Unit 7: Equilibrium And Acids

Section 3: Equilibrium Constant (Pressure)

* Equilibrium And Pressure
* Relationship Between K and Kp
* Using the Equilibrium Constant

Equilibrim and Pressure

Earlier in this unit, K was defined as the equilibrium constant. In this
unit, we are going to define the Kp. The Kp is the equilibrium constant
using pressure. When using Kp, we don't use concentration, instead we use
the partial pressures of each substance. (Note: the only substances put int
the equilibrium constant when using pressures are gases.) So, the rule for
the equilibrium constant with pressure is:

Kp = (Pc)l (Pd)m / (Pa)j (Pb)k

for the reaction of:

j A + k B <=====> l C + m D

Relationship Between K and Kp

The relationship between K and Kp is:

Kp = K (RT)delta n

where R is .08206 L * atm / (K * mol) T is the temperature in Kelvin delta n
is the difference between the gaseous mole of product and the gaseous moles
of the reactants

Using the Equilibrium Constant

Right now, you are probably saying, FINALLY we get to use the equilibrium
constant. Well you are right. Now you get to see how all the pieces of the
puzzle fit together. First, when we know the equilibrium constant, we can
determine if the equilibrium will have more product or reactants in it. If
the K is less that 1, the the equilibrium position will probably contain a
lot of reactants. If the K is greater that 1, then the equilibrium position
will probably contain a lot of products.

Secondly, if you know to equilibrium constant and in the problem, they gave
you all the concentrations or pressures you can calculate the reaction
quotient (Q). The "Q" is exactly the same equation as you equilibirium
constant, except the numbers that you put in is the concentrations or
pressures that you were given in the equation. Once you have solved for the
"Q", you can compare it to the equilibrium contant.

1. If Q = K, the reaction is at equilibrium
2. If Q is greater that K, the products will be consumed to produce more
reactants and the reaction will shift left.
3. If Q is less that K, the reactants will be consumed to produce more
products and the reaction will shift right.