redox

• Oxidation definition(in terms of electrons)

  Loss of electrons.
• Reduction definition(in terms of electrons).

  Gaining of electrons.
• Oxidising agent


  Involves an increase in oxidation state-gains electron.
• Reducing agent


  Involves a decrease in oxidation number.Loses an electron.
• What are the rules for oxidation states?

  • group 1=+1
  • group 2=+2
  • Al=+3
  • H=+1(but is -1 when it is a hydride-hydrogen bonded with a metal.)
  • O=-2(but is -1 in peroxides and F compounds)
  • Cl=-1 (but is ve+ in O or F compounds)
  • elements on their own=0
  • sum of compounds in oxidation states in compound is always 0.
  • sum of oxidation states ions=charge of ion.
  • Most electronegative is negative-this is where you start from when balancing.
• State all the oxidation states in Cr ₂O²⁻ ₇

  1)Begin with O-7 oxygens that are 2-=-14 charge.

  2)Balance Cr2 to ensure all add up to 2-charge.To make it -2 you add 12 to 14.As there is two chromiums divide by 2 to get +6 for one chromium.
• What are the steps to balancing a half equation?


  1)Split ions into charges and multiply by their coefficients.

  2)If one of the charges stays the same it's a spectator ion-not included in half or ionic equation.

  3)Balance out the charges by adding electrons.Each half equation made is for each oxidation state change.

  4) To make the ionic equation first check electrons are balanced-multiply one of the reactions if you need to put all reactants on one side and put products on the other-exclude the electrons.
• How can you tell if it is an oxidation half equation?


  There is an electron lost from an atom/compound by the end of balancing the reaction.
• How can you tell if it is a reduction half-equation?


  If there is an electron gained by the end of balancing the reaction.
• How to balance a half equation using oxidation states.

  1)Balance all atoms except except for O and H.

  2)Add H2o to balance O (if needed)

  3)Add H+ to balance H(if needed)

  4)Add e- to balance out charges.

  5)Construct ionic equation using combined half equation but ensure there is equal no of electrons before making ionic equation.
• Balance  Cr ₂O²⁻ ₇.→Cr^3+


  1)Balance Cr (x2)

  2)Balance oxygen-(x 7H20)

  3)Balance Hydrogens (x 14H+)

  4)Balance charges-Cr ₂O²⁻ ₇. and 14H+ is +2 and 2Cr^3 is +6(3x2)(check 2cr^3 to cr3+ change)(H20 is neutral).Add 6e- on reactants side.
• the oxidising agent is the compound whos oxidation state increases from the reactants to products)
• the reducing agent is the compound whose oxidation state decreases from the reactants to products
• If they ask for an overall equation they mean combine both half equations
• How do you if you should balance with oxidation statess?

  they ask to balance with redox
• How to balance a compound

  most electronegative atom -negative charge

  do the other element you know

  then deduce the last element's oxidation state.
• If they ask to deduce a half equation from an original reaction don't use the coefficients from that equation,add your own when balancing.