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Reversible Reactions

Introduces reversible chemical reactions.

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Reversible Reaction

Adding hydrochloric acid or water can change the color of a cobalt solution

Credit: User:Chemicalinterest/Wikipedia
Source: http://commons.wikimedia.org/wiki/File:Cobalt_chloride_equilibrium.JPG
License: CC BY-NC 3.0

How do you change the color of a solution?

A solution of cobalt chloride in water is pink due to the presence of the solvated Co2+ ion. If sufficient HCl is added, the solution turns blue as the CoCl42- ion forms. The reaction can be shifted back to the pink form if more water is added to the solution.

Reversible Reactions

Up until this point, we have written the equations for chemical reactions in a way that would seem to indicate that all reactions proceed completely until all the reactants have been converted into products. In reality, a great many chemical reactions do not proceed entirely to completion. A reversible reaction is a reaction in which the conversion of reactants to products and the conversion of products to reactants occur simultaneously. One example of a reversible reaction is the reaction of hydrogen gas and iodine vapor to from hydrogen iodide. The forward and reverse reactions can be written as follows.

\begin{align*}& \text{Forward reaction}: \quad \text{H}_2(g)+\text{I}_2(g) \rightarrow 2\text{HI}(g) \\ & \text{Reverse reaction}: \quad 2\text{HI}(g) \rightarrow \text{H}_2(g)+\text{I}_2(g)\end{align*}Forward reaction:H2(g)+I2(g)2HI(g)Reverse reaction:2HI(g)H2(g)+I2(g)

In the forward reaction, hydrogen and iodine combine to form hydrogen iodide. In the reverse reaction, hydrogen iodide decomposes back into hydrogen and iodine. The two reactions can be combined into one equation by the use of a double arrow.

\begin{align*}\text{H}_2(g)+\text{I}_2(g) \rightleftarrows 2\text{HI}(g)\end{align*}

The double arrow is the indication that the reaction is reversible.

When hydrogen and iodine gases are mixed in a sealed container, they begin to react and form hydrogen iodide. At first, only the forward reaction occurs because no HI is present. As the forward reaction proceeds, it begins to slow down as the concentrations of the H2 and the I2 decrease. As soon as some HI has formed, it begins to decompose back into H2 and I2. The rate of the reverse reaction starts out slow because the concentration of HI is low. Gradually, the rate of the forward reaction decreases while the rate of the reverse reaction increases. Eventually the rate of combination of H2 and I2 to produce HI becomes equal to the rate of decomposition of HI into H2 and I2. When the rates of the forward and reverse reactions have become equal to one another, the reaction has achieved a state of balance.


  • A reversible reaction is defined.


  1. Do all chemical reactions proceed to completion?
  2. Why is the reaction given above considered a reversible reaction?
  3. How do we designate a reversible reaction?

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Image Attributions

  1. [1]^ Credit: User:Chemicalinterest/Wikipedia; Source: http://commons.wikimedia.org/wiki/File:Cobalt_chloride_equilibrium.JPG; License: CC BY-NC 3.0

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