Question

Given the standard enthalpy changes for the following two reactions:

(1) 4C(s) + 5H₂(g)-------> C₄H₁₀(g)...... ΔH° = -125.6 kJ

(2) C₂H₂(g) ----> 2C(s) + H₂(g)......ΔH° = -226.7 kJ

what is the standard enthalpy change for the reaction:

(3) 2C₂H₂(g) + 3H₂(g)--------->C₄H₁₀(g)......ΔH° = ? kj

Answer 1

2 * (2)
2 C₂H₂ (g) ---------> 4C (s) + 2H₂ (g) ; ΔH° = -453.4 kJ
Add equation (1) to the above equation then you will obtain equation (3) i.e)
2 C₂H₂ (g) + 3 H₂ (g) ---------> C₄H₁₀ (g) ......... ΔH° = -125.6kJ + (-453.4)kJ
ΔH° = -579 kJ ------ [ for (3) ]