Electrical and Chemical Energy Interconversion ELECTROCHEMISTRY Electrical and Chemical Energy Interconversion

REDOX REACTION Reduction Oxidation Reducing Agent Oxidizing Agent

ELECTRICAL CONDUCTION Electrons Conducting medium electron transfer (flow) conductance (mho) Resistance (ohm) Copper – solid medium Aqueous solution – liquid medium

Electrochemical Cell e- V Zn Cu ZnSO4 CuSO4 Cl- SALT BRIDGE K+ ANODE Electrodes – where reaction takes place OA – CR Salt bridge – hinder build up of excess positive and negative charges on either solution Electron movement from anode to cathode Cl- SALT BRIDGE K+ ANODE CATHODE

ELECTRODES Active Electrodes Inert Electrodes

CELLS Cell Cell Reaction Ecell GALVANIC SPONTANEOUS + ELECTROLYTIC NON-SPONTANEOUS -

SAMPLE REACTION Zn(s)  Zn2+ + 2e- 2e- + Cu2+  Cu(s) Zn(s) + Cu2+  Zn2+ +Cu(s) E°cell = E°cathode - E°anode Based on STANDARD CELL POTENTIAL – standard hydrogen electrode

SPONTANEITY E°cell > 0 SPONTANEOUS at standard state E°cell < 0 NON SPONTANEOUS at standard state w = -nFE ΔG = -nFE ΔG° = -nFE° n = moles of electrons F = Faraday’s constant = 96485 C/mole of e-

SAMPLE PROBLEM Evaluate the standard cell potential of each of the following cells and indicate whether the cell is GALVANIC or ELECTROLYTIC. AuCl4- + Fe2+  Au(s) + 4Cl- +Fe3+ K+ + H2O(l)  O2(g) + H+ + K(s)

SEATWORK Complete the cell reaction for: Pb(s) + MnO4-  Pb2+ + Mn2+ Identify the oxidation and reduction half reaction and evaluate the Standard Cell Potential.

CELL REPRESENTATION Anode | Oxidation || Reduction | Reduction Anode Before Cathode! Zn(s) | Zn2+(aq) || Cu2+(aq) | Cu(s)

SEATWORK 5Pb(s) + 16H+ + 2MnO4-  Pb2+ 2Mn2+ +8H2O Write the cell representation for the following reaction.

Pt(s) | Fe2+,Fe3+ || Cu2+ |Cu(s) QUIZ Pt(s) | Fe2+,Fe3+ || Cu2+ |Cu(s) Draw the cell. (Include the direction of the flow of electrons) Identify the half-cell reaction and the complete cell reaction. Evaluate the standard electron cell potential. Identify whether the cell is Galvanic or Electrolytic.

EFFECT OF CONCENTRATION ΔG = ΔG° + RT ln Q -nFEcell = -nFE°cell + RT ln Q Ecell = E°cell – RT/nF ln Q At 25°C E = E°cell – 0.05916/n log Q

SAMPLE PROBLEM Is the cell Pt(s) | Fe2+ (0.10M),Fe3+ (0.06M)|| Cu2+ (0.05M)|Cu(s) Galvanic or Electrolytic?

SEATWORK Is the cell Pb(s) | Pb2+(0.0025M) || H+(0.0020M), MnO4- (0.0030M), Mn2+ (0.010M) | Pt(s) Galvanic or Electrolytic?

SAMPLE PROBLEM Given the cell: Pt | Fe2+, Fe3+ || Mn2+, H+, MnO4- | Pt [Fe2+] = 0.20M [Fe3+] = 0.0020M [Mn2+] = 0.0010M [MnO4-] = 0.60M Calculate the pH at which the reaction will occur spontaneously.

APPLICATION On the basis of electrochemistry, provide an explanation on how galvanized iron (iron coated with zinc) prevents corrosion of iron.

EVALUATION OF EQUILIBRIUM CONSTANT ΔG = 0 ΔG = ΔG° + RT ln Q ΔG° = -RT ln Keq -nFE° = -RT ln Keq Keq = e ^ (nFE°/RT) Keq = 10 ^ (nE°/0.0592)

SAMPLE PROBLEM Calculate the Ksp of AgCl AgCl(s) + e-  Ag(s) + Cl-(aq) E°=0.22V Ag(s)  Ag+(aq) + e- E°=0.80V Ksp = 1.57 x 10^-10

Electrical Conduction Electron / Metallic Conduction – metals Electrolytic Conduction – salts in solution (electrolytes in general) Conductance (L) = 1/resistance Molar conductance (siemens) = 1000Ls/C

ELECTROLYSIS - + A+ B-

MECHANISM OF ELECTROLYSIS Migration of ions to electrode Cation to cathode Anion to Anode Reaction Cathode – reduction Anode - Oxidation

FACTORS Nature of Electrodes Nature of Electrolytes Active Inert Molten (one anion, one cation) Aqueous (competition )

FARADAY’s Law of ELECTROLYSIS Q = It Q – charge in Coulombs I – current in Ampere t – time in seconds 1 mol of e- carries a charge of 96485C F = 96485 C / mol e-

SAMPLE PROBLEM 500 C of electricity is applied to a molten solution of NaI. What are the substances formed at each electrode? Write the reactions. Determine the mass of products produced. How about for aqueous solution of NaI? How about when a current of 10 amperes is applied to molten NaI for 1 hour.

SEATWORK Calculate the mass (if solid) or volume at STP (if liquid) of products from the electrolysis of molten CaCl2 between Platinum electrodes using 5.0A for 8 mins.

QUIZ A spoon with an area of 3.5cm2 is plated with silver from a silver nitrate aqueous solution using a current of 0.60A for 1.6 hours. How many grams of silver are plated if the current efficiency is 100%? Recalculate for a current efficiency of only 80%. What is the thickness of the silver plate formed if the density of silver is 10.5 g/cm3?