Equilibrium explanation

Question

I'm learning the topic of equilibrium right now and can't seem to wrap my head around the topic. Could someone explain the concept of Kc/Qc (what does it represent) and why/how it is affected (or lack of) by change in temperature, pressure and concentration.

Answer 1

Kc and Qc are very similar in that they both represent the ratio of the concentration of the products to that of the reactants. Kc (equilibrium constant) is the value at equilibrium and Qc is the value at any other point in the reaction (it is sometimes called the reaction quotient).

Example:

aA + bB ==>cC + dD

Kc = [C]^c[D]^d / [A]^a[B]^b and Qc takes the same form, i.e. Qc = [C]^c[D]^d / [A]^a[B]^b

So, you see it represents how much product there is relative to how much reactant there is at some point in time (Kc at equilibrium, Qc at some other time). If the value is greater than 1, the reaction favors product formation. If less than 1, the reaction favors reactants. Does that make sense?

The value of Kc is a constant and as such is affected by temperature so values of Kc are determined at a given temperature and will have a different value if the temperature is different. As far as pressure and concentration, the Kc is the same and not affected by these variable. However, the POSITION of the equilibrium will be affected by concentration and/or pressure. For more about this, you might want to look up and read about Le Chatelier's Principle.

If you need additional help, let me know.