Electrochemistry

Electrochemistry

Electrochemistry deals with the inter-conversion of electrical energy and chemical energy. A flow of electricity through a substance may produce a chemical change (redox reaction)

and also a chemical change (redox reaction) may cause a flow of electricity through some external circuit. The former involves the study of electrolysis and conductance while the latter,

the measurement of electromotive force.


Electrochemical reaction are broadly of two types.

1. One in which current is produced as a result of a chemical change^- electrochemical cell.

2. The other in which chemical change results due to the passage of electric current - electrolysis.


Faraday's Laws of Electrolysis :

1st Law : The weight of a substance deposited is directly proportional to the quantity of electricity passed through the electrolyte.

W Q, where Q = I × t

W = ZQ.
Z is a constant known as electro chemical equivalent. It is the weight in grams of a substance deposited by one coulomb of electricity.



IInd Law: When the same quantity of electricity is passed through different solutions, the weights of different substances deposited at the electrode are directly proportional to their

chemical equivalent weights.

W E, or W_A/W_B = E_A / E_B

Combining the two laws, we get

W QE. or W = QE/F

Approach of the Problems:

Number of equivalents of a substance discharged at electrode = Number of Faradays Passed through the electrolyte =


Some Important Electrodes

1. Metal-metal ion electrodes:- It consists of a metal in contact with its ions in solution.

e.g. Silver metal immersed in a solution of silver nitrate^- Ag⁺ | Ag(s)


2. Gas-ion electrodes:- It uses a gas in contact with its anion or cation in solution and the gas is bubbled into the solution and electrical contact is made by means of a piece of inert

metal usually platinum.

e.g. Hydrogen - hydrogen ion electrode. It consists of platinum immersed in one molar solution of H⁺ ions and hydrogen gas from a cylinder is bubbled into a solution around it at a

pressure of 1 atm at 25^oC. It is also known as the Standard Hydrogen Electrode.

The electrode potential of any half cell can be determined by coupling it to form a cell with standard hydogen electrode which has been arbitrarily assigned zero potential.

The diagram for this electrode functioning as a cathode is H⁺ | H₂ (g) | Pt


3. Metal-insoluble salt anion electrodes:- This type of electrode consists of a metal in contact with one of its insoluble salts and also with a solution containing the anion of the salt.

e.g. Silver - Silver chloride electrode

Cl^- | AgCl(s) | Ag(s)

AgCl(s) + e^- Ag(s) + Cl^-.

In this electrode a silver wire is coated with a paste of silver chloride and immersed in the chloride ion containing solution.

4. Inert, ''oxidation reduction''; electrodes:- It consists of a strip or wire of an inert metal like platinum in contact with a solution which contain ions of a substance in two different

oxidation states.

e.g. Ferrous - Ferric electrode

Fe⁺³, Fe⁺² | Pt(s)

Where comma indicates that both ions are in the solution phase.

Nernst Equation for Electrode Potential and Cell Potential:


1. For a general electrode reaction:

Mⁿ⁺ + ne^- M

oxidised reduced

state state

E(Mⁿ⁺ | M) = E^o (Mⁿ⁺ | M) +


2. For a general redox cell reaction involving the transference of n electrons

aA + bB cC + dD, E_cell =

Note: i) Concentration of solids (metals) is taken to be unity

ii) Concentration of ionic species are taken in mol L^-1

iii) Concentration of gases are expressed in terms of their respective partial pressures inatmospheres.

iv) Value of 2.303 RT/F at 18^oC, 25^oC and 30^oC are nearly 0.058, 0.059 and 0.060 respectively.


Standard Electrode Potentials

Elements	Electrode reaction	volts
Li	Li+(aq) + e- Li(s)	-3.05
K	K+(aq) + e- K(s)	-2.93
Ba	Ba+2(aq) + 2e- Ba(s)	-2.90
Ca	Ca+2(aq) + 2e- Ca(s)	-2.87
Na	Na+(aq) + e- Na(s)	-2.71
Mg	Mg2+(aq) + 2e- Mg(s)	-2.37
Al	Al3+(aq) + 3e- Al(s)	-1.66
Zn	Zn+2(aq) + 2e- Zn(s)	-0.76
Cr	Cr3+(aq) + 3e- Cr(s)	-0.74
Fe	Fe2+(aq) + 2e- Fe(s)	-0.44
	H2O(l) + e- 1/2H2(g) + OH-(aq)	-0.41
Cd	Cd2+(aq) + 2e Cd(s)	-0.40
Pb	PbSO4(s) + 2e- Pb(s) + SO42-(aq)	-0.31
Co	Co+(aq) + 2e- Co(s)	-0.28
Ni	Ni+(aq) + 2e- Ni(s)	-0.25
Sn	Sn+(aq) + 2e- Sn(s)	-0.14
Pb	Pb+ (aq) + 2e- Pb(s)	-0.13
H2	2H+ + 2e H2(g)	0.00
Cu	Cu2+(aq) + 2e- Cu(s)	+0.34
I2	I2(s) + 2e- 2I -(aq)	+0.54
Fe	Fe3+(aq) + e- Fe2+(aq)	+0.77
Hg	+ 2e- 2Hg(l)	+0.79
Ag	Ag+(aq) + e- Ag(s)	+0.80
Hg	Hg2+(aq) + 2e- Hg(l)	+0.85
N2	NO3- + 4H+ + 3e- NO(g) + 2H2O	+0.97
Br2	Br2(aq) + 2e- 2Br-(aq)	+1.08
O2	O2(g) + 2H3O+(aq) + 2e- 3H2O	+1.23
Cr	+ 14H+ + e- 2Cr+3 + 7H2O	+1.33
Cl2	Cl2(g) + 2e- 2Cl-(aq)	+1.36
Au	Au3+(aq) + 3e- Au(s)	+1.42
Mn	MnO4-(aq) + 8H3O+(aq) + 5e- Mn+2(aq) + 12H2O(l)	+1.51
F2	F2(g) + 2e- 2F-(aq)	+2.87

Application of Nernst Equation

1. Calculation of equilibrium constant

2. Calculation of pH

3. Calculation of solubility product

4. Calculation of G, H and S

G = -nFE^o H = nF

= Temp. Coefficient



IUPAC notation of a cell


The galvanic cell is represented in a short IUPAC cell notation as follows:




It is important to note that: `
1. First of all the anode (electrode of the anode half cell) is written. In the above case, it is Zn.

2. After the anode, the electrolyte of the anode should be written with concentration. In this case it is ZnSO₄ with concentration as C₁.

3. A slash (|) is put in between the Zn rod and the electrolyte. This slash denotes a surface barrier between the two as they exist in different phases.

6. Finally we write the cathode electrode of the cathode half - cell .

7. A slash (|) between the electrolyte and the electrode in the cathode half - cell.

In case of a gas, the gas to be indicated after the electrode in case of anode and before the electrode in case of cathode. Example: Pt, H₂/H⁺ or H⁺|H₂, Pt.