Electrochemistry

Electrochemistry It is a branch of science, which deals with production of electricity from energy released during spontaneous chemical reactions and electrical energy produced is used to bring about non-spontaneous chemical transformations. The basis of these types of processes is redox reactions. 1) Redox Reactions: Redox reaction means a reaction that involves both oxidation and reduction reactions simultaneously. Oxidation is a process, which involves the loss of electrons, and reduction is a process, which involves gain of electrons.

The substance which can lose one or more electrons (i.e. get oxidized) is called reducing agent while the substance which can gain one or more electron (i.e. get reduced) is called oxidizing agent. So, in redox reactions one substance acts as reducing agent and it gets oxidized while another substance acts as oxidizing agent and gets reduced by itself. For example the reaction between zinc and copper (II) salt occurring in a battery is an example of redox reaction. In this reaction, zinc loses electrons and gets oxidized whereas Cu2+ ions gain electrons and gets oxidized.

In the above equation zinc acts as a reducing agent while Cu2+ ions act as oxidizing agent. Some other examples are:

2) Metallic and electrolytic conductance: Those substances, which allow the electric current to pass through them, are called conductors. The best conductors are metals like copper, silver, tin etc. While those substances, which don’t allow the passage of electric current through them are called non- conductors or insulators like glass, ceramic, wood, wax, rubber etc.

Types of Conductors: The conductors are classified into two categories: a) Metallic conductors or electronic conductors: These are the substances, which allow the electricity to pass through them without undergoing any chemical change. The best examples are metals and their alloys. The flow of electric current through metallic conductors is due to the flow of electrons in the metal atoms. The electronic conductance depends upon i) Nature and structure of the metal ii) Number of valence electrons per atom iii) Density of metal iv) Temperature b) Electrolytes or electrolytic conductors: These are the substances which allow the electricity to pass through them in their molten state or in the form of their aqueous solutions and undergo chemical decomposition e.g. acids, bases and salts. In this type of conduction, charge is carried by ions. Therefore it is also called ionic conductance.

Non - electrolytes are the substances, which don’t conduct electricity, either in their molten state or through their aqueous solutions e.g. sugar, glucose, ethyl alcohol, urea etc.

Classification of electrolytes: All the electrolytes don’t ionize to the same extent in solution. On this basis these electrolytes are divided into two categories. a) Strong electrolytes: These are the electrolytes, which dissociate completely into ions in the solution. e.g. NaCl, KCl, HCl, NaOH, KNO3. b) Weak electrolytes: The electrolytes which don’t dissociate completely in the solution are weak electrolytes e.g. CH3COOH, H2CO3, H3BO3, HCN, HgCl2, NH4OH etc. The extent of ionization of a weak electrolyte is expressed in terms of degree of ionization or degree of dissociation (α), which is defined as the fraction of total number of molecules of the electrolyte, which ionize in the solution. For strong electrolytes, (α) is equal to 1 and for weak electrolytes, it is less than 1.

Factors that effect electrical conductivity of electrolytic solutions: 1) Interionic interactions: Interionic interactions mean solute-solute interactions. If these interactions are strong, the extent of dissociation will be less. If dissociation is less, then number of ions available will be less. So conductivity will also be less. This factor is also responsible for the classification of the electrolytes as strong electrolytes and weak electrolytes. 2) Solvation of ions: It means solute-solvent interaction. These interactions depend upon the interactions between number of ions of solute and that of solvent. If these interactions are strong, then the ions of the solute are highly solvate i.e. strongly attracted by the solvent molecules and not able to move freely for the electrical conductance. Hence conductivity will be less. 3) Viscosity of the solvent: Viscosity of the solvent depends upon the solvent – solvent interactions. Larger the interactions, larger will be the viscosity and lesser will be the electrical conductivity. This is because ions of the solute are not able to move freely in a viscous medium. If these ions will not be able to move easily, then conductivity of the electrolyte will be less.

All the above factors decrease with increase in temperature. Therefore, the average kinetic energy of the ions of the electrolyte increases with the increases in temperature. Consequently, the conductance of solutions increases with rise in temperature. But this behaviour is totally different in the metallic conductance i.e. it decreases with the increase in temperature.

