Redox Reactions Questions and Answers

Answer: D

The reaction that is not a redox reaction is BaCl₂ + H₂SO₄ → BaSO₄ + 2HCl.

It is a double displacement (precipitation) reaction: barium stays +2, chlorine -1, sulfur +6, oxygen -2 and hydrogen +1 on both sides. No oxidation numbers change.

The others are redox reactions. In CuO + H₂, copper is reduced from +2 to 0 and hydrogen oxidised from 0 to +1. In Fe₂O₃ + 3CO, iron goes from +3 to 0 and carbon from +2 to +4. In 2K + F₂, potassium is oxidised from 0 to +1 and fluorine reduced from 0 to -1.

Answer: B

One needs to add 2e^- to the right side.

The atoms are already balanced: one S, four O and two H on each side. The charges are not: the left side carries -2 (the sulfite ion) and the right side -2 + 2(+1) = 0. Sulfur is oxidised from +4 in sulfite to +6 in sulfate, losing two electrons, so this is an oxidation half-reaction and the electrons appear as products:

\[ \begin{aligned} &\mathrm{SO_{3}^{2-} + H_{2}O} \\ &\rightarrow \mathrm{SO_{4}^{2-} + 2H^{+} + 2e^{-}} \end{aligned} \]

Now both sides carry -2. Adding electrons to the left would make the charge -4 there. Adding water would unbalance the atoms, and multiplying by 2 balances nothing.

Answer: D

The observation would be No reaction.

A metal can displace another from solution only if it is more reactive, that is, if it has a more negative reduction potential. Copper (E° Cu²⁺/Cu = +0.34 V) lies well below zinc (E° Zn²⁺/Zn = -0.76 V), so copper cannot reduce Zn²⁺. The reverse reaction, zinc displacing copper from CuSO₄, is the one that happens.

A blue solution would require Cu²⁺ ions to form, which needs copper to be oxidised. Effervescence would need a gas such as hydrogen, which copper does not release from a neutral salt solution, and no white solid is formed.