Question

Given the standard reaction enthalpies for these two reactions:

(1) 2C(s) + 2H2(g) = C2H4(g)...... ΔrH° = 52.3 kJ/mol

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

calculate the standard reaction enthalpy for the reaction: (3) C2H4(g) + H2(g) = C2H6(g)......ΔrH° = ?

Answer 1

Lets number the reaction as 1, 2, 3 from top to bottom

required reaction should be written in terms of other reaction

This is Hess Law

required reaction can be written as:

reaction 3 = -1 * (reaction 1) +1 * (reaction 2)

So, deltaHo rxn for required reaction will be:

deltaHo rxn = -1 * deltaHo rxn(reaction 1) +1 * deltaHo rxn(reaction 2)

= -1 * (52.3) +1 * (-84.7)

= -137 KJ

Answer: -137 KJ