Equilibrium Concentrations

Question

The research and development unit of a chemical company is studying the reaction of CH4 and H2S, two components of natural gas: CH4(g) + 2H2S(g) ⇌ CS2(g) + 4H2(g) In one experiment, 1.00 mol of CH4, 1.00 mol of CS2, 2.00 mol of H2S, and 2.00 mol of H2 are mixed in a 250-mL vessel at 960 oC. At this temperature, Kc = 0.036. If [CH4] = 5.56 M at equilibrium, what are the equilibrium concentrations of H2?

Answer 1

In this case, I would suggest writing down the reaction and do an equilibrium table. The first row will contain the initial concentration. The second row will have the concentration change at equilibrium (the stoichiometric coefficient multiplied by an unknown x). The third row (equilibrium concentration) will be the sum of the first 2 rows.

CH4(g) + 2H2S(g) ⇌ CS2(g) + 4H2(g)

4 M 8 M 4M 8M

-x -2x x 4x

4-x 8-2x 4-x 8+4x

First you need to determine if the reaction is moving forward or backward (if required) (recommended but not needed). You can do this by calculating K using (8)^4*4/8^2*4=64>0.036. Since You would need to solve for x by equating the CH4 at equilibrium as 4-x=5.56 thus x=-1.89, then replacing x in the equilibrium value for H2 =8+4x=8+4*(-1.89)= 0.44 M.