A-LEVEL OCR ChEMISTRY NOTES
Redox
Oxidation Numbers
- Oxidation number is a number representing the number of electrons lost or gained by an atom in a compound.
- Oxidation is a loss of electrons during a reaction or an increase in oxidation number.
- Reduction is a gain of electrons during a reaction or a decrease in oxidation number
- The rules for assigning oxidation numbers:
- An uncombined element has an oxidation number of 0
- A simple ion (of a single element) has an oxidation number equal to the charge on the ion
- The sum of oxidation numbers of the elements in a compound is equal to the overall charge of the compound
- The charge on a complex ion, e.g. NH4+, is equal to the sum of the oxidation numbers
- The most electronegative element in a compound always has a negative oxidation number
- Oxygen is always -2 except in peroxides where its -1
- Hydrogen is +1 except in metal hydrides where its -1
- Oxidation numbers are represented by Roman numerals when naming compounds
- Oxidation numbers can be used to write formulae for a compound
Redox Reactions
- Reduction: The gain of electrons and decrease in oxidation number of an element
- Oxidation: The loss of electrons and increase in oxidation number of an element
- Redox reactions involve both oxidation and reduction
- Oxidising agents cause oxidation of other species, and so are themselves reduced
- Reducing agents cause reduction of other species, and so are themselves oxidised
- In the reaction below, H is reduced, Na is oxidised.
2 HCl + 2 Na→ 2 NaCl + H2
+1 0 +1 0
+1 0 +1 0