3.1.7Additional lessons
Oxidation States — IB Chemistry SL Revision Notes & Topical Questions
5 min read 6 parts Lesson 43 of 98
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Part 1 of 6Rules for Assigning
Oxidation states are a bookkeeping tool that tracks electrons in compounds.
Elements
- Elements Atoms in an uncombined element, such as Na, O₂ or P₄, have an oxidation state of 0.
Simple ions
- Simple ions The oxidation state of a simple ion equals its charge, for example Mg²⁺ is +2 and Cl⁻ is −1.
Usual values
- Usual values Hydrogen is +1 (−1 in metal hydrides) and oxygen is −2 (−1 in peroxides), and fluorine is always −1.
Sum rule
- Sum rule The oxidation states in a neutral compound add to 0, and in an ion add to the charge on the ion.
Part 2 of 6Worked Assignments
Worked Assignments
SO₄²⁻
- SO₄²⁻ Four oxygens give −8, so sulfur must be +6 to make a total of −2.
SO₃²⁻
- SO₃²⁻ Three oxygens give −6, so sulfur is +4.
MnO₄⁻
- MnO₄⁻ Four oxygens give −8, so manganese is +7 for a total of −1.
Cr₂O₇²⁻
- Cr₂O₇²⁻ Seven oxygens give −14, so the two chromium atoms are +12 together, which is +6 each.
| Species | Element | Oxidation state |
|---|---|---|
| H₂SO₄ | S | +6 |
| NO₃⁻ | N | +5 |
| Fe₂O₃ | Fe | +3 |
| CH₄ | C | −4 |
Part 3 of 6Using Oxidation States
1
Oxidation is an increase in oxidation state and reduction is a decrease in oxidation state.
2
Identify the atom whose oxidation state changes in an equation, for example Mg from 0 to +2 in Mg + 2HCl → MgCl₂ + H₂.
3
The species that is oxidised loses electrons and is the reducing agent.
4
The species that is reduced gains electrons and is the oxidising agent.
5
Roman numerals in names show the oxidation state, so iron(III) chloride contains Fe³⁺ and copper(II) sulfate contains Cu²⁺.
Part 4 of 6Checks and Limits
Fractional values
- Fractional values An average oxidation state may be fractional, as in Fe₃O₄ where iron averages +8/3.
Not real charge
- Not real charge In covalent compounds the oxidation state is an assigned value, not the actual charge on an atom.
Redox test
- Redox test If no element changes oxidation state, the reaction is not redox.
Disproportionation
- Disproportionation In some reactions the same element is both oxidised and reduced.
Part 5 of 6Worked Example
Question Determine the oxidation state of manganese in KMnO₄.
- K is a Group 1 metal in a compound, so its oxidation state is +1.
- Each O is −2, so four O atoms contribute −8.
- The compound is neutral+1 + x + (−8) = 0, so x = +7.
Answer Manganese is +7.
Part 6 of 6Exam Checklist & Tips
3.1.7 – Key Points for the Exam
- Oxidation state The charge an atom would have if all bonds were treated as fully ionic.
- Oxidation An increase in oxidation state, corresponding to loss of electrons.
- Reduction A decrease in oxidation state, corresponding to gain of electrons.
- Redox A reaction in which oxidation and reduction occur together.
Common Mistakes
- Forgetting that the sum of oxidation states in a polyatomic ion equals its charge.
- Writing the oxidation state as 2+ instead of +2.
- Assigning oxygen −2 in peroxides like H₂O₂, where it is −1.
- Confusing the reducing agent with the species being reduced.
Last updated:
\n