Transcript 2/23 Lecture Notes

Chem. 133 – 2/23 Lecture
Announcements I
• Homework Set 1.2 – I posted solutions on Friday, but
then found a few errors (on 1.2.2 and 1.2.5) when
grading the problems. These now have been corrected.
• Return Q2 and HW1.2
• Exam 1
– is scheduled for next Tuesday
– will cover Electronics plus some of electrochemistry (what I’m
planning on covering today)
– format: part multiple choice/short answer, part problems
– see S’15 class website for example exam (link now working on
class website)
– will review topics on Thursday
– Can have a review session Monday 11 to 12? or 12 to 1?
Announcements II
• Electronics Lab Reports – due today
• Term Project Topics due (again, no penalty for being
late, I can give a brief description of possible STORC
projects for those interested)
• Today’s Lecture
– Noise – just questions not gotten to last time
– Electrochemistry
•
•
•
•
•
intro
redox balancing
definitions
galvanic cells
electrolytic cells
Noise
Questions
1.
2.
3.
4.
5.
6.
7.
What type of noise is likely to be present when using
thermocouples to measure temperature?
Why is modulation normally required to reduce 1/f
noise?
What is the percent noise on a current producing
transducer which generates signal over a 1000 Hz
band if the signal is 10 nA? if the signal is 2.0 pA?
What specific type of noise is reduced best by
shielding electronics?
How would use of a low pass filter reduce shot noise?
Suggest one method for reducing thermal noise.
What type of noise is not effectively reduced by using
a low pass filter?
Electronics
Additional Questions
250
200
a)
Voltage (mV)
• Answer the questions 1-3 from the
following plots which were obtained
from background measurements
(instrument noise):
150
100
50
0
1. Which plot is most likely shows 1/f
noise: ______________________
-50
0
50
100
150
Time (ms)
180
160
will produce a plot with a peak at 55
Hz: ______________
140
120
Voltage (mV)
2. Which plot when Fourier transformed b)
100
80
60
40
20
0
0
40
60
80
100
120
140
160
time (ms)
120
100
Voltage (mV)
3. If plot c) shows noise from a GC signal
in which peaks typically are on the
c)
order of 2 s (2000 ms) wide, what can
be done to reduce the noise?
20
80
60
40
20
0
0
50
100
Time (ms)
150
Electrochemistry
Overview
• Applications
– quantitative analysis
• potential measurement methods (e.g. pH electrode)
• current based measurements (amperometry)
– qualitative analysis (voltammetry)
– note: potential normally gives qualitative information
and current quantitative measurements
– HPLC/IC detectors
• Why Use?
– lower cost (both for instruments and supplies)
– high sensitivity possible (particularly mass sensitivity)
– simpler equipment, more useful for field, in-situ type
measurements
Electrochemistry
Redox Reactions
• Reduction = loss of charge
– e.g. Fe3+ + e- → Fe2+
• Oxidation = gain in charge
– e.g. Pb2+ + 2H2O → PbO2(s) + 4H+ + 2e- (Pb goes
from +2 to +4)
• Balancing reactions
– review steps in general chemistry book
– example: Zn(s) + Cr2O72- → Zn2+ + Cr3+
– note: based on methods used for problems in this
book, full cell balancing may not be needed
Electrochemistry
Fundamental Equations
• Relationship between charge, energy and
current
– redox reactions involve the exchange of electrons
– when the exchange occurs on an electrode surface,
current can be measured
– Total charge transfer = q = nF, where
• n = moles of electrons in reaction and
• F = Faraday’s constant = 96500 C/moles e• F = NAvogadro·e (e = elementary charge = 1.6 x 10-19 C)
– Current Produced = I = dq/dt
• or q = ∫I·dt (or = I·t under constant current conditions)
• can be used to determine battery lifetime
Electrochemistry
Fundamental Equations
• Relationship between charge, energy and
current (continued)
– Electrical work = E·q (E = potential in volts)
– and ΔG = -E·q = -nFE
– under standard conditions (1 M reactant/product
conc., 298K, etc.), ΔGº = -nFEº
– ΔGº are given in Tables and allows calculation of K
values
– Eº, standard reduction potential, also given in Tables
(see Appendix H), but for “half-reactions”
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)