If you need to reverse the following reaction and multiply it by 2 in order for it to be an intermediate reaction in a Hess's law problem, what would be the final value for the enthalpy of reaction you use for this intermediate reaction? H2 + 0.5 O2 H2O, H = - 286 kJ

We have that the total enthalpy of the reaction changes with the quantity of the reactants and it is proportional to them. Also, the reverse of a reaction has the opposite enthalpy. Hence, since we need to multiply by 2, the reactants are double and thus the value of the enthalpy is 2 as big. Also, since we are using the inverse reaction, we must also invert the sign. Thus, for this reaction we must use the value H=572 kJ.