Redox

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Lesson notes

Oxidation & Reduction (Oxygen Definition)

Oxidation is the gain of oxygen by an element or compound.
Reduction is the loss of oxygen from an element or compound.
A redox reaction is one where oxidation and reduction occur simultaneously.
Example: ZnO + C → Zn + CO – zinc oxide is reduced (loses O), carbon is oxidised (gains O).

Reduction and oxidation shown for zinc oxide + carbon

Oxidation & Reduction (Electron Definition – Extended)

Oxidation is the loss of electrons; reduction is the gain of electrons (OIL RIG).
Oxidation increases the oxidation number; reduction decreases it.
Half-equations show electron transfer: e.g., Ag → Ag⁺ + e⁻ (oxidation); O₂ + 4e⁻ → 2O²⁻ (reduction).
Example: Fe + Cu²⁺ → Fe²⁺ + Cu – Fe loses electrons (oxidised), Cu²⁺ gains electrons (reduced).

Redox reaction between Fe and Cu2+ shown by electron transfer

Oxidation Numbers & Roman Numerals

Oxidation number (state) shows the charge an atom would have if electrons were completely transferred.
Roman numerals in names indicate oxidation number: e.g., iron(II) = Fe²⁺, iron(III) = Fe³⁺.
Rules: uncombined elements = 0; monatomic ion = its charge; sum in compound = 0; sum in ion = charge.
Group 1 always +1, Group 2 always +2, F always –1, H usually +1, O usually –2.

Bohr model of iron atom showing electron shells. Iron can lose 2 electrons (Fe²⁺) or 3 electrons (Fe³⁺), corresponding to oxidation numbers +2 and +3.

Identifying Redox Reactions by Colour Changes

Potassium manganate(VII) (KMnO₄) is a purple oxidising agent; with a reducing agent it turns colourless.
Potassium iodide (KI) is a colourless reducing agent; with an oxidising agent it turns red-brown (I₂ formed).
These colour tests are used to detect oxidising or reducing agents.

Colour-change tests for oxidising and reducing agents

Oxidising & Reducing Agents (Extended)

An oxidising agent oxidises another substance and is itself reduced (gains electrons).
A reducing agent reduces another substance and is itself oxidised (loses electrons).
Common oxidising agents: O₂, Cl₂, KMnO₄, H₂O₂. Common reducing agents: C, H₂, KI.
Example: CuO + H₂ → Cu + H₂O – CuO is oxidising agent (reduced), H₂ is reducing agent (oxidised).

Displacement Reactions & Redox

In displacement reactions, the more reactive metal loses electrons (oxidised) and the less reactive metal ion gains electrons (reduced).
Example: Fe + CuSO₄ → FeSO₄ + Cu – Fe is oxidised, Cu²⁺ is reduced.
Ionic equation: Fe + Cu²⁺ → Fe²⁺ + Cu.

Half-Equations & Balancing (Extended)

Half-equations show either oxidation or reduction separately with electrons.
Oxidation half: atoms/ions lose electrons (e.g., Ni → Ni²⁺ + 2e⁻).
Reduction half: atoms/ions gain electrons (e.g., Ni³⁺ + e⁻ → Ni²⁺).
Balance atoms and charge: e.g., O₂ + 4e⁻ → 2O²⁻.

Extraction of Metals & Redox

Carbon is used to reduce metal oxides (e.g., ZnO + C → Zn + CO).
The metal oxide is reduced (loses oxygen), carbon is oxidised (gains oxygen).
Metals below carbon in the reactivity series can be extracted by reduction with carbon.

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Practice questions

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1.In terms of oxygen, what is oxidation?
Easy
- AGain of oxygen
- BLoss of oxygen
- CLoss of electrons
- DGain of electrons
2.In terms of oxygen, what is reduction?
Easy
- ALoss of oxygen
- BGain of oxygen
- CLoss of electrons
- DGain of electrons
3.What is a redox reaction?
Easy
- AA reaction where oxidation and reduction occur simultaneously
- BA reaction where only oxidation occurs
- CA reaction where only reduction occurs
- DA reaction where no electron transfer occurs
4.In the reaction ZnO + C → Zn + CO, which substance is reduced?
Easy
- AZinc oxide (ZnO)
- BCarbon (C)
- CZinc (Zn)
- DCarbon monoxide (CO)
5.In terms of electron transfer, oxidation is the:
Easy
- ALoss of electrons
- BGain of electrons
- CGain of oxygen
- DLoss of oxygen
6.What is the oxidation number of iron in iron(III) oxide (Fe₂O₃)?
Easy
- A+3
- B+2
- C-3
- D0
7.Which change in oxidation number indicates that the species has been reduced?
Easy
- ADecrease from +3 to +2
- BIncrease from 0 to +2
- CIncrease from -1 to 0
- DNo change
8.In the reaction Cl₂ + 2KI → 2KCl + I₂, which species is reduced?
Medium
- AChlorine (Cl₂)
- BPotassium iodide (KI)
- CIodine (I₂)
- DPotassium chloride (KCl)

