The molar heats of combustion of C₂H₂ (g), C (graphite) and H₂ (g) are 210.62 Kcal, 84.05 Kcal and 69.32 Kcal respectively. Calculate the standard heat of formation of C₂ H₂ (g)

The required equation is
2C + H₂ ———-> C₂H₅
Standard heat of formation i.e. ΔH =?

Writing the thermo-chemical equation of the given data

(i) C₂ H₂ + 5/2 O₂ ————> 2CO₂ +
H₂ O ΔH = –210.62 kcal (given)

(ii) C + O₂ —————-> CO₂ ΔH = –84.05 kcal (given)

(iii) H₂ + 1/2 O₂ ——–> H₂O ΔH = –69.32 kcal (given)

Now, multiplying (ii) eq. by 2 and then adding (iii) eq. and subtracting (i) eq. i.e.
i.e. (ii) 2 C + 2 O₂ —————-> 2 CO₂ ΔH = –168.1 kcal

Hence after adding adding (iii) eq. and subtracting (i) eq. we will get following equation:

2C + H₂ —————–> C₂ H₂ ΔH = 26.80 kcal

Hence, Standard heat of formation of C₂ H₂ i.e. ΔH = 26.80 kcal.