2.3.2Unit 2. Energetics, Group Chemistry, Halogenoalkanes and Alcohols
Oxidation and Reduction — IAL Edexcel Linear (Chemistry) Revision Notes & Topical Questions
5 min read 5 parts Lesson 22 of 53 Unit quiz
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.
Section 1 of 5Three Ways to Define the Terms
The same reaction can be described using oxygen, electrons or oxidation numbers, and the last two are the most general.
Oxygen and hydrogen
- Oxygen and hydrogen Oxidation is gain of oxygen or loss of hydrogen, while reduction is loss of oxygen or gain of hydrogen.
Electrons
- Electrons Oxidation is loss of electrons and reduction is gain of electrons, remembered by OIL RIG.
Oxidation number
- Oxidation number Oxidation is an increase in oxidation number and reduction is a decrease in oxidation number.
Redox
- Redox Oxidation and reduction always happen together, because electrons lost by one species are gained by another.
Section 2 of 5Oxidising and Reducing Agents
Oxidising and Reducing Agents
Oxidising agent
- Oxidising agent This species takes electrons from another, so it is itself reduced and its oxidation number falls.
Reducing agent
- Reducing agent This species gives electrons to another, so it is itself oxidised and its oxidation number rises.
Common oxidising agents
- Common oxidising agents Halogens, acidified potassium manganate(VII) and acidified potassium dichromate(VI) are typical oxidising agents.
Common reducing agents
- Common reducing agents Metals, iodide ions and iron(II) ions readily donate electrons and act as reducing agents.
Section 3 of 5Analysing a Redox Equation
1
Write the oxidation number of every element above its symbol on both sides of the equation.
2
Find the element whose oxidation number increases; that species has been oxidised and is the reducing agent.
3
Find the element whose oxidation number decreases; that species has been reduced and is the oxidising agent.
4
Check that the total increase in oxidation number equals the total decrease, which confirms electrons are conserved.
5
State the answer using the correct terms, for example that magnesium is oxidised from 0 to +2 as it loses electrons.
Section 4 of 5Worked Example
Question In the reaction MnO₂(s) + 4HCl(aq) → MnCl₂(aq) + Cl₂(g) + 2H₂O(l), identify what is oxidised, what is reduced and the oxidising agent.
- Manganese in MnO₂ is +4, because the two oxygens give -4.
- Manganese in MnCl₂ is +2, so the oxidation number falls and Mn is reduced.
- Chlorine in HCl is -1 and chlorine in Cl₂ is 0, so the oxidation number rises and Cl is oxidised.
- The species that gets reduced is MnO₂, so it takes electrons from the chloride ions.
Answer Chloride in HCl is oxidised (-1 to 0), manganese is reduced (+4 to +2), so MnO₂ is the oxidising agent and HCl is the reducing agent.
Section 5 of 5Exam Checklist & Tips
2.3.2 – Key Points for the Exam
- Oxidation Loss of electrons, or an increase in oxidation number.
- Reduction Gain of electrons, or a decrease in oxidation number.
- Oxidising agent A species that accepts electrons and is itself reduced.
- Reducing agent A species that donates electrons and is itself oxidised.
- Redox reaction A reaction in which oxidation and reduction occur together.
Common Mistakes
- Saying the oxidising agent is oxidised; it causes oxidation but is reduced itself.
- Describing oxidation as gain of electrons because it sounds like gain; it is loss.
- Deciding what is oxidised or reduced without comparing the oxidation numbers of the element on both sides of the equation.
- Stating oxidation number changes without the sign, such as writing 0 to 2 instead of 0 to +2.
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