Chem. 133 – 2/25 Lecture
Announcements I
• Exam 1
– is scheduled for Tuesday
– will cover Electronics plus some of electrochemistry (up to but
not including the Nernst Equation)
– will review topics after covering the parts of electrochemistry on
Exam 1
– no help session – but I can have an additional office hour Friday
9 to 10
• Homework Set 1.3
–
–
–
–
Due Date is 3/1 (Tuesday for exam)
Nothing collected
Shortened (last problem is now Harris 13.10)
Solutions have been posted
Announcements II
• Lab
– Finishing Set 2 Period 1 labs today (still make up day if needed)
– On Tuesday we start the Set 2 Period 2 labs
– I will Add to the table (for Period 3 and Period 4 labs)
• Today’s Lecture
– Electrochemistry Topics on Exam 1
• example: charge of battery problem
• galvanic cells
• electrolytic cells
– Review of material on Exam 1
– Electrochemistry Topics not on Exam I (if time)
• the Nernst equation and its applications
Electrochemistry
Fundamental Equations
• Example problem:
A NiCad battery contains 12.0 g of Cd that is
oxidized to Cd(OH)2. How long should the
battery last if a motor is drawing 421 mA?
Assume 100% efficiency.
Electrochemistry
Galvanic Cells
• What are galvanic cells?
GALVANIC CELL
– Cells that use chemical
reactions to generate electrical
energy
– Batteries are examples of
Zn(s)
useful galvanic cells
– Example reaction
voltmeter
Ag(s)
Zn(s) + 2Ag+ → Zn2+ + 2Ag(s)
– If reactants are placed in a
beaker, only products + heat
are produced
– When half reactions are
isolated on electrodes,
ZnSO4(aq)
electrical work can be
produced
AgNO3(aq)
Salt Bridge
Electrochemistry
Galvanic Cells
• Description of how example cell
works
GALVANIC CELL
– Reaction on anode = oxidation
voltmeter
– Anode = Zn electrode (as the Eº
for Zn2+ is less than for that for
Ag+)
Zn(s)
– 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
ZnSO4(aq)
Zn(s) → Zn2+ + 2e-
Ag+ + e- → Ag(s)
+
Salt Bridge
Ag(s)
AgNO3(aq)
Electrochemistry
Galvanic Cells
• Cell notation
GALVANIC CELL
– Example Cell:
voltmeter
Zn(s)|ZnSO4(aq)||AgNO3(aq)|Ag(s)
Zn(s)
Ag(s)
“|” means phase
boundary
left side for
anode (right
side for
“||” means salt bridge
cathode)
AgNO3(aq)
ZnSO4(aq)
Salt Bridge
Electrochemistry
Galvanic Cells
• Given the following
cell, write the cell
notation:
GALVANIC CELL
voltmeter – reads
+0.43 V
Pt(s)
+
–
Ag(s)
AgCl(s)
FeSO4 (aq),
Fe2(SO4)3(aq)
NaCl(aq)
Salt Bridge
Electrochemistry
Galvanic Cells
• Example Questions
– Given the following cell, answer the following
question:
MnO2(s)|Mn2+(aq)||Cr3+(aq)|Cr(s)
– What compound is used for the anode?
– What compound is used for the cathode?
– Write out both half-cell reactions and a net reaction
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
AgNO3(aq))
Pt(s)
• Standard reduction potential (Eº) is
cell potential when reducing
electrode is coupled to standard
hydrogen electrode (oxidation
electrode)
• Large + Eº means easily reduced
compounds on electrode
H2(g)
• Large – Eº means easily oxidized
H+(aq)
compounds on anode
Ag(s)
AgNO3(aq)
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.
B.
Instrument Performance Measures
know main measures discussed in class (e.g. sensitivity, selectivity,
accuracy, etc.)
Electronics
1.
2.
3.
4.
DC circuits (know and be able to apply: Kirchhoff’s Laws,
Ohm’s Law, Power Law)
AC Circuits and Fourier Transforms (be able to determine
frequency, have qualitative understanding of Fourier
Transformation of time dependence to frequency dependence)
RC Circuits (know quantitatively for for step changes in
voltage, know qualitative effects for other changes such as
noise)
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,
significance of input range, know A/D performance
parameters)
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 general uses)
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; know ways to reduce noise)
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.
Exam 1
Equations Given On Exam
1.
2.
3.
4.
5.
VR in response to step change in DVin in RC circuit (VR
= DVin·e-t/RC
Equation for standard deviation
Equation for converting between voltage to A/D board
and decimal number recorded
(decimal # = (V – Vmin)2n/(Vmax – Vmin)
Equations for thermal and shot noise
Definition of minimum observable signal (e.g. 3s for
use in calculating limit of detection)
You are responsible for all other equations (e.g. V = IR, P
= IV, etc.)
Constants will also be provided
Electrochemistry
The Nernst Equation
• The Nernst Equation relates thermodynamic quantities to
electrical quantities for a cell reaction
• Thermodynamics:
– ΔG = ΔGº + RTlnQ ΔG = free energy, Q = reaction quotient
– so, -nFE = -nFEº + RTlnQ, or E = Eº – (RT/nF)lnQ
– more often seen as: E = Eº – (0.05916/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 Ecell = E+
– E- where + and – refer to voltmeter leads
– Best to use activities in Q (even though we will just use
concentrations)
Electrochemistry
The Nernst Equation
• Example: Determine the voltage for a
Ag/AgCl electrode when [Cl-] = 0.010 M if
Eº = 0.222 V (at T = 25°C)?
Electrochemistry
Applications of The Nernst Equation
• Examples:
– The following electrode,
Cd(s)|CdC2O4(s)|C2O42- is used to determine
[C2O42-]. It is paired with a reference
electrode that has an E value of 0.197 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 -0.522
V, and the measured voltage is 0.647 V, what
is [C2O42-]?