Our objective is to study the shift of equilibrium between ferric ions and thiocyanate ions by increasing the concentration of either of them.
Let's see how chemical equilibria behave when concentration changes.
Many chemical reactions involve a state of equilibrium. Equilibrium is said to be dynamic when the rate of forward reaction become equal to that of the backward reaction.
Consider a general reversible reaction;
With the passage of time, there is a depletion of the reactants A and B and an accumulation of the products C and D. As a result, the rate of forward reaction decreases and that of backward reaction increases. Eventually, the two reactions occur at the same rate and the system reaches a state of equilibrium.
Norwegian chemists Cato Maxmillian Guldberg and Peter Waage proposed that for a chemical reaction;
The rate of reaction in either direction is proportional to the active mass (concentration) of the reactants.
By applying the Law of Mass Action;
The rate of forward reaction;
The rate of back ward reaction;
Where [A], [B], [C] and [D] are the equilibrium concentrations of A, B, C and D respectively.
a, b, c, and d represents the stoichiometric coefficients of A, B, C and D respectively.
Kf and Kb are the rate constants of forward and backward reaction.
However, at equilibrium,
Rate of forward reaction = Rate of backward reaction.
Kc is called the equilibrium constant expressed in terms of molar concentrations and the above equation is known as the Law of Chemical Equilibrium.
The Law of Chemical Equilibrium is defined as, the ratio of product of concentration of the products to the product of concentration of the reactants, with each concentration term is raised to the power by its coefficient in overall balanced chemical equation, is a constant quantity at a given temperature and it is called equilibrium constant.
If a system is subjected to a change in concentration of one or more reactants, or a change in temperature or pressure, the equilibrium condition of the system is altered. A net reaction will take place in some direction until a new equilibrium state is attained.
In 1884, a French Chemist and Engineer, Le Chatelier, showed that in every such case, the new equilibrium state partially reduces the effect of change that brought it about. This principle is known as Le Chatelier’s Principle.
The principle states that, if a system at equilibrium is subjected to a change of pressure or temperature or number of moles of the component, there will be a tendency for a net reaction in the direction that reduces the effect of this change.
According to Le Chatelier’s principle, when the concentration of any of the reactants or products in a reaction at equilibrium is changed, the composition of the equilibrium mixture changes so as to minimize the effect of concentration changes.
To illustrate the effect of change in concentration on equilibrium, consider the equilibrium between ferric ions and thiocyanate ions;
The equilibrium constant for the above reaction can be written as,
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.
It affects the equilibrium between potassium ions and thiocyanate ions.
As the concentration of potassium ions increases, more thiocyanate ions react with potassium ions to form more potassium thiocyanate and the above equilibrium shift in the backward direction occurs. As a result, the concentration of thiocyanate ions decreases and the equilibrium 1 shifts in the backward direction. In other words, some of the [Fe(SCN)2+] complex dissociates to give Fe3+ and SCN- ions.
As a result, the concentration of [Fe(SCN)2+] decreases and therefore the intensity of red colour decreases. Thus the increase in concentration of K+ ions shifts the equilibrium in the backward direction.
Generally we can say that;
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