What is the standard enthalpy change for the reaction?

Question

(1) 2C(s) + 2H₂(g)C₂H₄(g)...... ΔH° = 52.3 kJ

(2) 2C(s) + 3H₂(g)C₂H₆(g)......ΔH° = -84.7 kJ

(3) C₂H₄(g) + H₂(g)C₂H₆(g)......ΔH° = _kJ?

Answer 1

HESS' LAW:

C2H4(g) + H2(g) ==> C2H6(g) TARGET EQUATION

Given:

eq.1. 2C(s) + 2H2(g) ==> C2H4(g) ...... ΔH° = 52.3 kJ

eq. 2. 2C(s) + 3H2(g) ==> C2H6(g) ...... ΔH° = -84.7 kJ

Process:

reverse eq. 1: C2H4 ==> 2C + 2H2 ...... ∆Hº = -52.3 kJ

copy eq. 2: 2C(s) + 3H2(g) ==> C2H6(g) ...... ΔH° = -84.7 kJ

Add them: ------------------------------------------------------------------

C2H4 + 2C + 3H2 ==> 2C + 2H2 + C2H6 ... ∆Hº = -52.3 kJ + (-84.7 kJ) = -137 kJ

Combine and/or cancel like species:

C2H4 + H2 ==> C2H6 ... TARGET EQUATION ... ∆H = -137 kJ