Oxidation numbers and redox
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The teaching part (1 min) is free to watch. A free account opens the whole lecture: 5 worked examples, 6 practice questions, 2 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
- Rust, fire and batteries
- Three definitions
- The rules
- Usual values
Read the transcript of the teaching part
Last time
Before we start: last time, in Rates of reaction and collision theory, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.
Rust, fire and batteries
Rust forming on a nail, a candle burning, a battery powering your phone. They look quite different, but all three are redox reactions: reduction and oxidation happening together. To spot them, chemists give every atom a number that shows how many electrons it has gained or lost. That is its oxidation number.
Three definitions
Oxidation is gain of oxygen, or loss of electrons. Reduction is loss of oxygen, or gain of electrons. Remember OIL RIG: Oxidation Is Loss of electrons, Reduction Is Gain. A redox reaction is one where reduction and oxidation both happen. One substance cannot be oxidised unless another is reduced.
The rules
An uncombined element has an oxidation number of zero. That includes O 2, Fe and Cl 2. A simple ion has an oxidation number equal to its charge. Mg²⁺ is plus two. Cl⁻ is minus one. In a compound the numbers add up to zero. In an ion, they add up to the charge.
Usual values
Oxygen is usually minus two. Hydrogen is usually plus one. Group I metals are plus one. Group II metals are plus two. Halogens in a halide are minus one. Then use the sum rule to find the one that you do not know.
How often it comes up
How often Cambridge has asked each part, counted by ExamYard from every past paper in its question bank: Identifying oxidation and reduction · Oxidation numbers · Oxidation and reduction.
The whole chapter: Chemical Reactions, Chemical Reactions.
Chapter 6: Chemical reactions
- Collision theory and rate graphs 6 min
- Dynamic equilibrium, position of equilibrium and industry 6 min
- Oxidation numbers and redox 6 min
- Oxidising and reducing agents 6 min
Also for IGCSE Chemistry 0620
Which topics come up most in IGCSE Chemistry 0620 past papers, counted from every paper since 2004.