Conductivity of electrolytic (ionic) solutions also depends upon the following factors: 1. Nature of electrolyte: The conductance of an electrolyte depends upon the presence of number of ions in the solution. Greater the no. of ions in the solution, greater will be the conductance. Large numbers of ions are produced by those electrolytes, which dissociate completely. Only strong electrolytes dissociates completely. So, strong electrolytes have high conductance as compared to weak electrolytes. 2. Nature of the solvent: Electrolytes ionize more in polar solvents. So, those solvents which have greater polarity are highly ionisable. Hence conductance will also be large. 3. Size of the ions produced and their salvation: If the ions are strongly solvated i.e. attracted by solvent molecules more strongly, then their effective size will be large. Hence their conductance will decrease. 4. Concentration of the electrolytic solution: Concentration of electrolyte affects the molar conductance of the electrolytic solution. In other words the molar conductance of an electrolyte increases with decrease in concentration or increase in dilution. 5. Temperature: The conductivity of an electrolyte depends upon the temperature; it increases with the increase in temperature.

Electrolytic Conductance: Electrolytic conductance occurs when a voltage is applied to the electrode dipped into an electrolyte solution, ions of the electrolyte move and electric current flows through the electrolytic solution. This power of the electrolyte to conduct electricity is known as conductance or conductivity. Electrolytic solutions also obey Ohm’s Law just like metallic conductors. 1) Ohm’s Law: It states that the current flowing through a conductor is directly proportional to the potential difference across it i.e.

Ohm’s law can also be defined as the strength of current flowing through a conductor is directly proportional to the potential difference applied across the conductor and inversely proportional; to the resistance of the conductor. Some basic terms: a) Resistance: It is the measure of the obstruction to the flow of current. It is directly proportional to the length (l) and inversely proportional to the area of cross section (a).

b) Resistivity or specific resistance:

c) Conductance: It is a measure of the ease with which current flows through a conductor. It is expressed as ‘G.’ It is the reciprocal of the electrical resistance i.e.

c) Conductivity (Specific Conductance): The reciprocal of resistivity is called conductivity. It is represented by symbol, К (kappa). It can also be defined as the conductance of a solution of 1 cm length and having 1 sq. cm as the area of cross- section or conductivity is the conductance of one-centimeter cube of a solution of an electrolyte.

Molar Conductivity (Λm): It is defined as the conducting power of all the ions produced by dissolving one mole of an electrolyte in solution. It is denoted by Λm (lambda).

Relation between Conductivity and Molar Conductivity:

Suppose solution is diluted to 100 cc. There are now 100cm cubes of the solution. The conductance of each 1 cm cube will be its conductivity so that conductance of solution would be 100 times of its conductivity. But the solution contains 1 gram mole of the electrolyte. Therefore the measured conductance will be the molar conductivity. Thus,

Equivalent Conductivity: It is defined as the conducting power of all the ions reduced by dissolving 1 gram equivalent of an electrolyte solution. It is denoted as Λe

Variation of conductivity and molar conductivity with concentration: Both conductivity and molar conductivity change with concentration of an electrolyte. Conductivity always decreases with decrease in concentration for both weak and strong electrolyte. This is because conductance of ions is due to the presence of ions in the solution. The greater the number of ions, greater is the conductance. As with dilution, more ions are produced in the solutions so conductance also increases on dilution. Conductivity of an electrolyte always decreases with decrease in concentration for weak as well as strong electrolyte whereas molar conductivity increases with decrease in concentration. The reason can be explained as follows: •

Conductivity is the conductance of 1 cm. cube of the solution. On diluting the solution the concentration of ions per cubic.cm decreases and therefore conductivity decreases.

•

On the other hand the increase in molar conductivity on dilution is due to the fact that it is the product of conductivity (k) and the volume (V) of the solution containing 1 mole of the electrolyte.

•

On dilution conductivity decreases but volume containing 1 mole of an electrolyte increases and it is found that the increase in volume on dilution is much more than the decrease in conductivity. As a result molar conductivity increases with dilution.

•

Like molar conductivity equivalent conductivity also increases with dilution because of the increase in volume containing 1 gram equivalent of the electrolyte.

Variation of molar conductivity with concentration for strong and weak electrolyte: a) Strong Electrolyte: Molar conductivity increases slowly with dilution and there is a tendency for molar conductivity to approach a certain limiting value when the concentration approaches zero i.e. when dilution is infinite. The molar conductivity when the concentration approaches zero (infinite dilution) is called molar conductivity at infinite dilution or limiting molar conductivity. It is denoted by Λ0m.

