Textbook Q 4D.15

[Angela Hung 1H]

I am confused on how to do textbook question 4D.15. I know the equation for change in standard enthalpy is the sum of enthalpy of products minus the sum of the enthalpy of the reactants, but everytime I plug in the standard enthalpy values given I do not get the right answer. I also made sure to multiple the enthalpy of H2 by 2, since there is a coefficient of 2 in the balanced chemical equation.

[Rachel 1J]

To find combined reaction enthalpies, you want to use Hess' Law. Hess' Law states that the reaction enthalpy of the combined equation is the SUM of all the individual reactions. One caveat is that you must first recognize that the ΔH values given are for each individual reaction moving in the forward direction. In order to calculate the correct reaction enthalpy from the sum of all 3 reactions, you must flip one of the reactions. This results in the following final equation: -1300 + 1560 + 2(-286) = -312 kJ/mol.

If this doesn't fully make sense, there's a solution in the solution manual on BruinLearn that shows each individual combustion reaction.