Electrochemistry(Types of Electrodes, Applications of EMF, Determination of pH)(Faradays law of Electrolysis, Electrolysis of NaCl, Ionic Strength)Dr.S.SURESHAssistant ProfessorEmail:avitsureshindia@gmail.com

Types of Electrodes

Different Types of ElectrodesAn electrochemical cell consists of two electrodes, positive and negative. Each electrode along with the electrolyte constitutes a half cell. The commonly used electrodes in different electrochemical cells areMetal-Metal ion electrodesMetal-Amalgam electrodesMetal insoluble metal salt electrodesGas electrodesOxidation-reduction electrodes

Metal-Metal ion electrodesAn electrode of this type consists of a metal rod (M) dipping into a solution of its metal ions(Mn+). This is represented asM/Mn+Example:Cu rod dipping in Copper sulphate solution (Cu/Cu2+)

Metal-Metal ion electrodesThe electrode reaction may be represented asMn+ + ne MIf the metal rod behaves as negative electrode (i.e the reaction involves oxidation) the equilibrium will shift to the left and hence the concentration of the Metal ions in the solution will increase. On the other hand, if the metal rod behaves as positive electrode (i.e the reaction involves reduction), and the equilibrium will shift to the right and hence the concentration of the metal ions will decrease.

Metal-Amalgam ElectrodesThe activities and the electrode potentials of highly reactive metals such as sodium, potassium, etc. are difficult to measure in aqueous solution. Hence, the activity of the metal is lowered by dilution with mercury.M(Hg)/Mn+ExampleNa, Hg (C1)/Na+(C2)

Metal-Insoluble metal salt electrodesThis type of electrode consists of a metal (M) covered by a layer of sparingly soluble salt (MX) dipping into a solution containing a common anion (X). These electrodes are represented as M/MX // X (a)Example:(a)Calomel Electode: It consists of Mercury-Mercurous chloride in contact with a solution of potassium chloride (Hg/Hg2Cl2, KCl)

Gas ElectrodesIn a gas electrode hydrogen gas is continuously bubbled through a 1M solution of the acid. A inert metal like platinum is used, since it is not attacked by acid. The inert metal in electrode does not participate in the electrode reaction but helps in making electrical contact. Let the gas bubbled be X2, then the electrode is denoted as Pt, X2 (1atm) /X+(a)Example:Standard hydrogen electrode: Pt, H2 (1atm)/H+(1M)

Oxidation-Reduction electrodes (or) Red-Ox electrodes

In this type of electrode the potential is developed due to the presence of ions of the same substance in two different valence(i.e. oxidation) states.This electrode is set up by inserting an inert metal like platinum in an appropriate solution containing a mixture of ferrous (Fe2+) and Ferric (Fe3+) ions. In this electrode the potential is due to the tendency of the ions to change from one oxidation state to the other more stable oxidation state.These electrodes are represented asPt/Mn1+ (a1), M n2+ (a2)

Applications of EMF measurements

Applications of EMF measurements

(i) Determination of valency of ion in doubtful casesThe valency of mercurous ion can be determined by determining the EMF of a concentration cell of the type given below.Hg/ Hg2(NO3)2 (C1) // Hg2(NO3)2(C2) / MercuryThe EMF of the cell, E, is given by the expression E = It was found that when C2/C1 was 10, (i.e. C2 = 1M and C1 = 0.1M) the EMF was 0.0295V. Therefore, the valency of mercurous ion is 2, and it should be represented as .

Applications of EMF measurements(ii) Determination of solubility product of sparingly soluble salt:The solubility product constant of a sparingly soluble salt is a kind of equilibrium constant. Consider the salt MX in equilibrium with its ions in a saturated solution.MX(s) +The solubility product of the salt is given by Ksp = [M+] [X]

The cell is represented asM, M+ X(sat.sol.) // MX(s), MR.H.E MX(s) + e M + XL.H.E M + eOverall reaction MX(s) + E = We know that G = nFEAnd G = 2.303 RT log Ksp log Ksp = E = =

