Electrode potential

• Why do cells and batteries need to be disposed of separately?

  -To prevent pollution of the environment by toxic or dangerous substances

  -Can recycle valuable components
• State the disadvantages of a hydrogen fuel cell

  1) Need a constant supply of fuels (fuel cells have limited lifetime)

  2) Hydrogen may need to be made using an energy source that is not 'carbon neutral'.

  Most hydrogen is derived from:

  crude oil

  electrolysis of water (which uses electricity often derived from burning fossil fuels)

  3) Hydrogen is flammable and explosive

  4) Hydrogen is difficult to store (takes up a lot of space so must be stored in pressurized vessels or solid metal hydrides)

  5) Expensive
• State the advantages of a hydrogen fuel cell

  1) The only waste product is water (NO/ SO2 etc. not produced)

  2) Doesn't need to be re-charged: can be used continuously

  3) Very efficient:

  -Hydrogen is more energy dense than petrol

  -Less energy is dissipated as heat compared to combustion
• Draw a diagram of a hydrogen fuel cell

  
• What reactions occur in an alkaline hydrogen fuel cell?

  Anode: 2H2 +4OH- à 4H2O + 4e-

  Cathode: O2 + 2H2O + 4e- à 4OH-

  Overall: 2H2 + O2 à 2H2O
• State the disadvantages of rechargeable cells

  1) Electricity for recharging the cell may come from burning (fossil) fuel in power stations which releases carbon dioxide2) Some waste issues (depends on cell used)
• State the advantages of rechargeable cells

  -Metal compounds are reused (resources are not depleted)-Less waste and environmental impact: does not leak / no landfill problems / less mining / less energy to extract metals / less waste-Cheaper than non-rechargeable cells in the long run
• How are rechargeable cells recharged?

  The reactions are reversed by running a higher voltage through the cell than the cell's emf
• What reactions occur on discharge in lithium-ion cells?

  Anode: Li à Li+ + e-

  Cathode: Li+ CoO2 +e- à Li+[CoO2]-
• What reactions occur in a nickel/cadmium battery?

  Anode: Cd(s) + 2OH- (aq) àCd(OH)2(s) + 2e-

  Cathode: NiO(OH)(s) + H2O(l ) + e- à Ni(OH)2(s) + OH-(aq)
• Give examples of rechargeable cells

  Nickel/ cadmium cells (rechargeable AA batteries)

  Lithium-ion cells (mobile phones/ laptops)
• Why are some electrochemical cells non-rechargeable?

  Reaction(s) not reversible
• What reactions occur in a lead/acid battery?

  Anode: Pb(s) + SO42-(aq) à PbSO4(s) + 2e-Cathode: PbO2(s) + 4H+(aq) + SO42-(aq)+2e- à PbSO4(s) + 2H2O(l)
• What are lead/acid batteries used as?

  Car batteries
• What are the 2 reactions that take place in zinc/ carbon cells

  Anode: Zn(s) → Zn2+(aq) + 2e-

  Cathode (carbon electrode):

  2NH4+(aq) + 2e- → 2NH3(aq) + H2(g)
• What are zinc/carbon cells more commonly known as?

  Disposable batteries
• What was the first commercial cell (Daniell cell) made from?

  Zn/Cu(II)
• What is the strongest reducing agent in the electrochemical series?

  The species on the RHS of the half cell with the most negative electrode potential (best electron donor)
• What is the strongest oxidising agent in the electrochemical series?

  The species on the LHS of the half cell with the most positive electrode potential (best electron acceptor)
• Why is a metal electrode rubbed with sandpaper prior to use?

  To remove the oxide layer
• How can the e.m.f of a cell be kept constant?

  -Reagents supplied constantly so concentrations of ions are kept constant

  -Temperature and pressure controlled
• Why might a calculated value of the e.m.f differ from an experimentally determined value?

  -Non-standard conditions

  -Current flow- this would lower the voltage

  -Side reactions (only if applies to question)
• What would happen if a voltmeter was replaced by a low resistance component/ it short circuits?

  Current would flow.

  The reactions will occur separately at each electrode.

  The e.m.f will fall to zero as the reactants are used up.
• Why is a high resistance voltmeter used in electrochemical cells to measure the emf?

  To stop current flow- this allows the maximum potential difference to be measured.
• Describe the platinum electrodes.

  Platinum electrodes are made by coating a porous ceramic material with platinum (rather than by using platinum rods) to increase the surface area
• Why are platinum electrodes used in non-metal electrodes?

  To allow transfer of electrons (good electrical conductors) and provide a reaction surface

  Platinum is inert so it won't react with the half cells
• What is the conventional representation for a redox electrode?

  Pt(s) | Xa+(aq), Xb+(aq)
• What is the conventional representation for a gas electrode?

  Pt(s) | H2(g) | H+(aq)
• What is the conventional representation for a metal electrode?

  X(s) | Xa+(aq)
• What happens at the right-hand electrode?

  Reduction(has the mostpositiveE value)
• What happens at the left-hand electrode?

  Oxidation(has the mostnegativeE value)
• What species is written furthest from the || in standard notation?

  The most reduced species
• When is a redox reaction feasible?

  A reaction is feasible when the emf is positive (ERHS is more positive than ELHS)
• How do you calculate the e.m.f of an electrochemical cell?

  RHS - LHS

  Eocell = EoRED - EoOX
• Define: electromotive force (e.m.f)

  The maximum voltage an electrochemical cell can produce. (This can only be observed when negligible current flows.)
• How can the electrode potential of a half cell be measured?

  The electrode potential of a half cell cannot be measured directly; it is determined by connecting the half cell to a standard hydrogen electrode.
• Why might you use other standard electrodes?

  They are cheaper (platinum is expensive)
• Draw the standard hydrogen electrode

  
• Why is the potential of the hydrogen electrode exactly 0.00 V under standard conditions?

  It is 0V by definition
• What is the half equation for the standard hydrogen electrode?

  2H+(aq) + 2e- → H2(g) = 0V