Electrochemistry Notes - UNIT 1 2023-24 PDF

Unit – I ELECTROCHEMISTRY  Unit Syllabus ⮚Electrodes- Electrode potential, standard electrode potential. ⮚Types of electrodes – calomel, quinhydrone and glass electrode. ⮚Single electrode potential and its determination. ⮚Electrochemical cells. ⮚Nernst equation - dete...

Unit – I ELECTROCHEMISTRY  Unit Syllabus ⮚Electrodes- Electrode potential, standard electrode potential. ⮚Types of electrodes – calomel, quinhydrone and glass electrode. ⮚Single electrode potential and its determination. ⮚Electrochemical cells. ⮚Nernst equation - determination of pH of a solution by using quinhydrone and glass electrode. ⮚Electrochemical series and its applications. ⮚Batteries – Primary (dry battery) and secondary batteries (Lead – acid storage battery and Lithium ion battery) and next generation batteries  Introduction ⮚ Electrochemistry is a branch of chemistry. ⮚ It deals with the chemical reactions produced by passing electric current through an electrolyte or production of electric current through a chemical reaction Conductors Conductor is a material which allows free flow of electricity. Example: All metals, graphite, fused salts, solution of electrolytes Non-conductors (Insulators) Insulators are materials which donot conduct electrical current Example: Wood, plastics, most of non metals.  Types of conductors (i) Metallic conductors : The solid material, which conduct electric current due to the movement of electron from one end to the other end without producing chemical reaction. Examples : All metals & graphite. (ii) Electrolytic conductors : They conduct electric current due to the movement of ions from one electrode to another electrode in solution or in fused state. This process is accompanied by a chemical reaction. Examples : Metal ions dissolved solvent Origin of electrode potential When a metal (M) is placed in a solution of its own salt (Mn+) one of the two processes are possible (i) Metal atoms go into solution in the form of ions. Example : M → Mn+ + ne- (Oxidation) (ii) Metal ions from solution may deposit on the metal Mn+ + ne- → M (reduction) e- e- Electrode Potential Zn Cu Zn2+ Cu2+ Zn → Zn2+ + 2e- Cu2+ + 2e-→ Cu  Electrode Potential At equilibrium, the potential difference becomes a constant value which is known as the electrode potential of the metal. Thus the tendency of the electrode to lose electrons is called Oxidation potential and tendency of an electrode to gain electrons is called reduction potential. Single electrode potential (E) : It is the tendency of a metallic electrode to lose or gain electrons when it is in contact with a solution of its own salt. Standard electrode potential (Eo) : It is the tendency of a metallic electrode to lose or gain electrons when it is in contact with a solution of its own salt of 1M concentration at 25oC.  Types of Electrodes Reference Electrode: A reference electrode is that electrode whose potential is known and remain constant. e.g. Saturated calomel electrode (ESCE = 0.242 V) Indicator Electrode: An indicator electrode is that electrode whose potential depends on the activity of ions being titrated or estimated. e.g. To carry out acid-base potentiometric titration Hydrogen gas. Quinhydrone electrode and glass electrodes are used as indicator electrode.  Measurement of Single Electrode Potential And Its Applications. Measurement of single electrode potential It is impossible to evaluate the absolute value of a single electrode potential. Reference (or) Standard electrode The potential of unknown electrode can be measured by coupling it with another electrode, called reference electrode whose electrode potential is already known. Examples : Standard hydrogen electrode, Standard calomel electrodes. Standard hydrogen electrode (SHE) It is also called as Primary reference electrode because. The potential developed by this electrode is arbitrarily fixed as zero  Construction ⮚ It consists of a platinum foil that is connected to a platinum wire sealed in a glass tube. ⮚The Pt foil is dipped in 1M HCl. H2 gas of 1 atm pressure is passed through the side of glass tube. H 2 (g) → 2H+ + 2e- The standard electrode potential of SHE is arbitrarily fix as zero Pt , H2 (1atm) / H+(1M) ; E0 = 0 V  Limitations (or) drawbacks of SHE ⮚ It is difficult to get pure hydrogen gas. ⮚ The pressure of hydrogen is to be kept 1 atm all the time. ⮚ It is difficult to set up and transport. ⮚ Hydrogen gas reduces many ions like Ag+ and affects compounds of Hg, Ag etc ⮚ A large volume of test solution is required. ⮚ It cannot be used in solutions of redox systems, the solution may poison platinum surface.  