Hess' Law & Standard enthalpy subtr. 7.13A Self-test

[RaquelAvalos1K]

Hey everyone!

In 7.13A, why is the second equation & change in enthalpy subtracted from the first equation & change in enthalpy?
When does this occur versus addition?


Thank you!!

[Terri Shih 3D]

When you flip the second equation, you change the sign of the change in enthalpy

This occurs when you're trying to cancel out components that are not in the final equation (when it's in the product of one eq and reactant of another in equal quantities), and when you're trying to get the component in the final equation to be on the right side (either product or reactant)
In this case, you want to form carbon monoxide and water

(1) 2 C8H8 + 25 O2 -> 16 CO2 + 18 H2O
(2) 2 CO + O2 -> 2 CO2

In this case, by flipping around the second equation and multiplying it by 8:

(1) 2 C8H8 + 25 O2 -> 16 CO2 + 18 H2O
(2) 16 CO2 -> 16 CO + 8 O2

16CO2 cancels out
You subtract to get 17O2
You get 16 CO

And, unchanged, you have 2 C8H18 and 18 H2O from the first equation, giving you the final equation as stated in the answer
So the calculations are (1) + 8(2) = (-10942) + 8(566) = -6414 kJ