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Question

Calculate the enthalpy of formation of ethyl alcohol from the following data:
C2H5OH(l)+3O2(g)2CO2(g)+3H2O(l); ΔrH=1368.0kJ
C(s)+O2(g)CO2(g); ΔrH=393.5kJ
H2(g)+12O2(g)H2O(l); ΔrH=286.0kJ

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Solution

C2H5OH(l)+3O2(g)2CO2(g)+3H2O(l)ΔH=1368kJ
2CO2(g)+3H2O(l)C2H5OH(l)+3O2(g)ΔH=1368kJ.....(1)
C(s)+O2(g)CO2(g)ΔH=393.5kJ
2×[C(s)+O2(g)CO2(g)ΔH=393.5kJ]
2C(s)+2O2(g)2CO2(g)ΔH=787kJ.....(2)
H2(g)+12O2(g)H2O(l)ΔH=286.0kJ
3×[H2(g)+12O2(g)H2O(l)ΔH=286.0kJ]
3H2(g)+32O2(g)3H2O(l)ΔH=858.0kJ.....(3)
Adding eqn(1),(2)&(3), we have
2C(s)+3H2(g)+12O2(g)C2H5OH(l)ΔH=277
Hence the enthalpy of formation of ethyl alcohol is 277kJ.

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