Redox

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Oxidation & Reduction in Terms of Oxygen

Oxidation is the addition of oxygen to an element or compound.
Reduction is the removal of oxygen from an element or compound.
A redox reaction involves both oxidation and reduction occurring simultaneously.
Example: ZnO + C → Zn + CO – zinc oxide is reduced (loses oxygen), carbon is oxidised (gains oxygen).

Oxidation and reduction shown for zinc oxide + carbon

Oxidation & Reduction in Terms of Electrons (Extended)

Oxidation is the loss of electrons; the oxidation number increases.
Reduction is the gain of electrons; the oxidation number decreases.
Mnemonic: OIL RIG – Oxidation Is Loss, Reduction Is Gain.
Half-equations show electron transfer, e.g. Ag → Ag⁺ + e⁻ (oxidation) and O₂ + 4e⁻ → 2O²⁻ (reduction).
Example: when iron reacts with copper sulfate, Fe + Cu²⁺ → Fe²⁺ + Cu – iron loses electrons (oxidised), copper(II) ions gain electrons (reduced).

Redox reaction between Fe and Cu²⁺ shown by electron transfer

Oxidation Numbers & Roman Numerals

Transition metals can form ions with different charges; the charge is shown by a Roman numeral in the name.
The Roman numeral equals the oxidation number of the metal ion.
Example: Fe²⁺ is iron(II); Fe³⁺ is iron(III).
Example: FeO is iron(II) oxide; Fe₂O₃ is iron(III) oxide.
In CoCl₂, chloride ions are each 1−, so cobalt is 2+ → cobalt(II) chloride.

Bohr model of iron atom showing electron shells

Identifying Redox Reactions

In a redox reaction, one species is oxidised and another is reduced.
Example: Fe₂O₃ + 3CO → 2Fe + 3CO₂ – Fe₂O₃ is reduced (loses oxygen), CO is oxidised (gains oxygen).
In ionic equations, spectator ions are removed to show the redox change.
Example: Fe + Cu²⁺ → Fe²⁺ + Cu – Fe is oxidised (loses electrons), Cu²⁺ is reduced (gains electrons).
The reducing agent is the species that is oxidised (it loses electrons); the oxidising agent is the species that is reduced (it gains electrons).

Oxidation and reduction shown for iron(III) oxide + carbon monoxide

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Câu hỏi luyện tập

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1.State the meaning of the mnemonic OIL-RIG in terms of electron transfer.
Easy
2.In the reaction between zinc oxide and carbon, which substance is reduced?
Medium
- Azinc oxide
- Bcarbon
- Czinc
- Dcarbon dioxide
3.Complete the sentence using the words 'oxidation' or 'reduction'.
Easy
In terms of oxygen, ____ is the gain of oxygen and ____ is the loss of oxygen.
4.What is the oxidation number of the transition metal in silver(I) chloride, AgCl?
Easy
5.In the reaction V³+ + Fe³+ → V⁴+ + Fe²+, which species is oxidised?
Medium
6.Which half-equation represents reduction?
Medium
- AAg → Ag+ + e-
- BO2 + 4e- → 2O2-
- CFe → Fe2+ + 2e-
- DCu → Cu2+ + 2e-
7.Match each term with its correct definition in terms of oxygen.
Easy
- Oxidation
- Reduction
- Gain of oxygen
- Loss of oxygen
8.Arrange the following steps in the correct order to show the process of identifying oxidation in the reaction V³+ + Fe³+ → V⁴+ + Fe²+.
Medium
- Identify changes for each species
- Determine oxidation or reduction
- Identify species that loses electrons
- Write the overall equation

