Electrochemistry

Electrochemistry Conductivity – Electrodes & Electrolytes Electrochemical Cell Electrolytic Cell Applications of Redox Reactions Electrochemical Cell Galavanic or Voltaic Cells Anode/Cathode/Salt Bridge Cell Notations Determining Cell Potential/Cell Voltage/Electromotive force (emf) Electromotive Force (emf) Water only spontaneously flows one way in a waterfall. Likewise, electrons only spontaneously flow one way in a redox reaction—from higher to lower potential energy. Standard Electrode Potential To measure relative electrode potentials, we must establish an arbitrary standard. That standard is the Standard Hydrogen Electrode (SHE). The SHE is assigned an arbitrary voltage of 0.000000… V The Zinc-SHE Cell For this cell the components are: A Zn strip immersed in 1.0 M zinc (II) sulfate. The other electrode is the Standard Hydrogen Electrode. A wire and a salt bridge to complete the circuit. The initial cell voltage is 0.763 volts. The cathode is the Standard Hydrogen Electrode. In other words Zn reduces H+ to H2. The anode is Zn metal. Zn metal is oxidized to Zn2+ ions. The Copper-SHE Cell The cell components are: A Cu strip immersed in 1.0 M copper (II) sulfate. The other electrode is a Standard Hydrogen Electrode. A wire and a salt bridge to complete the circuit. The initial cell voltage is 0.337 volts. In this cell the SHE is the anode The Cu2+ ions oxidize H2 to H+. The Cu is the cathode. The Cu2+ ions are reduced to Cu metal. Uses of Standard Electrode Potentials Electrodes that force the SHE to act as an anode are assigned positive standard reduction potentials. Electrodes that force the SHE to act as the cathode are assigned negative standard reduction potentials. Standard electrode (reduction) potentials tell us the tendencies of half-reactions to occur as written. For example, the half-reaction for the standard potassium electrode is: Previous Year IIT Questions IIT-2009 (Paper II) 1. Multiple Choice Question- more than one correct Solution: IIT-2007 (Paper II) Linked Comprehension Type This section contains 2 paragraphs C36-38 and C39-41. Based upon each paragraph, 3 multiple choice questions have to be answered. Each question has 4 choices (A), (B), (C) and (D), out of which ONLY ONE is correct. Comprehension Redox reactions play a pivotal role in chemistry and biology. The values of standard redox potential (Eo) of two half-cell reactions decide which way the reaction is expected to proceed. A simple example is a Daniel cell in which zinc goes into solution and copper gets deposited. Given below are a set of half-cell reactions (acidic medium) along with their Eo (V with respect to normal hydrogen electrode) values. Using this data obtain the correct explanations to Questions 2 & 3. I2 + 2e- → 2I- Eo = 0.54 Cl2 + 2e- → 2Cl- Eo = 1.36 Mn3+ + e- → Mn2+ Eo = 1.50 Fe3+ + e- → Fe2+ Eo = 0.77 O2 + 4H+ + 4e- → 2H2O Eo = 1.23 2. Among the following, identify the correct statement. (A) Chloride ion is oxidized by O2 (B) Fe2+ is oxidized by iodine (C) Iodide ion is oxidized by chlorine (D) Mn2+ is oxidized by chlorine Solution . (C) Reduction potential of I2 is less than Cl2. 3. While Fe3+ is stable, Mn3+ is not stable in acid solution because (A) O2 oxidises Mn2+ to Mn3+ (B) O2 oxidises both Mn2+ and Fe2+ to Fe3+ (C) Fe3+ oxidizes H2O to O2 (D) Mn3+ oxidises H2O to O2 Solution. (D) Reaction of Mn3+ with H2O is spontaneous. Comprehension IV IIT-2006 Tollen’s reagent is used for the detection of aldehyde when a solution of AgNO3 is added to glucose with NH4OH then gluconic acid is formed 4. 2Ag++C6H12O6 +H2O → 2Ag (s) + C6H12O7 + 2H+ Find ln K of this reaction. (A) 66.13 (B) 58.38 (C) 28.30 (D) 46.29 Solution: The Catalyst The Catalyst www.thecatalystgroup.co.in www.thecatalystgroup.co.in The Catalyst Electrochemistry www.thecatalystgroup.co.in Anode Electrode is oxidized M(s) ( Mx+(aq) + x e- Reducing agent Mx+ Salt Bridge (KNO3) voltmeter – e – e – e Mred(s) Oxidizing agent My+ Cathode Ions are reduced My+(aq) + y e- ( M(s) Mox(s) – e Anode Electrode is oxidized Zn(s) ( Zn2+(aq) + 2e- Reducing agent Zn2+ Salt Bridge (KNO3) +1.10V – e – e – e Cu(s) Oxidizing agent Cu2+ Cathode Ions are reduced Cu2+(aq) + 2e- ( Cu(s) Zn(s) – e So: Cell potential298K: Q, reaction quotient: n = #moles of e- transferred So: So: From thermodynamics: From electrochemistry: At equilibrium: G0 = 0 and Keq = Q Connection to work: G0, E0, and K E0cell cell potential under standard conditions (reference tables) elements in standard states: s, l, g solutions: 1 M gases: 1 atm Relative to standard hydrogen electrode, “SHE” 2H+(aq) + 2 e- ( H2(g) E0cell = 0.00 V Overall E0cell: combine E0’s for half-reactions Standard Reduction Potentials, E0 Voltage difference in energy of the e- on the metals or relative difference in metals’ abilities to give e- different metals → different e- energy → different “push” on e- Electromotive force (EMF; cell potential), Ecell Driving force on electrons Measured voltage = potential difference Higher Ecell = larger “drive” Nonstandard conditions: Nernst Equation Non-standard conditions:298K: Standard Conditions and at Equilibrium: Remember: Ecell is proportional to (G Summary of Key Equations 0 2. Dependence of potential on concentration Metal1 = Metal2 c1 ( c2 Use 0.1 M solution with 1(10-5 to 1(10-1 M solutions Measure Ecell for each Plot Ecell vs. log(cdil/cconc) Compare slope to Nernst equation Experimental Overview 1. Dependence of potential on metal type Metal1 ( Metal2 c1 = c2 Use 0.1 M solutions and electrodes of different metals Measure Ecell for each (= E0cell) Compare experimental vs. literature values