C2.1.5

• The oxidation number is the number of electrons involved in bonding to a different element and is derived from a set of rules
• In a pure element the oxidation number of each atom is 0
• What is the oxidation number of O in O2?

  0
• Groups 1 , 2 , 3 , 7 have the same oxidation number as their ions
• What is the oxidation number of group 1 elements?

  +1
• Combined oxygen usually has an oxidation number of -2 except in a peroxide when it has an oxidation number of -1
• What is the oxidation number of O in H2O2?

  -1
• Combined hydrogen usually has an oxidation number of +1 except in a hydride when it has an oxidation number of -1
• What is the oxidation number of hydrogen in NaH?

  -1
• What is the oxidation number of hydrogen in CaH2?

  -1
• Combined fluorine has an oxidation number of -1 in a compound
• What are the oxidation numbers of fluorine and oxygen in the compound F2O?

  • F = -1
  • O = +2
• The sum of the oxidation numbers in a compound is equal to the overall charge
• Roman numerals show the oxidation numbers of ions with changing oxidation numbers ( eg the transition metals )
• What are the oxidation numbers in the Nitrate (III) ion And what is its symbol?

  • N = +3
  • O = -2
  • NO2-
• What are the oxidation numbers in the Nitrate (V) ion and what is it’s symbol?

  • N= +5
  • O = -2
  • NO3-
• When writing oxidation numbers where does the positive/negative sign go?

  in front
• Define oxidation

  Oxidation is the loss of electrons and increase in oxidation number
• Define reduction

  Reduction is the gain of electrons and decrease in oxidation number
• Define redox reaction

  A reaction in which oxidation and reduction occur simultaneously
• Define oxidising agent

  The substance that accepts electrons and is reduced
• Define reducing agent

  The substance that donates electrons and is oxidised
• Metal + acid → salt + hydrogen
• What is happening at a?

  hydrogen is reduced

  
• What is happening at B?

  aluminium is oxidised

  
• What is the oxidation state of aluminium at c?

  0

  
• What is the oxidation state of hydrogen at d?

  +1

  
• What is the oxidation state of aluminium at e?

  +3

  
• What is the oxidation state of hydrogen at f?

  0

  
• Describe what is happening to aluminium in the equation:

  • aluminium has been oxidised as its oxidation number has changed from 0 to +3 meaning it has lost 3 electrons

  
• Describe what is happening to hydrogen in the equation:

  • hydrogen has been reduced as its oxidation number has changed from +1 to 0 meaning it has gained 1 electron