Where, [Fe(SCN)2+], [ Fe3+] and [ SCN-] are the equilibrium concentrations of the respective species and Kc is the equilibrium constant.
[Fe(SCN)2+]/[ Fe3+][ SCN-] is known as the reaction quotient as is denoted by Qc. It can be expressed in the same way as equilibrium constant Kc except that the concentrationsin Qc are not necessarily the equilibrium values.
In this reaction, ferric chloride reacts with potassium thiocyanate to form a deep red-coloured ferric thiocyanate complex. The intensity of the red colour becomes constant on attaining equilibrium.
When the concentration of any of these species is changed, the equilibrium is disturbed and the reaction quotient remains no longer equal to equilibrium value. In order to re-establish the equilibrium, the ions interact in such a way that the reaction quotient again becomes equal to the equilibrium constant.
If we add ferric chloride solution to the deep red-coloured solution containing ferric ions, thiocyanate ions and ferric thiocyanate complex, the concentration of ferric ions increases. As a result, more thiocyanate ions combine with ferric ions to increase the [Fe(SCN)2+] complex and therefore the colour intensity of the red solution increases. The increase in concentration of ferric ions shifts the equilibrium in the forward direction.
If we increase the concentration of thiocyanate, more ferric ions react with thiocyanate ions to increase the [Fe(SCN)2+] complex. As a result, the colour intensity of the red solution increases. Thus, the increase in concentration of thiocyanate ions shifts the equilibrium in the forward direction.
If we add potassium chloride to the solution, the concentration of potassium ions increases.