Law of mass action

It states that: Rate of a reaction is directly proportional to concentration of reactants raised to their respective moles.

Consider a reaction: aA + bB –> cC + dD

According to this law:

At equilibrium: Rate of forward reaction = rate of backward reaction

That is: K_f[A]^a[B]^b = K_b[C]^c[D]^d

But, K_c=K_f/K_b (where K_c is equilibrium constant)

K_c = [A]^a[B]^b/[C]^c[D]^d

At particular instant of time: The equilibrium constant is called as reaction quotient (Q).

That is: Q=[C]^c[D]^d /  [A]^a[B]^b

At equilibrium Q=K_c

For example:

Please note:

• If change in gaseous moles=0 then k_p = k_c
• If change in gaseous moles is greater than 1 ,then K_p= k_c RT or k_p > k_c
• If change in gaseous moles is less than zero, then k_p < k_c

Characteristics of Equilibrium constant (k)

1. Its value depends upon temperature and is independent of concentration of reactants or products, with which we start reaction.
2. If the reaction reverses, the value of k also gets reversed.

For example: If for forward reaction k =4 then, on reversing the value of k will be ¼ .

3. If whole reaction is divided by 2, then the value of k also gets root of K.
4. If whole reaction is multiplied by 2, then value of k also gets multiplied by the same number 2 or 3 i.e Kⁿ.
5. If the reaction takes place in two or more steps, then the value of k for complete reaction is given by k=k₁.k

Characteristics of chemical equilibrium

1. The Rate of forward reaction = Rate of reverse reaction.
2. Catalyst does not affect the equilibrium .It just helps to achieve equilibrium faster.
3. The Equilibrium is attained only is closed vessel.
4. Equilibrium can be achieved from either direction.
5. At equilibrium concentration of reactants and products becomes equal.

Effect of temperature on K

We know that K=K_f/K_b

Now if temperature is increased, then the rate of reaction also increases. The extent of reaction depends upon activation energy of reactants i.e R_f and R_b will be different and also the value of K_f and K_b .So, the value of K will also change.

• For endothermic reactions: K_f>K_b. Therefore, K increases.
• For exothermic reactions: K_f<k_b .Therefore, K decreases.

Units of k

K_c = (mol/L)^{change in gaseous moles}

K_p = (atm)^{change in number of gaseous moles}