Question

Determine the change in enthalpy for the third reaction:

A + B → C ΔH = 18kJ

Q + B → 2C ΔH = -67kJ

Q → 2A + B ΔH = ?

Answer 1

Try solve it using the following procedure:
1) You many multiply any number on the stoichiometric numbers as long you make sure to multiply all of them and the enthalpy.
2) You many inverse all the reactions due to the following rule:
For a reaction \(\large \sf A \to B\) it follows that:
\[\Large \Delta H(A \to B)=- \Delta H(B \to A)\]3) You obtain the correct reaction when you can cancel out so only the wanted reaction is there. From that you then add together due to rule 1) and 2).

Good luck.