A-Level Chemistry OCR Notes

2.1.5 Redox

Redox
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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

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