Gas Equilibrium: Physical & Chemical Characteristics

[djinn_Aria]

Hey everyone, I'm in a little pickle if I must say.
I've got this question on my assignment asking me to explain what the physical and chemical characteristics of a "gas" equilibrium whilst explaining the forward/reverse reactions and their dynamic nature.

My problem, what is a "gas" equilibrium? Is it the same as a normal equilibrium? I'm also not sure what characteristics are, is it when the forward reaction is equal to the reverse reaction?

Also on another question, it asks to explain how the equilibrium constant is calculated and I'm fine with that, but its sort of bugging me how they came up with the equation. where K=[products]/[reactants]
Can anyone explain it to me in not so difficult terms?

 

[Yanick]

Gas equilibria are treated, Mathematically, the same as aqueous equilibria. The difference is units. The classic example I've seen in textbooks is the Haber Process. My undergrad General Chemistry text started off with equilibria in the gas phase then introduced aqueous equilibria and acid/base Chemistry. You might try looking in yours or finding a decent one in a library.

As for the second part you can think of it as such.

At equilibrium the forward rate of reaction equals the reverse rate of reaction:

A \rightleftharpoons B

Rate_fwd=Rate_rev

k_fwd[A] = K_rev

k_fwd/k_rev = /[A]

where k_fwd/k_rev = K_eq

Then you have k_eq = /[A]

 

[djinn_Aria]

wow that was explained nicely, thank you very much Yanick :)
But what do you mean by difference is the units?

 

[Borek]

What are units of Kc and Kp constants for reactions

A(aq) <-> B(aq) + C(aq)

and

A(g) <-> B(g) + C(g) ?

 

[djinn_Aria]

would the units of Kc be in moles/L because it uses the concentrations in calculations?
I'm not sure about the Kp but I'd say in atm or Pa?

 

[Borek]

You are on the right track.

Note that this is a little bit more complicated, as exact units of the K_whatever constant depend on the reaction. For a simple reaction like

A -> B

K_c = \frac {<b>}{[A]}</b>

units cancel out, but for reaction like

A -> 2B

K_c = \frac {[A]^2}{<b>}</b>

we are left with mol/L (it works the same way for K_p).

In general it is better to treat K as dimensionless and all expressions present as dimensionless activities of the substances present. Quite often we need a logarithm of K and units become a problem.