Electrochemical, Voltaic Cell
Electro-chemical cell is a seat of electrical power. Infect the electro chemical cell is a device by which electric current energy is generated at the cost of chemical energy due to chemical action taking place in the cell electro – chemical cells are of two types (primary cells (ii) secondary cells.
Primary cell is that cell in which electrical energy is produced due to chemical energy. The chemical reaction in the cell is irreversible. For example; voltaic cell banal cell, leclanche cell dry cell etc.
Secondary cell is that cell in which the electrical energy is first strode up as chemical energy and when current is drawn from the cell them the chemical energy is reconverted into electrical energy. The chemical reaction is reversible in the cell. These cells are also known as storage cells or accumulators. Examples acid or lead accumulators and A alkali or Edison cells.
The initial cost of a primary cell is relatively low vis-à-vis a secondary cell,. But its operating cost is quite high in contrast the running cost of a secondary cell is very low in the long run.
Voltaic cell was invented by Alexandra be Volta in 1800. It consists of two rods (called electrodes) one of copper and another of zone. Partly immersed in dilute sulphuric acid (called electrolyte) contained in a glass vessel the copper rod acts as positive electrode and zinc rod acts as negative electrode.
Action of voltaic cell, in electrolyte (dilute H2SO4) due to dissociation there exist positively charged hydrogen ions (H+) and negatively charged sulphate ions (SO4) as represented below
H2SO4 --> 2H++ SO4
When the copper and zine rode are immersed into the electrolyte some of the zine atoms from zinc rod go into the solution as zine ions (Zn++)
Zn---> Zn ++ +2e-
For each zinc ion so produced two electrons are left on the zinc rod. Due to it the zinc rod attains negative charge and goes at lower potential w. r. t. to electrolyte. The positively charged zinc ions which enter the solution repel the positively charged hydrogen ions on to the copper. These hydrogen ions (H+) on reaching the copper rod extract electrons from there and become neutral hydrogen gas, leaving the copper rod positively charged due to it, the copper rod attains higher potential w. r. t. to electrolyte, inside the solution the Zn++ combine with SO4 forming ZnSO4
Zn ++ + SO4 ---> Zn SO4
This process continues till the positive charge acquired by the copper rod is sufficient or repel the positively charged hydrogen ions with a force equal in magnitude but opposite in direction to the force excreted by the positive zinc ions on them thus a maximum potential difference is set up between copper and zinc rods copper being at higher potential than zinc rod. The maximum potential of zinc rod w. r .t. electrolyte is -0.62V. The maximum V. this maximum potential difference between the two electrodes of a cell in open circuit is called its electromotive force. The e. m. f of a voltaic cell = 0.46 – (-0.62) = 1.08 V.
When the electrodes are connected to an external resistor the circuit is completed there will be flow of electrons from the negatively charged zinc rod to the positively charged copper rod through the external resistor now the conventional electric current is said to flow from copper to zinc. As the electric current is drawn from the cell, the potential difference between the electrodes is reduced to some extent now more zinc goes into solution and the original potential difference is again built up. Thus the electric current can be continuously drawn from the cell till the whole zinc is dissolved or the sulphuirc acid is consumed.
We know that in a closed circuit current flow from anode to cathode inside the cell the electric current (the motion of positive charge) is from zinc electrode to copper electrode hence the zinc rod is the anode and the copper rod is the cathode.
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