redox reactions

• Why do we use oxidation numbers?

  when we can't easily write 1⁄2 equations to help us work out what has been oxidised and reduced
• State oxidation number of an uncombined element

  0
• State the oxidation number of a monatomic ion in an ionic compound

  equal to its charge
• State the sum of the oxidation numbers of the atoms in a

  neutral compound

  0
• State the sum of the oxidation numbers in a polyatomic ion

  equals the charge on the ion
• Which elements nearly always have the same oxidation number in their compounds and act as reference points in assigning oxidation numbers to other elements?

  Al Ca Cl F H K Mg Na O
• Exception for H oxidation number differing

  in metal hydrides
• Exception for Cl oxidation number differing

  compounds with O or F
• Exception for O oxidation number differing

  peroxides, superoxides and fluoride
• General formula for half equations


  X⁺₍ₐ₎ + e⁻ → X₍ₐ₎
• Define reduction

  gain of electrons
• Define oxidation

  loss of electrons
• Define redox reaction


  reactions in which electrons are transferred from 1 species to another
• Define half equation

  an equation redox reaction showing 1 species involved and shows the electrons transferred to or from it
• Define spectator ions

  ions that are unchanged during a chemical reaction
• Define oxidation states


  the number of electrons lost or gained by an atom in a compound compared to the uncombined atom
• Rule for varying oxidation numbers

  the less electronegative element has a positive charge
• What does the name of a compound show?

  the oxidation number in the bracket next to the element
• Define reduction in terms of oxidation number

  oxidation number has become more negative
• Define oxidation in terms of oxidation number

  oxidation number has become more positive
• State the rules for constructing half equations

  1. write the formulae for the reagents and products 2. balance any atoms that aren't hydrogen or oxygen 3. add water to balance oxygen 4. add H+ ions to balance hydrogen 5. add electrons to balance charge 6. add state symbols
• State the rules for combining half equations

  1. write down the 2 half equations 2. multiply the half equations to balance the electrons 3. add the half equations 4. cancel the e⁻, H₂O and H⁺ where necessary 5. check atoms and charges are balances 6. add state symbols
• Define oxidising agent


  species that remove electrons from another
• Define reducing agent


  species that give electrons to another