Untitled - Kelly Walsh High School

206 CHEMISTRY FOR THE UTTERLY CONFUSED
Don’t Forget!
Careful!
the concentrations of solids or pure liquids (not in solution) that appear in the
equilibrium expression are assumed to be 1, since their concentrations do not
change.
The concentrations of pure solids and liquids do not appear in the equilibrium
constant expression.
Consider the Haber process for the production of ammonia: N 2(g) 3 H 2(g) K
2 NH 3(g).
[NH3 ]
The equilibrium constant expression is: Kc
2
[N2 ][H2 ] 3
If the partial pressures of the gases were used, then the K p would be written in
the following form:
2 PNH3 Kp
There is a relationship between the Kc and the Kp: Kp Kc (RT) where R is
the ideal gas constant and ng is the change in the number of moles of gas in the
reaction.
n 3 P P N2 H2
g
Be sure that your value of R is consistent with the units chosen for the partial
pressures of the gases.
The numerical value of the equilibrium constant gives an indication of the
extent of the reaction after reaching equilibrium. If K c is large, then that means
the numerator is much larger than the denominator and the reaction has produced
a relatively large amount of products (reaction lies far to the right). If K c
is small, then the numerator is much smaller than the denominator and not
much product has been formed (reaction lies far to the left).