More questions

9.What colour change is observed when acidified potassium manganate(VII) is added to a reducing agent?
Easy
- APurple to colourless
- BColourless to purple
- CRed to blue
- DYellow to red
10.Which of the following is an oxidising agent?
Easy
- APotassium manganate(VII)
- BPotassium iodide
- CCarbon
- DHydrogen
11.In the reaction Fe + Cu²⁺ → Fe²⁺ + Cu, which species is the reducing agent?
Medium
- AFe
- BCu²⁺
- CFe²⁺
- DCu
12.What is the oxidation number of oxygen in hydrogen peroxide (H₂O₂)?
Medium
- A-1
- B-2
- C0
- D+1
13.The oxidation number of chromium in Cr₂O₇²⁻ is:
Medium
- A+6
- B+3
- C+7
- D+2
14.In the reaction 2FeCl₃ + SnCl₂ → 2FeCl₂ + SnCl₄, which species is oxidised?
Medium
- AFe³⁺
- BSn²⁺
- CCl⁻
- DFe²⁺
15.What is the oxidation number of nitrogen in NH₄NO₃?
Hard
- A-3 and +5
- B+5 and -3
- C+3 and -5
- D-3 and +3
16.Which colour change is observed when acidified potassium manganate(VII) is added to a solution of iron(II) sulfate?
Medium
- Apurple to colourless
- Bcolourless to purple
- Cpurple to green
- Dgreen to purple
17.In the reaction between zinc and copper(II) sulfate, Zn + CuSO₄ → ZnSO₄ + Cu, which statement is correct?
Medium
- AZinc is oxidised and copper is reduced
- BZinc is reduced and copper is oxidised
- CBoth are oxidised
- DBoth are reduced
18.What is the oxidation number of sulfur in H₂SO₄?
Medium
- A+6
- B+4
- C+2
- D0
19.In the reaction Co₂O₃ + 6HCl → 2CoCl₂ + Cl₂ + 3H₂O, which element is reduced?
Medium
- ACobalt
- BChlorine
- COxygen
- DHydrogen
20.Which change represents reduction?
Medium
- ACr³⁺ → Cr₂O₇²⁻
- BMnO₄⁻ → Mn²⁺
- CFe²⁺ → Fe³⁺
- DCl⁻ → Cl₂
21.What is the oxidation number of chlorine in Cl₂?
Easy
- A0
- B-1
- C+1
- D+2
22.In the reaction 2H₂S + SO₂ → 3S + 2H₂O, which substance is the oxidising agent?
Medium
- AH₂S
- BSO₂
- CS
- DH₂O
23.Which of the following is a redox reaction?
Medium
- ANaOH + HCl → NaCl + H₂O
- BZn + CuSO₄ → ZnSO₄ + Cu
- CAgNO₃ + NaCl → AgCl + NaNO₃
- DCaCO₃ → CaO + CO₂
24.What is the oxidation number of iron in iron(II) oxide (FeO)?
Easy
- A0
- B+2
- C+3
- D-2
25.In the reaction 2Fe + 3Cl₂ → 2FeCl₃, which element is reduced?
Easy
- AIron
- BChlorine
- CBoth iron and chlorine
- DNeither element is reduced
26.What is the oxidation number of chromium in Cr₂O₇²⁻?
Medium
- A+3
- B+6
- C+7
- D+2
27.In the reaction 2MnO₄⁻ + 5SO₂ + 2H₂O → 2Mn²⁺ + 5SO₄²⁻ + 4H⁺, which species is the oxidising agent?
Medium
- AMnO₄⁻
- BSO₂
- CMn²⁺
- DSO₄²⁻
28.Which change represents oxidation in terms of electron transfer?
Easy
- AGain of electrons
- BLoss of electrons
- CGain of oxygen
- DLoss of oxygen
29.In terms of oxygen, what is the definition of reduction?
Easy
- AGain of oxygen
- BLoss of oxygen
- CGain of electrons
- DLoss of electrons
30.What is the oxidation number of iron in iron(III) oxide, Fe₂O₃?
Easy
- A+2
- B+3
- C-2
- D0
31.Which statement correctly describes a redox reaction?
Easy
- AA reaction where oxidation occurs alone
- BA reaction where reduction occurs alone
- CA reaction where oxidation and reduction occur simultaneously
- DA reaction where no change in oxidation numbers occurs
32.In the reaction Cl₂ + 2KI → 2KCl + I₂, which substance is reduced?
Medium
- ACl₂
- BKI
- CKCl
- DI₂
33.In the reaction Co₂O₃ + 6HCl → 2CoCl₂ + Cl₂ + 3H₂O, the oxidation number of cobalt changes from:
Medium
- A+3 to +2
- B+2 to +3
- C+3 to +1
- D+1 to +2
34.When 25.0 cm³ of 0.0200 mol/dm³ acidified KMnO₄ reacts completely with Fe²⁺ according to the equation: MnO₄⁻ + 5Fe²⁺ + 8H⁺ → Mn²⁺ + 5Fe³⁺ + 4H₂O, how many moles of Fe²⁺ are oxidised?
