7 Cells

Cells Electrochemical cells can be used as a commercial source of electrical energy Cells can be non-rechargeable (irreversible), rechargeable and fuel cells You should be able to work out Ecell for given half reactions.  N Goalby chemrevise.org 9 Example primary non rechargeable cells Dry Cell Zn2+(aq) + 2e-
Zn(s) E = – 0.76 V 2MnO2 (s) + 2NH4 + (aq) + 2 e- → Mn2O3 (s) + 2NH3 (aq) + H2O(l) E = 0.75 V More negative half equation will oxidise Overall reaction 2MnO2 + 2NH4 ++ Zn → Mn2O3 + 2NH3 + H2O + Zn2+ Ecell =+1.51V You do not need to learn the details of these cells. Relevant cell information will be given. You should be able to convert between standard electrode potential half cells, full cell reactions and be able to calculate potentials from given data Cells are non-rechargeable when the reactions that occur with in them are non-reversible Example secondary Nickel–cadmium cells are used to power electrical equipment such as drills and shavers. They are rechargeable cells. The electrode reactions are shown below. NiO(OH) + H2O + e-
Ni(OH)2 + OH– E = +0.52 V (Ni will reduce changing oxidation state from 3 to 2) Cd(OH)2 + 2e-
Cd + 2OH– E = –0.88 V (Cd will oxidise changing oxidation state from 0 to 2) Overall reaction discharge 2NiO(OH) + Cd + 2H2O
2Ni(OH)2 + Cd(OH)2 E= +1.40V Ecell = E red- Eox = +0.52 – – 0.88 = + 1.40 V The overall reaction would be reversed in the recharging state 2Ni(OH)2 + Cd(OH)2
2NiO(OH) + Cd + 2H2O