Saturated calomel electrode (SCE) (Secondary reference electrode) ⮚ Glass tube containing pure Hg at the bottom over which mercurous chloride is placed. The remaining portion of the tube is filled with saturated solution of KCl. ⮚ The bottom of the tube is sealed with a platinum wire. The side tube is used for making electrical contact with a salt bridge. Hg | Hg2 Cl2(s) | KCl (Saturated, Solution) Eº = 0.2422V 2Hg(l) + 2Cl- → Hg2Cl2(s) + 2e- HgCl2 + 2e- → 2 Hg+ + Cl- KCl (v) 0.1N 0.3335 V 1N 0.281 V Saturated 0.2422 V Measurement of single electrode potential using a reference electrode (saturated calomel electrode) Hg 2 Cl 2 (s) → 2Hg+ (s ) + 2Cl- The emf of the cell is measured using a potentiometer. The value of Ecell = 1.0025 volt. Ecell = E oright----- Eoleft Ecell = E calo --- EoZn E oZn = E ocal ---- Eocell = + 0.2422 – 1.0025 = – 0.7603 V  Cell Terminology 1. Current: Flow of electrons through a conductor. 2. Electrode: Electrode is a material (rod, bar, strip) which conducts electrons. 3. Anode: Electrode at which oxidation occurs. 4. Cathode: Electrode at which reduction occurs. 5. Electrolyte: Water soluble substance forming ions in solution and conducts electric current 6. Anode compartment: Compartment of the cell in which the oxidation half reaction occurs. It contains the anode 7. Cathode compartment: Compartment of the cell in which the reduction half reaction occurs. It contains the cathode 8. Half–cell: It is the part of a cell, which contains an electrode dipped in an electrolyte. If oxidation occurs in this half-cell, then it is called the oxidation half cell. If reduction occurs at the cell, it is called the reduction half-cell. 9. Cell: Device consisting of two half cell. The two half cells are connected through one wire. 10. Salt bridge: Contains solutions of a salt (KCl, KNO3 or NH4NO3) that literally serve as a bridge to completed the circuit, maintain electro neutrality of electrolyte and minimize. For precise measurement of potential a salt bridge is used.  Types of cells A cell is a device consisting of two half cells. Each half cell consists of an electrode dipped in an electrolyte solution. The two half cells are connected through one wire. S.No Electrolytic cell Electrochemical cell 1 Electrical energy converted to Chemical energy converted to chemical energy. electrical energy. Example: Electrolysis, Example: Daniel Cell electroplating. 2 Anode carries +ve charge. Anode carries –ve charge 3 Cathode carries – ve charge. Cathode carries +ve charge. 4 Electrons are supplied to the cell Electrons are drawn from the cell from an external source. 5 Amount of electricity is measured Emf produced is measured by by coulometer potentiometer 6 Extent of chemical change is Emf of the cell depends on the governed by Faraday’s laws. concentration of the electrolyte and the nature of the electrode. Electrolytic cell - Example : Electrolysis of HCl. At anode : 2Cl– → Cl2 + 2e- (oxidation) At cathode : 2H+ + 2e– → H2 (reduction) Electrochemical cell - Example : Daniel cell (at anode) (at cathode) Cu2+ + Zn → Zn2+ + Cu (net cell reaction)  Components of a Cell At anode : Oxidation of Zn to Zn2+ place with the liberation of electrons. At cathode : Reduction of Cu2+ to Cu place by the acceptance of electrons. The electrons liberated in oxidation reaction flow through external wire and are consumed by the copper ions at the cathode. Salt bridge : It consists of a U-tube containing a saturated solution of KCl or (NH4)2NO3 agar–agar gel. It connects the two half cells. Functions ⮚ It eliminates liquid junction potential. ⮚ It provides a path for the flow of electrons between two half cells.  Representation (Cell diagram) 1. Galvanic cell consists of two electrodes, anode and cathode 1. Anode is written on the LHS and cathode on RHS 1. The anode is written with the metal first and then the electrolyte which are separated by a vertical line Examples : Zn/Zn2+ (or) Zn/ZnSO4 1. The cathode is written with the electrolyte first and then the metal. Examples : Cu2+/Cu (or) CuSO4/Cu 5. The two half cells are separated by a salt bridge, which is indicated by two vertical lines. Cell is represented as Zn/ZnSO4 (1M) // ) CuSO4 (1M) /Cu Nernst equation for electrode potential Ref: Sri Krishna Hitech Publishing Company Pvt. Ltd.  