Electro-Chemistry PDF

The branch of chemistry which deals with the study of the production of electricity from energy released during spontaneous chemical reaction is called electro chemistry. The process associated with chemical changes which occur by using electrical energy is called electrolysis and the device...

The branch of chemistry which deals with the study of the production of electricity from energy released during spontaneous chemical reaction is called electro chemistry. The process associated with chemical changes which occur by using electrical energy is called electrolysis and the device used for electrolysis is called electrolytic cell. : These are the chemical reactions in which both oxidation and reduction reaction takes place simultaneously. In this reaction, one substance loses electron and undergoes oxidation and at the same time other substances gains those electron and under goes reduction. And the net balanced reaction is neutral. : According to electronic concept, oxidising agent is a substance which accepts electron where as reducing agent is a substance which involves in loses of electrons during a redox reaction. Therefore, oxidising and reducing agents can be summarised by the following equilibrium system. : It is a substance that dissociates in solution to produce ions and hence conduct electricity in dissolved or in molten state. : −NaOH, HCl, KCl (Strong electrolyte) $%& $''%, (%) '% (Weak electrolyte) * : These are the substances that fully dissociate into ions either in molten or in aqueous solution and carry larger quantity of current. The degree of dissociation for strong electrolyte (+) is equal to unity (+ = 1). : −HCl, H. SO) (Strong acid) NaOH, KOH (Strong base) NaCl, KCl (Ionic salts) 0 1 : These are the substances that dissociated into ions either in molten or in aqueous solution and carry lesser quantity of current. The degree of dissociation for strong electrolyte (+) is equal less than unity (+ < 1). : −HCN, R − COOH, HF (Weak acid) NH) OH, R − NH. (Weak base) HgCl. , SnCl) (Inorganic salts) 7 88 * 9 : 8 9 : i. ; 9 8 : These are attractive interactions between the ions of obtained from the electrolyte in solutions. These are known as solute-solute interactions. Lesser the interaction greater will be the freedom of movement of ions and higher will be the conductance. 1 ii. : 8 : These are known as solute-solvent interactions, indicating there by interaction between ions produced from solute and solvent interaction. Larger the magnitude of this interaction greater is the extent of salvation and lower will be electric conductivity of the solution. iii. < 8 9 : The property gives rise to solvent-solvent interaction, larger the magnitude of this type of interaction greater will be the resistance offered by the solvent to the flow of ions and hence lesser will be the electric conductivity. iv. => 9 : The temperature has a direct bearing on the electric conductivity of the solution. As the temperature of the solution rises, inter ionic attraction decreases and thus electrical conductivity also increases. v. 8 9 : Higher the concentration of solution lesser will be its conductance because inter ionic attractions are of greater magnitude. Thus a dilute solution has more electrical conductivity. ; − : Substances which do not allow the electric current to pass through their aqueous solutions are non-electrolytes. : −Cane sugar, urea etc 9 : ’ @0: It states that, “at constant temperature the current flowing through a conductor of uniform area of cross-section is directly proportional to the difference of potential across the two ends of a conductor.” A. C. D< <  = Where, E = FGHCIHAJK LAMMCNCIOC GM HPG CILQ GM OGILROHGN A = ORNNCIH AI OGILROHGN 8 \ = : The Daniel cell is represented as: ¸I (Q)| ¸I. ($^ )||$R. ($. )|$R(Q) a. Anode half-cell is written