Chemistry Practice Test: Oxidation and Reduction (Redox)

Oxidation is the loss of electrons; losing negative charge makes the oxidation number go up. Gaining electrons is reduction, which lowers the oxidation number.

Hydrogen is +1 and oxygen is −2, so 2(+1) + S + 4(−2) = 0, which gives S = +6. The value −2 is sulfur’s charge in sulfides, not in sulfate.

Potassium is +1 and oxygen is −2: +1 + Mn + 4(−2) = 0, so Mn = +7. The value +4 is manganese’s oxidation number in MnO₂, a different compound.

The sum must equal the ion’s charge: N + 4(+1) = +1, so N = −3. A common error is setting the sum to zero, which gives −4.

Zinc goes from 0 to +2, so it loses two electrons and is oxidized. Cu²⁺ gains those electrons and is reduced to copper metal; sulfate is a spectator ion.

Chlorine takes electrons from sodium and is reduced from 0 to −1; the substance that gets reduced is the oxidizing agent. Sodium is oxidized, which makes it the reducing agent.

Magnesium goes from 0 to +2 and oxygen from 0 to −2, so electrons are transferred. In the other three reactions every element keeps the same oxidation number; they are neutralization, precipitation and decomposition without electron transfer.

Each aluminum atom loses 3 electrons, and 2 × 3 = 6; each Cu²⁺ gains 2, and 3 × 2 = 6, so the totals agree. Counting only one aluminum atom gives 3.

Oxidation always happens at the anode and reduction at the cathode ("an ox, red cat"). The salt bridge only lets ions move to keep the solutions neutral.

Electrons are released by oxidation at the zinc anode and travel through the wire to the copper cathode, where Cu²⁺ ions use them. Electrons never travel through the salt bridge; ions do.