To calculate the total enthalpy of a final reaction from a set of related reactions, we can apply Hess’s law.
- For each related reaction, look for similar formula units that are present in the final reaction.
- If such a formula unit exists, try to manipulate the equation so that the formula unit is on the same side as the final reaction. Change the of the related reaction accordingly (i.e. if the formula unit is not on the same side as the final reaction, multiply the original by ).
- If the coefficient of this formula unit does not match the unit (and this equation has the only occurrence of such formula unit out of all other related reactions), multiply (or divide) the coefficients of this whole equation and the by the same number so that the numbers match.
- Do so for every equation. Add the equations together (product side with product side, reactant with reactant side, enthalpies with enthalpies). If the sum does not match the final equation, modify the coefficients and try again.