of formation we write all the elements that are present in the reaction in their molar quantities. But in this combustion one, they split into CO2 and H20 because it's the products of combustion I get that part as well. But what is the 3O2 on both the arrows where did that come from And for the oxygen it's not balanced im so confused with this also y are the arrows pointing down in this one and not one going to reactants and one to product like the previous page?