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Kp

the equation is:

PCl5(s) <----> PCl3(l) + Cl2(g)

surely the Kp expression is:

Kp = P(Cl2)

but the answers says that Kp = P(PCl3) x P(Cl2) / P(PCl5)

can anyone expand on this because the equilibrium constant only includes solutes and gases not liquid and solids....

Reply 1

charco

Study Forum Helper

Original post by JEvans1234

you are correct - the equation is wrong for the given kp expression.

Reply 2

Borek

IMHO both answers are in a way right. Both PCl₅ and PCl₃ are volatile enough to build a substantial partial pressure and react in gaseous phase, so the answer given makes sense. However, you approach is also OK, although it will yield a different Kp value. It would be possible to convert between both Kp values knowing pressures of the saturated vapor over PCl₅(s) and PCl₃(l) (as these pressures are constant as long as the liquid and solid are present, they can be moved into the Kp value).

Question is ambiguous.

(edited 10 years ago)

Reply 3

apple112

Hi. Kp indicates that the equilibrium of gases where only reactants and products which are in gaseous states are included in the expression, hence accounting for the ans. From your understanding of equilibrium constant, it applies to Kc where products and reactants are in different states while for Kp it only applies to gases only.

Hope this clarifies any confusion about your ans.

(edited 10 years ago)

Reply 4

JEvans1234

thanks everyone for your feedback, being me i didnt read the information in the question thoroughly and found that it said that at this certain temperature all the compounds were gases