Redox Reactions & Transport Numbers
Electron transfer, oxidation states, and the science of ionic current distribution
🔬 Interactive Ion Migration Simulation
Cations (blue) move toward the cathode (right); anions (red) move toward the anode (left). Adjust the slider to change the cation transport number and observe how the speeds change.
The speed ratio is proportional to the transport numbers. Faster ions carry a larger fraction of the current.
1. Oxidation Reaction
Oxidation can be defined in two complementary ways:
- Electron loss: Loss of electrons by a substance (increase in oxidation state).
- Classical definition: Addition of oxygen or a more electronegative element, or removal of hydrogen or a more electropositive element.
2S(s) + O₂(g) → SO₂(g)
CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(l)
In the second reaction, carbon is oxidized from −4 to +4, while oxygen is reduced from 0 to −2.
2. Reduction Reaction
Reduction is the opposite of oxidation:
- Electron gain: Gain of electrons by a substance (decrease in oxidation state).
- Classical definition: Addition of hydrogen or a more electropositive element, or removal of oxygen or a more electronegative element.
2CH₂=CH₂(g) + H₂(g) → CH₃–CH₃(g) (hydrogenation)
2FeCl₃(aq) + H₂(g) → 2FeCl₂(aq) + 2HCl(aq) (iron reduced from +3 to +2)
3. Redox Reactions: Electron Transfer
A redox reaction is a chemical reaction in which electrons are transferred between two species. The species that loses electrons is oxidized; the species that gains electrons is reduced. These two processes always occur together.
Electron transfer between A (reducing agent) and B (oxidizing agent).
A → A⁺ + e⁻ (oxidation); B + e⁻ → B⁻ (reduction).
Accepts electrons; undergoes reduction.
Example: KMnO₄, O₂, F₂.
Donates electrons; undergoes oxidation.
Example: Zn, NaBH₄, H₂.
4. Types of Redox Reactions
Compound breaks down into simpler substances.
Examples: 2NaH → 2Na + H₂; 2H₂O → 2H₂ + O₂.
General form: AB → A + B.
Two or more substances combine to form a single product.
Examples: H₂ + Cl₂ → 2HCl; C + O₂ → CO₂.
General form: A + B → AB.
One element displaces another from its compound.
General: X + YZ → XZ + Y.
Metal displacement: CuSO₄ + Zn → Cu + ZnSO₄.
Non‑metal displacement: 2NaBr + Cl₂ → 2NaCl + Br₂.
A single substance is simultaneously oxidized and reduced.
Example: P₄ + 3NaOH + 3H₂O → 3NaH₂PO₂ + PH₃.
Chlorine in water: Cl₂ + H₂O → HCl + HOCl (Cl from 0 to –1 and +1).
5. Worked Examples of Redox Reactions
Example 1: Hydrogen + Fluorine
Oxidation half: H₂ → 2H⁺ + 2e⁻ (H from 0 to +1)
Reduction half: F₂ + 2e⁻ → 2F⁻ (F from 0 to –1)
Overall: H₂ + F₂ → 2H⁺ + 2F⁻ → 2HF
Example 2: Zinc + Copper(II) Sulfate (Metal displacement)
Oxidation: Zn → Zn²⁺ + 2e⁻ (Zn is reducing agent, oxidized)
Reduction: Cu²⁺ + 2e⁻ → Cu (Cu²⁺ is oxidizing agent, reduced)
Net ionic: Zn + Cu²⁺ → Zn²⁺ + Cu
6. Balancing Redox Equations
In acidic or basic media, half‑reaction method is used:
- Write separate half‑reactions (oxidation and reduction).
- Balance atoms other than H and O.
- Balance O by adding H₂O, balance H by adding H⁺ (acidic) or OH⁻ (basic).
- Balance charge by adding electrons.
- Multiply half‑reactions so that electrons cancel, then add them.
