Help with an equilibrium constant question?

Question

At a certain temperature the equilibrium
constant, Kc, equals 0.11 for the reaction:
2 ICl(g) ? I2(g) + Cl2(g).

What is the equilibrium concentration of ICl
if 0.45 mol of I2 and 0.45 mol of Cl2 are
initially mixed in a 2.0-L flask?

 

 

For this problem, since it is asking for the reactant do I still set up as [C][D]/[A] or would it be inverted?

 

I am aware that mols need to be converted to M first, which is .45mol/2L = .225 M for Cl2 and for I2.

 

Do I set up as .11 = [.225][.225]/[ICI]^2 and solve for ICI or do I do something different?

 

Thank you!!

 

 

 

 

 

 

 

Answer 1

You still use the form [C][D]/[A]²=0.11

 

However, you must take into account that once the equilibrium is reached the initial concentrations have decreased:

 

                  start                equilibrium

ICl                 0                       x

Cl₂                0.225                 0.225-x/2    (we divide by 2 because for each mol of Cl₂ 2 mol of ICl are formed)

I₂                 0.225                  0.225-x/2 

 

Now we can set up the equation as 0.11=(0.225-x/2)(0.225-x/2)/x²

 

Solve for x to get the equilibrium concentration of ICl.

Note that as this is a quadratic equation you will get two solutions. But one of them does not make sense since it would be an impossibly high concentration!

 

Hope this helps!