What a learner can do afterwards
- Assigns oxidation numbers in a compound or a polyatomic ion using the standard rules
- Identifies what is oxidised and what is reduced from the changes in oxidation number
- Balances a half equation in acid solution using water and hydrogen ions
- Combines two half equations so that the electrons cancel exactly
1 · Read
An oxidation number is a bookkeeping charge. Use the standard rules: oxygen is usually -2, hydrogen is usually +1, Group 1 is +1, Group 2 is +2, fluorine is -1, and an uncombined element is 0. Peroxides and metal hydrides are exceptions. The numbers add to the overall charge.
In HNO3, hydrogen contributes +1 and oxygen contributes -6. The molecule is neutral, so nitrogen is +5. In PO4 3-, oxygen contributes -8 and the ion has charge -3, so phosphorus is +5.
When an oxidation number rises, electrons have been lost and the species is oxidised. When it falls, electrons have been gained and the species is reduced. When magnesium meets silver ions, magnesium changes from 0 to +2, so it is oxidised, and silver changes from +1 to 0, so it is reduced.
In acid, balance atoms other than O and H first. Then add water to balance oxygen, H+ ions to balance hydrogen, and electrons to balance charge. For dichromate becoming Cr3+, write Cr2O7 2- -> 2Cr3+, add 7H2O on the right and 14H+ on the left. The left charge is -2 + 14 = +12 and the right is +6, so add 6e- on the left: Cr2O7 2- + 14H+ + 6e- -> 2Cr3+ + 7H2O. To combine two halves, multiply each until the electron counts match, add them, and cancel the electrons.
Oxidation numbers track electron changes. Balance acid half equations in order, then match and cancel electrons.
Original chemistry lesson and assessment text authored for os-taxonomy.
The readings on balancing half equations and reaction families were written for this dataset. The reading on oxidation-number rules is original writing for os-taxonomy. The two videos are linked Khan Academy videos, not adaptations.
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Where it sits
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.