Saturday, 4 December 2010

Chemistry : Chemical Cell I

Simple Chemical cells
Primary and secondary cells refers to not rechargeable and rechargeable cells.
Anode is the electrode from which e- flows into the external circuit.
Cathode is the electrode from which e- flows from the external circuit.
Electrolyte is the medium to allow ionic conduction between the two electrodes.
Capacity is the quantity of electricity delivered under certain conditions, with unit mAh.
Discharge is the conversion from chemical energy to electrical energy. There’re two typical discharge curve “flat”: voltage remains steady during discharge; “sloping” voltage drop gradually during discharge.
Service life is the useful period of the cell before the voltage drops to a certain value (0.8V).
Cycle life refers to the number of recharge available before it can deliver sufficient energy by a secondary cell.
Shell life is the duration of storage for a (primary) cell to maintain the same capacity.
A simple chemical cell model:
Mg and Cu strip in a CuSO4 solution/inserted in a lemon/put a filter paper soaked with NaCl and put between the two metals, and connected to the external circuit.
In all three cases:
-          Mg dissolves in the solution, that is, Mg→Mg2++2e-.
-          e- from Mg goes to the external circuit and flows to the Cu strip. Excess e- in Cu strip is obtained by the Cu2+, and they are reduced to metal form: Cu2++2e-→Cu
-          Overall equation: Cu2++Mg→Mg2++Cu, the e- flow produces current.
-          Identifying: Mg is anode, Cu is cathode, and CuSO4 is the electrolyte.
Voltage obtained by the cell is given by the difference of position in electrochemical series. Larger difference gives larger voltage. Note that the negative terminal of voltmeter is connected to cathode if the voltage is negative.
However there’s direct displacement between Mg and Cu in the above cells, then less mobile e- flows to the circuit, which make the cells less efficient.
We improve the cells by separating two half cells, containing different solution and one electrode, connected by a salt bridge (usually filter paper soaked with KNO3 or NaCl)
Roles of salt bridge:
-          Allow ions to move between the two half cells to complete the circuit.
-          Provide ions for the half cells to prevent excess charged ions accumulating and cause the reaction to stop.
Inert electrodes (usually graphite or Pt): they do not react with the solutions, but the chemical cells still happen. e.g., one of the beaker is Fe2(SO4)3 and another beaker is KI. 2I-→I2+2e-, the e- flows to the external circuit and goes to Fe2(SO4)3 solution, and iron(III) ion is reduced to iron(II) ion: Fe3++e-→Fe2+. We can predict the preference of losing e- by E.C.S.
Daniell cell is the first portable cells developed in 1835. The cell is constructed by a two half cells, one inside another and separated by a porous pot; Zn and ZnSO4 at anode while Cu and CuSO4 at cathode. The overall reaction is Zn+Cu2+→Zn2++Cu. Note that the porous pot does not provide ions, but it prevents direct displacement and allow ions to move through the small holes in the porous pot.

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