Oxidation and Reduction
Edwin Ogie Library
Oxidation and Reduction – Redox E‑Book
Objectives:
• Identify the various forms of expressing oxidation and reduction.
• Classify chemical reactions in terms of oxidation or reduction.
• Balance redox reaction equations.
• Deduce the oxidation numbers of chemical species.
• Compute the number of electrons transferred.
• Distinguish between oxidizing and reducing agents.
• Apply oxidation numbers in naming inorganic compounds.
• Relate reagents to their oxidizing and reducing abilities.
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Page 1: Introduction to Oxidation and Reduction (Redox)
Redox reactions are chemical reactions that involve the transfer of electrons between species. They can be expressed in several ways:
- Oxidation as the addition of oxygen or the removal of hydrogen.
- Reduction as the removal of oxygen or the addition of hydrogen.
- Oxidation and reduction in terms of electron transfer.
- Change in oxidation numbers.
Page 2: Oxidation
Oxidation can be defined in several ways:
- Addition of oxygen: When a substance gains oxygen atoms.
- Removal of hydrogen: When a substance loses hydrogen atoms.
- Electron loss: When a substance loses electrons (oxidation means an increase in oxidation number).
For example, the reaction of iron with oxygen to form rust (Fe₂O₃) involves oxidation.
Example OX1: Oxidation by Oxygen Addition
Solution:
Fe + O₂ → Fe₂O₃ (balanced as 4Fe + 3O₂ → 2Fe₂O₃). Here, iron is oxidized by the addition of oxygen.
Example OX2: Oxidation by Hydrogen Removal
Solution:
CH₄ → CO₂ + H₂O (in combustion, hydrogen is removed along with oxygen being added, resulting in oxidation of carbon).
Example OX3: Oxidation as Electron Loss
Solution:
Zn → Zn²⁺ + 2e⁻. Zinc loses electrons, so it is oxidized.
Page 3: Reduction
Reduction is defined as:
- Removal of oxygen: When a substance loses oxygen atoms.
- Addition of hydrogen: When a substance gains hydrogen atoms.
- Electron gain: When a substance gains electrons (reduction means a decrease in oxidation number).
For example, in the conversion of CuO to Cu, copper is reduced by removal of oxygen.
Example RD1: Reduction by Oxygen Removal
Solution:
CuO + H₂ → Cu + H₂O. Oxygen is removed from copper oxide, so copper is reduced.
Example RD2: Reduction by Hydrogen Addition
Solution:
C₂H₄ + H₂ → C₂H₆. Hydrogen is added to ethene, reducing it to ethane.
Example RD3: Reduction as Electron Gain
Solution:
Cl₂ + 2e⁻ → 2Cl⁻. Chlorine gains electrons and is reduced.
Page 4: Redox in Terms of Electron Transfer & Oxidation Numbers
Redox reactions are fundamentally about electron transfer. In a redox reaction, one species loses electrons (oxidation) while another gains electrons (reduction). Oxidation numbers are assigned to atoms in a compound to keep track of these changes.
For example, in the reaction: Zn + 2HCl → ZnCl₂ + H₂, zinc goes from 0 to +2 (oxidation) and hydrogen goes from +1 to 0 (reduction).
Example EN1: Assigning Oxidation Numbers
Solution:
In H₂O, O is -2 and H is +1. In HCl, H is +1 and Cl is -1. In ZnCl₂, Zn must be +2 because 2×(-1) from Cl gives -2.
Example EN2: Identifying Electron Transfer
Solution:
In the reaction Fe²⁺ → Fe³⁺ + e⁻, iron loses one electron (oxidation). The electron is gained by another species.
Example EN3: Change in Oxidation Number
Solution:
In the reaction: Cu + 2Ag⁺ → Cu²⁺ + 2Ag, copper goes from 0 to +2 (oxidation) and silver goes from +1 to 0 (reduction). Total electron transfer: 2 electrons lost by Cu.
Page 5: Balancing Redox Equations Using Oxidation Numbers
To balance a redox equation:
- Assign oxidation numbers to all atoms.
- Identify the atoms that change oxidation number (oxidized and reduced).
- Balance the electrons lost and gained.
- Balance the overall equation by adjusting coefficients.
Example BR1: Balancing a Simple Redox Equation
Solution:
For Zn + HCl → ZnCl₂ + H₂, assign oxidation numbers:
Zn: 0 → +2; H: +1 → 0.
