Chem. 133 – 2/26 Lecture

Announcements Lab –Finish up Set 2 Period 1 Labs –I will have sign ups for Period 3 and 4 Exam 1 –Help Session Monday 12 to 1 (in place of office hours) –Parts of Electrochemistry covered: through Galvanic Cells and Standard Potentials, but not including use of Nernst Equation (through Ch in text) Today’s Lecture –Galvanic Cells and Standard Potential –Review of Material on Exam 1 –Nernst Equation and Applications

Electrochemistry Galvanic Cells What are galvanic cells? –Cells that use chemical reactions to generate electrical energy –Batteries are examples of useful galvanic cells –Example reaction –If reactants are placed in a beaker, only products + heat are produced –When half reactions are isolated on electrodes, electrical work can be produced Zn(s) + 2Ag + → Zn Ag(s) Salt Bridge voltmeter Zn(s) ZnSO 4 (aq) Ag(s) AgNO 3 (aq) GALVANIC CELL

Electrochemistry Galvanic Cells Description of how example cell works –Reaction on anode = oxidation –Anode = Zn electrode (as the E º for Zn 2+ is less than for that for Ag + ) –So, reaction on cathode must be reduction and involve Ag –Oxidation produces e -, so anode has ( – ) charge (galvanic cells only); current runs from cathode to anode –Salt bridge allows replenishment of ions as cations migrate to cathode and anions toward anodes Salt Bridge voltmeter Zn(s) ZnSO 4 (aq) Ag(s) AgNO 3 (aq) GALVANIC CELL Zn(s) → Zn e - Ag + + e - → Ag(s) – +

Electrochemistry Galvanic Cells Cell notation –Example Cell: Zn(s)|ZnSO 4 (aq)||AgNO 3 (aq)|Ag(s) Salt Bridge voltmeter Zn(s) ZnSO 4 (aq) Ag(s) AgNO 3 (aq) GALVANIC CELL left side for anode (right side for cathode) “|” means phase boundary “||” means salt bridge

Electrochemistry Galvanic Cells Given the following cell, write the cell notation: Salt Bridge voltmeter – reads V Pt(s) FeSO 4 (aq), Fe 2 (SO 4 ) 3 (aq) Ag(s) NaCl(aq) GALVANIC CELL AgCl(s) + –

Electrochemistry Standard Reduction Potential A half cell or electrode, is half of a galvanic cell A standard electrode is one under standard conditions (e.g. 1 M AgNO 3 (aq)) Standard reduction potential (E º ) is cell potential when reducing electrode is coupled to standard hydrogen electrode (oxidation electrode) Large + E º means easily reduced compounds on electrode Large – E º means easily oxidized compounds on anode Ag(s) AgNO 3 (aq) Pt(s) H + (aq) H 2 (g)

Electrochemistry Electrolytic Cells Used in more advanced electrochemical analysis (not covered in detail) Uses voltage to drive (unfavorable) chemical reactions Example: use of voltage to oxidize phenol in an HPLC electrochemical detector (E° of 0 to 0.5 V needed) anode (note: oxidation driven by voltage, but now + charge) cathode (reduction, - charge)

Exam 1 Topics to Know A.Instrument Performance Measures know main measures discussed in class (e.g. sensitivity, selectivity, accuracy, etc.) B.Electronics 1.DC circuits (know and be able to apply: Kirchhoff ’ s Laws, Ohm ’ s Law, Power Law) 2.AC Circuits and Fourier Transforms (be able to determine frequency, have qualitative understanding of Fourier Transformation of time dependence to frequency dependence) 3.RC Circuits (know quantitatively for DC signals and for step changes in voltage, know qualitative effects for other changes such as noise) 4.Analog to Digital Signal Conversion (know how to convert between decimal and binary, and between digital signal and voltage, be able to estimate uncertainty in digitizer)

Exam 1 Topics to Know – cont. B.Electronics – cont. 5.Measurements with digital voltmeters (know how these can be used for current and resistance measurements; know errors associated with measurements) 6.Transducers (know how a few of each type work + how signal is measured) 7.Operational Amplifiers (know some uses of these, know how ideal op amp behaves, be able to solve simple circuit problems covered) 8.Noise (know how to calculate S/N and limit of detection; know main types of noise; know how to calculate thermal or shot noise; know effect of signal averaging)

Exam 1 Topics to Know – cont. C. Electrochemistry (Ch. 13) 1.Redox reaction knowledge (be able to identify elements being oxidized and reduced, be able to balance half and full reactions). 2.Be able to relate charge to moles of redox reactants consumed and to current and time.* 3.Know how to calculate electrical energy and relate it to chemical energy.* 4.Be able to identify anodes/cathodes in galvanic or electrolytic cells and their charge. 5.Know what the standard potential is a measure of.

Electrochemistry The Nernst Equation The Nernst Equation relates thermodynamic quantities to electrical quantities for a cell reaction Thermodynamics: –ΔG = ΔG º + RTlnQ ΔG = free energy, Q = reaction quotient –so, -nFE = -nFE º + RTlnQ, or E = E º – (RT/nF)lnQ –more often seen as: E = E º – ( /n)logQ (although only valid at T = 298K) –Note: in calculations, E is for reductions (even if oxidation actually occurs at that electrode) –Equation for electrodes or full cells, although text uses E cell = E + – E - where + and – refer to voltmeter leads

Electrochemistry The Nernst Equation Example: Determine the voltage for a Ag/AgCl electrode when [Cl - ] = M if Eº = V (at T = 25°C)?

Electrochemistry Applications of The Nernst Equation Examples: –The following electrode, Cd(s)|CdC 2 O 4 (s)|C 2 O 4 2- is used to determine [C 2 O 4 2- ]. It is paired with a reference electrode that has an E value of V (vs. the S.H.E.) with the reference electrode connected to the + end of the voltmeter. If Eº for the above reduction reaction is V, and the measured voltage is V, what is [C 2 O 4 2- ]?

Electrochemistry Applications of The Nernst Equation Application of Nernst Equation is most common in potentiometry In potentiometry –measured voltage is related to log[x] (where x is the analyte) –this provides a method to analyze analytes over a broad concentration range (e.g. pH electrodes function well from about pH 1 to pH 11 or over 10 orders of magnitude)

Electrochemistry Applications of The Nernst Equation Relating the Nernst Equation to Equilibrium Equations –Example problem: It is desired to use the reaction Zn(CN) 2 (s) + 2e - ↔ Zn(s) + 2CN - to measure [CN - ] in suspected poisoned drinks. However, the Eº value is not available. Given that Eº = V for Zn e - ↔ Zn(s), and K sp = 3.0 x for Zn(CN) 2 (s) ↔ Zn CN -, calculate Eº for the first reaction.