Question

Given the thermochemical equations A(g) — Bg AH = 70 kJ B(g) — C(g) AH = -140 kJ find the enthalpy changes for each reaction 3 A(g) — 3Bg AH = B(g) Ag) AH = A(g) -C(O) AH =

Answer 1

1)

This reaction is 3*1st reaction

So,

ΔHo rxn = 3*given ΔHo rxn

= 3*(70)

= 210 KJ

Answer: 210 KJ

2)

This reaction is -1*1st reaction

So,

ΔHo rxn = -1*given ΔHo rxn

= -1*(70)

= -70 KJ

Answer: -70 KJ

3)

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, ΔHo rxn for required reaction will be:

ΔHo rxn = +1 * ΔHo rxn(reaction 1) +1 * ΔHo rxn(reaction 2)

= +1 * (70) +1 * (-140)

= -70 KJ

Answer: -70 KJ