Chemistry Practice Problems Hess's Law Practice Problems Solution: Given the following dataCa(s) + 2C (graphite) → Ca...

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Solution: Given the following dataCa(s) + 2C (graphite) → CaC2 (s)                      ΔH = -62.8 kJCa(s) + 1/2 O2 (g) → CaO (s)                              ΔH = -635.5 kJCaO (s) + H2O (l) → Ca(OH)2 (aq)                     ΔH = -653.1 kJC2H2 (g) + 5/2O2 (g) → 2CO2 (g) + H2O (l)      ΔH = -1300. kJC(graphite) + O2 (g) → CO2 (g)                        ΔH = -393.5 kJCalculate ΔH for the reactionCaC2 (s) + 2H2O (l) → Ca(OH)2 (aq) + C2H2 (g)

Problem

Given the following data

Ca(s) + 2C (graphite) → CaC(s)                      ΔH = -62.8 kJ

Ca(s) + 1/2 O2 (g) → CaO (s)                              ΔH = -635.5 kJ

CaO (s) + H2O (l) → Ca(OH)(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)(aq) + C2H2 (g)

Solution

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)

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