CBSE Class 11-science Answered
Formation reaction for ethanol is given as:
2C(S) + 3H2(g) + (1/2) O2(g) → C2H5OH(l) ΔfH = ? ….. (i)
The enthalpy of combustion of 1 mole of ethanol is,
C2H5OH(l) + 3O2(g) → 2CO2(g) + 3H2O ΔCH = -1365.6 kJ ….. (ii)
The enthalpy of formation of 1 mole of CO2 is,
C(S) + O2(g) → CO2(g) ΔfH = -393.5 kJ / mol ….. (iii)
The enthalpy of formation of 1 mole of H2O is,
H2(g) + (1/2) O2(g) → H2O(l) ΔfH = -285.83 kJ / mol ….. (iv)
Multiply eq. (iii) by 2 & (iv) by 3 we will get,
2C(S) + 2O2(g) → 2CO2(g) ΔfH = -787 kJ / mol ….. (v)
3H2(g) + (3/2) O2(g) → 3H2O(l) ΔfH = -857.49 kJ / mol ….. (vi)
Add (v) & (vi),
2C(S) + 3H2(g) + (7/2)O2(g) → 2CO2(g) + 3H2O(l) ΔfH = -1644.49 kJ / mol ….. (vii)
Now, reverse eq (ii),
2CO2(g) + 3H2O → C2H5OH(l) + 3O2(g) ΔCH = 1365.6 kJ ….. (viii)
Add (vii) & (viii), we will get,
2C(S) + 3H2(g) + (1/2) O2(g) → C2H5OH(l) ΔfH = 278.89 kJ / mol