3. (a) sodium nitrate potassium chloride copper(II) sulfate Samples of three salts are provided on clock glasses as shown above - Leaving Cert Chemistry - Question 3 - 2013

As a result of the reaction, the following observations would have been made:

• A layer of copper, characterized by a brown/red-black deposit, would appear on the zinc rod (which is the result of a displacement reaction).
• There would be visual signs of bubbles forming in the solution, indicating some reaction had occurred.

The reaction can be explained in terms of electron transfer as follows:

• Zinc (Zn) is oxidized to zinc ions (Zn²⁺), losing electrons: $\text{Zn} \rightarrow \text{Zn}^{2+} + 2e^-$
• The copper(II) ions (Cu²⁺) in solution are reduced to copper metal by gaining electrons: $\text{Cu}^{2+} + 2e^- \rightarrow \text{Cu}$ This transfer of electrons from zinc to copper(II) ions is what drives the reaction, leading to deposition of copper on the zinc rod.

To confirm the presence of chloride ions in an aqueous solution of potassium chloride, perform the following steps:

1. Add Silver Nitrate Solution: Add a few drops of silver nitrate (AgNO₃) to the solution.
2. Observation: A white precipitate of silver chloride (AgCl) is formed, indicating the presence of chloride ions.

The reaction can be represented as follows: $\text{Ag}^+ + \text{Cl}^- \rightarrow \text{AgCl (s)}$

To confirm the presence of nitrate ions in an aqueous solution of sodium nitrate, carry out these steps:

1. Add Freshly Prepared Iron(II) Sulfate Solution: Add a few drops of freshly prepared iron(II) sulfate (FeSO₄) solution to the nitrate solution.
2. Add Concentrated Sulfuric Acid: Add concentrated sulfuric acid (H₂SO₄) carefully, avoiding mixing the layers.
3. Look for a Brown Ring: If a brown ring appears at the interface of the two layers, this indicates the presence of nitrate ions.

The reaction responsible for this observation is: $\text{Fe}^{2+} + 2\text{NO}_3^- + 2\text{H}_2\text{SO}_4 \rightarrow ext{Fe(NO}_3)_2 + \text{H}_2\text{O} + \text{NO}$ This forms a colored complex, confirming the presence of nitrate ions.