Fundamentals of Electrochemistry Introduction 1.)Electrical Measurements of Chemical Processes  Redox Reaction involves transfer of electrons from one species to another. - Chemicals are separated  Can monitor redox reaction when electrons flow through an electric current - Electric current is proportional to rate of reaction - Cell voltage is proportional to free-energy change  Batteries produce a direct current by converting chemical energy to electrical energy. - Common applications run the gamut from cars to ipods to laptops

Fundamentals of Electrochemistry Basic Concepts 1.)A Redox titration is an analytical technique based on the transfer of electrons between analyte and titrant  Reduction-oxidation reaction  A substance is reduced when it gains electrons from another substance - gain of e - net decrease in charge of species - Oxidizing agent (oxidant)  A substance is oxidized when it loses electrons to another substance - loss of e - net increase in charge of species - Reducing agent (reductant) (Reduction) (Oxidation) Oxidizing Agent Reducing Agent

Fundamentals of Electrochemistry Basic Concepts 2.)The first two reactions are known as “1/2 cell reactions”  Include electrons in their equation 3.) The net reaction is known as the total cell reaction  No free electrons in its equation 4.)In order for a redox reaction to occur, both reduction of one compound and oxidation of another must take place simultaneously  Total number of electrons is constant ½ cell reactions: Net Reaction:

Fundamentals of Electrochemistry Basic Concepts 5.)Electric Charge (q)  Measured in coulombs (C)  Charge of a single electron is 1.602x C  Faraday constant (F) – 9.649x10 4 C is the charge of a mole of electrons 6.)Electric current  Quantity of charge flowing each second through a circuit - Ampere: unit of current (C/sec) Relation between charge and moles: Coulombs moles

Fundamentals of Electrochemistry Basic Concepts 7.)Electric Potential (E)  Measured in volts (V)  Work (energy) needed when moving an electric charge from one point to another - Measure of force pushing on electrons Relation between free energy, work and voltage: Joules Volts Coulombs Higher potential difference Higher potential difference requires more work to lift water (electrons) to higher trough

Fundamentals of Electrochemistry Basic Concepts 7.)Electric Potential (E)  Combining definition of electrical charge and potential Relation between free energy difference and electric potential difference: Describes the voltage that can be generated by a chemical reaction

Fundamentals of Electrochemistry Basic Concepts 8.)Ohm’s Law  Current ( I ) is directly proportional to the potential difference (voltage) across a circuit and inversely proportional to the resistance (R) - Ohms (  ) - units of resistance 9.)Power (P)  Work done per unit time - Units: joules per second J/sec or watts (W)

Fundamentals of Electrochemistry Galvanic Cells 1.)Galvanic or Voltaic cell  Spontaneous chemical reaction to generate electricity - One reagent oxidized the other reduced - two reagents cannot be in contact  Electrons flow from reducing agent to oxidizing agent - Flow through external circuit to go from one reagent to the other Net Reaction: Reduction: Oxidation: AgCl(s) is reduced to Ag(s) Ag deposited on electrode and Cl - goes into solution Electrons travel from Cd electrode to Ag electrode Cd(s) is oxidized to Cd 2+ Cd 2+ goes into solution

Fundamentals of Electrochemistry Galvanic Cells 1.)Galvanic or Voltaic cell  Example: Calculate the voltage for the following chemical reaction  G = -150kJ/mol of Cd Solution: n – number of moles of electrons

Fundamentals of Electrochemistry Galvanic Cells 2.)Cell Potentials vs.  G  Reaction is spontaneous if it does not require external energy

Fundamentals of Electrochemistry Galvanic Cells 2.)Cell Potentials vs.  G  Reaction is spontaneous if it does not require external energy Potential of overall cell = measure of the tendency of this reaction to proceed to equilibrium ˆ Larger the potential, the further the reaction is from equilibrium and the greater the driving force that exists Similar in concept to balls sitting at different heights along a hill

Fundamentals of Electrochemistry Galvanic Cells 3.)Electrodes Cathode: electrode where reduction takes place Anode: electrode where oxidation takes place

Fundamentals of Electrochemistry Galvanic Cells 4.)Salt Bridge  Connects & separates two half-cell reactions  Prevents charge build-up and allows counter-ion migration Two half-cell reactions Salt Bridge  Contains electrolytes not involved in redox reaction.  K + (and Cd 2+ ) moves to cathode with e - through salt bridge (counter balances –charge build-up  NO 3 - moves to anode (counter balances +charge build-up)  Completes circuit

Zn|ZnSO 4 ( a ZN 2+ = )||CuSO 4 ( a Cu 2+ = )|Cu anode Phase boundary Electrode/solution interface Solution in contact with anode & its concentration Solution in contact with cathode & its concentration 2 liquid junctions due to salt bridge cathode Fundamentals of Electrochemistry Galvanic Cells 5.)Short-Hand Notation  Representation of Cells: by convention start with anode on left

