Question

Consider the reaction: NH4HS (s) --> NH3 (g) + H2S (g). At a certain temperature, Kc = 8.5 * 10^-3. A reaction mixture at this temperature containing solid NH4HS has [NH3] = 0.166 M and [H2S] = 0.166 M. Will more of the solid form or will some of the existing solid decompose as equilibrium is reached?

Answer 1

Q= reaction quotient, it is calculated the same way Keq is calculated. The difference is that Keq uses concentrations of reagents and product at equilibrium, and Q uses concentrations at any moment. We can compare Q with Keq and know which way the reaction will go.

Q=Keq ----> the reaction is in equilibrium

Q>Keq ----> the reaction will go to the side of reagents, products are consumed.

Q<Keq ----> the reaction will go to the right, the reagents are consumed.

Let´s see what happen with our problem:

Q= [NH3][H2S], the NH4HS is not included because it is a solid (liquids are ignored too).

Q= (0.166)(0.166)= 0.0276

Keq= 0.0085 ------> Q>Keq-----> the reaction will consume the product to form reagent, so more solid will form.