Thêm câu hỏi

9.In the ionic equation Fe + Cu2+ → Fe2+ + Cu, which species is the oxidising agent?
Medium
10.In terms of oxygen, what is oxidation?
Easy
- AGain of oxygen
- BLoss of oxygen
- CGain of electrons
- DLoss of electrons
11.In the reaction between zinc oxide and carbon, what happens to zinc oxide?
Medium
- AIt is oxidised because it gains oxygen
- BIt is reduced because it loses oxygen
- CIt is oxidised because it loses oxygen
- DIt is reduced because it gains oxygen
12.Match each term with its correct definition in terms of electron transfer.
Medium
- Oxidation
- Reduction
- Redox
- Gain of electrons
- Loss of electrons
- Both oxidation and reduction occur together
13.Arrange the following steps in the correct order to show the process of identifying oxidation in the reaction V³⁺ + Fe³⁺ → V⁴⁺ + Fe²⁺.
Hard
- Identify that V³⁺ loses one electron to become V⁴⁺
- Determine that V³⁺ is oxidised
- Compare oxidation numbers: V goes from +3 to +4, Fe goes from +3 to +2
14.Which of the following are true about the reaction Fe + Cu²⁺ → Fe²⁺ + Cu? (Select all that apply)
Hard
- AIron is oxidised
- BCopper(II) ions are reduced
- CIron is the oxidising agent
- DCopper(II) ions are the oxidising agent
- EIron gains electrons
15.In the reaction between zinc oxide and carbon, which substance is oxidised?
Medium
- Azinc oxide
- Bcarbon
- Czinc
- Dcarbon dioxide
16.In the reaction V³⁺ + Fe³⁺ → V⁴⁺ + Fe²⁺, which species is oxidised?
Medium
- AV³⁺
- BFe³⁺
- CV⁴⁺
- DFe²⁺
17.In the ionic equation Fe + Cu²⁺ → Fe²⁺ + Cu, which species is the oxidising agent?
Medium
- AFe
- BCu²⁺
- CFe²⁺
- DCu
18.In the reaction 2Mg + O₂ → 2MgO, which statement about the oxide ions (O²⁻) is correct?
Medium
- AThey are formed by oxidation of oxygen atoms.
- BThey are formed by reduction of oxygen atoms.
- CThey are formed by oxidation of oxygen molecules.
- DThey are formed by reduction of oxygen molecules.
19.In a redox reaction, the substance that gains oxygen is reduced.
Easy
True or false?
20.In the reaction 2FeCl₃ + SnCl₂ → 2FeCl₂ + SnCl₄, which species is reduced?
Medium
- AFe³⁺
- BSn²⁺
- CCl⁻
- DSn⁴⁺
21.In the reaction Zn + CuSO₄ → ZnSO₄ + Cu, which of the following are true? (Select all that apply)
Hard
- AZinc is oxidised.
- BCopper is reduced.
- CThe oxidation number of zinc increases from 0 to +2.
- DThe oxidation number of copper increases from +2 to 0.
22.Arrange the following steps in the correct order to determine oxidation and reduction in the reaction V³⁺ + Fe³⁺ → V⁴⁺ + Fe²⁺.
Hard
- Identify changes in oxidation numbers.
- Write the half-equations.
- Determine which species loses electrons (oxidation).
- Determine which species gains electrons (reduction).
23.In terms of oxygen, what is reduction?
Easy
- AGain of oxygen
- BLoss of oxygen
- CGain of electrons
- DLoss of electrons
24.In a redox reaction, oxidation and reduction occur separately.
Easy
True or false?
25.What is the oxidation number of iron in iron(III) oxide, Fe₂O₃?
Easy
- A2
- B3
- C6
- D1
26.In the reaction Fe + CuSO₄ → FeSO₄ + Cu, the iron atom is reduced.
Easy
True or false?
27.Match each definition with the correct term in terms of oxygen.
Easy
- Oxidation
- Reduction
- Redox reaction
- Gain of oxygen
- Loss of oxygen
- Both oxidation and reduction occur together
28.In the reaction between zinc oxide and carbon: ZnO + C → Zn + CO. Which of the following are true? (Select all that apply)
Medium
- AZinc oxide is reduced
- BCarbon is oxidised
- CZinc is oxidised
- DCarbon is reduced
- EThe reaction is a redox reaction
29.In a redox reaction, the substance that gains electrons is oxidised.
Easy
True or false?
30.In the ionic equation Fe + Cu²⁺ → Fe²⁺ + Cu, which species is the reducing agent?
Medium
- AFe
- BCu²⁺
- CFe²⁺
- DCu
31.Match each half-equation with the correct process (oxidation or reduction).
Medium
- Na → Na⁺ + e⁻
- Cl₂ + 2e⁻ → 2Cl⁻
- Mg → Mg²⁺ + 2e⁻
- O₂ + 4e⁻ → 2O²⁻
- Oxidation
- Reduction
- Oxidation
- Reduction
32.In the reaction between iron(III) oxide and carbon monoxide: Fe₂O₃ + 3CO → 2Fe + 3CO₂, which substance is oxidised?
Medium
- AFe₂O₃
- BCO
- CFe
- DCO₂
33.A student carries out an experiment to investigate the reaction between magnesium and dilute sulfuric acid. The equation is: Mg + H₂SO₄ → MgSO₄ + H₂. Which of the following statements are correct? (Select all that apply)
Hard
- AMagnesium is oxidised because it loses electrons.
- BHydrogen ions are reduced because they gain electrons.
- CSulfate ions act as the oxidising agent.
- DThe oxidation number of magnesium changes from 0 to +2.
- EThe half-equation for oxidation is: Mg → Mg²⁺ + 2e⁻
34.Match each species with its role in the reaction: Zn + Cu²⁺ → Zn²⁺ + Cu.
Medium
- Zn
- Cu²⁺
- Zn²⁺
- Cu
- oxidised
- reduced
- oxidising agent
- reducing agent
35.In the reaction between zinc oxide and carbon: ZnO + C → Zn + CO, which substance is reduced?
Medium
- AZinc oxide
- BCarbon
- CZinc
- DCarbon monoxide
36.In the reaction Fe + Cu²⁺ → Fe²⁺ + Cu, which of the following are true? (Select all that apply)
Medium
- AIron is oxidised
- BCopper(II) ions are reduced
- CIron is the oxidising agent
- DCopper(II) ions are the oxidising agent
37.Arrange the following steps in the correct order to determine whether V³⁺ is oxidised or reduced in the reaction V³⁺ + Fe³⁺ → V⁴⁺ + Fe²⁺.
Hard
- Compare oxidation numbers before and after reaction
- Identify initial oxidation number of V
- Identify final oxidation number of V
- Determine that V is oxidised if oxidation number increases
38.A student reacts magnesium with dilute hydrochloric acid. The equation is Mg + 2HCl → MgCl₂ + H₂. Which statement is correct?
Medium
- AMagnesium is reduced because it gains electrons.
- BHydrogen ions are oxidised because they lose electrons.
- CMagnesium is oxidised because its oxidation number increases from 0 to +2.
- DChlorine is reduced because its oxidation number decreases.
39.What is the oxidation number of cobalt in cobalt(II) chloride, CoCl₂?
Easy
40.Complete the sentence using the mnemonic OIL-RIG.
Easy
OIL-RIG stands for: Oxidation Is ____, Reduction Is ____.
41.Complete the sentence using the words 'oxidised' or 'reduced'.
Easy
In the reaction Fe + CuSO₄ → FeSO₄ + Cu, the iron atom is ____ and the copper ion is ____.
42.Which half-equation represents reduction? (a) Na → Na⁺ + e⁻, (b) Cl₂ + 2e⁻ → 2Cl⁻
Easy
43.Write the half-equation for the reduction of chlorine gas to chloride ions.
Easy
44.Complete the sentence by filling in the blank.
Easy
In terms of electron transfer, oxidation is the ____ of electrons.
45.Explain which chemicals are reduced and oxidised in the reaction between iron(III) oxide and carbon monoxide: Fe₂O₃ + 3CO → 2Fe + 3CO₂.
Medium
46.Write the half-equation for the oxidation of silver when it reacts with chlorine, and state whether this represents oxidation or reduction.
Medium
47.In the reaction Fe + Cu²⁺ → Fe²⁺ + Cu, identify the species that is oxidised and the species that is reduced, and explain your answer in terms of electron transfer.
Medium
48.A student reacts iron filings with copper(II) sulfate solution. After the reaction, the solution changes from blue to pale green and a brown solid forms. Explain, in terms of electron transfer, which species is oxidised and which is reduced.
Hard
49.What is the oxidation number of cobalt in cobalt(II) chloride, CoCl₂?
Easy
- A3
- B2
- C1
- D4

