To understand the shift in equilibrium between ferric ions and thiocyanate ions by either increasing/decreasing the concentration of the ions.
Chemical equilibrium refers to the state of a system in which the concentration of the reactant and the concentration of the products do not change with time and the system does not display any further change in properties.
The equilibrium reaction between potassium thiocyanate and ferric chloride through the change in the concentration of colour of the solution, also the dissociation occurs until the balance is restored on both sides. The equation of the formation of ferric thiocyanate is,
In this equation the Fe3+ act as a cation, and SCN- act as an anion and they combine to form the coordination complex of the [Fe (SCN)]2+ ions.
The formula gives the equilibrium constant is,
The equilibrium constant for the reaction between potassium thiocyanate and ferric chloride is,
At a constant temperature, the value of K also remains constant.
Factors involving in the equilibrium of the ferric and thiocyanate ions.
According to the Le Chatelier’s principle the increasing amount of the concentration of the Fe3+ and the SCN- reflects the equilibrium in the right to form more of the ferric thiocyanate complex, conversely, the amount of increasing the concentration of the ferric thiocyanate complex reflects the equilibrium in the left to consume the added of complex ions.
Also, the shift in equilibrium between ferric ions (Fe3+) and thiocyanate ions (SCN-) can be influenced by the presence of other ions such as potassium ions (K+) from the potassium chloride (KCl) and potassium thiocyanate (KSCN)
When adding the potassium ions as KCl or KSCN, leads to the increasing the concentration of SCN- ions, and the equilibrium will shift to the left to consume and maintain the equilibrium constant. This will lead to a decrease in the concentration of the ferric thiocyanate complex ion [Fe (SCN) ]2+ and an increase in the concentration of ferric ions (Fe3+).
As a result, the concentration of thiocyanate ions decreases, and equilibrium shifts in the backwards direction. In other words, some of the [Fe (SCN)2+] complex dissociates to give Fe3+ and SCN- ions.