Which reaction is NOT a redox reaction?

The correct answer is: BaCl₂ + Na₂SO₄ → BaSO₄ + 2NaCl

A redox (reduction-oxidation) reaction is a chemical reaction that involves a transfer of electrons between chemical species. This transfer is detected by a change in the oxidation states (oxidation numbers) of the atoms involved in the reaction.

Let's analyze the oxidation states of the elements in each of the given reactions to determine which one is not a redox reaction:

1. Zn + CuSO₄ → ZnSO₄ + Cu
- Zinc (Zn) changes its oxidation state from 0 in its elemental form to +2 in ZnSO₄ (oxidation).
- Copper (Cu) changes its oxidation state from +2 in CuSO₄ to 0 in its elemental form (reduction).
Since there are changes in oxidation states, this is a redox reaction.

2. 2KClO₃ + I₂ �� 2KIO₃ + Cl₂
- Iodine (I) changes its oxidation state from 0 in I₂ to +5 in KIO₃ (oxidation).
- Chlorine (Cl) changes its oxidation state from +5 in KClO₃ to 0 in Cl₂ (reduction).
Since there are changes in oxidation states, this is a redox reaction.

3. H₂ + Cl₂ → 2HCl
- Hydrogen (H) changes its oxidation state from 0 in H₂ to +1 in HCl (oxidation).
- Chlorine (Cl) changes its oxidation state from 0 in Cl₂ to -1 in HCl (reduction).
Since there are changes in oxidation states, this is a redox reaction.

4. BaCl₂ + Na₂SO₄ → BaSO₄ + 2NaCl
Let us look at the oxidation state of each element before and after the reaction:
- Barium (Ba): Remains at +2 in both BaCl₂ and BaSO₄.
- Chlorine (Cl): Remains at -1 in both BaCl₂ and NaCl.
- Sodium (Na): Remains at +1 in both Na₂SO₄ and NaCl.
- Sulfur (S): Remains at +6 in the sulfate ion (SO₄^2-) on both sides.
- Oxygen (O): Remains at -2 in the sulfate ion (SO₄^2-) on both sides.
Because none of the atoms change their oxidation states during the process, this is a double displacement (precipitation) reaction rather than a redox reaction.