Unit 4 · Topic 4.9 Beta

Oxidation-Reduction (Redox) Reactions

In a redox reaction electrons move from the species oxidized (oxidation number rises) to the species reduced (oxidation number falls).

Practice 1: Models and RepresentationsPractice 5: Mathematical Routines

Question set for this topic

Part 1 · Hook

Why this matters

A battery, a rusting bike chain, a burning match and the cells in your body releasing energy from food all run on one kind of reaction: electrons passing from one substance to another. Chemists track those electrons with a simple bookkeeping number, and the bookkeeping tells you what is oxidized, what is reduced and how to balance the equation.

Part 2 · Before you start

What this builds on

Part 3 · Prerequisite check

Quick check before you start

1. How many valence electrons does a neutral zinc atom lose to form Zn²⁺?

  1. 2
  2. 1
  3. 0
  4. 3
Show the answer

The 2+ charge means two more protons than electrons: two electrons were lost.

  • Correct: 2:
  • 1:
  • 0:
  • 3:

2. In which reaction do electrons move from one particle to another?

  1. Zn(s) + Cu²⁺(aq) → Zn²⁺(aq) + Cu(s)
  2. H⁺(aq) + OH⁻(aq) → H₂O(l)
  3. Ag⁺(aq) + Cl⁻(aq) → AgCl(s)
  4. H₂O(s) → H₂O(l)
Show the answer

Zinc atoms become ions and copper ions become atoms: electrons moved from zinc to copper.

  • Correct: Zn(s) + Cu²⁺(aq) → Zn²⁺(aq) + Cu(s):
  • H⁺(aq) + OH⁻(aq) → H₂O(l):
  • Ag⁺(aq) + Cl⁻(aq) → AgCl(s):
  • H₂O(s) → H₂O(l):

3. Which element is more electronegative, O or H?

  1. O
  2. H
  3. They are equal
  4. It depends on the compound
Show the answer

Oxygen has a much larger effective nuclear charge acting on its valence electrons.

  • Correct: O:
  • H:
  • They are equal:
  • It depends on the compound:

Part 4 · See it

See it first

A zinc atom passes two electrons to a Cu²⁺ ion, giving a Zn²⁺ ion and a copper atom. Oxidation numbers: Zn 0, Cu²⁺ +2, Zn²⁺ +2, Cu 0. Zinc goes from 0 to +2 and loses two electrons: it is oxidized and is the reducing agent. Copper goes from +2 to 0 and gains two electrons: it is reduced, and Cu²⁺ is the oxidizing agent. Half-reactions: Zn(s) → Zn²⁺(aq) + 2e⁻ and Cu²⁺(aq) + 2e⁻ → Cu(s). Electrons lost equal electrons gained.
Zinc in copper(II) sulfate. Each zinc atom gives two electrons to a Cu²⁺ ion. Zinc is oxidized (its oxidation number rises from 0 to +2); copper is reduced (+2 to 0). LevlPrep original diagram.

Part 5 · Step by step

How it works, step by step

  1. One substance gives up electrons and another takes themoxidation and reduction always happen together
  2. Oxidation numbers track electrons as if every bond were ionica rise shows oxidation and a fall shows reduction
  3. The species that takes electrons causes the other to be oxidizedthe oxidizing agent is the species reduced, and the reducing agent is the one oxidized
  4. Electrons are neither created nor destroyedhalf-reactions are multiplied so electrons lost equal electrons gained

Part 6 · Key ideas

Key ideas

  • Oxidation is loss of electrons (oxidation number goes up); reduction is gain (it goes down).
  • Assign an oxidation number: elements 0; monatomic ions = charge; O usually −2; H usually +1; the sum equals the charge.
  • The oxidizing agent is reduced; the reducing agent is oxidized.
  • Write each half-reaction, multiply so electrons lost = electrons gained, add, and check atoms and charge.

Part 7 · Misconception

A common mistake

The wrong idea: Oxidation means a reaction with oxygen, and the oxidizing agent is the substance that is oxidized.

What actually happens: Oxidation is loss of electrons, with or without oxygen (Zn + Cu²⁺ has none). The oxidizing agent takes electrons, so it is the species that is reduced.

Part 8 · Check yourself

Check yourself

Exam-style questions. Anything you miss goes into your review queue.

Data table

Metals in metal-ion solutions

A student places a clean strip of each metal into 1 M solutions of the nitrates of the other metals and watches for a coating or other change after 10 minutes. Nitrate ions take no part in any of the reactions.

Observations (✓ = coating forms and metal strip slowly dissolves; – = no change)
Metal stripCu²⁺(aq)Zn²⁺(aq)Ag⁺(aq)Mg²⁺(aq)
Cu–✓ (silver-gray)–
Zn✓ (red-brown)✓ (silver-gray)–
Ag–––
Mg✓ (red-brown)✓ (gray)✓ (silver-gray)

1. Which balanced net ionic equation describes the copper strip in the Ag⁺ solution?

  1. Cu(s) + 2Ag⁺(aq) → Cu²⁺(aq) + 2Ag(s)
  2. Cu(s) + Ag⁺(aq) → Cu²⁺(aq) + Ag(s)
  3. Cu²⁺(aq) + 2Ag(s) → Cu(s) + 2Ag⁺(aq)
  4. Cu(s) + 2Ag⁺(aq) → Cu²⁺(aq) + Ag₂(s)
Show the answer

Copper atoms lose 2 electrons each (Cu → Cu²⁺ + 2e⁻) and each Ag⁺ gains 1 (Ag⁺ + e⁻ → Ag). Two Ag⁺ are needed per Cu so electrons lost equal electrons gained; charge is +2 on both sides.