For strong electrolytes Λm increases slowly with dilution and can be represented by

The above equation is called DEBYE HUCKEL ONSAGER equation and is found to hold good at low concentration. The variation of molar conductivity can be seen with the help of a graph plotted between Λ and C1/2

From the above graph, which is plotted for KCl and HCl , it has been noted that the variation of Λm with concentration, C1/2 is small so that the plots can be extrapolated to zero concentration. The intercept is equal to Λ0m and slope is –A. The value of constant A for a given solvent and temperature depends on the type of electrolyte i.e. the charges on the cation and anion produced on dissociation of the electrolyte in the solution. b) Weak Electrolyte: In weak electrolyte like acetic acid they have low degree of dissociation as compared to strong electrolyte. Therefore the molar conductivity is low as compared to strong electrolyte. However, the variation of Λm with C1/2 is very large and we can’t obtain molar conductance at infinite dilution (Λ0m) by extrapolation of Λm vs. C1/2 plots.

Explanation for the variation of Molar Conductivity with concentration 1) Conductance behaviour of strong electrolyte: There is no increase in the number of the ions with the dilution because the strong electrolytes are completely ionized in the solution at all concentrations. However, in concentrated solutions of strong electrolytes there are strong forces of attraction between the ions of the opposite charges called interionic forces. Due to these forces the conducting ability of the ions is less in concentrated solutions. With dilution the ions become far apart from one another and inter ionic forces decrease. As a result, molar conductivity increases with dilution. When concentration of the solution becomes very low, the interionic interaction becomes almost negligible and molar conductance approaches the limiting value called limiting molar conductivity or molar conductance at infinite dilution. 2) Conductance behaviour of weak electrolyte: The variation of Λm with dilution can be explained on the basis of number of ions in the solution. The number of ions produced by an electrolyte in solution depends upon the degree of dissociation with dilution. On increasing dilution the degree of dissociation also increases as a result molar conductance increases. The limiting value of molar conductance corresponds to degree of dissociation equal to one. It means electrolyte dissociates completely.

KOHLRAUSCH’S LAW Kohlrausch observed that the difference of Λo of different pairs of electrolytes having a common cation or anion was almost same. For example, the difference between the molar conductances of K+ & Na+ is 23.4 ohm-1 cm2mol-1 irrespective of the anion.

In the same way, the difference between the molar conductivities of chloride and nitrate ions is 4.9 ohm-1 cm2 mol-1 irrespective of the cation

Thus it may be concluded that each ion makes definite contribution to the molar conductivity at infinite dilution irrespective of their ions. So Kohlrausch can be stated as: At infinite dilution when the dissociation complete, each ion makes a definite contribution towards molar conductance of the electrolyte irrespective of the nature of the other ion with which it is associated. It means that the molar conductivity at infinite dilution for a given salt can be expressed as the sum of the individual contributions from the ions of the electrolyte. If molar conductivity for cation is represented by Λo+ and that of anion as Λo-, then the law of independent migration of ions is:

For example: For NaCl For KNO3

Λo(NaCl) Λo(KNO3)

For MgCl2

Λo(MgCl2)

For Al2(SO4)3 Λo{Al2(SO4)3}

= λo+ (Na+)+ λo-(Cl-) = λo+ (K+)+ λo-(NO3-) = =

λo+ (Mg2+)+ 2λo-(Cl-) 2λo+ (Al3+)+ 3λo-(SO42-)

Limiting molar conductivity for some ions in water at 298K

Applications of Kohlrausch’s Law 1) Calculation of Molar Conductance at Infinite Dilution for weak Electrolytes: We know that, at infinite dilution the value of limiting molar conductivity for weak electrolyte can not be determined by extrapolation of λ vs. C ½. But it can be calculated by using Kohlrausch law. For example, limiting molar conductivity for acetic acid (CH3COOH). According to kohlrausch’s law,

This equation can be obtained by the knowledge of molar conductivity at infinite dilution for some strong electrolytes. E.g. consider strong electrolytes HCl, NaCl and CH3 COONa. From Kohlrausch law, it is clear that

2) Calculation of Degree of weak Electrolyte: Molar conductance of a weak electrolyte depends upon its degree of dissociation. Higher the degree of dissociation, larger is the molar conductance. With increase in dilution, the conductance increases and at infinite dilution, the electrolyte is completely dissociated so that degree of dissociation becomes one i.e. Λ = Λo (at C →0).

3) Calculation of dissociation constant of weak electrolyte: Dissociation constant for weak electrolyte is given by

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