Electrochemistry deals with the conversion of electrical energy into chemical energy and vice versa. When electric current is passed through an aqueous solution of certain substances or through molten salts, it causes a chemical reaction to occur. On the other hand, in dry cells, button cells or lead acid batteries chemical reactions occur which produce electrical energy. In this lesson you will study some aspects of these processes.
OBJECTIVES
After seen this video, you will be able to:
- understand oxidation and reduction in terms of electron transfer concept;
- calculate oxidation number (ON) of an atom in a molecule or ion;
- balance the chemical equation for redox reaction;
- explain electrolytic conduction, conductance and molar conductivity;
- describe the effect of dilution on conductivity and molar conductivity of an electrolyte; z
- differentiate between electrolytic and Galvanic cell;
- state Faraday’s laws of electrolysis;
- predict and justify the products of electrolysis of some common electrolytes;
- state standard electrode potential and use it for calculation of standard electrode potential of a cell;
- explain standard Hydrogen electrode;
- describe electrochemical series and its application;
- state effect of concentration on electrode potential (Nernst equation);
- solve numericals based on Nernst equation and
- find relationship between emf and Gibbs energy change.
OXIDATION AND REDUCTION AS ELECTRON TRANSFER PROCESS
Oxidation and reduction reactions constitute a very important class of chemical reaction. The electronic concept looks at oxidation and reduction in terms of electron transfer : process in which an atom or ion looses one or more electron to the other is called oxidation and the process in which an atom or ion gains one or more electron is termed as reduction. In the formation of NaCl from Na and Cl.
Na → Na+ + e– (loss of e– by Na; oxidation)
Cl + e– → Cl– (gain of e– by Cl; reduction)
Sodium undergoes oxidation and chlorine undergoes reduction. Here, sodium helps chlorine to undergo reduction and therefore it is called a reducing agent or reductant.
A reductant is a species in a chemical reaction which looses its electron to another reactant. Chlorine, on the other hand accepts electron, therefore it is an oxidising agent or oxidant. An oxidant is a species which accepts electrons in a chemical reaction.
It may be noted that oxidation and reduction processes do not take place independently but occur simultaneously and are thus called oxidation-reduction reaction or redox reactions. A redox reaction is a sum of oxidation and reduction half reactions in a chemical reaction.
OXIDATION NUMBER
It is easy to identify species undergoing oxidation or reduction in simple molecules. However, in polyatomic molecules, it is difficult to do the same. In the example of NaCl taken earlier it was easy to identify as sodium undergoing oxidation and chlorine undergoing reduction but in the reaction involving ferrous sulphate with potassium permanganate (KMnO4 ) it is difficult. Therefore, a new term called Oxidation number has been introduced, Oxidation number is the apparent charge which an atom appears to have when each pair of electrons is counted with more elecronegative atom. Oxidation number is always assigned to an atom. It is a number written with +ve or – ve sign. The number indicates the number of electrons that has been shifted from an atom towards a more electro-negative atom, in a hetronuclear covalent bond. The +ve sign for the atom shifting its electron away from itself and –ve is given to more electro –ve atom. The concept of Oxidation Number is based on the assumption that in a polyatomic covalent bonding, shared pair of electrons belongs to more electro–ve atom. Oxidation state (OS) is also used for Oxidation Number.
Rules for Assigning Oxidation Number
There are certain rules that are followed for computing the oxidation number of an atom in a molecule or ion.
BALANCING REDOX REACTION
The redox reaction can be balanced by any of the following methods :
(a) Oxidation number method.
(b) Ion electron method.
Balancing by Oxidation Number method
The steps involved in balancing redox reactions by this method are as follows:
1. Write the skeletal equation of reaction i.e. chemical equation without the stoichiometric coefficient.
2. Write the oxidation number of each atom above its symbol in the equation.
3. Identify the atoms undergoing change in oxidation number.
4. Calculate the increase or decrease in oxidation number per atom for the atom undergoing a change in oxidation number. If more than one atom is involved, multiply the increase or decrease in number with the number of atoms undergoing the change to determine the total change in oxidation number.
5. Equate the increase and decrease in oxidation number on the reactant side by multiplying the formulae of the oxidising and reducing agents suitably.
6. Balance the equation with respect to all the atoms except hydrogen and oxygen.
7. Finally balance H and O also.
8. If the reaction is taking place in acidic medium balance the O atoms by adding required number of H2 O molecule on the side where O atoms are less in number. Balance the H atoms by adding H+ to the side deficient in H atoms.
9. In the basic medium by add required number of negative charges by adding required number of OH– ions to the side deficient in the magnitude of charges, then add H2 O molecules to balance OH– ions.
For example : When Phosphorus is treated with nitric acid, nitric oxide is formed.
Balancing by Ion Electron Method
This method is based on the principle that electrons lost during oxidation half reaction is equal to the electrons gained in the reduction half reaction. The steps involved are
1. Write the skeleton equation.
2. Write the oxidation number of all the atoms above their symbols in the skeletal equation.
3. Find the atoms undergoing change in Oxidation Number. Thus find out the species getting oxidised and reduced respectively.
4. Split the whole (net) equation into two half reactions i.e. oxidation half reaction and reduction half reaction.
5. Balance the atoms, undergoing change in oxidation number in each half reaction.
6. Calculate the total change in oxidation number in each half reaction which is equal to total number of electron transfer.
7. Add total number of electron transfer as calculated above on the reactant side in reduction half and on the right hand side on the oxidation half reaction.
8. Balance the charges by adding H+ (for reactions in acidic medium) or OH– (reactions basic medium) either on left or right of equation.
9. Finally balance H and O by adding H2 O on the required side of the reaction.
10. Add the two half reactions such that total number of electrons cancel out on both sides. To do so half reactions may be required to multiplied by some numbers to make the number of electrons equal on both sides.
Example of Balancing
Example 13.1 : Balance the following skeletal reaction by ion electron method
Step VI & VII : Write the total number of electron transfer taking place. Here each atom undergoes change in ON by 3 therefore two Cr atoms undergoes change in Oxidation Number by 6.
ELECTROLYTIC CONDUCTION
When electricity is passed through an aqueous solution, it may or may not conduct current. The chemical substances whose aqueous solutions conduct electricity are called electrolytes and those which do not conduct current are called as nonelectrolytes. This phenomenon of conduction of current through a solution is called electrolytic conduction.
Electrolytic conduction takes place due to the movement of cations and anions in a solution. The electrical conductance of a solution, depends upon (a) nature of solute (b) valency of its ion, (c) the concentration in solution and (d) the temperature. In this section we will learn about various ways of expressing the conductance of electrolytes and the factors affecting them.
Conductance and Conductivity
Like solid conductors, electrolytic solutions also obey Ohm’s Law. When a current of I amperes flows through a solution which offers a resistance of R ohms and a potential difference of V volts is applied, then according to ohm’s law.
V = I . R
If the solution is taken in a conductivity cell which has two parallel electrodes l cm apart and each having an area of cross section A cm2 , the resistance R of the electrolyte is found to be directly proportional to l and inversely proportional to A i.e.
Where ρ “rho” is a constant of proportionality and is called specific resistance or resistivity. It is characteristic of the nature of electrolyte, its concentration and temperature.
In case of solutions, it is preferred to discuss their conductance and conductivity rather than their resistance and specific resistance. The conductance is reciprocal of resistance and the conductivity is reciprocal of specific resistance.
Conductance is denoted by L and is measured in the unit of ohm–1 which has now been named as siemens, S. The conductivity is denoted by k “kappa”. Thus by definition.
L = 1 /R and k = 1/ ρ
The units of k can be worked out from relation (i) as under :
The inverse of (i) is,
Measurement of Conductance
The conductance of an electolyte is measured with the help of a conductivity cell. Conductivity cell is a device which has two parallel platinum electrodes coated with platinum black.
Molar Conductivity
The electrolytic conductivity of a solution depends on the concentration of the electrolyte in the solution. Therefore, the conductivity of an electrolyte is normally expressed as molar conductivity.
Molar conductivity is the conducting power of all the ions furnished by one mole of an electrolyte in a solution of specified concentration.
FACTORS AFFECTING CONDUCTIVITY
As mentioned the conductivity of an electrolyte depends upon the following aspects of the electrolyte.
(a) Nature of Electrolyte : Conductivity of an electrolyte depends upon the nature of electrolyte on the following points :
(i) Weak or strong electrolyte : A weak electrolyte furnishes fewer ions therefore it has lower conductivity than a strong electrolyte of same concentration.
(ii) Valency of the ions : The ions with higher valency carry more charge and therefore they conduct more charge than the ion of lower valency. Thus higher the valency of the ion greater is the conducting power.
(iii) Speed of the ion : The ion which can move faster will carry the charge also faster and therefore has more conducting power.
(b) Temperature : Conductivity of an electrolyte generally increases by 2–3 percent for each degree rise in temperature. With increase in temperature the viscosity of the solvent decreases and thus ion can move faster. In case of weak electrolyte, when the temperature is increased its degree of dissociation increases, thus conductivity increases.
(c) Concentration :
(i) Variation of conductvity (k) with concentration. When the solution is diluted its conductivity also decreases. It is because k is the conducting power of all the ions present per cm3 of the solution. When the solution is diluted the number of ions per cm3 also decreases, hence k decreases.
Variation of Molar and Equivalent conductivity with concentration: As the solution is diluted its molar conductivity increases. Λm is given as
ELECTROCHEMISTRY SHORT NOTES :
It is the study of production of electricity from energy released during spontaneous
chemical reaction and the used of electrical energy to bring about non-spontaneous
chemical transformation.
Types of Cell:
- Electrochemical cell
- Electrolytic cell
Electrode potential: Potential difference develop between the electrode and electrolytic is called electrode potential.
Standard electrode potential: When the conc. of all the species involve in a half cell is unity than the electrode potential is known as standard electrode potential. According to IUPAC convention standard reduction are called standard electrode potential.
Anode: In a galvanic cell in which oxidation take place is called anode.
Cathode: In a galvanic cell in which reduction take place is called cathode.
Cell potential: The potential difference between the two electrodes of a galvanic cell is called potential.
- Nernst Equation:
➢ Measurement of the Conductivity of ionic solution: For measuring the resistance of an ionic solution we face two problem.
1. Passing direct current changes the composition of solution.
2. A solution cannot be connecting like a metallic wire.
- The first difficulty is resolved by using an alternating current (AC) source by power.
- The second problem is solved by using a specially design vessel called conductivity cell.
➢ Faraday’s laws of Electrolysis:
