Wednesday, February 3, 2016

5. Electrochemistry - JEE Main - CBSE Class XII - Core Revision Points

Importance of  Core Revision Points: Core Revision Points are important because if you remember them strongly, many more points related to them will come out of your memory and help you to answer question and problems. Read them many times and make sure you remember them very strongly.

Sections in the Chapter

5.1 Electrochemical changes: electrolytic and galvanic cells
5.2 Electrolysis and laws of electrolysis
5.3 Metallic and electrolytic conductance
5.4 Electrolytic conduction
5.5 Factors for the variation of molar conductance
5.6 Kohlrausch’s law
5.7 Electrochemical cell or galvanic cell
5.8 Representation of an electrochemical cell
5.9 Electrode potential and E.M.F. of a galvanic cell
5.10 Standard Electrode Potential
5.11 Electrochemical series
5.12 Differences between Galvanic cell and electrolytic cell
5.13 Dependence of electrode and cell potentials on concentration: Nernst Equations
5.14 Equilibrium constant form Nernst Equation
5.15 Electrochemical cell and free energy
5.16 Some commercial cells
5.17 Electrode potential electrolysis and criteria for product formation
5.18 Corrosion
5.19 Commercial production of chemicals
5.20 Manufacture of some important metals an chemical compounds

Sections in the Chapter

5.1 Electrochemical changes: electrolytic and galvanic cells

5.2 Electrolysis and laws of electrolysis

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Quantitative Relationships in Electrolytic Cells

Determining the amount of electrical energy necessary for accumulating a given amount material from the electrolytic cell.

First law: It states that the amount of any substance that is liberated at an electrode during electrolysis is directly proportional to the quantity of electricity passed through the electrolyte.

W α Q (w = weight of substance deposited and Q is charge = ampere * time)

Second law: It states tht when the same quantity of electricity is passed through different electrolytes amount of different substances liberated or deposited at the different electrodes are directly proportional to the chemical equivalents9i.e., equivalent weight) of substances.

One faraday (F) is the amount of electrical energy required for flow of 1 mole of electrons.

To three significant digits, 1 faraday equals 96,500 coulombs(coul).

Current flow is measured in amperes (A)which is coulombs/seconds or coul/s,

Electrolytic conductance, specific, equivalent and molar conductance,

Electrolytic conductance

The flow of electric current through an electrolytic solution is known as electrolytic conduction.

Electrolytic conduction also follows Ohm's law.

V = I/R

R = ρ* l/a

ρ is called specific resistance.
The reciprocal of specific resistance is termed specific conductance. It may be defined as the conductance of a solution of 1 cm length and having 1 sq.cm as the area of cross section.

Specific conductance is the conductance of one centimetre cube of a solution of an electrolyte. It is denoted by k (kappa)

κ = 1/ρ

The equivalent conductivity of an electrolyte may be defined as the conductance of a volume of solution containing one equivalent mass of a dissolved substance when placed between two parallel electrodes which are at a unit distance apart, and large enough to contain between them the whole solution.

The molar conductivity of a solution gives the conducting power of ions produced by one molar mass of an electrolyte at any particular concentration.

It is denoted by Λm (Lambda).

Λm = κ/M

where M is the molar concentration

5.4 Electrolytic conduction
5.5 Factors for the variation of molar conductance

5.6 Kohlrausch’s law

Kohlrausch's Law on the independence of migrating ions: The molar conductivity of an electrolyte equals the sum of the molar conductivities of the cations and the anions; n = number of anions or cations.

Λ = v+Λ+ + vˉΛˉ

According to this law, the molar conductance of infinite dilution for a given salt can be expressed as the sum of the contributions from each ion of the electrolyte. If molar conductivity of the cation is denoted by Λˉ and anion by Λ+,and vˉ and v+ are number of cations and anions respectively, total molar conductance will be given by Λ.

Revision

1. Calculation of molar conductance at infinite dilution for weak electrolytes
2. Calculation of degree of dissociation of weak electrolytes

5.7 Electrochemical cell or galvanic cell

5.8 Representation of an electrochemical cell

5.9 Electrode potential and E.M.F. of a galvanic cell

The difference between the electrode potentials of the two electrodes constituting an electrochemical cell is known as electromotive force or cell potential of a cell.

5.10 Standard Electrode Potential

5.11 Electrochemical series

The electrochemical series is built up by arranging various redox equilibria in order of their standard electrode potentials (redox potentials). The most negative E° values are placed at the top of the electrochemical series, and the most positive at the bottom.

The electrochemical series

equilibrium E° (volts)
Li-3.03
K -2.92
Ca -2.87
Na -2.71
Mg -2.37
Al -1.66
Zn -0.76
Fe-0.44
Pb -0.13
H 0
Cu +0.34
Ag+0.80
Au +1.50

Remember that in terms of electrons:

OIL RIG

Oxidation is loss Reduction is gain

Reducing agents and oxidising agents

reducing agent reduces something else. That must mean that it gives electrons to it.

Magnesium is good at giving away electrons to form its ions. Magnesium must be a good reducing agent.

An oxidising agent oxidises something else. That must mean that it takes electrons from it.

Copper doesn't form its ions very readily, and its ions easily pick up electrons from somewhere to revert to metallic copper. Copper(II) ions must be good oxidising agents.

5.12 Differences between Galvanic cell and electrolytic cell

5.13 Dependence of electrode and cell potentials on concentration: Nernst Equations

Nernst Equation: The cell potential of a half cell (as well as that of a complete cell) depends upon the concentrations of involved ions, pressure of the gaseous species (if involved) and the temperature. The relation connecting them is given by the Nernst equation.

It is expressed as

E = E° - (RT/nF)ln Q°

Q° = Product of concentration (or pressure) of products each raised to the corresponding stochiometric number/Product of concentration (or pressure) of reactants each raised to the corresponding stochiometric number

n = number of electrons involved in the hall cell reaction

5.14 Equilibrium constant form Nernst Equation
5.15 Electrochemical cell and free energy
5.16 Some commercial cells
5.17 Electrode potential electrolysis and criteria for product formation
5.18 Corrosion
5.19 Commercial production of chemicals
5.20 Manufacture of some important metals an chemical compounds

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Updated 3 Feb 2016,  22 May 2015