Changing concentration
In The Equilibrium Law we learned that Kc for an equilibrium system is a constant at constant temperature. In this topic we're going to explore what happens if we change the concentration of one of the components of the equilibrium mixture.
Iron(III) ions and thiocyanate ions react to form an equilibrium mixture containing iron thiocyanate. This is a good equilibrium to explore because a solution containing iron(III) ions is yellow while a solution containing iron thiocyanate is blood-red.
Fe3+(aq) + SCN–(aq) [FeSCN]2+
(pale yellow) (colourless) (red)
An equilibrium mixture is made by mixing solutions of iron(II) chloride and potassium thiocyanate. It is then divided into four parts labelled A, B, C and D.
A drop of iron(III) chloride solution is added to tube A.
A drop of potassium thiocyanate solution is added to tube B.
Some solid ammonium chloride is added to tube C.
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What happens to the colour of the mixture when the concentration of Fe3+ ions is increased in tube A?
The mixture becomes red.
What happened to the equilibrium position as a result of increasing the concentration of Fe3+ ions?
The equilibrium shifts to the right.
What happens to the colour of the mixture when the concentration of SCN– ions is increased in tube B?
The mixture becomes red.
What happens to the position of equilibrium as a result of increasing the concentration of SCN– ions?
The equilibrium shifts to the right.
The effect of adding ammonium chloride to tube C is to reduce the concentration of Fe3+ ions due to the formation of [FeCl4]–.
What happens to the colour of the mixture in tube C when the concentration of Fe3+ ions is reduced?
The mixture becomes yellow.
What happens to the position of equilibrium when the concentration of Fe3+ ions is reduced in tube C?
The equilibrium shifts to the left.
Why is it useful to have tube D even though nothing is added to it?
It enable us to compare the other tubes with the original colour of the equilibrium mixture to see more clearly what changes have occurred.
How, in general, does changing the concentration of one of the components of an equilibrium mixture affect the position of the equilibrium?
Increasing the concentration of a component on one side of the system pushes the equilbrium in the direction in which this component reacts. Decreasing the concentration of a component on one side of the system pushes the equilibrium in the direction in which this component is produced.