Explain, in terms of particles, concentration, and reaction rate, what you expect to happen when methane gas (CH4) and hydrogen sulfide gas (H2S) are sealed in a flask and reach a state of equilibrium.

Question

CH4 (g) +2 H2S (g) CS2 (g) + 4 H2 (g

Answer 1

Your lack of an arrow is frustrating. How are we to know the reactants from the products?
Wouldn't you think the concn of CH4 and H2S would get smaller and the concns of CS2 and H2 would get larger?

Answer 2

CH4 and H2S will start to react, so their concentration falls. As this falls, so does the reaction rate. As the products are formed, CS2 and H2, they will being to react to form CH4 and H2S again ie the backward reaction. The more products, the faster the backward rate.

This will continue until rate forwards = rate backwards and both products and reactants are in dynamic equilibrium., and their concentrations remain constant although they are still reacting.