Chemsheets AS006 (Electron arrangement) 22/09/2018 ELECTROCHEMISTRY

Galvanic Cell or Voltaic Animation (2 min) https://www.youtube.com/watch?v=C26pH8kC_Wk

Electrochemical Cells spontaneous redox reaction

Zn2+(aq) + 2 e–  Zn(s)

Electrochemical Cells Cell Diagram Zn (s) + Cu2+ (aq) Cu (s) + Zn2+ (aq) [Cu2+] = 1 M & [Zn2+] = 1 M Zn (s) | Zn2+ (1 M) || Cu2+ (1 M) | Cu (s) anode cathode

Zn  Zn2+ + 2 e- oxidation Cu2+ + 2 e-  Cu reduction - electrode anode oxidation + electrode cathode reduction electron flow At this electrode the metal loses electrons and so is oxidised to metal ions. These electrons make the electrode negative. At this electrode the metal ions gain electrons and so is reduced to metal atoms. As electrons are used up, this makes the electrode positive. Zn Cu Zn  Zn2+ + 2 e- oxidation Cu2+ + 2 e-  Cu reduction

½ Equations and e- flow University Berkley (6 min) https://www.youtube.com/watch?v=0Ot2WbfIeMQ

Standard Conditions Concentration 1.0 mol dm-3 (ions involved in ½ equation) Temperature 298 K Pressure 100 kPa (if gases involved in ½ equation) Current Zero (use high resistance voltmeter)

S tandard H ydrogen E lectrode

University of Berkley S.H.E. 3 (min) https://www.youtube.com/watch?v=pfpNo10xcTw

Emf = E = Eright - Eleft

Pt(s) | H2(g) | H+(aq) || Cu2+(aq) | Cu(s)

Non-rechargeable (primary) cells – Zinc-carbon -0.80 V Zn(NH3)22+ + 2 e-  Zn + 2 NH3 +0.70 V 2 MnO2 + 2 H+ + 2 e-  Mn2O3 + H2O Standard cell Short life Determine: a) cell emf b) overall reaction during discharge

Batteries Dry cell Leclanché cell Anode: Zn (s) Zn2+ (aq) + 2e- Cathode: 2NH4 (aq) + 2MnO2 (s) + 2e- Mn2O3 (s) + 2NH3 (aq) + H2O (l) + Zn (s) + 2NH4 (aq) + 2MnO2 (s) Zn2+ (aq) + 2NH3 (aq) + H2O (l) + Mn2O3 (s)

Non-rechargeable (primary) cells – alkaline -0.76 V Zn2+ + 2 e-  Zn +0.84 V MnO2 + H2O + e-  MnO(OH) + OH- Determine: a) cell emf b) overall reaction during discharge Longer life

Non-rechargeable (primary) cells – lithium Very long life High voltage Determine: a) cell emf b) overall reaction during discharge

Batteries Mercury Battery Anode: Zn(Hg) + 2OH- (aq) ZnO (s) + H2O (l) + 2e- Cathode: HgO (s) + H2O (l) + 2e- Hg (l) + 2OH- (aq) Zn(Hg) + HgO (s) ZnO (s) + Hg (l)

Rechargeable (secondary) cells In non-rechargeable (primary) cells, the chemicals are used up so the voltage drops In rechargeable (secondary) cells the reactions are reversible – they are reversed by applying an external current. It is important that the products from the forward reaction stick to the electrodes and are not dispersed into the electrolyte.

Rechargeable (secondary) cells – Li ion +0.60 V Li+ + CoO2 + e-  LiCoO2 -3.00 V Li+ + e-  Li Rechargeable Most common rechargeable cell Determine: a) cell emf b) overall reaction during discharge c) overall reaction during re-charge

Rechargeable (secondary) cells – lead-acid +1.68 V PbO2 + 3 H+ + HSO4- + 2 e-  PbSO4 + 2 H2O -0.36 V PbSO4 + H+ + 2 e-  Pb + HSO4- Determine: a) cell emf b) overall reaction during discharge c) overall reaction during re-charge Used in sealed car batteries (6 cells giving about 12 V overall)

Rechargeable (secondary) cells – nickel-cadmium +0.52 V NiO(OH) + 2 H2O + 2 e-  Ni(OH)2 + 2 OH- -0.88 V Cd(OH)2 + 2 e-  Cd + 2 OH- Determine: a) cell emf b) overall reaction during discharge c) overall reaction during re-charge

FUEL CELLS High efficiency (more efficient than burning hydrogen) How is H2 made? Input of H2/O2 to replenish so no need to recharge +0.40 V O2 + 2 H2O + 4 e-  4 OH- -0.83 V 2 H2O + 2 e-  H2 + 2 OH- Determine: a) cell emf b) overall reaction

From wikipedia (public domain)

Batteries A FUEL CELLis an electrochemical cell that requires a continuous supply of reactants to keep functioning Anode: 2H2 (g) + 4OH- (aq) 4H2O (l) + 4e- Cathode: O2 (g) + 2H2O (l) + 4e- 4OH- (aq) 2H2 (g) + O2 (g) 2H2O (l)

Pros & cons of cells + portable source of electricity Pros & cons of non-rechargeable cells + cheap, small – waste issues Pros & cons of rechargeable cells + less waste, cheaper in long run – still some waste issues Pros & cons of fuel cells + water is only product – most H2 is made using fossil fuels, fuels cells expensive