Balance electrons: 1 Zn gives 2 electrons, so 2H⁺ (from 2 HCl) gain 2 e⁻ to form H₂.
Balanced equation: Zn + 2HCl → ZnCl₂ + H₂.
Example BR2: Balancing in Acidic Medium
Solution:
Consider MnO₄⁻ + Fe²⁺ → Mn²⁺ + Fe³⁺ in acidic medium.
Assign oxidation numbers:
Mn in MnO₄⁻: +7 → +2; Fe: +2 → +3.
Balance electrons: Mn gains 5e⁻, Fe loses 1e⁻. Multiply Fe²⁺ by 5:
Balanced: MnO₄⁻ + 5Fe²⁺ + 8H⁺ → Mn²⁺ + 5Fe³⁺ + 4H₂O.
Example BR3: Balancing in Basic Medium
Solution:
For a basic redox equation, first balance as in acid and then add OH⁻ to neutralize H⁺.
(Detailed steps would be provided based on a chosen reaction.)
Page 6: IUPAC Nomenclature of Inorganic Compounds
Oxidation numbers are used in naming inorganic compounds. The oxidation state of the metal is indicated in Roman numerals in parentheses.
For example, Fe₂O₃ is named iron(III) oxide because each iron atom is in the +3 oxidation state.
Example IN1: Naming a Compound
Solution:
In CuO, copper has an oxidation number of +2, so the compound is named copper(II) oxide.
Example IN2: Another Naming Example
Solution:
In FeCl₃, iron is in the +3 state, and the compound is named iron(III) chloride.
Example IN3: Complex Nomenclature
Solution:
In KMnO₄, manganese is in the +7 state, so the compound is named potassium permanganate.
Page 7: Tests for Oxidizing and Reducing Agents
Various chemical tests are used to identify oxidizing and reducing agents. Common tests include:
- Potassium permanganate (KMnO₄) test: In acidic solution, a purple KMnO₄ solution is decolorized by reducing agents.
- Iodine-starch test: Iodine is reduced by strong reducing agents, turning the blue starch solution colorless.
- Diphenylamine test: Used for nitrates, it changes color in the presence of oxidizing agents.
Example RD4: KMnO₄ Test
Solution:
When a few drops of a reducing agent (e.g., oxalic acid) are added to an acidic purple solution of KMnO₄, the solution loses its purple color as MnO₄⁻ is reduced to Mn²⁺.
Example RD5: Iodine-Starch Test
Solution:
A blue starch solution turns colorless when iodine is reduced by a strong reducing agent such as sulfite ions.
Example RD6: Diphenylamine Test
Solution:
In the presence of oxidizing agents like nitrates, diphenylamine reagent gives a characteristic blue color.
Page 8: Additional Worked Examples on Redox Reactions
Example RX1: Identifying Oxidation and Reduction
Solution:
In the reaction: 2Fe²⁺ + H₂O₂ + 2H⁺ → 2Fe³⁺ + 2H₂O, iron is oxidized from +2 to +3 (losing 1 e⁻ each) and hydrogen peroxide is reduced.
Example RX2: Electron Transfer Calculation
Solution:
In the oxidation of Zn: Zn → Zn²⁺ + 2e⁻, two electrons are transferred per atom of Zn.
Example RX3: Balancing a Redox Equation
Solution:
For the reaction: MnO₄⁻ + Fe²⁺ → Mn²⁺ + Fe³⁺, in acidic medium, balance it by first assigning oxidation numbers, then equalize electrons, and finally add H⁺ and H₂O:
Balanced: MnO₄⁻ + 5Fe²⁺ + 8H⁺ → Mn²⁺ + 5Fe³⁺ + 4H₂O.
Page 9: IUPAC Nomenclature and Oxidation Numbers
Oxidation numbers are used in the systematic naming of inorganic compounds. The oxidation state of a metal is indicated by a Roman numeral in parentheses after the name.
For example, Fe₂O₃ is named iron(III) oxide.
Example IN4: Naming Using Oxidation Numbers
Solution:
In CuO, copper is assigned an oxidation number of +2, so the compound is named copper(II) oxide.
Example IN5: Naming a Mixed Oxide
Solution:
For CrO₃, chromium has an oxidation state of +6, and the compound is named chromium(VI) oxide.
Example IN6: Naming a Complex Salt
Solution:
In KMnO₄, manganese is in the +7 state, so the compound is named potassium permanganate.
