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Equilibrium: Concentration and Color

Skills to develop

Describe colour changes of a system in terms of equilibrium.
Apply equilibrium concepts to describe colour changes.

Coloured Chemicals and Equilibrium

Many chemicals are coloured substances. The presence of these materials in flowers, birds, rocks, animals make the world interesting.

A slight change in the structure of a substance may cause a dramatic change in its colour. Substances having different colours in acidic and basic solutions have been used as indicators for acidity. A mixture of indicators are used to indicate the pH of solutions. Thus, coloured substances are indeed very interesting.

The color of a bulk material is determined by the dominate form of molecules. If the majority of molecules is in the red form, its solution is red. When the pH changes, the H⁺ ions in the solution cause the forms of the indicators to change. If the yellow form become the dominate species, then the color is yellow.

As we all know that the computer screen emits red, green and blue (R G & B) lights. The relative intensities of R G and B determines the color of the dots appear to us. The intensity is another matter. Here are some interesting colour ratios.

In the following table, charactrers in the row have different red intensities. Characters in the second row have various ratio of Red to Blue, and characters in the last row have different blue intensities. Two tables are given, the first one in a black background, and the second in a white background.

Red Red Red Red Red Red Red Red Red Red Red Red Red Red Red
RnB RnB RnB RnB RnB RnB RnB RnB RnB RnB RnB RnB RnB RnB RnB
Blue Blu Blue Blu Blue Blu Blue Blu Blue Blu Blue Blu Blue Blu Blue

Red Red Red Red Red Red Red Red Red Red Red Red Red Red Red
RnB RnB RnB RnB RnB RnB RnB RnB RnB RnB RnB RnB RnB RnB RnB
Blue Blu Blue Blu Blue Blu Blue Blu Blue Blu Blue Blu Blue Blu Blue

The Colour of Solution

Here are some facts about the colour of solutions.

True solutions are usually transparent if the solvent is transparent. Small particles in coloids and suspensions in a solution scatter lights making them opaque. When these particles reflects all color, they appear as white such as milk. Milk is a coloid. Students often ask if there is a white solution that is transparent. How would you answer?

If molecules of a dissolved substance absorb visible light of certain energy, then the solution appears to have color due to the fact that we see what is not absorbed. For these solutions, the intensity of the color is related to the concentration.

If the species involved in the equilibrium have colours, the color of the solution is determined by the dominated species of molecules or ions. The intensity of the color, again, is determined by the concentration, and the colour is determined by the relative amounts of various coloured species. Since the equilibrium constant determines the relative amounts of the reactants and products, its values will affect the color of the solution.

For example, for the reaction

AB = A + B The equilibrium constant is, [A] [B]
K = -----------
[AB] The fraction of AB dissociated into A and B will depend on [AB] and on the K value.

Confidence Building Questions

In an equilibrium mixture of A + B = AB how do concentrations of A and B change as more AB is added to the system?

Hint increase
Consider...
As you add more AB to the solution, more will dissociate.
Assume that the equilibrium constant K = 0.01 for the equilibrium AB = A + B What is the color of a very dilute solution of AB (say 0.01 M)?
(a) color of A
(b) color of B
(c) color of AB
(d) color due to an equal mixture of A and B.
Hint...
For this system, [AB] = 0.0038 M; [A] = [B] = 0.00618 of [A] and [B]. What do you think is the color?
Chemists produce brilliant solutions... if you have a sense of humour.

Next page: Dynamic equilibria

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