1.07 Oxidation, Reduction and Redox

• What is an oxidation number?

  The number of electrons an atom uses to bond with any other atom
• What is the oxidation number of an uncombined element such as C, H, or O2?

  0
• What is the oxidation number of combined oxygen such as in H2O?

  -2
• What is the oxidation number of oxygen in peroxides e.g H2O2?

  -1
• What is the oxidation number of combined hydrogen such as in NH3 or H2S?

  +1
• What is the oxidation number of hydrogen in metal hydrides such as LiH?

  -1
• What is the oxidation number of combined fluorine such as in NaF, CaF2?

  -1
• When an element has more than one stable oxidation number, how is it indicated?

  It is written as a roman numeral
• What are oxyanions?

  Negative ions that have an element along with oxygen
• What is the oxidation number of S in $SO_4^{\ \ 2-}$?


  +6

  • Because combined oxygen has an oxidation number of -2
  • 4 x -2 =-8
  • The charge on the compound is -2. Sum of oxidation numbers must equal -2
  • So -2 - (-8) = +6
• Define oxidation in terms of electron transfer and oxidation number

  Oxidation is:

  • Loss of electrons
  • Increase in oxidation number
• Define reduction in terms of electron transfer and oxidation number

  Reduction is:

  • Gain of electrons
  • Decrease in oxidation number
• What is a redox reaction?

  A reaction in which both oxidation and reduction takes place
• What is the oxidation number of a metal?

  0, because it is an uncombined element
• How do you form half equations?

  1. Balance all species excluding oxygen and hydrogen
  2. Balance oxygen using H2O
  3. Balance hydrogen using H+ ions
  4. Balance charges using e-
• What is an example of forming a half equation?


  .

  
• How do you combine half equations?

  In order to combine half equations, make sure the number of electrons are the same for both half equations to determine the overall redox reaction
• What is an oxidising agent?

  Accepts electrons from the species that is being oxidised.

  • Therefore, it gains electrons and is reduced, which is shown by a decrease in oxidation number
• What is a reducing agent?

  Donates electrons to the species that is being reduced.

  • Therefore it loses electrons and is oxidised, which is shown by an increase in oxidation number