Redox Fundamentals and Oxidation Numbers
Quick Notes
- Oxidation = loss of electrons
- Reduction = gain of electrons
- Oxidation and reduction occur simultaneously in redox reactions.
- Oxidising agent = accepts electrons (gets reduced)
- Reducing agent = donates electrons (gets oxidised).
- Oxidation number (oxidation state): a number assigned to atoms to keep track of electron transfer.
- Rules for oxidation states:
- Elements in their natural state have an oxidation number of 0.
- Oxygen is usually −2 (except in peroxides where it is −1).
- Hydrogen is usually +1 (except in metal hydrides where it is −1).
- Group 1 metals are +1, Group 2 metals are +2.
- The sum of oxidation states in a neutral compound is 0.
- The sum of oxidation states in an ion equals the charge of the ion.
- Oxidation = increase in oxidation number
- Reduction = decrease in oxidation number
- Roman numerals are used to show oxidation number in names
(e.g. iron(III), manganese(VII)).
Full Notes
Note this is a recap page from redox I (see here).
Redox in Terms of Electron Transfer
Redox (reduction–oxidation) reactions involve the movement of electrons between species.
- Oxidation: A species loses electrons.
- Reduction: A species gains electrons.
Example Electron transfer pair
Na → Na⁺ + e⁻ (oxidation)
Cl₂ + 2e⁻ → 2Cl⁻ (reduction)
In this redox pair, sodium is oxidised, chlorine is reduced.
Oxidation Numbers
Oxidation states help track electron transfer in reactions. It is straightforward to see how atoms have lost or gained electrons when ions get formed, however it can be harder to see how atoms have lost or gained electron density when dealing with molecules.
For example, carbon is oxidised to form carbon dioxide when combusted. However, no ions get formed, meaning it isn’t immediately clear how electrons are involved!
To help, we consider each atom to have an ‘imaginary’ charge, described as its oxidation number (or state).
Rules for assigning oxidation states
- Uncombined elements (e.g., O₂, N₂, Fe) have an oxidation state of 0.
- Group 1 metals = +1, Group 2 metals = +2.
- Oxygen is −2, except:
- In peroxides (O₂²⁻), oxygen is −1.
- With fluorine (OF₂), oxygen is +2.
- Hydrogen is +1, except in metal hydrides (e.g., NaH), where it is −1.
- In a neutral compound, the sum of oxidation states = 0.
- In polyatomic ions, the sum of oxidation states = charge of the ion.
| Rule | Details |
|---|---|
| Uncombined elements | Uncombined elements (e.g., O₂, N₂, Fe) have an oxidation state of 0. |
| Group 1 and Group 2 | Group 1 metals = +1, Group 2 metals = +2. |
| Oxygen | Oxygen is −2, except in peroxides (O₂²⁻) where it is −1, and in OF₂ where O is +2. |
| Hydrogen | Hydrogen is +1, except in metal hydrides (e.g., NaH), where it is −1. |
| Neutral compounds | Sum of oxidation states in a neutral compound is 0. |
| Polyatomic ions | Sum of oxidation states equals the overall ion charge. |
Using these rules, we can see now how carbon gets oxidised from an oxidation state of 0 in C(s) to +4 in CO₂(g).
An increase in oxidation number (gets more positive) means oxidation has occurred.
A decrease in oxidation number (gets more negative) means reduction has occurred.
Assign oxidation states in H2SO4 (sulfuric acid).
- H = +1 (there are 2 H, total +2).
- O = −2 (there are 4 O, total −8).
- The total charge must be 0, so S must be +6 to balance the equation:
2(+1) + S + 4(−2) = 0 → S = +6.
Roman Numerals in Names
Oxidation numbers are shown in Roman numerals in the names of compounds — particularly for transition metals and other elements with variable oxidation states.
Examples:
- FeCl₂ is named iron(II) chloride because Fe is +2
- FeCl₃ is named iron(III) chloride because Fe is +3
- MnO₄⁻ is named manganese(VII) oxide because Mn is +7
Summary
- Oxidation is loss of electrons and reduction is gain of electrons.
- Oxidising agents accept electrons and reducing agents donate electrons.
- Use oxidation numbers to track electron transfer using the standard rules.
- Oxidation increases oxidation number and reduction decreases oxidation number.
- Roman numerals in names show the oxidation number of elements with variable states.