NECO 2009 Chemistry · Question 31

The following equations represent redox reactions EXCEPT

Options

A.Ba2+(aq) + SO2-4 (aq) → BaSO4(s)
B.Cl2(g) + 2Br- (aq) → 2CI - (aq) + Br2(g)
C.Fe2+(aq) + ½ C I2(aq) → Fe3+ (aq) + CI-(aq)
D.2Fe3+(aq) + Sn2+(aq) → 2Fe2+(aq) Sn4+ (aq)
E.Zn(s) + CU2+(aq) → Zn2+(aq) + CU(s)

Answer: A. Ba2+(aq) + SO2-4 (aq) → BaSO4(s)

Explanation written with AI and checked against the answer key

Analyze the question: We need the equation that is NOT a redox reaction. A redox reaction involves a change in oxidation numbers (electron transfer). The odd one out will show no change in oxidation states.

Concept: Oxidation is loss of electrons (oxidation number increases). Reduction is gain of electrons (oxidation number decreases). A precipitation reaction, where ions simply combine, is not redox.

Checking each option:

A. Ba²⁺ + SO₄²⁻ → BaSO₄ Ba stays +2, S stays +6, O stays -2. There is no change in oxidation numbers, so this is not redox. It is a precipitation reaction.

B. Cl₂ + 2Br⁻ → 2Cl⁻ + Br₂ Cl goes from 0 to -1 (reduced). Br goes from -1 to 0 (oxidized). This is redox.

C. Fe²⁺ + ½Cl₂ → Fe³⁺ + Cl⁻ Fe goes from +2 to +3 (oxidized). Cl goes from 0 to -1 (reduced). This is redox. (The "CI₂" in the question is a typing error for Cl₂.)

D. 2Fe³⁺ + Sn²⁺ → 2Fe²⁺ + Sn⁴⁺ Fe goes from +3 to +2 (reduced). Sn goes from +2 to +4 (oxidized). This is redox.

E. Zn + Cu²⁺ → Zn²⁺ + Cu Zn goes from 0 to +2 (oxidized). Cu goes from +2 to 0 (reduced). This is redox.

Answer: Only option A shows no change in oxidation numbers, so it is the one that is not a redox reaction.

Tip: When you see "EXCEPT" in a redox question, look for a reaction where ions simply combine to form a precipitate (like BaSO₄, AgCl, or PbSO₄). These are usually double-decomposition (precipitation) reactions with no electron transfer. Also, any reaction involving a free element (like Zn, Cl₂, Br₂, Cu) as a reactant or product is almost always redox.

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