on left had side while cathode half-cell on right hand side. b. A single vertical line separates the metal from aqueous solution of its own ions. ¸I (Q)| ¸I. (Jy) $R. (Jy)|$R(Q) JIGLAO OℎJVeCN OJHℎGLAO OℎJVeCN c. A double vertical line represents salt bridge which allows the passage of ions through it but prevents the mixing of two solutions. d. The molar concentration (C) or activity (a) is paled in brackets after the formula of the corresponding ion. e. The value of e.m.f of the cell is written on the extreme right of cell. ¸I (Q)| ¸I. (1M)||$R. (1M)|$R(Q) ´…U = +1.1E f. If an inert electrode like platinum is involved in the construction of the cell. It may be written along the working electrode in bracket. ¸I (Q)| ¸I. ($^ )||% ($. )|%. (ÊH) 9 : i. An electrochemical cell converts chemical energy into electrical energy. ii. In an electrochemical cell, anode is that electrode at which oxidation half reaction takes place. iii. Cathode is that electrode at which reduction half reaction takes place. iv. Electrons in the external circuit flow anode to cathode. v. Representation of electrochemical cell: Anode salt bridge cathode. ¸I | ¸I Ë('& $R. |$R 10  > :- In an electrochemical cell, oxidation occurs at one electrode and reduction at the other. In other words, one electrode has the tendency to lose electrons while the other has the tendency to gain electrons. The tendency of an electrode to loss or gain electrons is called electrode potential. The electrode potential is further called oxidation potential if oxidation takes place at the electrode. If the reduction takes place at the electrode, then it is called reduction potential. > : The electrode potential developed in the half cell when an electrode is immersed in a solution of its ions, the concentration being 1 mole Ìl^ at 298o is called standard electrode potential. It is denoted by ´ {. The standard conditions for a gas electrode is 1 atmosphere pressure and 298K temperature. i. Absolute value of electrode potential cannot be determined because neither oxidation nor reduction takes independently i.e. no half-cell works independently. ii. The electrode potential of an electrode is always determine with respect to a standard electrode. iii. The electrode potential of an electrode changes with the change in concentration of ions in solution, in contact with metals otherwise, Reduction potential of electrode + concentration of ions 1 'ŸALJHAGI FGHCIHAJK GM CKCOHNGLC + OGIOCIHNJHAGI GM AGIQ = : i. It is difficult to maintain unit molar conc. of H ions and also 1 atmospheric pressure uniform for a long time. ii. The hydrogen electrode gets poisoned in presence of impurities. > (..7 8 ) :- An electrochemical cell is made up of two half cells, i.e. two electrodes. One of these electrodes must have a higher electrodes potential (higher tendency to lose electrons) than the other electrode. It’s a result of this potential difference, the electrons flow from one electrode at higher potential to an electrode at lower potential. Thus, the difference between the electrode potential of two half cells is known as electromotive force (e.m.f) of the cell or the cell potential or voltage. It may be also defined as the force which causes the flow of electrons from one electrodes to another and thus, results in the flow of current as an electrochemical cell is combination of two half cells. It is also equal to the potential difference between the two electrodes when no current is drawn from the cell. It is the maximum voltage obtainable in a cell. The difference between reduction potential of two electrodes of a cell is called as emf of the cell. Cell potential = 9= 8 w \ 8 > 11  = > (. >) ( ) + 9 > (. ]) ( \ ) =.] –.] \ =.] \ –.] E.M.F of a cell is measured of overall tendency which determines the spontaneity of a reaction to take place. The E.M.F of