Question

Consider the reaction for the decomposition of hydrogen disulfide:

2H2S(g)⇌2H2(g)+S2(g),                              Kc=1.67×10−7 at 800∘C

The reaction is carried out at the same temperature with the following initial concentrations:

[H2S] =4.80 x 10^-4 M

[H2] = 0.00 M

[S2]= 0.00 M

Find the equilibrium concentration of S2 .

Express the concentration to three significant figures and include the appropriate units.

Answer 1

You need to analize the equilibrium concentrations in the reaction:

You have at the beginnig a concentration of H2S (C)

From the stoichiometry, at the equilibrium the concentration of S2 is half of the concentration of H2, assume (H2=X) equal to X

And also from the stoichiometry the concentration of the H2S is C-X

Now from the mass action equation:

You can make the assuption that at the equilibrium C is bigger tha X (C>>X), since the Kc value is low, it means that the concentration of the denominator shold be bigger than the product of the concentrations in the numerator.

and the equation yields: