Tiêu đề 2A.ELECTRO CHEMISTRY FINAL (VIJ) FINAL.pdf ✅

ELECTRO CHEMISTRY JEE ADVANCED NISHITH Multimedia India (Pvt.) Ltd., 3 9 JEE ADVANCED - VOL - VI REDOX REACTIONS & ELECTROCHEMISTRY NISHITH Multimedia India (Pvt.) Ltd., SYNOPSIS * CONDUCTOMETRIC TITRATION The electrical conductivity of a solution serves as means of determining the end point in a chemical reaction, involved in the titration of acids and bases or precipitation. The measurment of conductance of resulting solution form an acid and base titration is used to calculate equivalence point if there is a regular change in the conductance and sharp change at equivalence point. a. Strong acid (HCl) vs strong base (NaOH) S S A B /  When as drop of NaOH is added to a certain volume of HCl solution taken in a conducivity cell, NaOH reacts with equivalent amount of HCl. NaOH HCl Na Cl H O2      Before NaOH is added the conductance of HCl has a high value due to the presence of highly mobile H  ions. As NaOH is added, H  ions combine with OH  ions to form undissociated H O2 , and thus faster moving H  ions are replaced by relatively slower moving Na ions. Hence the conductance of the solution decreases upto equivalence point. The addition of further NaOH increases the conductance due to release of fast moving OH  ions. b. Strong acid (HCl) vs weak base NH OH S S 4  A B /  On addition of NH OH 4 there is replacement of fast moving H  ions by slow moving NH4  ions as follows. H Cl NH OH NH Cl H O 4 4 2          conductance End Point v NH4OH therefore, the conductance decreases. A slight curvature is noticed near the end point. It is due to the following reaction. 4 2 4 3 NH H O NH OH H O 2      c. Weak acid CH COOH 3  vs strong base NaOH  conductance End Point VNaOH Initially the solution shows poor conductance due to feeble ionization of CH COOH 3 . B u t soon the conductance starts increasing as the REDOX REACTIONS & ELECTROCHEMISTRY
REDOX REACTIONS & ELECTROCHEMISTRY 4 0 NISHITH Multimedia India (Pvt.) Ltd., JEE ADVANCED - VOL - VI NISHITH Multimedia India (Pvt.) Ltd., addition of NaOH neutralizes the undissociated HAc to Na Ac   upto equivalence point. The graph near the equivalence point is curved due to the hydrolysis of salt NaAc. d.Weak acid CH COOH 3  vs Weak base NH OH W W 4  A B /  conductance End Point VNH4OH After the equivalence point, conductance virtually remains the same as weak base which is being added is feebly ionized. e. Mixture of weak and strong acids HCl CH COOH  3  vs Strong base NaOH  co nd u cta n ce V1 V2 In this case, strong acid is first neutralized after that weak acid starts neutralizing. Thus, we have the neutralization reaction in two stages and the curve contains two breaks. * CONCENTRATION CELLS The cells whose o Ecell is zero are called concentration cells. This means that the two compartments (cathode and anode) of the electrochemical cell involve same chemical species but the concentrations of the chemical species in the two compartments are different. The concentration cells are of basically two types. (a) Electrode concentration cells and (b) Electrolyte concentration cells (a) ELECTRODE CONCENTRATION CELLS In such cells, two similar electrodes at different concentrations/pressures are dipped in same solution with similar concentration. Let us have an electrochemical cell represented as Pt | H2 (P1 atm) | H+ (c M) || H+ (c M) | H2 (P2 atm) | Pt For the given cell, the reactions occuring are At cathode : 2H 2e H (P ) C 2 2     At anode : H (P ) 2H 2e 2 1 A     ____________________ Net cell reaction : H (P ) H (P ) 2 1 2 2  Since the H+ concentration at the anode and cathode are same, so the net reaction independent of the concentration of the electrolyte. Applying Nernst equation to the net reaction gives. C 2 2 A 2 1 o o o 2 cell H |H (P ) H |H (P ) 1 0.059 P E (25 C) E E log 2 P            o o 2 cell cell 1 0.059 P E (25 C) E log 2 P         or o 2 cell 1 0.059 P E (25 C) log 2 P         (since, E 0 cell  )       o 1 cell 2 0.059 P E (25 C)= log 2 P The EMF of the given cell would be positive when P1 > P2 and the cell reaction would spontaneous. An other example of the electrode concentration cell is that of an amalgam with two different concentrations of the same metal dipped in same electrolyte solution. The cell is represented as Hg - Pb (c1 M) | PbSO4 (c M) | Hg - Pb (c2 M)
