Chemical and Biomolecular Engineering

Official blog of the Lehigh University Chemical Engineers

Equilibrium Reactions — Part 2

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In a previous post, I illustrated a Chem Lab that centered around equilibrium reactions and Le Chatelier’s Principle. Different stresses were applied to a system at equilibrium and we were able to determine what happened based on qualitative results (ie. color changes). These stresses included; adding compounds and changing the temperature of the reaction. The final two reactions studied in the lab were complex ion equilibria, meaning different ions, composed of multiple atoms, formed in the systems.

The first reaction is a cobalt chloride solution. Here is the chemical equation:

Pink                       Blue

Pink                                                                       Blue

The initial solution in the test tube is pink and then we added HCl. This increased the concentration of Cl-, which pushed the reaction toward the products (right). In turn, the solution became blue.

Cobalt pink blue

Next, we studied the effect of changing the temperature of the system. When the test tube was placed in boiling water (heat added), it became blue. Then, when it was allowed to cool and then placed in an ice water bath, the solution reverted to its original pink color. This indicates that the reaction is endothermic. When heat is added, the reaction proceeds to the right, the solution turns blue. And then when heat is taken out of the system (in an ice bath), reactants are formed. This means that the enthalpy of the reaction is positive or endothermic (needs heat to progress).

For the final reaction, we explored the oxidation of Iodide (I-) by Copper II (Cu2+). The first reaction is:

pale blue green

pale blue                                                                   green

Potassium iodide (KI) is added to a solution with copper ions (Cu2+). This increases the concentration of I-, which forces equilibrium to the right (products). The solution turns yellow and then becomes a pale green color. Next, cyclohexane is added to the solution. Iodine (I2) is more soluble in cyclohexane than in water, and cyclohexane is less dense than water. Therefore, after the solution is mixed, the Iodine dissolves into the cyclohexane which sits on top of the water solution.

Iodine in cyclohexane

Iodine in cyclohexane

Lastly, ammonia (NH3) is added to the copper solution. This addition forces the reaction to the products and is visually depicted by the pale blue copper solution becoming a deep blue with the addition of ammonia. Check out the video of this reaction!

This lab was a great opportunity to work with chemicals and really understand Le Chatelier’s Principle and the role it plays in all reactions.


Author: Ben Dunmire

I am a sophomore Chemical Engineering major at Lehigh. On campus, I am the president of Club Baseball and a member of AIChE and NSCS. Outside of school, I enjoy the outdoors, fishing, and any type of athletic activity. You can reach me at

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