Redox

• oxidation is the loss of electrons and reduction is the gain of electrons
• a redox reaction is where oxidation and reduction take place at the same time. the number of electrons lost must equal the number of electrons gained
• magnesium and hydrochloric acid

  Mg (s) + 2 HCl (aq) -> MgCl2 (aq) + H2 (g)
• oxidation of magnesium


  Mg -> Mg 2+ + 2e-
• reduction of hydrogen ions

  2H+ 2e- -> H2
• what is the oxidation state of an element?


  the number of electrons involved when an atom forms a bond to atoms of a different element. the oxidation state refers to one atom of the element in a compound or ion. the oxidation state has a number and a sign (positive or negative)
• Rule 1 for assigning oxidation states

  An uncombined element has an oxidation state of zero, e.g. O2
• Rule 2


  some elements have the same oxidation state in all their compounds. Group 1 elements like Li, Na, K have a +1 state, Group 2 all have 2+, Group 3 are all 3+.
• Exceptions to rule 2


  H oxidation is usually +1, like H2O. But in metal hydrides like CaH2, H has a -1. F always -1. O usually -2 but in H2O2, its -1 and in OF2 its +2. Cl is mostly -1 but not in chlorate ions ClOx +1,+3,+5,+7
• Rule 3


  For a compound, the oxidation number of each atom in the compound's formula counts separately and the numerical sum is zero. e.g. H2O, NH3
• Rule 4


  For an element existing as an ion, the oxidation state is the charge on the ion e.g. Na+, Fe2+
• Rule 5


  For compound ions the sum of the oxidation states of the atoms is the charge on the ion, e.g. SO4 2-
• Roman numerals in chemical names are used to indicate the oxidation state of the appropriate element. FeO is called iron (ii) oxide so the o.s state of iron is +2
• sulfuric (vi) acid


  H2SO4
• Charges of common oxo-anions

  -1: nitrates, chlorates, iodates

  -2:sulfates

  -3:phosphates
• oxidation is an increase in oxidation state

  reduction is a decrease in oxidation state
• oxidising agent


  a species that accepts electrons
• reducing agent


  a species that donates electrons
• in the reaction of magnesium and oxygen, which is the oxidising and reducing agent?

  Magnesium is the reducing agent (donates e-). Oxygen is the oxidising agent (accepts e-).
• what are rules used when constructing complex half equations?

  • Write down the formulae for the reactants and products and balance the atoms undergoing redox
  • balance any oxygen atoms by adding water
  • balance hydrogen atoms by adding h+
  • balance charges by adding e-
• the conversion of nitric acid to nitrogen dioxide gas


  HNO3 + H+ + e- -> NO2 + H2O
• How to combine half equations

  one equation must show reduction, the other shows oxidation. the number of e- being transferred must be the same so you may have to multiply one or both of the half equations. The 2 half equations can then be added and any species that appears on both sides of the equation must be cancelled
• Construct the redox equation for the reaction between dichromate (VI) ions and iron (ii) ions


  Fe2+ -> Fe3+ + e- (x6)

  Cr2O7 2- + 14H+ + 6e- -> 2Cr3+ + 7H2O