Hard
- A0.00250 mol
- B0.00100 mol
- C0.00500 mol
- D0.000500 mol
35.Which of the following describes the colour change observed when acidified potassium manganate(VII) is added to a solution containing iron(II) ions?
Medium
- APurple to colourless
- BColourless to purple
- CRed-brown to colourless
- DPurple to red-brown
36.In the reaction: 2FeCl₃ + SnCl₂ → 2FeCl₂ + SnCl₄, which species is oxidised?
Medium
- AFe³⁺
- BCl⁻
- CSn²⁺
- DSn⁴⁺
37.In terms of oxygen, reduction is the:
Easy
- ALoss of oxygen
- BGain of oxygen
- CLoss of electrons
- DGain of electrons
38.In the reaction: 2H₂S + SO₂ → 3S + 2H₂O, which substance is the oxidising agent?
Medium
- AH₂S
- BSO₂
- CS
- DH₂O
39.A redox reaction is one in which oxidation and reduction occur simultaneously.
Easy
True or false?
40.What does the Roman numeral in iron(III) oxide indicate?
Easy
- AThe number of iron atoms
- BThe oxidation number of iron
- CThe number of oxygen atoms
- DThe charge on the oxide ion
41.The oxidation number of an uncombined element is always zero.
Easy
True or false?
42.Which of the following are correct definitions of reduction? (Select all that apply)
Medium
- AGain of oxygen
- BLoss of oxygen
- CGain of electrons
- DLoss of electrons
- EDecrease in oxidation number
43.Match each term with its correct definition.
Medium
- Oxidising agent
- Reducing agent
- Redox reaction
- Substance that gains electrons and is reduced
- Substance that loses electrons and is oxidised
- Reaction where oxidation and reduction occur together
44.Arrange the following steps in the correct order to determine if a reaction is redox using oxidation numbers.
Medium
- Assign oxidation numbers to all atoms
- Identify changes in oxidation numbers
- If any oxidation numbers change, the reaction is redox
- Check if the sum of oxidation numbers equals the charge
45.The reaction between copper(II) oxide and hydrogen is shown: CuO + H₂ → Cu + H₂O. Which statement is correct?
Medium
- ACopper(II) oxide is oxidised because it loses oxygen.
- BHydrogen is reduced because it gains oxygen.
- CCopper(II) oxide is reduced because it loses oxygen.
- DHydrogen is oxidised because it loses electrons.
46.In the reaction 2Fe + 3Cl₂ → 2FeCl₃, what is the oxidation number of iron in FeCl₃?
Easy
- A+2
- B+3
- C-2
- D-3
47.Which of the following reactions is a redox reaction?
Medium
- ANaOH + HCl → NaCl + H₂O
- BCaCO₃ → CaO + CO₂
- C2Mg + O₂ → 2MgO
- DAgNO₃ + NaCl → AgCl + NaNO₃
48.Complete the sentence using the words 'oxidised' or 'reduced'.
Easy
When a substance loses oxygen in a reaction, it is said to be ____.
49.In terms of electron transfer, what is oxidation?
Easy
50.Complete the sentences using the words 'oxidised' or 'reduced'.
Easy
In terms of oxygen, when a substance gains oxygen it is ____. When a substance loses oxygen it is ____.
51.What is the oxidation number of sulfur in the sulfate ion, SO₄²⁻?
Medium
52.In the reaction 2MnO₄⁻ + 5SO₂ + 2H₂O → 2Mn²⁺ + 5SO₄²⁻ + 4H⁺, what is the change in oxidation number of manganese?
Medium
53.25.0 cm³ of 0.0200 mol/dm³ acidified KMnO₄ reacts completely with Fe²⁺ according to the equation: MnO₄⁻ + 5Fe²⁺ + 8H⁺ → Mn²⁺ + 5Fe³⁺ + 4H₂O. Calculate the number of moles of Fe²⁺ oxidised.
Hard
54.Explain, in terms of electron transfer, what happens to the oxidising agent in a redox reaction.
Medium
55.In the reaction 2MnO₄⁻ + 5SO₂ + 2H₂O → 2Mn²⁺ + 5SO₄²⁻ + 4H⁺, what is the change in oxidation number of manganese?
Medium
- A5
- B6
- C4
- D7

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