(iii)Free energy, enthalpy and entropy changes in electrochemical reactions:The standard free energy can be calculated as followsG = nFEn= number of electrons, F = 96500 C, E = EMF of the cell.By knowing the standard EMF of a cell we can calculate the entropy change using the equationS = n F Then the enthalpy change can be calculated using the equation G = H TS

Determination of pH

Determination of pHBy using a glass electrode:It consists of thin walled glass bulb (made of special type of glass having low melting point and high electrical conductivity) containing a Pt wire in 0.1M HCl. The thin walled glass bulb functions as an ion-exchange resin, and an equilibrium is set up between the sodium ions of glass and hydrogen ions in solution. The potential difference varies with the hydrogen ion concentration, and it given by

Determination of pH by using a glass electrodeThe cell may be represented as Pt, 0.1M HCl/ Glass// KCl(Satd. Soln.)/ Hg2Cl2(s). HgThe EMF of the cell is measured byECell = ECell = = 0.2422 V = 0.2422 V pH =

Faradays Law of Electrolysis

Faradays Law of ElectrolysisFirst Law: According to it during electrolysis, the amount of any substance deposited or evolved at any electrode is proportional to the quantity of electricity passedThe quantity of electricity (Q) is equal to the product of the current strength and the time for which it is passed.Q = current strength time

Faradays Law of ElectrolysisIf W is the weight of a substance liberated/deposited at an electrode during electrolysis, then from first law, we get:W QBut Q = itW itW = Zit

Faradays Law of ElectrolysisSecond Law: According to it the weights of different substance evolved/deposited by the passage of same quantity of electricity are proportional to their chemical equivalent weightsW EWhere W = Weight of the substance liberated or deposited.E = Chemical equivalent weight of the substance liberated/deposited.

Electrolysis of aqueous sodium chlorideWhen sodium chloride is dissolved in water, it ionises as NaCl Na+ + ClWater also dissociates asH2O H+ + OHWhen electric current is passed through aqueous sodium chloride solution using platinum electrodes

Electrolysis of aqueous sodium chlorideH+ ions move towards the cathode. The H+ ions gain electrons and change into neutral atoms. Hydrogen atom is unstable and combines with another atom to form stable hydrogen molecule.Hydrogen atom is unstable and combines with another atom to form stable hydrogen molecule H+ + e H (atom unstable)H + H H2 (stable)

Electrolysis of aqueous sodium chlorideCl ions move towards anode. These Cl ions lose electrons and change into neutral atoms, chlorine atom is unstable and combines with another atom to form stable chlorine molecule.At Anode:Cl e Cl atom (unstable)Cl + Cl Cl2 (stable)Hence in the electrolysis of aqueous solution of sodium chloride, hydrogen is liberated at cathode while chlorine is liberated at anode.

Electrolysis of aqueous copper sulphate solution (using pt electrode)

When copper sulphate is dissolved in water, it ionizes as CuSO4 Cu2+ + SO42 H2O H+ + OH (slightly ionized)When electric current is passed through copper sulphate solution using platinum (Pt) electrodes:(a) Cu2+ ions move towards cathode. These Cu2+ ions gain electrons and change into neutral atoms and get deposited at cathode.Cu2+ + 2e Cu (deposited)

At CathodeOH ions move towards anode. These OH ions lose electrons and change into neutral hydroxyl groups.At Anode: 2OH e 2OH (neutral)The neutral hydroxyl groups being unstable react with other neutral OH groups to form water and oxygen. 2OH H2O + O O + O O2Hence during electrolysis of copper sulphate solution using platinum electrodes, copper and oxygen are liberated.

Ionic Strength

Ionic strength of solutionsIonic strength is a measure of the concentration of ions in that solution. Ionic strength may be expressed as

Ionic strength Calculate the ionic strength of 0.1M solution of NaCl

For Na+ C = 0.1 and Z = 1For Cl C = 0.1 and Z = 1

Ionic strength Calculate the ionic strength of 0.1M solution of Na2SO4, MgCl2 and MgSO4 respectively.Na2SO4 MgCl2 MgSO4

Ionic strength The ionic strength of a solution containing more than one electrolyte is the sum of the ionic strength of the individual salts comprising the solution.Hence, the ionic strength of a solution containing Na2SO4, MgCl2 and MgSO4 each at a concentration of 0.1M is 1.0.