Equilibrium Constant

Question

The basic question I am asking is how does varying the initial concentrations of reactant(s) in a chemical reaction affect the equilibrium constant. What I have so far is:

 

Using this basic equation for equilibrium constant:

Kc = ((A)^w(B)^x)/((C)^y(D)^z)

where C and D are the concentrations of the products and A and B are the concentrations of the reactants, we can see that if we increase the concentration of any of the reactants the rate constant would be the same because the concentration of the products would also be larger. Qualitatively, we know that matter can be neither created nor destroyed, so the amount of matter added to the reactants would be the same as the products.

 

Am I correct in assuming this or am I misunderstanding the idea of concentration?

Answer 1

Hi Adam

 

Although you are right that the concentration affects the overall rate of the reaction, in a chemical equilibrium, the rate of the forward reaction is equal to the rate of the reverse reaction. In this case concentration does not affect the reaction rate. To determine how the initial concentration affects chemical equilibrium you need to calculate the reaction quotient, Q, which is similar to the equilibrium constant expression expect for the reaction quotient you work with initial concentration. For the general reaction

 

wA + xB <=> yC + zD

 

the reaction quotient is

 

Q = [C]₀^y[D]₀^z/[A]^w₀[B]^x₀

 

To determine the equilibrium position you compare the value of Q to K.

 

If Q = K, the system is at equilibrium so no shift in the equilibrium position will occur.

If Q > K, the system will shift to the right since the ratio of products to reactants is large

If Q < K, the system will shift to the left since the ratio of products to reactants is small

 

Hope this answers your question.