Hess’s Law: Two Reactions

Question

1) Given the standard enthalpy changes for the following two reactions:

(1) 2Fe(s) + O2(g)2FeO(s)...... ΔH° = -544.0 kJ

(2) 2Hg(l) + O2(g)2HgO(s)......ΔH° = -181.6 kJ

what is the standard enthalpy change for the reaction:

(3) FeO(s) + Hg(l)Fe(s) + HgO(s)......ΔH° = ?

2) Given the standard enthalpy changes for the following two reactions:

(1) N2(g) + O2(g)2NO(g)...... ΔH° = 181.8 kJ

(2) N2(g) + 2O2(g)2NO2(g)......ΔH° = 66.4 kJ

what is the standard enthalpy change for the reaction:

(3) 2NO(g) + O2(g)2NO2(g)......ΔH° = ?

Answer 1

FeO(s) + Hg(l) ==> Fe(s) + HgO(s) ... TARGET EQUATION

(1) 2Fe(s) + O₂(g) ==> 2FeO(s)...... ΔH° = -544.0 kJ

(2) 2Hg(l) + O₂(g) ==> 2HgO(s)......ΔH° = -181.6 kJ

reverse (1) and divide by 2 to get FeO(s) ==> Fe(s) + 1/2 O₂(g) ... ∆Hº = +272 kJ

copy (2) and divide by 2 to get Hg(l) + 1/2 O₂(g) ==> HgO(s) ... ∆Hº = -90.8

add new equations to get

FeO(s) + Hg(l) ==> Fe(s) + HgO(s) ... Target equation

Adding the ∆Hº values we get +272 - 90.8 = +181.2 kJ

The second problem follows the same principles as this first problem. You should be able to do it. If not, post it as a separate question.