Oxidation Numbers & Redox Equations — Practice · Grade 11 Science

Chapter: Redox, Cells & Nuclear Chemistry

What this practice covers

By the end of Oxidation Numbers & Redox Equations you should be able to:

8 questions, numeric and written answer, each with a worked explanation.

Vocabulary used: oxidation, reduction, oxidizing agent.

Problem 1

Find nitrogen oxidation number in NO₃⁻ with O=−2.
Hints

Write the relationship first; substitute with consistent units.

Answer and explanation

5

x−6=−1, x=+5.

Problem 2

Identify the reducing agent in Zn+Cu²⁺→Zn²⁺+Cu.
Answer and explanation

Zn donates electrons and is oxidized, so it is the reducing agent.

Response criteria

  • State a scientifically accurate conclusion.
  • Show the calculation or mechanism that supports it.
  • Include units, conditions, or limits where relevant.

Problem 3

Find carbon oxidation number in CO₂ using O=−2.
Hints

List the given quantities, select a relationship, and check units before substituting.

Answer and explanation

4

x−4=0, x=+4.

Problem 4

How many electrons are lost when 2 Mg atoms become 2 Mg²⁺ ions?
Hints

Write the relationship first; substitute with consistent units.

Answer and explanation

4 electrons

Each loses 2, total 4.

Problem 5

In 2Mg+O₂→2MgO, which element is reduced? Show the reasoning, not just the conclusion.
Hints

Oxidation and reduction occur together in the complete reaction.

Answer and explanation

Oxygen, changing from 0 to −2 oxidation state; magnesium is oxidized.

Problem 6

What charge-balance check should a redox equation satisfy?
Hints

State what the supplied observations or conditions establish, and separate that from any assumption.

Answer and explanation

Total charge must match on both sides, along with atom counts and balanced electron loss/gain.

Problem 7

Half-reaction balancing includes atoms, charge and the stated mediumHalf-reaction balancing includes atoms, charge and the stated mediumAcidic-medium reduction exampleMnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂OAtoms: Mn 1; O 4; H 8 on each sideCharge: −1 +8 −5 = +2 on each sideBasic-medium balance requires the corresponding OH⁻ account.
Original selected standard acidic half-reaction bookkeeping. It is a paper analysis, not a procedure for handling permanganate or concentrated acid.
Interpret the supplied visual. How does the charge check show that five electrons are required in the displayed acidic half-reaction?
Answer and explanation

The left side without electrons has charge +7, while Mn²⁺ on the right has +2. Five negative electron charges make both +2.

A half-reaction method first balances non-H/O atoms, then uses water and the medium’s proton/hydroxide account to balance O and H, and electrons to balance charge. In the selected acidic permanganate reduction, five electrons are required. A basic-medium answer should not leave an inappropriate net inventory of free H⁺; converting an acidic balance uses matched OH⁻ additions and water cancellation. The method is formal conservation, not proof that the equation is an elementary microscopic step.

The left side without electrons has charge +7, while Mn²⁺ on the right has +2. Five negative electron charges make both +2.

Response criteria

  • Interpret labels, structures, axes, units and directions correctly.
  • Explain the chemistry relationship or mechanism.
  • Respect stated assumptions and distinguish observation from inference.

Problem 8

For Mg+2Ag⁺→Mg²⁺+2Ag, identify oxidation, reduction, both agents and the electron amount per mole of Mg reacted.
Answer and explanation

Mg goes 0→+2 and is oxidized/reducing agent. Ag⁺ goes +1→0 and is reduced/oxidizing agent. One mole Mg transfers two moles electrons, consumed by two moles Ag⁺ in the selected complete account.

Mg goes 0→+2 and is oxidized/reducing agent. Ag⁺ goes +1→0 and is reduced/oxidizing agent. One mole Mg transfers two moles electrons, consumed by two moles Ag⁺ in the selected complete account.

Response criteria

  • Use the stated measurements or conditions, with correct units.
  • Connect the result to the chemical mechanism or evidence relationship.
  • Identify the assumption, model limit or further evidence requested.

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