Question

X4A10 + 6 H2A → 4 H3XA4
Calculate the ΔHo for the reaction reaction above using the enthalpy data provided below.

4 X + 5 A2 → X4A10 ΔHo = -2984 kJ

A2 + 2 H2 → 2 H2A ΔHo = -571.7 kJ

3H2 + 4 A2 + 2 X → 2 H3XA4 ΔHo = -2534 kJ

Answer 1

Reverse eqn 1 and change sign of dHo.
Multiply eqn 2 by 3, reverse, change sign of dHo
Multiply eqn 3 by 2
Add all the equations and the new dHo values.
It is important you add the equations, cancel any substances that occur on both sides and MAKE SURE THE EQUATION IS THE SAME AS IN THE QUESTION. I DID THIS IN MY HEAD AND I MAY HAVE MADE AN ERROR SO YOU MUST CHECK IT CAREFULLY.