a cell measured by connecting the volt meter between two electrodes of a cell. The magnitude of E.M.F depends upon the nature of electrodes and the concentration of the solution in the two half cells. = ( \ ) − ( ) = ( ) − ( \ ) = ( \ ) + ( ) For examples, The E.M.F of a Daniell cell in which concentration of an aqueous solution of zinc sulphate and copper sulphate in the two half cells in 1 M at 298 K is 1.10 volt (a) In term of oxidation potential: = − \ (b) In terms of reduction potential: = \ − \ = : The series of electrodes arranged in the increasing order of their standard reduction potential is called electrochemical series. @>> : i. Predicting relative reactivity of elements. ii. Constructing galvanic cell. iii. Determining feasibility of a reaction. iv. Calculating EMF of a cell. v. Extraction of metals by using the principle of ‘Hydrometallurgy’. ; ; ÏZ@ ;: The value of electrode potential at different concentration and temperature can be obtained by using Nernst equation.  Nernst gave a relation between the reduction potential of an electrode and the concentration of the ions in the electrode reaction. For this purpose the electrode reaction is written as reduction, i.e. … (Jy) + I C l → …(Q) C.. ¸I. + 2C l Zn Then the Nernst equation is applied as follows, SÑ [… ] ´ = ´{ − KI IM [… ] Where, ´ = electrode potential under given concentration of … , ion and temperature ‘Ñ’. ´ { = Standard electrode potential S = Real gas constant Ñ =Temperature in Kelvin U = 1 Faraday I =Number of electrons involved in the electrode reaction When concentration of solid ‘M’ is taken as unity, 12  SÑ 1 ´ = ´{ − KI IM [… ] hÒ ^  ´ = ´{ − x 2.303 KG [¹ÔÕ] º Putting, S = 8.314 Jo l^ VGKl^ U = 96500 OGRKGVeQ Ñ = 298 o We get, ×.&^) ”.Ã× ^  ´ = ´{ − x 2.303 KG [¹ÔÕ] ÃÄÅ{{ ¦ ¦.¦³±[ [  = − *[ Õ] While applying Nernst equation, electrode potential is always taken as reduction potential. @>> : ; r9 8 =8 8 : Let’s consider an example of a Denial cell in which the cell reaction is ¸I(Q) + $R. (Jy) → ¸I. (Jy) + $R(Q) For cathode: ƒ qÕ ƒ qÕ hÒ ^ ´ Ø Ù = ´{ Ø Ù− KI [ƒ qÕ ƒ ƒ.Ú (_u)] For anode:  qÕ Â qÕ hÒ ^ ´ Ø Ù = ´{ Ø Ù− KI [ qÕ Â Â.Ú (_u)] The cell potential: ƒ qÕ Â qÕ  ´|t`` = ´ Ø Ù− ´ Ø Ù ƒ  ƒ qÕ hÒ ^  qÕ hÒ ^  ´|t`` = Ø´ { Ø Ù− KI [ƒ qÕ Ù − Ø´ { Ø Ù− KI [ qÕ Ù ƒ.Ú (_u)] Â.Ú (_u)] ƒ qÕ hÒ ^  qÕ hÒ ^  ´|t`` = ´ { Ø Ù− KI [ƒ qÕ(_u)] − ´ { Ø Ù+ KI [ qÕ ƒ.Ú Â.Ú (_u)] ƒ qÕ Â qÕ hÒ ^ hÒ ^  ´|t`` = ´ { Ø Ù − ´{ Ø Ù− KI [ƒ qÕ(_u)] + KI [ qÕ ƒ Â.Ú.Ú (_u)] ¦ - §Õ  = − Û Ü §7 9§Õ It can be seen that ´|t`` depends on the concentration of both $R. and ¸I. ions. It increases with increase in the concentration of $R. ions and decreases in the concentration of ¸I. ions. By converting the natural logarithm in equation to the base 10 and substituting the values of R,F and T it reduces to {.{ÅÃ^  qÕ ´ = ´{ − x KG Û Ü ƒ qÕ t Ý Taking a general electro chemical reaction of the type: J˜ + eš Þ⎯à O$ + Lá Nernst equation can be written as ¦ [ ] [â]  = − [@] [ã]v 7 13  v w ä vv’ * =8 8 :  It is known that Gibb’s energy is a measure of chemical useful work that is available from a chemical reaction. We write ∆æ = VJŸAVRV PGNË  If the potential of two electrodes differ by ´|t`` then the maximum work done is given as, VJŸAVRV PGNË = IM ´|t`` Where, n is the number of moles of electrons transferred in them and F is the faraday constant.  In a denial cell, the work is done on the surroundings because electrical energy flows through the external circuit.  Such a work is given a negative sign (by convention) ∴ …JŸAVRV PGNË = −IM ´|t`` = ∆æ  For comparing cell voltage, standard electrode potentials (´ { ) are used.  Thus, ∆ä = − 8 ¦ Where, ∆æ { = QHJILJNL Aee¨ Q CICN d GM J NCJOHAGI * 8 : The above reaction is used to predict feasibility of a reaction. If ∆æ { is negative, the above reaction is spontaneous or feasible otherwise not. 14