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Solution Colors

Chemical Equilibrium: Previous page: Equilibrium Constants for Special Systems

Skills to Develop

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

Colored Chemicals and Equilibrium

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

A slight change in the structure of a substance may cause a dramatic change in its color. Substances having different colors 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, colored substances are indeed very interesting.

The color of a bulk material is determined by the dominant form of molecules. If the majority of molecules is in the red form, its solution is red. When the pH changes, the \(\ce{H+}\) ions in the solution cause the forms of the indicators to change. If the yellow form becomes the dominant species, then the color is yellow.

As we all know, computer screens emit red, green and blue (R G & B) lights. The relative intensities of R G and B determine how the colors of the dots appear to us. The intensity is another matter. Here are some interesting color ratios.

In the following table, characters in the row have different red intensities. Characters in the second row have various ratios 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 Color of Solution

Here are some facts about the color 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 reflect 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 colors, the color of the solution is determined by the dominant species of molecules or ions. The intensity of the color, again, is determined by the concentration, and the color is determined by the relative amounts of various colored 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

\(\mathrm{ {\color{Red} AB} \rightleftharpoons {\color{Green} A} + {\color{Blue} B}}\) ,

the equilibrium constant is

\(K = \mathrm{\dfrac{ {\color{Green} [A]} {\color{Blue} [B]}}{ {\color{Red} [AB]}}}\)

The fraction of \(\mathrm{ {\color{Red} AB}}\) dissociated into \(\mathrm{ {\color{Green} A}}\) and \(\mathrm{ {\color{Blue} B}}\) will depend on \(\mathrm{ {\color{Red} [AB]}}\) and on the K value.

Questions

  1. In an equilibrium mixture of

    \(\ce{A + B \rightleftharpoons AB}\)

    how do concentrations of \(\ce{A}\) and \(\ce{B}\) change as more \(\ce{AB}\) is added to the system?

  2. Assume that the equilibrium constant K = 0.01 for the equilibrium

    \(\mathrm{ {\color{Red} AB} \rightleftharpoons {\color{Green} A} + {\color{Blue} B}}\)

    What is the color of a very dilute solution of \(\mathrm{ {\color{Red} AB}}\) (say 0.01 M)?
    1. color of \(\mathrm{ {\color{Green} A}}\)
    2. color of \(\mathrm{ {\color{Blue} B}}\)
    3. color of \(\mathrm{ {\color{Red} AB}}\)
    4. color due to an equal mixture of \(\mathrm{ {\color{Cyan} A\: and\: B}}\)

Solutions

  1. increase
    Consider...
    As you add more \(\ce{AB}\) to the solution, more will dissociate.

  2. For this system, \(\mathrm{ {\color{Red} [AB] = 0.0038\: M}}\); \(\mathrm{ {\color{Green} [A]} = {\color{Blue} [B]} = {\color{Cyan} 0.00618\: of\: [A]\: and\: [B]}}\). What do you think is the color?
    Chemists produce brilliant solutions... if you have a sense of humor.

Next page: Dynamic Equilibria

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