CHEM 205 lecture 6

• pH


  Concentration scale for acids and bases
• Base


  pOH = -log [OH-]
• Calculating pH for 0.635 M NaOH

  1. [OH-] = 6.35x10-1 M

  2. pOH = -log (6.35x10-1) = 0.197

  3. pH = 14 - 0.197 = 13.803
• Acid


  pH = -log [H3O+]
• Calculating pH for 1.0 M HCl


  1. [H+] = 1.0 M

  2. pH = -log (1.0x100) = 0.00
• Gas-forming reactions


  Net formation of gas, redox (e- transfer)
• Gas-forming reactions


  Certain metal + acid → dissolved metals + gas
• Oxidation-reduction reactions


  Electron transfer
• Oxidation


  Loss of electrons, gaining oxygen
• Reduction


  Gain of electrons, loss of oxygen
• How to know if electron transfer: Compounds formed from elements or vice-versa, but can also involved compounds
• Oxidation state


  • Single free element/atom: 0
  • Monoatomic ion: their charge (if - charge = extra electron, if + charge = missing electron)
  • Covalent bond, non-polar: 0 = equal sharing between atoms, no charges
  • Fluorine: -1
  • Oxygen: -2, unless bonded to fluorine or called peroxide/superoxide
  • Hydrogens when bonded to non metals: +1
  • Hydrogens when bonded to metals: -1
  • Halogens: -1
  • Alkaline earth metals: +2
• Combustion


  Burning substances using oxygen
• Corrosion


  Deterioration of metals by electron-transfer reactions (rusting)
• Half-reaction method for balancing redox reactions

  1. Separate into reduction and oxidation half reactions

  2. Balance separately: all elements except H and O, balance changes using e-, balance O using h2o and H using H+, check net charges

  3. Multiply to get equal on all sides

  4. Add half reactions

  5. Check for balance
• Balancing redox reactions in acidic environment

  1. Balance equation

  2. Assign oxidation state

  3. Bracket half reaction (break down each molecule)

  4. Balance oxygen by adding H2O

  5. Balance hydrogen by adding H+

  6. Balance charges by adding electrons

  7. Make number electrons equal by multiplying
• Balancing redox reactions in basic environment

  1. Balance as if in an acidic solution

  2. For every H+ add an OH- to both sides of the equation

  3. Form water H+ + OH- → H2O, simplify H2O on either sides, try cancelling out as many as possible

  4. Simplify like usual

  5. Check balance