Question

Find ΔH° for the formation of solid vanillin, C8H8O3(s), using the data below. C(s) + O2(g) → CO2(g) ΔH°= -393.52 kJ/mol H2(g) + ½O2(g) → H2O(l) ΔH°= -285.83 kJ/mol C8H8O3(s) + 8½O2(g) → 8CO2(g) + 4H2O(l) ΔH°= -3827.88 kJ/mol The heat of formation of solid vanillin is _______Kj mol-1

Answer 1

C(s) + O2(g) → CO2(g) ΔH°= -393.52 kJ/mol ------(1)

H2(g) + ½O2(g) → H2O(l) ΔH°= -285.83 kJ/mol ------(2)

C8H8O3(s) + 8½O2(g) → 8CO2(g) + 4H2O(l) ΔH°= -3827.88 kJ/mol -------(3)

8CO2(g) + 4H2O(l) → C8H8O3(s) + 8½O2(g) ΔH°= +3827.88 kJ/mol -------(4) ( Reverse reaction of eqn 3)

4H2(g) + 2O2(g) → 4H2O(l) ΔH°= 4*-285.83 kJ/mol ------(5) ( eqn 2*4)

8C(s) + 8O2(g) → 8CO2(g) ΔH°= 8* -393.52 kJ/mol ------(6) ( eqn 1*8)

--------------------------------------------------------------------------------------------------------- add eqn 4+5+6

8C(s) + 4H2(g) + 3/2 O2(g) → C8H8O3(s)   ΔH°= +3827.88 + 4*-285.83 +  8* -393.52 => -463.6 kJ/mol.

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Answer : -463.6 kJ/mol