Chemical Equilibrium with Cobalt Chloride

Chemical equilibrium is a dynamic balance between forward and reverse reactions — a concept that can seem abstract until students see it in action. The reversible colour change of cobalt chloride provides a striking demonstration of how temperature and concentration shifts affect equilibrium, perfectly illustrating Le Chatelier’s Principle.

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The Experiment

Reaction System:

$$\text{[Co(H₂O)₆]}^{2+} (aq) + 4Cl^- (aq) \rightleftharpoons \text{[CoCl₄]}^{2-} (aq) + 6H₂O (l)$$

Visible Change:

• The pink complex $\text{[Co(H₂O)₆]}^{2+}$ dominates in cold conditions.

• The blue complex $\text{[CoCl₄]}^{2-}$ forms when the solution is heated or when chloride concentration increases.

Method:

1. Place a few drops of cobalt(II) chloride solution into a small test tube.

2. Add concentrated hydrochloric acid until the solution turns blue.

3. Split the mixture into two tubes.

4. Warm one tube gently in hot water — it turns a deeper blue.

5. Cool the other in ice — it returns to pink.

6. Alternate heating and cooling to show the reversible colour change.

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The Science

This reaction is endothermic in the forward direction (blue complex formation).

• Heating shifts the equilibrium to the right (more blue).

• Cooling shifts it back to the left (pink).

• Adding chloride ions (from HCl) also favours the blue complex, as the system counteracts the added ion concentration.

Le Chatelier’s Principle states that a system at equilibrium will adjust to oppose any change in conditions — and this reaction provides instant visual proof.

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Skills Highlight

• Observing qualitative changes in a dynamic equilibrium

• Linking colour change to molecular composition and energy changes

• Applying Le Chatelier’s Principle to temperature and concentration

• Connecting equilibrium to industrial processes such as the Haber and Contact reactions

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Why It Works in Teaching

Few experiments make equilibrium as clear as cobalt chloride. The reversible pink–blue colour shift lets students see how reactions respond to change, turning a theoretical idea into an elegant, memorable demonstration of chemical balance.