Transcript Document

Chapter 14 – Fundamentals of Electrochemistry
Homework -
Due Friday, April 1
Problems: 14-4, 14-5, 14-8, 14-12, 14-15,
14-17, 14-18, 14-25, 14-26, 14-41,
CHM 320 - Lecture 23 Chapt 14
Electrochemistry
Review of the Basics
• Oxidation
• Reduction
– Loss of electrons
– Always occurs at the
anode
– Happens because of
the action of a
reducing agent
– Gain of electrons
(charge is reduced)
– Always occurs at the
cathode
– Happens because of
the action of the
oxidizing agent
Fe+3 + e- =
CHM 320 - Lecture 23 Chapt 14
Fe+2
Redox Reaction
ox1 +
Example:
red2 = red1
+
ox2
2(e- + Fe+3 = Fe+2)
+
Sn+2 = Sn+4 + 2 e-----------------------------------------2 Fe+3 + Sn+2 = 2 Fe+2 + Sn+4
So…
+2, red =Fe+2,
ox1= Fe+3 CHM
, 320red
=
Sn
- Lecture
2 23 Chapt 14
1
ox =Sn+4
Electric Charge (in Coulombs) and Work
• Voltage represents electrical
• The charge in
potential (potential to do
coulombs (q) is equal
work)
to the number of
moles of electrons (n) • If some total charge in
coulombs (q) is moved
times the Faraday
through some electrical
Constant (F)
potential (E, in volts V) then
work is done!
q (coulombs)  n (moles) x F (Faraday Constant)
4 Coulombs
F  9.649E10
mole of e Work (joules)  E (volts) x q (coulombs)
CHM 320 - Lecture 23 Chapt 14
Ohm’s Law and Power
• Ohm’s law relates electrical resistance, current and
potential!
• Power is the work done in some unit time (e.g.
joules of work per second)
• The units of Power are Watts (W)
• Ohm’s law and power are related!
E(potential)  I (current) x R (resistanc e, in Ohms, )
Work (joules) E x q
Power (in Watts) 

second
s
Exq
q
E x E xI
s
s
CHM 320 - Lecture 23 Chapt 14
Let’s Work Some Problems
• A 6.00V battery is connected across a
2.00 K resistor, how many electrons
flow through the circuit per second?
• How many joules of heat (heat is work)
are produced per electron?
• What voltage would the battery need to
be to deliver a power at 100.0 Watts?
CHM 320 - Lecture 23 Chapt 14
Electrochemical Cells
• A complete cell
contains:
– anode
– cathode
– completed circuit (for
electrons to flow)
– a salt bridge
(usually!)
– an electrolyte solution
– chemical species that
undergo reaction.
• There are two basic
electrochemical cells:
– A GALVANIC cell
uses spontaneous chemical
reactions to generate
electricity
– A ELECTROYLTIC
cell requires an electrical
potential to be applied to
the cell to drive some
reaction.
CHM 320 - Lecture 23 Chapt 14
Galvanic Cell
Cells and Cell Reactions
Overall Cell Reaction
Zn(s) + Cu+2(aq) ---> Zn+2(aq) + Cu(s)
oxidation half reaction
anode
Zn(s) ---> Zn+2(aq) + 2 ereduction half reaction
cathode Cu+2(aq) + 2 e- ---> Cu(s)
CHM 320 - Lecture 23 Chapt 14
CHM 320 - Lecture 23 Chapt 14
CHM 320 - Lecture 23 Chapt 14
What is happening at the
electrode(s) and how do we
describe the cell?
Anode half - rxn : Zn 0(s)  Zn 2 (aq)  2 e 2
Cathode half - rxn : Cu(aq)
 2 e -  Cu0(s)
Complete Cell reaction : Zn (s)  Cu2 (aq)  Zn 2 (aq)  Cu0 (s)
in shorthand, we use symbols!
a single vertical line marks the phase difference
a double vertical line marks the salt bridge
Anode on the left, cathode on the right
Including the counter - ions tells us something about the solutions
Zn (s) | ZnSO 4(aq) || CuSO4(aq) | Cu(s)
CHM 320 - Lecture 23 Chapt 14
The Standard Hydrogen Electrode (SHE)
• The basis by which all other measurements are made.
• Assigned a potential of zero by definition!
• Not practical for regular use
Hydrogen Half-Cell
H2(g) = 2 H+(aq) + 2 ereversible reaction
SHE consists of a platinum
electrode covered with a
fine powder of platinum
around which H2(g) is
bubbled. Its potential is
defined as zero volts.
CHM 320 - Lecture 23 Chapt 14
CHM 320 - Lecture 23 Chapt 14
CHM 320 - Lecture 23 Chapt 14
Standard Potentials
• Standardized potentials (Eo), listed as reductions, for all
half-reactions
• Measured versus the S.H.E (0)
• Used in predicting the action in either a galvanic cell or
how much energy would be needed to force a specific
reaction in a non-spontaneous cell
• Assumes an activity of one for the species of interest
(usually a fair approximation) at a known temperature
in a cell with the S.H.E.
• Assumes that the cell of interest is connected to the (+)
terminal of the potentiometer (voltmeter) and the
S.H.E. is connected to the (-) terminal
CHM 320 - Lecture 23 Chapt 14
Better Oxidizing Agents
in upper left hand corner.
CHM 320 - Lecture 23 Chapt 14
Better Reducing
Agents in lower
Right hand corner