Mastering Electrochemistry (Chemistry-Class XII)

Type of cells:

1. Electrochemical cell / Galvanic cell- Convert chemical energy of a spontaneous redox

reaction into electrical energy, example: Daniel cell

2. Electrolytic cell: Convert electrical energy into chemical energy, example: two copper strips

dipped in an aqueous solution of copper sulphate. Copper is dissolved (oxidised) at the anode

and is deposited (reduced) at the cathode.

Some important terms:

Electrode potential Potential difference developed between the electrode and the electrolyte

Standard electrode potential Electrode potential when the concentration of the species of the half-cell is unity

Anode Electrode where oxidation takes place

Cathode Electrode where reduction takes place

Cell potential Potential difference between the cathode and the anode

Cell electromotive force (emf) Potential difference between cathode and anode when no current is drawn

SHE ( Standard Hydrogen Electrode): It has zero potential at all temperatures.

Nernst equation: For a general electrochemical reaction

Nernst equation is given by

Relation between cell potential and Gibbs energy:

Relation between equilibrium constant and Gibbs energy:

Resistance(R) : Directly proportional to length (l) and inversely proportional to cross-sectional

area (A), unit: Ohm

Resistivity : Resistance of a substance when it is 1 metre long and has 1 m2 cross-sectional area

Conductance: Reciprocal of resistance

Specific conductance / Conductivity: Conductance of a substance when it is 1 m long and has 1

m2 cross-sectional area, its SI unit is S m

1, but is often expressed in S cm

1.

Molar conductivity: Molar conductivity of a solution at a given concentration is the

conductance of volume V of a solution containing 1 mole of the electrolyte, kept between two

electrodes with the cross-sectional area of A and distance of unit length. It is denoted as m.

Molar conductivity = Specific conductivity V

Molar conductivity,

Molar Conductivity for Strong Electrolytes:

Molar Conductivity for Weak Electrolytes: The plot of against is given below.

Kohlrausch law of independent migration of ions: This law states that the limiting molar

conductivity of an electrolyte can be represented as the sum of the individual contributions of its

anion and cation.

Faradays law of electrolysis:

First law The amount of chemical reaction which occurs at any electrode during electrolysis by a current is proportional to the quantity of electricity passed through the electrolytic solution

or melt.

Second law The amount of different substances liberated by the same quantity of electricity passing through the electrolytic solution is proportional to their chemical equivalent weights.

Q (coulomb) = I (ampere) t (second)

1F (Faraday) = 96487 C mol1

96500 C mol1

Types of electrodes:

1. Inert: They act only as source or sink for electrons instead of participating in the reaction.

Example: Pt, Au

2. Active: They participate in the reaction.

Batteries: Battery is a galvanic cell in which chemical energy of the redox reaction is converted

into electrical energy.

Types of batteries:

1. Primary Batteries- In primary batteries, reaction occurs only once and cannot be reused.

Example: Dry cell (or Leclanche cell), Mercury cell

2. Secondary Batteries- Secondary batteries can be recharged again by passing current through

them in the opposite direction. Example: Lead storage battery, Nickel-cadmium cell

Corrosion: It is the process of slow conversion of metals into their oxides (undesirable) by the

action of moisture, oxygen and other gases of the atmosphere. In the process of corrosion, metals

lose electrons to oxygen and get oxidised.

Hydrogen Economy- Use of hydrogen as a renewable and non-polluting source of energy

Questions asked previously

Q. Express the relation among cell constant, resistance of the solution in the cell and

conductivity of the solution. How is molar conductivity of a solution related to its conductivity?

(2012 Delhi Set 3)

Q. The molar conductivity of a 1.5M solution of an electrolyte is found to be 138.9 S cm2 mol

-1.

