Transcript Document
Chemistry
Session
Electrochemistry - 3
Session Objective
• Nernst equation • Equilibrium constant and Nernst equation • Primary cell(Batteries) • Secondary cell(Batteries) • Fuel Cell • Corrosion and its prevention.
Nernst Equation
For a general reduction reaction, M n ne M(s) The Nernst equation can be written as E M n+ /M = E 0 M n+ /M - 2.303
RT nF log [M 1 n+ ] E M n+ /M = E o M n+ /M 0.059
log n 1 M n+ (At 298K) Where n = Number of electrons involved [M n+ ] = molar concentrations at 298K
Illustrative Example
Calculate the electrode potential at a copper electrode dipped in a 0.1M solution of copper sulphate at 25 0 C . The standard potential of Cu 2+ /Cu system is 0.34 volt at 298 K.
Solution:
Cu 2+ + 2e Cu We know that E Cu 2 / Cu E 0 Cu 2 / Cu 0.0591
n Putting the values of E 0 red E red 0.34
0.0591
2 0.34
10 2 2 0.31045 volt
Equilibrium constant from Nernst equation
Consider the following cell reaction in equilibrium then Zn 2+ + Cu(s) Then Nernst equation is: 2.303RT
2F log 2.303RT
2F log Reduction half reaction Equilibrium point At equilibrium Therefore, = K c E (cell) = 0 2.303RT
2F 2.303RT
2F logK logK c Oxidation half reaction Progress of reaction
Illustrative Example
Calculate the equilibrium constant of the reaction: Cu(s)+2Ag + (aq.) Cu 2+ (aq.)+Cu(s) E 0 =0.46 V
Solution :
E (cell) = 0.059
2 0.46 ×2 0.059
log K c = 15.6
K = 4×10 15
Electrochemical cell and Gibbs Energy of the reaction
e.g. for cell reaction If the reactants are in standard state then, 0 r r 0 r = -nF × RT nF lnK r
Illustrative Example
Calculate D G° for Zn-Cu cell at standard state conditions E o Zn 2+ /Zn E 0 Cu 2+ /Cu
Solution
E cell E Cu 2 / Cu E Zn 2 / Zn = + 0.34 V – (–0.76 V) = 1.10 V D G ° = –nFE° = 2×96500×1.10
= –212.3 kJ mol – 1
Commercial Cells
Primary Cell Dry Cell
Commercial Cells
The oxidation taking place at the negative zinc electrode. Anode: Zn(s) The reduction takes place at positive electrode Cathode: 2MnO +H O + 2e 2 2 Mn O + 2OH 2 3 The net cell reaction is 2 2 Zn 2 Mn O 2 3 The emf of the cell is about 1.45 V.
2OH
Secondary Cells
Lead storage battery Net cell reaction is reversible . Hence, it can be recharged .
Lead storage battery
At anode: 2 4 PbSO 4 At cathode: 2 4 Overall reaction: 2 4 + 2e 2 2
Secondary Cell
In the above equation H 2 SO 4 is used up during the discharge.During recharging the reactions are the reverse of those that occurs during discharge.
At cathode: PbSO 4 2e SO 2 4 At anode: PbO 2 SO 2 4 2e Overall reaction: 2SO 2 4
Fuel Cells
Galvanic cells which converts energy of combustion of fuel like hydrogen, methane and methanol etc. directly into electrical energy are called fuel cells.
Example
One of the most successful fuel cell uses hydrogen and oxygen reaction to form water.
At cathode: O 2 (g)+2H 2 O(l)+4e 4OH (aq.) At anode: 4H 2 O(l)+4e 2H 2 + 4OH (aq.) Overall cell reaction is: 2H 2 (g)+O 2 (g) 2H 2 O(l) Efficiency of fuel cell is 70% much more as compared thermal plants(40%).
Corrosion
Process of slowly eating away of the metal due to attack of atmospheric gases on the surface of the metal.
Examples of corrosion • • • Rusting of iron Tarnishing of silver Development of green coating on copper and bronze, etc.
Corrosion
Methods of preventing corrosion
• • • Barrier protection Using anti rust solutions Sacrificial protection For example iron surface is covered with a metal which has higher tendency to get oxidized (larger negative value of standard reduction potential) than iron.
Zinc is used for covering iron and the process is called galvanization.
Illustrative Example
The standard reduction potentials of Sn +2 /Sn and Zn +2 /Zn are respectively –0.14V, -0.76V. Predict whether the corrosion of tin can be prevented by coating with zinc or not.
Solution :
Zinc lies above tin in the electrochemical series, therefore it has a lower reduction potential than tin.This property is employed to prevent corrosion of tin by coating it with zinc as zinc acts as a sacrificial electrode.