Ag + + e - » Ag(s)E o = V Fundamentals of Electrochemistry Standard Hydrogen Electrode (S.H.E) Hydrogen gas is bubbled over a Pt electrode Pt(s)|H 2 (g)( a H 2 = 1)|H + (aq) (a H + = 1) || Standard Potentials 1.) Predict voltage observed when two half-cells are connected  Standard reduction potential (E o ) the measured potential of a half-cell reduction reaction relative to a standard oxidation reaction - Potential arbitrary set to 0 for standard electrode - Potential of cell = Potential of ½ reaction  Potentials measured at standard conditions - All concentrations (or activities) = 1M - 25 o C, 1 atm pressure

Fundamentals of Electrochemistry Standard Potentials 1.) Predict voltage observed when two half-cells are connected As E o increases, the more favorable the reaction and the more easily the compound is reduced (better oxidizing agent). Reactions always written as reduction Appendix H contains a more extensive list

Fundamentals of Electrochemistry Standard Potentials 2.) When combining two ½ cell reaction together to get a complete net reaction, the total cell potential (E cell ) is given by: Where:E + = the reduction potential for the ½ cell reaction at the positive electrode E + = electrode where reduction occurs (cathode) E - = the reduction potential for the ½ cell reaction at the negative electrode E - = electrode where oxidation occurs (anode) Electrons always flow towards more positive potential Place values on number line to determine the potential difference

Fundamentals of Electrochemistry Standard Potentials 3.) Example: Calculate E o, and  G o for the following reaction:

Fundamentals of Electrochemistry Nernst Equation 1.) Reduction Potential under Non-standard Conditions  E determined using Nernst Equation  Concentrations not-equal to 1M aA + ne - » bB E o For the given reaction: The ½ cell reduction potential is given by: Where:E = actual ½ cell reduction potential E o = standard ½ cell reduction potential n = number of electrons in reaction T = temperature (K) R = ideal gas law constant (8.314J/(K-mol) F = Faraday’s constant (9.649x10 4 C/mol) A = activity of A or B at 25 o C

Fundamentals of Electrochemistry Nernst Equation 2.) Example:  Calculate the cell voltage if the concentration of NaF and KCl were each 0.10 M in the following cell: Pb(s) | PbF 2 (s) | F - (aq) || Cl - (aq) | AgCl(s) | Ag(s)

Fundamentals of Electrochemistry E o and the Equilibrium Constant 1.) A Galvanic Cell Produces Electricity because the Cell Reaction is NOT at Equilibrium  Concentration in two cells change with current  Concentration will continue to change until Equilibrium is reached - E = 0V at equilibrium - Battery is “dead” aA + ne - » cC E + o dD + ne - » bB E - o Consider the following ½ cell reactions: Cell potential in terms of Nernst Equation is: Simplify:

Fundamentals of Electrochemistry E o and the Equilibrium Constant 1.) A Galvanic Cell Produces Electricity because the Cell Reaction is NOT at Equilibrium Since E o =E + o - E - o : At equilibrium E cell =0: Definition of equilibrium constant at 25 o C

Fundamentals of Electrochemistry E o and the Equilibrium Constant 2.) Example:  Calculate the equilibrium constant (K) for the following reaction:

Fundamentals of Electrochemistry Cells as Chemical Probes 1.) Two Types of Equilibrium in Galvanic Cells  Equilibrium between the two half-cells  Equilibrium within each half-cell If a Galvanic Cell has a nonzero voltage then the net cell reaction is not at equilibrium For a potential to exist, electrons must flow from one cell to the other which requires the reaction to proceed  not at equilibrium. Conversely, a chemical reaction within a ½ cell will reach and remain at equilibrium.

Fundamentals of Electrochemistry Ni(s)|NiSO 4 (0.0025M)||KIO 3 (0.10 M)|Cu(IO 3 ) 2 (s)|Cu(s) Cells as Chemical Probes 2.) Example:  If the voltage for the following cell is 0.512V, find K sp for Cu(IO 3 ) 2 :

Fundamentals of Electrochemistry Biochemists Use E o ´ 1.) Redox Potentials Containing Acids or Bases are pH Dependent  Standard potential  all concentrations = 1 M  pH=0 for [H + ] = 1M 2.) pH Inside of a Plant or Animal Cell is ~ 7  Standard potentials at pH =0 not appropriate for biological systems - Reduction or oxidation strength may be reversed at pH 0 compared to pH 7 Metabolic Pathways

Fundamentals of Electrochemistry Biochemists Use E o ´ 3.) Formal Potential  Reduction potential that applies under a specified set of conditions  Formal potential at pH 7 is E o ´ Need to express concentrations as function of K a and [H + ]. Cannot use formal concentrations! E o ´ (V)