Kp and Pressure Change

name         Sophia
status       student
grade        other
location     HI

Question -   I read one sentence that confuses me a lot.
Kp will remain the same even when the change
of pressure causes the change of concentration.
Taking this for example:
3H2 + N2 ---> 2NH3
When the pressure goes upwards, I suppose there
will be more NH3 than before, so in my opinion
the Kp will become larger. But I know this is
false, could you kindly explain it to me? 
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Sophia,

Look the le Chatelier Principle. Essentially what this principle 
summarizes is that when a system that is in equilibrium is perturbed 
- as is the case when we decrease the volume of a container 
containing gaseous reactants and products - that the equilibrium 
will shift to reacquire equilibrium.

Thus, in the particular example you talked about: 3H2(g) + N2(g) 
---> 2NH3(g), if this system is already at equilibrium then: Kp = 
(pNH3)^2/((pH2)^3 * (pN2)). The initial effect of reducing the 
volume of the container is to raise the pressures of all the gases - 
at this point the right side of the equation will not be equal to 
Kp. The ratio will be smaller than Kp. There are essentially 2 moles 
of gas on the product (numerator) side as opposed to 4 moles of 
reactant (denominator) side - this means that the decrease in volume 
will have a higher effect on the reactant side (consequently 
increasing the reactant contribution to the pressure). However, the 
Le Chatelier Principle expresses the system will reacquire 
equilibrium. This means that, in this case, there will be an 
increase in the forward rate of reaction so that more product will 
be formed. This will continue until the pressures are such that the 
ratio is once again equal to Kp.

Remember that the Keq is a function of the ratio of the forward and 
reverse rate constants and only changes in the rate constants change Keq.
Changes in concentrations or pressures result in a perturbation of 
the equilibrium such that the system is momentarily not at 
equilibrium. Such changes do not change Keq.

Greg (Roberto Gregorius)
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