J.R. S. answered 07/22/19
Ph.D. University Professor with 10+ years Tutoring Experience
Hess' Law:
Target equation is Ca(OH)2(s) ==> CaO(s) + H2O(g)
copy rx.1: Ca(s) + 1/2O2(g) ===> CaO ∆H = -635
copy rx.2: Ca(OH)2(s) ===> Ca(s) + O2(g) + H2(g) ∆H = +986
reverse rx3. H2(g) + 1/2O2(g) ===> H2O(g) ∆H = -242 (NOTE change of sign since reversed the rx)
Add them together to get...
Ca(s) + 1/2O2(g) + Ca(OH)2(s) + H2(g) + 1/2O2(g) ===> CaO(s) + Ca(s) + O2(g) + H2(g) + H2O(g)
Cancel like terms on both sides of equation to end up with the target equation...
Ca(OH)2(s) ===> CaO(s) + H2O(g) ∆H = -635 + 986 + (-242) = +109 kJ/mole

J.R. S.
07/22/19
Tasha P.
@J.R. S. Thank you! Quickly checked up Hess' law and understood in a heartbeat.07/22/19