# Chemical equilibrium

1. Mar 1, 2009

### Anna_Kwong

1. The problem statement, all variables and given/known data
It is a chem equilibrium question, involving the ICE table.

The following reaction:
S2(g) + 2H2 (g) ←→ 2SH2(g)
takes place at high temperatures. If the initial amount of H2 and SH2 is 1.75 g and 1.95 g, respectively, and at equilibrium the amount of S2(g) is found to be 0.00188 atm, what is the value of the equilibrium constant? Please give your answer to three significant figures. Given: T = 1670 K and Reaction Volume = 0.500 L. (Hint: you will need to use the ideal gas law).

2. Relevant equations

pv=nRT, ICE table

3. The attempt at a solution
What I did was find the the mols of H2 and SH2 since we are given n=m/M. So I have those 2 for "I" initial.
Then I found the equilibrium mols of S2, by plugging into the formula pv=nRt, solving for n=pv/Rt and I get 2.00E-5.

Essentially, I know K = [products]/[reactants]. But I only know the "I" of H2 and SH2, and I only know the "E" of S2.

2. Mar 1, 2009

### Lchan1

K = P(product)/ P(reactant) when everything is in gas state.

3. Mar 1, 2009

### Anna_Kwong

how do I figure out the "C" in change? or is that even necessary..

4. Mar 1, 2009

### Charlie_Brown

remember when dealing with gases, use partial pressure. when dealing with solutions, use concentration.

5. Mar 1, 2009

### Anna_Kwong

ok so, kp= pSH2^2/ pH2^2 x pS2 , i know the value of pS2, which is 0.00188atm, how do I find the other 2 values needed to solve for the kp value? Thanks

6. Mar 1, 2009

### Anna_Kwong

My kp value is 0.580, can anyone confirm this please and thanks

7. Mar 1, 2009

### Staff: Mentor

Use stoichiometry of the reaction.

Kc vs Kp - it depends on what the question asks. When dealing with gases it is more likely that the answer expected is Kp, but it is also perfectly possible to calculate Kc and it will be a valid answer as well. Whether it will be accepted by your teacher... that's another story.

Seems to me like your Kp value is much too low. Show details of your work.

8. Mar 1, 2009

### Anna_Kwong

H2 :1.75g / 2.02g/mol = 0.866mol , use pv=nRT , p = nRT/v = (0.866mol)(0.082)(1670)/(0.5) = 237atm
SH2: 1.95g/34.06g/mol = 0.0572mol, use pv=nRT, p = nRT/v = (0.0572mol)(0.082)(1670)/(0.5)=15.7 atm
S2: 0.00188atm

kp= pSH2^2/pS2 x pH2^2 = (15.7)^2/((237)^2 x (0.00188)) = 2.33, eekk guess my calculations for the first run was incorrect, however, is the work done correctly?

9. Mar 2, 2009

### Staff: Mentor

Results looks much better now, although you have not accounted for the fact that your final mixture doesn't contain 1.75 g H2 nor 1.95 g of H2S - the first was produced together with S2, the latter was decomposed. You should use reaction stoichiometry for that.

Not that it will change much, but you should check it to be sure.