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: Kf[A]a[B]b = Kb[C]c[D]d
But, Kc=Kf/Kb (where Kc is equilibrium constant)
Kc = [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=Kc
- If change in gaseous moles=0 then kp = kc
- If change in gaseous moles is greater than 1 ,then Kp= kc RT or kp > kc
- If change in gaseous moles is less than zero, then kp < kc
Characteristics of Equilibrium constant (k)
- Its value depends upon temperature and is independent of concentration of reactants or products, with which we start reaction.
- 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 .
- If whole reaction is divided by 2, then the value of k also gets root of K.
- If whole reaction is multiplied by 2, then value of k also gets multiplied by the same number 2 or 3 i.e Kn.
- 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
- The Rate of forward reaction = Rate of reverse reaction.
- Catalyst does not affect the equilibrium .It just helps to achieve equilibrium faster.
- The Equilibrium is attained only is closed vessel.
- Equilibrium can be achieved from either direction.
- At equilibrium concentration of reactants and products becomes equal.
Effect of temperature on K
We know that K=Kf/Kb
Now if temperature is increased, then the rate of reaction also increases. The extent of reaction depends upon activation energy of reactants i.e Rf and Rb will be different and also the value of Kf and Kb .So, the value of K will also change.
- For endothermic reactions: Kf>Kb. Therefore, K increases.
- For exothermic reactions: Kf<kb .Therefore, K decreases.
Units of k
Kc = (mol/L)change in gaseous moles
Kp = (atm)change in number of gaseous moles