8.1a - Effects of Concentration Changes on the Equilibrium Mixture

Materials and Apparatus Required

Chemicals

• Sodium dichromate solution
• Dilute sodium hydroxide ($NaOH$) solution (2 M)
• Dilute hydrochloric acid ($HCl$) solution (2 M)

Apparatus

• Boiling tubes and racks
• Dropping pipettes
• Safety glasses

Safety Precautions

• Wear safety glasses throughout the experiment.
• Dilute hydrochloric acid and sodium hydroxide are corrosive. Avoid skin contact.
• Sodium dichromate is toxic and should be handled with care in a fume cupboard.

Method

1. Add sodium dichromate solution to a test tube (it will be orange). This establishes the equilibrium:

$$\text{Cr₂O₇²⁻ (orange)} + \text{H₂O} \rightleftharpoons 2\text{CrO₄²⁻ (yellow)} + 2\text{H⁺}$$

2. Test 1: Add sodium hydroxide ($NaOH$) to the solution.

3. The solution will turn yellow, as the removal of $H⁺$ions shifts the equilibrium to the right.

4. Test 2: Add hydrochloric acid ($HCl$) to the yellow solution.

5. The solution will turn orange, as the addition of $H⁺$ions shifts the equilibrium to the left.

6. Keep one tube as a control for comparison.

Results

• Effect of $NaOH$: The solution turns yellow, indicating the removal of $H⁺$ions shifts the equilibrium to the right.
• Effect of $HCl$: The solution turns orange, indicating the addition of $H⁺$ions shifts the equilibrium to the left.

Example Questions with Answers