Balanced: MnO₄⁻ + 5Fe²⁺ + 8H⁺ → Mn²⁺ + 5Fe³⁺ + 4H₂O
7. Applications & Importance of Redox Reactions
Galvanic cells convert chemical energy to electrical energy via spontaneous redox reactions (e.g., Zn–Cu cell).
Iron oxidizes in presence of oxygen and water, forming rust (Fe₂O₃·xH₂O).
Extraction of metals from ores: reduction of metal oxides with carbon or other metals.
All combustion reactions are redox: fuel + O₂ → CO₂ + H₂O.
Photosynthesis: CO₂ + H₂O → glucose + O₂ (water oxidized, CO₂ reduced).
Respiration: glucose + O₂ → CO₂ + H₂O + energy.
Redox titrations (e.g., permanganometry, iodometry) determine concentrations of oxidizing or reducing agents.
Bleaching, water disinfection (chlorine), electroplating, production of hydrogen and chlorine via electrolysis.
Removal of pollutants by redox reactions (e.g., chromium(VI) reduction to Cr(III)).
The concept of redox reactions is fundamental to understanding energy production, material degradation, and countless chemical transformations in nature and technology.
8. Transport Numbers (Transference Numbers) in Electrochemistry
The transport number (or transference number, also called Hittorf number) of an ion is the fraction of the total electric current carried by that ion in an electrolyte solution. Since cations and anions move at different speeds, they contribute differently to conductivity.
where t₊ is the cation transport number and t₋ is the anion transport number.
8.1 Mathematical Relationship
If u₊ and u₋ are ionic mobilities (drift velocity per unit electric field):
8.2 Hittorf’s Method (Concentration Change Method)
Hittorf’s method determines transport numbers by measuring the concentration changes in the anode and cathode compartments before and after electrolysis. For a salt like AgNO₃:
8.3 Moving Boundary Method
The moving boundary method is more accurate and directly measures the velocity of a boundary between two electrolyte solutions. The transport number is given by:
where ν = number of cations per formula unit, z = charge, F = Faraday constant, C = concentration, I = current, and dx/dt = boundary velocity.
8.4 Factors Affecting Transport Numbers
| Factor | Effect |
|---|---|
| Temperature | Increases mobility of slower ions; t₊ and t₋ tend toward 0.5. |
| Concentration | High concentration can cause ion pairing; abnormal transport numbers possible. |
| Nature of ions | H⁺ and OH⁻ have very high transport numbers due to the Grotthuss mechanism (tH+ ≈ 0.83 in HCl). |
| Hydration | Highly hydrated ions move slower and have lower transport numbers. |
8.5 Importance of Transport Numbers
- Battery design: High t₊ (e.g., Li⁺ in lithium batteries) ensures efficient charge transfer.
- Membrane science: Ion‑selective membranes are characterized by their transport numbers.
- Electrophoresis: Separation of ions based on mobility differences.
- Corrosion studies: Understanding ionic movement in electrolyte solutions.
9. Quick Reference: Key Terms
| Term | Definition |
|---|---|
| Oxidation | Loss of electrons, increase in oxidation state. |
| Reduction | Gain of electrons, decrease in oxidation state. |
| Oxidizing agent | Accepts electrons, gets reduced. |
| Reducing agent | Donates electrons, gets oxidized. |
| Transport number | Fraction of total current carried by a specific ion. |
| Hittorf method | Measures transport numbers via concentration changes. |
| Ionic mobility | Drift velocity of an ion per unit electric field. |
10. Summary
- Redox reactions involve electron transfer; oxidation = loss, reduction = gain (OIL RIG).
- Types: decomposition, combination, displacement, disproportionation.
- Transport numbers (t₊, t₋) describe current sharing between ions; t₊ + t₋ = 1.
- Determined by Hittorf’s method or moving boundary method.
- Both concepts are essential for understanding batteries, corrosion, and industrial electrolysis.
📺 Video Lectures (Redox Reactions)
Detailed explanations of oxidation, reduction, half-reactions, and types of redox reactions.
