Question

3. What name is used when the equilibrium constant, Kc, equals the product quotient, Qc?

Answer 1

Consider the following reaction

A + B -----> C + D ( Note that arrow is shown in only one direction)

When this reaction is not in equilibrium, the ratio of concentrations of products to reactants is termed as Qc ( Reaction Quotient)

Qc = [C] [D]/ [A] [ B]

Now consider the same reaction, but under equilibrium conditions

A + B <--------> C + D ( Note the reversible arrow)

So when the reaction is under equilibrium, Qc becomes equal to Kc.

Therefore we have Kc = [C] [D]/ [A] [ B] ---------- Equation (1)

Now consider forward reaction A + B ---> C + D

If we apply law of mass action to this reaction, we have

Rate of forward reaction = k1 [ A] [ B]

Consider reverse reaction C + D ----> A + B

Applying law of mass action to this reaction, we get

Rate of reverse reaction = k2 [ C] [D]

At equilibrium,

Rate of forward reaction = Rate of reverse reaction

k1 [A] [B] = k2 [C] [ D]

k1/k2 = [ C] [ D] / [A] [B] ( k1/k2 = K)

K = [ C] [D] / [A] [ B] which is similar to equation 1 we got earlier.

Therefore we can say , Equilibrium systems obey law of mass action

So when reaction quotient becomes equal to equilibrium constant, which indicates "Equilibrium"

The name used is "' Law of Mass Action""