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Question

Write short notes on : 
(i)Nernst equation
(ii)Corrosion.


Solution

(i) Nernst equation : The effect of change in temperature on EMF of cell can be studied by this equation. 
(a) Total change in free energy of cell can be calculated by this equations.
$$\triangle G^o = -nE^oF$$
(b) Total work done by cell can be calculated
Total work $$= nE^oF$$ Nernst studied the effect of concentration of solution and temperature on the electrode potential and gave a equation called Nernst equation.
Nernst equation is $$E=E^o+\dfrac{RT}{nF}ln [M^{n+}_{(aq)}]$$
Where, E= Electrode potential, $$E^o$$ = Standard electrode potential ($$M^{n+}$$ : Conc.] 1M and temp. 298K), R = Gas Constant, T = Temperature (in K), n = Valency of metal ion F = Faraday (96,500 coulomb). Concentration of pure solid and liquid are.taken as unity.
$$E=E^o+\dfrac{0.059}{n}\log 10 [M^{n+}_{(aq)}]$$
(ii) Corrosion : Process by which the layers of undesirable compounds are formed on the surface of a metal on its exposure to atmospheric conditions are called corrosion.
Factors affecting corrosion 
(1) Nature of metal : Reactive metal corrodes readily.
(2) Impurities in metal : Impure metal corrodes quickly to a greater extent.
(3) Environment : If environment around metal contains oxygens, carbon dioxide, m oisture, saltsand acidic gases Iike $$CO_2, SO_2, SO_3$$ etc. then corrosion occurs quickly.
Method to prevent corrosion 
(1) Barrier protection : In this method, the surface of the metal is coated with paint; oil or grease. Due to this the surface of the metal remain unexposed to atmospheric conditions and hence corrosion is prevented. Surface of the metal can also be protected by : 
(i) Coating metal surface with non-corroding metals like nickel and chromium is called electroplating : 
(ii) Dipping iron article in molten metal like zinc. This process is called galvanization.
(2) Sacrificial protection : The rusting of iron can be prevented by covering the iron with more electro positive metals like zinc. Zinc metal has more tendency to get oxidized as compared to iron. So, iron articles will not be harmed till the layer of zinc present on its surface hence zinc metal is called the sacrificial metal. 
(3) Antirust solution : The alkaline antirust solution are employed  to prevent rusting alkaline solution prevent the availability of $$H^+$$ ions.
In this method, iron articles are dipped in alkaline sodium phosphate of chromate solution. Due insoluble sticking film of iron phosphate is formed on the surface which prevents rusting.

Chemistry

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