Problem: What is the standard Gibbs free energy of formation, ΔG°f, of NH3(g) at 298 K?

FREE Expert Solution

Reaction: N2(g) + 3 H2(g) → 2 NH3(g)

Step 1: We can use the following equation to solve for ΔH˚rxn:


ΔH°rxn=ΔH°f, prod-ΔH°f, react


Note that we need to multiply each ΔH˚f by the stoichiometric coefficient since ΔH˚f is in kJ/mol. 

Also, note that ΔH˚f for elements in their standard state is 0.


We also need to convert ΔH˚rxn from kJ/mol to J/ mol so our units remain consistent.


ΔH°rxn=(2 mol NH3)-46 kJ1 mol NH3           -(1 mol N2)0 kJ1 mol N2+(3 mol H2)(0 kJ1 mol H2)

ΔH°rxn=-92 kJ×103 J1 kJ

ΔH˚rxn = –92000 J



Step 2: We can use the following equation to solve for ΔS˚rxn:


ΔS°rxn=S°f, prod-S°f, react


Note that we need to multiply each S˚ by the stoichiometric coefficient since S˚ is in J/mol • K.


ΔS°rxn=(2 mol NH3)193 Jmol NH3·K           -(1 mol N2)192 Jmol N2·K+(3 mol H2)(131 Jmol H2·K)

ΔS˚rxn = –199 J/K


View Complete Written Solution
Problem Details

What is the standard Gibbs free energy of formation, ΔG°f, of NH3(g) at 298 K?

Frequently Asked Questions

What scientific concept do you need to know in order to solve this problem?

Our tutors have indicated that to solve this problem you will need to apply the Gibbs Free Energy concept. You can view video lessons to learn Gibbs Free Energy. Or if you need more Gibbs Free Energy practice, you can also practice Gibbs Free Energy practice problems.