Redox

• Oxidation


  The process of electron loss
• Oxidation number


  Increases in oxidation
• Reduction


  The process of electron gain
• Oxidation number


  Decreases in reduction
• Rules for assigning oxidation numbers


  1. All uncombined elements have an oxidation number of zero

  2. The oxidation numbers of the elements in a compound add up to zero

  3. The oxidation number of a monoatomic ion is equal to the ionic charge

  4. In a polyatomic ion, the sum of the individual oxidation numbers of the elements adds up to the charge on the ion

  5. Several elements have invariable oxidation numbers in their common compounds
• • Group 1 metals = +1
  • Group 2 metals = +2
  • Al = +3
  • H = +1 (except in metal hydrides where it is –1 eg NaH)
  • F = -1
  • Cl, Br, I = –1 except in compounds with oxygen and fluorine
  • O = -2 except in peroxides (H2O2 ) where it is –1 and in compounds with fluorine
• Naming using Roman Numerals

  In IUPAC convention the various forms of sulphur, nitrogen and chlorine compounds where oxygen is combined are all called sulfates, nitrates and chlorates with relevant oxidation number given in roman numerals
• Redox Reactions


  • Metals generally form ions by losing electrons with an increase in oxidation number to form positive ions
  • Non-metals generally react by gaining electrons with a decrease in oxidation number to form negative ions
• Redox Reactions


  • 4Li + O2 → 2Li2O
  • WO3 + 3H2 → W + 3H2O
  • 2Sr(NO3)2 → 2SrO + 4NO2 + O2
• Oxidation


  Oxidation number is increasing
• Reduction


  Oxidation number is decreasing
• Redox Reactions of Metals and Acid

  1. 2HCl + Mg → MgCl2 + H2

  2. Fe + H2SO4 → FeSO4 + H2
• Observations: These reactions will effervesce because H2 gas is evolved and the metal will dissolve
• Reduction


  Gain of electrons
• Oxidation


  Loss of electrons
• Electrons are transferred when reduction and oxidation occurs
• Reducing agent


  Loses electrons but is itself oxidized
• Oxidizing agent


  Gains electrons and is itself reduced
• Calcium completely burning in oxygen is a redox reaction
• Oxidation number


  The charge assigned to an element in a compound
• Rules for assigning oxidation numbers


  • Uncombined elements have oxidation number 0
  • Ions have oxidation number equal to charge
  • Group 1 elements are +1
  • Group 2 elements are +2
  • Group 3 elements are +3
  • Hydrogen is +1 except in metal hydrides where it is -1
  • Chlorine is -1 except when bonded to F or O
  • Fluorine is always -1
  • Oxygen is usually -2, except in peroxides where it is -1
• Calculating oxidation numbers


  • NH3: N is -3, H is +1
  • H2S: S is -2
  • H2O2: O is -1
• Transition metal oxidation states

  • Fe2O3: Fe is +3
  • FeO: Fe is +2
  • V2O5: V is +5
  • VO2: V is +4
• Systematic name for chlorine compounds


  Add "-ate" to indicate chlorine bonded to oxygen, e.g. ClO2- is chlorate
• Sodium reacting with chlorine


  Sodium is oxidized from 0 to +1, chlorine is reduced from 0 to -1
• Metals reacting with acids


  Metal is oxidized, hydrogen gas is produced
• When metals react with acids, a salt and hydrogen gas are formed