Oxidising agents, reducing agents and disproportionation
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The teaching part (1 min) is free to watch. A free account opens the whole lecture: 1 worked example, 3 practice questions, 4 real past-paper questions with the answers explained, the mistakes that lose marks, the full-marks tip, a one-card summary and a quick quiz.
What this lecture covers
- Why iron rusts and bleach whitens
- Oxidation and reduction
- Oxidation number rules
Read the transcript of the teaching part
Last time
Before we start: last time, in Enthalpy changes, calorimetry and pathway diagrams, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.
Why iron rusts and bleach whitens
Iron rusting, bleach removing a stain, a battery powering a phone. In each case electrons are being transferred from one species to another. Whenever electrons move, one thing is oxidised and another is reduced, and oxidation numbers let us keep track of exactly which is which.
Oxidation and reduction
Oxidation is loss of electrons, and an increase in oxidation number. Reduction is gain of electrons, and a decrease in oxidation number. An oxidising agent accepts electrons from another species, so it is itself reduced. A reducing agent donates electrons to another species, so it is itself oxidised. Redox means that reduction and oxidation take place together in the same reaction.
Oxidation number rules
An uncombined element has oxidation number zero. A simple ion has an oxidation number equal to its charge. The sum of oxidation numbers in a neutral compound is zero. In an ion it equals the charge on the ion. Oxygen is minus 2, except in peroxides. Hydrogen is plus 1, except in metal hydrides. Fluorine is always minus 1. Write a plus or minus sign in front of the number, and use Roman numerals only in names, as in manganese(IV) oxide.
How often it comes up
How often Cambridge has asked each part, counted by ExamYard from every past paper in its question bank: Disproportionation · Oxidising and reducing agents.
The whole chapter: Electrochemistry.
Chapter 6: Redox reactions
- Oxidising agents, reducing agents and disproportionation 7 min
Also for A Level Chemistry 9701
Which topics come up most in A Level Chemistry 9701 past papers, counted from every paper since 2004: Paper 2 topic frequency, Paper 4 topic frequency.