Page 10: Tests for Oxidizing and Reducing Agents
Chemical tests help to identify oxidizing and reducing agents. Common tests include:
- KMnO₄ test: In acidic solution, a purple solution is decolorized by reducing agents.
- Iodine-starch test: The blue starch solution turns colorless in the presence of strong reducing agents.
- Diphenylamine test: Produces a blue color with oxidizing agents like nitrates.
Example RD4: KMnO₄ Test in Action
Solution:
When a reducing agent (e.g., oxalic acid) is added to an acidic purple KMnO₄ solution, the purple color fades as MnO₄⁻ is reduced to Mn²⁺.
Example RD5: Iodine-Starch Test
Solution:
A blue starch solution turns colorless when a reducing agent (such as sulfite) reduces iodine.
Example RD6: Diphenylamine Test
Solution:
When diphenylamine reagent is added to a solution containing oxidizing agents (e.g., nitrate), a blue color is produced.
Page 11: Electron Transfer and Redox Balancing
Redox reactions involve electron transfer. One species loses electrons (oxidation) while another gains electrons (reduction). The number of electrons lost must equal the number of electrons gained.
Example RX4: Balancing a Redox Reaction
Solution:
Consider: Cr₂O₇²⁻ + Fe²⁺ → Cr³⁺ + Fe³⁺ in acidic medium.
Balanced: Cr₂O₇²⁻ + 14H⁺ + 6Fe²⁺ → 2Cr³⁺ + 6Fe³⁺ + 7H₂O.
Example RX5: Electron Transfer Calculation
Solution:
In the oxidation of Zn: Zn → Zn²⁺ + 2e⁻, two electrons are transferred per atom of Zn.
Example RX6: Overall Redox Reaction
Solution:
In the reaction: Cu + 2Ag⁺ → Cu²⁺ + 2Ag, copper is oxidized (0 to +2) while silver is reduced (+1 to 0). Total electron transfer: 2 electrons from copper.
Page 12: IUPAC Nomenclature Using Oxidation Numbers
The oxidation number helps to name inorganic compounds. The oxidation state of the metal is indicated by a Roman numeral in parentheses.
Example IN7: Naming an Oxide
Solution:
Fe₂O₃ is named iron(III) oxide because each iron atom has an oxidation number of +3.
Example IN8: Naming a Halide
Solution:
In CuCl₂, copper has an oxidation state of +2, so the compound is named copper(II) chloride.
Example IN9: Naming a Complex Salt
Solution:
In KMnO₄, manganese is in the +7 state. The compound is named potassium permanganate.
Page 13: Additional Tests for Oxidizing and Reducing Agents
Several qualitative tests help identify oxidizing and reducing agents:
- KMnO₄ Test: A purple KMnO₄ solution in acid is decolorized by reducing agents.
- Iodine-Starch Test: A blue starch solution loses its color when iodine is reduced.
- Diphenylamine Test: Produces a blue color when reacting with oxidizing agents (e.g., nitrates).
Example RD7: Demonstrating the KMnO₄ Test
Solution:
When oxalic acid is added to an acidic purple solution of KMnO₄, the solution becomes colorless as MnO₄⁻ is reduced to Mn²⁺.
Example RD8: Iodine-Starch Test Example
Solution:
In the presence of a strong reducing agent, iodine is reduced and the starch indicator loses its blue color.
Example RD9: Diphenylamine Test Example
Solution:
When the diphenylamine reagent is added to a nitrate solution, a blue coloration appears, indicating the presence of an oxidizing agent.
Page 14: Extended Discussion and Applications
Redox reactions are vital in many fields: metallurgy (extraction of metals), energy (batteries and fuel cells), biochemistry (cellular respiration), and environmental science (corrosion). Understanding oxidation numbers, balancing redox equations, and knowing the tests for oxidizing/reducing agents are fundamental skills in chemistry.
Page 15: Final Summary and Quiz Introduction
- Oxidation is the addition of oxygen, removal of hydrogen, or loss of electrons (increase in oxidation number).
- Reduction is the removal of oxygen, addition of hydrogen, or gain of electrons (decrease in oxidation number).
- Oxidation numbers help track electron transfer and balance redox equations.
- IUPAC nomenclature uses oxidation numbers to name inorganic compounds.
- Qualitative tests can distinguish oxidizing and reducing agents.
Review these concepts and then test your understanding with the 30‑question CBT JAMB quiz below.
30 CBT JAMB Quiz on Oxidation and Reduction (Redox)
Click the "Start Quiz" button to begin. You will have 15 minutes to answer 30 questions.
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