Calculate the conductivity of this solution? (2012 Delhi Set 3)

Q. The electrical resistance of a column of 0.05M NaOH solution of diameter 1 cm and length

50 cm is 5.55103 ohm. Calculate its resistivity, conductivity and molar conductivity? (2012

Delhi Set 3)

Q. Express the relation between conductivity and molar conductivity of a solution held in a cell?

(2011 Delhi Set1)

Q. The chemistry of corrosion of iron is essentially an electrochemical phenomenon. Explain the

reactions occurring during the corrosion of iron in the atmosphere? (2011 Delhi Set 1)

Q. Determine the values of Go for the following reaction:

Ni(s) + 2Ag+(aq) Ni

2+(aq) + 2Ag(s) E

o = 1.05V ( 1F = 96500 C mol

-) (Delhi 2011 Set 1)

All the best,

Team Meritnation!

8 comments March 7th, 2013

Mastering Solutions (Class XII-Chemistry)

Laws to remember:

Raoults law: For a solution of volatile liquids, the partial vapour pressure of each

component in the solution is directly proportional to its mole fraction. p1 = x1p1

Daltons law of partial pressure: The total pressure over the solution phase in the container is the sum of the partial pressures of the components of the solution. ptotal = p1

+ p2

Henrys law: Mole fraction of a gas in solution is proportional to the partial pressure of

the gas over the solution. p = Khx

Formulae for numericals:

Mass % = (Mass of component in solution/ Total mass of the solution) * 100

Volume % = (Volume of component in solution/ Total volume of the solution) * 100

ppm =(No. of parts of the component/ Total no. of parts of all components of solution) * 100

Mole fraction = No. of moles of component/ Total no. of moles of all components

Molarity = (Moles of solute/ volume of solution in litres)

Molality = Moles of solute/ Mass of solvent in kg

Relative Lowering of vapour pressure

Elevation of boiling point

Depression of freezing point

Osmosis and Osmotic Pressure

vant Hoff Factor

Important graphs:

Questions which have been asked previously:

2007 Set-1 Delhi

Q. Define osmotic pressure.

Q. An antifreeze solution is prepared from 222.6 g of ethylene glycol [C2H4(OH)2] and 200 g of

water. Calculate the molality of the solution. If the density of this solution be 1.072 g mL1, what will be the molarity of the solution?

2008 Set-2 Delhi

Q. State Raoults law for solutions of volatile liquids. Taking suitable examples explain the meaning of positive and negative deviations from Raoults law.

Q. Define the term osmotic pressure. Describe how the molecular mass of a substance can be

determined by a method based on measurement of osmotic pressure?

2009 Set-1 Delhi

Q. Differentiate between molality and molarity of a solution. What is the effect of change in

temperature of a solution on its molality and molarity?

Q. 100 mg of a protein is dissolved in just enough water to make 10.0 mL of solution. If this

solution has an osmotic pressure of 13.3 mm Hg at 25C, what is the molar mass of the protein?

2010 Set-3 Delhi

Q. Non-ideal solutions exhibit either positive or negative deviations from Raoults law. What are these deviations and why are they caused? Explain with one example for each type.

Q. A solution prepared by dissolving 1.25 g of oil of winter green (methyl salicylate) in 99.0 g of

benzene has a boiling point of 80.31 C. Determine the molar mass of this compound. (B.P. of

pure benzene = 80.10 C and Kb for benzene = 2.53 C kg mol)

2011 Set-1 Delhi

Q. Differentiate between molarity and molality for a solution. How does a change in temperature

influence their values? Calculate the freezing point of an aqueous solution containing 10.50 g of

MgBr2 in 200 g of water. (Molar mass of MgBr2 = 184 g) (Kf for water = 1.86 K kg mol)

Q. Define the terms osmosis and osmotic pressure. Is the osmotic pressure of a solution a

colligative property? Explain. Calculate the boiling point of a solution prepared by adding 15.00

g of NaCl to 250.0 g of water. (Kb for water = 0.512 K kg mol), Molar mass of NaCl = 58.44 g)

2012 Set-1 Delhi

Q. A 1.00 molal aqueous solution of trichloroacetic acid (CCl3COOH) is heated to its boiling

point. the solution has the boiling point of 100.18 oC. Determine the vant Hoff factor for

trichloroacetic acid. (Kb for water = 0.512 kgmol-1

)