Textbook 5I.13 Part B: Which reaction and Kc is being used?

[Narhayne Nacana 3B]

Hi ya'll! For this problem part b) states

"If 2.0 mmol of F2 was placed into the reaction vessel instead of the chlorine, what would be its equilibrium composition at 1000. K?"

I'm confused if we use the reaction F2 <--> 2F, and use that reactions Kc at 1000K (1.2 * 10^-4), or do we use a different reaction and use the Kc from part a (which is 1.2 * 10^-7)?

These Kcs were from the table linked in the problem in the textbook.

[Hope Galarneau 1A]

Yes! You are correct for both the reaction and Kc that you would use ( (F2<->2F) and the Kc you found for that ).

[Katie 3H]

When do we know when to use k vs kc when looking at the graph? Also, what is the explanation for part c?

[Mia Yamada 1J]

Hi Katie! We use K when we're working with partial pressures of gases since it's the equilibrium constant of partial pressures. And, then we use Kc when we're working with molar concentrations since Kc is the equilibrium constant of molar concentrations.

And then for part c, Cl2 is more stable, because if you compare the equilibrium constants of the two reactions, we can see that Kc for Cl2 is smaller which means that there's a higher concentration of reactants (Cl2) than products (2Cl) as compared to the F2 reaction.