Quinhydrone electrode ⮚ The quinhydrone electrode is a type of redox electrode which can be used to measure the hydrogen ion concentration (pH) of a solution. ⮚ The electrode consists of an inert metal electrode (usually a platinum wire) in contact with quinhydrone crystals and a water- based solution. ⮚ Quinhydrone is slightly soluble in water, dissolving to form a mixture of two substances, quinone and hydroquinone ⮚ Each one of the two substances can easily be oxidised or reduced to the other. The potential at the inert electrode depends on the ratio of the activity of two substances. ⮚ Quinone + 2H+ +2e− ⇆ Hydroquinone From the Nernst equation: E= Eo + 2.303RT log [Q][H+]2 (or) 2F [QH2] E= Eo - 2.303RT log [QH2] 2F [Q] [H+]2 If quinone and hydroquinone are taken in equimolar concentrations, then [Q] = [QH2] then the above reaction reduces to E= Eo - 2.303RT log 1 = Eo - 2.303RT log 1 2F [H+]2 F [ H +] EQ = E0 - 0.0592v pH = 0.6994v - 0.0592v pH Construction and working QH electrode can very easily be set up by adding a pinch of quinhydrone powder to the experimental solution with stirring until the solution is saturated. Then indicator electrode usually a bright platinum is inserted in it. For determining the pH value, this half cell is combined with saturated calomel electrode and the emf of the cell is determine potentiometrically. Quinhydrone electrode in a cell. The complete cell may be represented as Hg2Cl2(s) | Hg+, KCl(sat), // H+(unknown, Pt | H2Q, Q, Ecell = EQH – Ecal Ecell = 0.6994v - 0.0592 pH - 0.2422v pH = 0.6994v – 0.2422v - Ecell 0.0592  Merits and Demerits of the electrode: Merits: i.Electrode is easy to set up. ii.It is also easy to handle. iii.It can be functioning satisfactorily also in highly acidic solution. iv.It is used to measure the pH of aqueous and non-aqueous solution. Demerits: i.This electrode is functioning only in the pH range of 1 to 8 ii.With the solution of pH greater than 8, the activity ratio is no longer remain equal to 1.  Glass electrode ⮚When two solutions of different pH values are separated by a thin glass membrane, there develops a difference of potential between the surfaces of the membrane. ⮚The potential value developed is proportional to the difference in pH of the test solution. ⮚The glass membrane functions as an ion exchange resin and an equilibrium is set up between the Na+ ions of the glass and H+ ions in the solution. ⮚For a particular type of glass the potential difference varies with H+ ion concentration, and is given by the expression EG = EoG + 0.0591 log [H+] EG = EoG - 0.0591 pH  Glass electrode. Construction The glass electrode assembly consists of a thin glass bulb filled with 0.1 N HCl and a silver wire coated with silver chloride immersed in it. The Ag/AgCl electrode here acts as the internal reference electrode. The glass electrode is represented as Ag | AgCl(s) | 0.1 M HCl | glass. Or Pt.0.1MHCl|Glass+ To carry out the determination of pH of a solution, the glass electrode is connected with a saturated calomel electrode. The emf of the cell is measured.  Glass electrode. The cell is therefore represented as; Ag, AgCl / HCl (0.1 N) / Glass // SCE Ecell = Eright – Eleft = Ecal - EG pH = EoG -0.2422v – Ecell 0.0592v The E0G value of a glass electrode can be determined by using a solution of known pH Advantages of Glass electrode i) To determine pH of any solution ii) Small quantity of solution is sufficient for determination iii) Used even in the presence of metallic ions and poisons iv) Equilibrium is easily reached  Merits and Demerits of the electrode: Merits: i.It provides a measure of pH in the pH range of 1 – 9. ii.Using a pH meter, pH of the solution can be directly read. iii.The electrode can be used in all aqueous solutions. iv.Electrode is not affected by oxidizing and reducing agents or by any organic compound. v.pH can be determined even for small volume of solution. Demerits: i.The electrode cannot function in highly acidic or alkaline medium ii.It cannot produce proper response with pH > 9 or