# Calculating thermodynamic quantities for hydrogenation of iron nitride

HW#6.3 If the reaction

$$\ce{Fe2N(s) + 3/2 H2(g) <=> 2 Fe(s) + NH3(g)}$$

comes to an equilibrium at a total pressure of $$\pu{1 bar},$$ analysis of the gas shows that at $$\pu{700 K}$$ and $$\pu{800 K},$$ $$p_\ce{NH3}/p_\ce{H2}$$ are $$2.165$$ and $$1.083,$$ respectively, if only $$\ce{H2(g)}$$ was initially in the gas phase and $$\ce{Fe2N(s)}$$ was in excess:

a. Calcualte $$K_p$$ at $$\pu{700 K}$$ and $$\pu{800 K}.$$
b. Calculate $$Δ_\mathrm{r}S^\circ$$ at $$\pu{700 K}$$ and $$\pu{800 K}$$ and $$Δ_\mathrm{r}H^\circ$$ assuming that it is independent of temperature.
c. Calculate $$Δ_\mathrm{r}G^\circ$$ for this reaction at $$\pu{298 K}.$$

At first I ignored the excess data, then I assumed that the pressure exerted by the solids is null and adding partial pressures I got an answer (sum of pressure of gases equals 1).

The problem is that part a in the book gives answer $$K_p = 1.777$$ at $$\pu{700 K}$$ which I think is wrong. Please help.

• I get 3.85 at 700K – Chet Miller Sep 3 at 18:29
• I got that too but the book says 1.777 guess I can't trust the back of the book now ,using Castellan Physical Chemistry – Esteban Soto Montijo Sep 3 at 19:42