To determine the ΔH for the reaction, use Hess Law. Manipulate the individual reactions such that when you add them you arrive to the required reaction.
Required reaction: CaC2(s) + 2H2O(l) → Ca(OH)2(aq) + C2H2(g)
Reaction 1: Ca(s) + 2 C (graphite) → CaC2(s) (reverse reaction) ΔH = -62.8 kJ (reverse sign)
(Reverse the reaction because CaC2 is on the product side in the required reaction while it is in the reactant side in Reaction 2)
Given the following data
Ca(s) + 2C (graphite) → CaC2 (s) ΔH = -62.8 kJ
Ca(s) + 1/2 O2 (g) → CaO (s) ΔH = -635.5 kJ
CaO (s) + H2O (l) → Ca(OH)2 (aq) ΔH = -653.1 kJ
C2H2 (g) + 5/2O2 (g) → 2CO2 (g) + H2O (l) ΔH = -1300. kJ
C(graphite) + O2 (g) → CO2 (g) ΔH = -393.5 kJ
Calculate ΔH for the reaction
CaC2 (s) + 2H2O (l) → Ca(OH)2 (aq) + C2H2 (g)
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