ELECTRO CHEMISTRY JEE ADVANCED NISHITH Multimedia India (Pvt.) Ltd., 4 1 JEE ADVANCED - VOL - VI REDOX REACTIONS & ELECTROCHEMISTRY NISHITH Multimedia India (Pvt.) Ltd., The reactions for the given cell are At cathode :   Pb (c) 2e Pb(c ) 2   2 At anode : 2 Pb(c ) Pb (c) 2e 1     ______________________ Net cell reaction : Pb(c ) Pb(c ) 1 2  Since, the concentration of Pb2+ for the two half cells is same as the electrolyte solution for the two compartments is same, so the net reaction is independent of the electrolyte concentration. Applying Nernst equation to the net cell reaction gives    2 2   2 1 o cell o o 2 Pb (c)|Pb(c ) Pb (c)|Pb(c ) 1 E (25 C) 0.059 c E E log 2 c o o 2 cell cell 1 0.059 c E (25 C) E log 2 c     0 2 cell 1 0.059 c E (25 C) log 2 c (since, o E 0 cell  )  0 1 cell 2 0.059 c E (25 C) log 2 c The net cell reaction would be spontaneous, when the EMF of the cell is positive, which is possible only when c1 > c2 . (b) ELECTROLYTE CONCENTRATION CELLS In such cells, two electrodes of the same metal are dipped in solutions of metal ions of different concentrations. Let us have an electrochemical cell represented as Pt | H2 (P atm) | HA (c1 M) || HB (c2 M) | H2 (P atm) | Pt In such cells, HA and HB would represent strong acids, if their Ka ’s are not given while they would be weak acids, if their Ka ’s are mentioned. For the given cell, the reactions occuring are At cathode : 2H (c ) 2e H (P) c 2 2     At anode : H (P) 2H (c ) 2e 2 A 1     _____________________ Net cell reaction :         2 1 c A C C 2H 2H (n 2) The net cell reaction is independent of the pressure terms as the pressure of H2 in the two half cells is same. Applying Nernst equation to the net cell reaction gives          c 2 A 2 o o o A cell H |H H |H C o 1 cell 2 0.059 [H ] E (25 C) E E log 1 [H ] c E 0.059log c o o 1 cell cell 2 c E (25 C) 0.059log (since E 0) c   The net cell reaction would be feasible spontaneously only when the EMF of the cell is positive, which is possible only when concentration of H+ in cathode compartment (c2 ) is greater than the concentration of H+ in anode compartment (c1 ). OTHER HALF CELLS (a) METAL-METAL ION HALF-CELL In this type of half-cell, the metal rod is dipped in a solution of the corresponding metal ion having concentration c M. The assembly is shown in figure. Metal-metal ion half-cell The representation of the half-cell when functions as anode can be represented as M(s) | Mn+ (c M) The anodic half-cell reaction is M(s)  Mn+ (aq) + ne-
REDOX REACTIONS & ELECTROCHEMISTRY 4 2 NISHITH Multimedia India (Pvt.) Ltd., JEE ADVANCED - VOL - VI NISHITH Multimedia India (Pvt.) Ltd., The representation of the half-cell when it functions as cathode is Mn+ (c M) | M (s) The cathodic half-cell reaction is Mn+ (aq) + ne-  M(s) Examples of this type of half-cell when used as cathode are Cu2+ | Cu, Zn2+ | Zn, Ag+ | Ag, Sn2+ | Sn etc. The half-cells or electrodes used in Daniel cell are of the metal-metal ion type. For such type of half-cells, very active metals cannot be used, because they react with water. For such highly reactive metals, an amalgam of the metal in mercury is used instead of pure metal. (b) GAS-GAS ION HALF-CELL In a gas-gas ion half-cell, a gas is bubbled into a solution of the gas ion, with a platinum rod having a sheet coated with platinum black being dipped in the gas ion solution. Platinum is used for making electrical contact because the gases are non- conducting and the platinum do not react with gas ions as it is an unreactive metal. Platinum black is used over the platinum sheet, which gives it a large surface area for the adsorption of gases. The half-cell assembly is shown in figure. Gas-gas ion half-cell The given half-cell when function as anode can be represented as Pt | Gas (P atm) | Gas ion (c M). Let us consider a hydrogen gas half-cell, functioning as anode. It is represented as Pt | H2 (P atm) | H+ (c M) and the anodic half-cell reaction is H2 (Pt)  2H+ (aq) + 2e- (Pt). The chlorine gas half-cell when functions as anode can be represented as Pt | Cl2 (P atm) | Cl- (c M) and the anodic half-cell reaction is 2Cl-  Cl2 + 2e- The oxygen gas half-cell when functions as anode can be represented as Pt | O2 (P atm) | OH- (c M) and the anodic half-cell reaction is 2 2 1 2OH O H O 2e 2      The gas-gas ion half-cell when functions as cathode can be represented as Gas ion (c M) | Gas (P atm) | Pt. Let us consider a hydrogen half-cell, functioning as cathode. It is represented as H+ (c M) | H2 (P atm) | Pt and the cathodic half-cell reaction is 2H+ (aq) + 2e- (Pt)  H2 (Pt). The hydrogen electrode is called standard hydrogen electrode when the concentration of H+ ion is 1 M and the pressure of H2 gas is 1 atm. The standard hydrogen electrode (SHE) functions as reference electrode and is used for the measurement of standard reduction potentials of other half-cells or couples. The standard reduction potential values obtained in this manner are arranged in the decreasing order to give electrochemical series. (c) REDOX HALF-CELL In redox half-cell, a platinum rod is dipped in a solution containing two different oxidation states of a metal ion ( M n1  and M n2  , where n2 is greater than n1 ). The half-cell assembly is shown in figure.