How the concentration effect the equilbrium?

Answer 1

The effect of the change in concentration on chemical equilibria can be predicted qualitatively by Le Chatelier's Principle. Consider the following reaction:

Fe3+(aq)+SCN-(aq)<---->Fe(SCN)2+(aq)

If you add Fe3+(aq), that is an increase in the concentration of Fe3+(aq), and the equilibrium position shifts to the left (i.e. the product side). That is because the Le Chatelier's Principle states, if the reaction condition of an equilibrium system is changed, the equilibrium position will shift in a way to REDUCE the effect of the change. When the equilibrium position shifts to the left in the above system, that means the forward reaction rate increases (the backward reaction rate remains the same). More product is hence produced, in this case, Fe(SCN)2+(aq).

Conversely, if you add Fe(SCN)2+(aq), the equilibrium position shifts to the right (i.e. the reactant side). This time the backward reaction rate increases while the forward reaction rate remains, hence more reactants (Fe3+(aq) and SCN-(aq)) are produced.

The concentration effect on equilirium can sometimes be observed. In the above reaction, Fe3+(aq) is yellow, SCN-(aq) is colourless and Fe(SCN)2+(aq)is deep red. If the equilibrium position shifts to the left due to the concentration effect, more Fe(SCN)2+(aq) is produced and the colour of the reaction mixture becomes darker, due to the colour of Fe(SCN)2+(aq). Conversely, if the equilibrium position shifts to the right, the colour becomes paler.

Answer 2

The meaning of equilibrium is an unchanged state (without external intervention).