Cambridge O Level Chemistry · Syllabus 5070 · Chemical Reactions
Oxidation Number
What is Oxidation Number?
A number assigned to an element in a compound or ion that records how many electrons it has effectively gained or lost compared with the uncombined element. It is found using four rules, and a change in it during a reaction is what identifies oxidation and reduction.
This definition is part of the Chemical Reactions chapter in Cambridge O Level Chemistry.
Oxidation Number in context
Chemical reactions are changes in which new substances form as bonds break and re-form between particles, and this chapter answers three questions about them. Rate asks how fast a reaction happens, and is governed by collision theory: particles must collide with combined energy at least equal to the activation energy, \(E_\mathrm{a}\), and factors such as concentration, pressure, surface area, temperature and catalysts all work by changing how often successful collisions occur. Equilibrium asks how far a reversible reaction goes before the forward and reverse rates become equal. Redox asks which species lost electrons and which gained them, tracked through oxidation numbers.
Common mistakes with Oxidation Number
- 20. “The Roman numeral is the charge on the ion.” Why temptingFor simple ions such as \(\mathrm{Fe^{3+}}\) in iron(III) chloride, the two do coincide. RepairThe Roman numeral is the oxidation number of one element. In manganate(VII) manganese is \(+7\) while the whole \(\mathrm{MnO_4^{\,-}}\) ion carries only \(1-\). TransferState the oxidation number of chromium in dichromate(VI) and the charge on the ion. \(+6\) per chromium atom; the ion is \(2-\).
Examiner tips on Oxidation Number
- Evidence first, name second. “Identify the oxidising agent” is asking for two things: which species it is, and how you know. Write the oxidation number change, or the electron transfer, and then name the agent. A bare name is an assertion; the evidence is what makes it an answer — and, since the inversion is so easy to get backwards, writing the evidence first is also the best way to catch your own error.
Questions students ask about Oxidation Number
How do you identify the oxidising agent and reducing agent in a reaction?
Track the oxidation number of each species. The species whose oxidation number falls has been reduced, so it is the oxidising agent; the species whose oxidation number rises has been oxidised, so it is the reducing agent. For \(\mathrm{Zn(s) + CuSO_4(aq) \rightarrow ZnSO_4(aq) + Cu(s)}\), zinc rises from \(0\) to \(+2\) so it is the reducing agent, and copper falls from \(+2\) to \(0\) so copper(II) ions are the oxidising agent.