Thẻ ghi nhớ

What is a redox reaction?

A redox reaction is where oxidation and reduction take place together at the same time in the same reaction.

Define oxidation in terms of oxygen.

Oxidation is the addition of oxygen to an element or a compound.

Define reduction in terms of oxygen.

Reduction is the removal of oxygen from an element or a compound.

Define oxidation in terms of electron transfer (extended tier).

Oxidation is the loss of electrons by an element, ion or compound; the oxidation number increases.

Define reduction in terms of electron transfer (extended tier).

Reduction is the gain of electrons by an element, ion or compound; the oxidation number decreases.

What does the mnemonic OIL-RIG stand for?

Oxidation Is Loss (of electrons), Reduction Is Gain (of electrons).

In the reaction Fe2O3 + 3CO → 2Fe + 3CO2, which substance is oxidised and which is reduced?

Carbon monoxide is oxidised (gains oxygen), iron(III) oxide is reduced (loses oxygen).

What is the oxidation number of the transition metal in silver(I) chloride, AgCl?

The oxidation number is 1 (the Roman numeral I indicates +1).

Name the compound with formula CoCl2 using Roman numerals.

Cobalt(II) chloride.

Write the half-equation for the oxidation of silver to silver ions.

Ag → Ag+ + e-

Write the half-equation for the reduction of oxygen to oxide ions.

O2 + 4e- → 2O2-

In the reaction Fe + CuSO4 → FeSO4 + Cu, which species is oxidised and which is reduced?

Iron is oxidised (loses electrons to become Fe2+), copper(II) ions are reduced (gain electrons to become Cu).

Write the ionic equation for the displacement reaction between iron and copper(II) sulfate, removing spectator ions.

Fe + Cu2+ → Fe2+ + Cu

In the reaction V3+ + Fe3+ → V4+ + Fe2+, which species is oxidised?

V3+ is oxidised to V4+ (loses one electron).

What is the oxidation number of iron in iron(II) oxide?

+2 (iron(II) indicates Fe2+).

What is the formula of iron(III) oxide?

Fe2O3.

What is the formula of iron(II) oxide?

FeO.

How do you show the oxidation number of a transition metal in a compound name?

Use a Roman numeral in parentheses after the element's name, e.g., iron(III) chloride.

What is the oxidation number of cobalt in cobalt(II) chloride?

+2 (since two chloride ions each have -1 charge, cobalt must be +2).

In terms of oxygen, what happens to zinc oxide in the reaction ZnO + C → Zn + CO?

Zinc oxide is reduced because it loses oxygen.

In terms of oxygen, what happens to carbon in the reaction ZnO + C → Zn + CO?

Carbon is oxidised because it gains oxygen.

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