redox questions

redox questions §

• redox reactions involve transfer of one or more electrons from one species to another.

electrochemistry in action! §

• what is electrolysis
  • a process where electricity is used to make a chemical change happen that wouldn’t happen otherwise. (non-spontaneous)
• types of galvanic cells
  • primary cell, secondary cell, fuel cell.
• primary cell definition
  • non-rechargeable galvanic cells
  • contains fixed amount of oxidant and reductant, which once consumed cannot be replaced/regenerated.
• secondary cell definition
  • rechargeable.
• how are secondary cells recharged?
  • applied dc voltage forcing current through cell in opposite direction to that it spontaneously reacted during discharge.
• describe the process of recharging
  • electrolysis where electrical energy is used to force the spontaneous discharge reactions to be reversed, and in the process, regenerate oxidant and reductant originally present in the charged cell.
  • tldr; reverses the original reactions and turns the products back into its original reactants.
• drawbacks of secondary cells?
  • there is a limit to number of recharge cycle,
  • as electrodes becomes physically degraded over time, and no longer able to be recharged
• what are fuel cells.
  • oxidants and reductant are continously being fed into the cell.
  • the cell is a combustible substance: fuel.
• examples of fuel cells
  • hydrogen, methane, methanol.
• what is usually oxidant of fuel cell
  • oxygen gas
• advantages of fuel cells
  • very long life
  • theoretically operate without limit as long as the oxidant and reductant are supplied into the cell.
• what is a dry cell
  • simplest, most inexpensive of primary cells (non-rechargeable cells)
• disadvantages of dry cell
  • produces max voltage of 1.5 V
  • voltage produced slowly decreases over lifetime of cell.
  • very low energy to mass ratio (energy density)
  • cannot be recharged (primary cell)
  • ammonium ions cause zinc anode to dissolve forming Zn2+ ions, thus it has low shelf life (~1 year)
• advantages of dry cell
  • inexpensive
  • ideal for use in flashlight, portable radios and calculators where low currents are required and decreasing voltage is not an issue.
  • materials used in dry cell (Zn, MnO2, Nh4Cl and C) pose negligible environmental impact. spent dry cells are considered non-hazardous waste and can be disposed of in normal household.
• materials used in dry cell
  • Zinc Zn
  • Manganese Dioxide MnO2
  • Ammonium Chloride NH4Cl
  • Carbon C
• types of primary cells
  • dry cells
  • alkaline cell
  • silver oxide button cells
  • lithium cells
• diagram of dry cell
  • 
• oxidation half reaction for dry cell
  • Zn(s)->Zn2+ + 2e-
• reduction half reaction for dry cell
  • 2MnO2 (s) + 2NH4+ (aq) + 2e- -> Mn2O3 (s) + 2NH3 (aq) + H2O (l)
• net reaction for dry cell
  • 2MnO2 (s) + 2NH4+ (aq) + Zn (s) -> Zn2+(aq) + Mn2O3 (s) + 2NH3 (aq) + H2O (l)
• what is reducing agent in alkaline cell
  • zinc (like dry cell)
• what is the oxidising agent in alkaline cell
  • MnO2 manganese dioxide
• difference dry cell and alkaline cell, and impact
  • instead of ammonium chloride electrolyte, it uses potassium hydroxide.
    • considerable improves shelf life by eliminating effect of acidic ammonium ions that ultimately dissolve zinc anode of dry cell.
  • zinc anode is powdered zinc, instead of zinc foil in dry cell.
    • provides more surface area for chemical reactions to take place compared to foil. lowers internal resistance of cell (?)
  • greater mass of reductant, Zn and oxidant, MnO2 than dry cell of similar size.
    • higher energy density and longer operating life.
• oxidation half reaction in alkaline cell
  • Zn (s) + 2OH- (aq) -> ZnO (s) + H2O (l) + 2e-
• reduction half reaction in alkaline cell
  • 2MnO2(s) + H2O(l) + 2e- -> Mn2O3 (s) + 2OH- (aq)
• overall redox reaction:
  • Zn(s) + 2MnO2(s) -> ZnO(s) + Mn2O3 (s)
• where is alkaline cells used
  • higher current flow devices
    • toys, portable radios, cd players, electronic games & torches.
• benefits of alkaline cells
  • minimal environmental impact (same as dry cells)
    • considered non-hazardous waste.
• disadvantages
  • disposed in normal household waste stream.
  • no economic process for recycling the materials in these cells. SILVER OXIDE BUTTON CELLS LITHIUM CELLS SECONDARY CELLS BLA BLAH