  • Correct: Cu(s) + 2Ag⁺(aq) → Cu²⁺(aq) + 2Ag(s): Right: 2 electrons lost and 2 gained; atoms and charge balance.
  • Cu(s) + Ag⁺(aq) → Cu²⁺(aq) + Ag(s): Atoms balance, but charge does not (+1 left, +2 right): electrons lost (2) do not equal electrons gained (1).
  • Cu²⁺(aq) + 2Ag(s) → Cu(s) + 2Ag⁺(aq): This is the reverse; the table shows silver metal does not react with Cu²⁺.
  • Cu(s) + 2Ag⁺(aq) → Cu²⁺(aq) + Ag₂(s): Silver metal is written as Ag(s); it does not form Ag₂ units.

2. Based on the table, which metal loses electrons most readily?

  1. Mg
  2. Zn
  3. Cu
  4. Ag
Show the answer

Magnesium reacts with every other metal ion, so it gives electrons to all of them. Silver reacts with none, so it gives up electrons least readily. Order: Mg > Zn > Cu > Ag.

  • Correct: Mg: Right: Mg gives electrons to Cu²⁺, Zn²⁺ and Ag⁺.
  • Zn: Zinc reacts with Cu²⁺ and Ag⁺ but not Mg²⁺, so magnesium gives up electrons more readily.
  • Cu: Copper reacts only with Ag⁺.
  • Ag: Silver reacts with none of the solutions, so it loses electrons least readily.

3. In the reaction of zinc metal with Cu²⁺(aq), which species is the oxidizing agent?

  1. Cu²⁺
  2. Zn
  3. Zn²⁺
  4. Cu
Show the answer

Cu²⁺ gains electrons (it is reduced to Cu), and by taking electrons it causes zinc to be oxidized. The species that is reduced is the oxidizing agent.

  • Correct: Cu²⁺: Right: Cu²⁺ is reduced, so it is the oxidizing agent.
  • Zn: Zinc loses electrons; it is oxidized, so it is the reducing agent.
  • Zn²⁺: Zn²⁺ is a product, formed when zinc is oxidized.
  • Cu: Copper metal is a product, formed when Cu²⁺ is reduced.

Data table

A redox titration

A student pipets 25.00 mL of an acidified Fe²⁺ solution into a flask and titrates it with 0.02000 M KMnO₄. The purple MnO₄⁻ ion is reduced to nearly colorless Mn²⁺, so the first lasting pale pink color marks the end point; no separate indicator is needed. The balanced net ionic equation is

MnO₄⁻(aq) + 5Fe²⁺(aq) + 8H⁺(aq) → Mn²⁺(aq) + 5Fe³⁺(aq) + 4H₂O(l)

Volume of KMnO₄ needed
TrialVolume of 0.02000 M KMnO₄ (mL)
121.63
221.66
321.61

4. What is the oxidation number of manganese in MnO₄⁻?

Type a number.

Show the answer

Oxygen is −2, so 4 O contribute −8. The sum must equal the ion's charge, −1: Mn + (−8) = −1, so Mn = +7.

  • Answer: 7

5. How many electrons does each MnO₄⁻ ion gain in this reaction?

  1. 5, because Mn goes from +7 to +2
  2. 7, because Mn starts at +7
  3. 2, because Mn ends as Mn²⁺
  4. 1, because the ion has a 1− charge
Show the answer

The oxidation number of Mn falls from +7 to +2, a gain of 5 electrons. That matches the 5 Fe²⁺ each losing 1 electron (Fe²⁺ → Fe³⁺).

  • Correct: 5, because Mn goes from +7 to +2: Right: +7 to +2 is a change of 5.
  • 7, because Mn starts at +7: Uses the starting oxidation number alone; the change is what counts.
  • 2, because Mn ends as Mn²⁺: Uses the final oxidation number alone; the change is 7 − 2.
  • 1, because the ion has a 1− charge: The charge of the ion is not the number of electrons transferred.

6. Using the average of the three trials, what is the molarity of Fe²⁺ in the sample?

Type a number and its unit.

Show the answer

Average volume = (21.63 + 21.66 + 21.61) mL ÷ 3 = 21.63 mL. Moles MnO₄⁻ = 0.02000 mol/L × 0.02163 L = 4.326 × 10⁻⁴ mol. Moles Fe²⁺ = 5 × 4.326 × 10⁻⁴ = 2.163 × 10⁻³ mol. Molarity = 2.163 × 10⁻³ mol ÷ 0.02500 L = 0.08652 M.

  • Answer: 0.08652 M

Part 9 · Summary

Summary

In a redox reaction electrons move from the species oxidized (oxidation number rises) to the species reduced (oxidation number falls). The oxidizing agent is reduced and the reducing agent is oxidized. Balance redox reactions with half-reactions so electrons lost equal electrons gained, then check atoms and charge.

Part 10 · Up next

What comes next

Part 11 · Connections

Connections