Download E - Waterford Public Schools

AP Chemistry
• As you watch the following video, answer the questions on the
sheet provided:
Decoding the Past - The Real Dr. Frankenstein
• Oxidation-reduction reaction (Redox)
• Involves a transfer of electrons from the reducing agent to the oxidizing agent
• Oxidation
• Loss of electrons
• LEO
• OIL
• Reduction
• Gain of electrons
• GER
• RIG
•
•
•
•
Oxidation number
Oxidizing agent
Reducing agent
Half-reactions
• Overall reaction is split into two half-reactions, one involving oxidation and one reduction
Work on the sample Review Problem
• Balance the following oxidation-reduction reaction IN BASIC
SOLUTION using the half-reaction method. Be sure to identify
the oxidizing agent and reducing agent.
HXeO4- (s)  XeO64- + Xe (g)
• Luigi Galvani
• Italian physician who observed a frog’s leg twitch
when it was touched with two different metals
• In attempting to explain what happened, Galvani thought
that the animal tissue in the frog’s leg was the source of
electricity
• Alessandro Volta
• Italian physicist who disputed Galvani’s hypothesis
• Resulting controversy resulted in discovery that electric
currents could be produced by chemical reactions
• Volta used this discovery to create the first chemical
battery
• In general, all chemistry is electrical in the sense
that it involves the behavior of electrons and
other charged particles
•The term electrochemistry is reserved
specifically for the study of the interchange of
chemical and electrical energy
• All of the following involve the principles of
electrochemistry:
• Remote controls for TVs, DVD players, CD players, stereos
• Itty bitty teeny tiny batteries
• Calculators
• Silverware
• Metal-plated jewelry
• Defined as a device in which
chemical energy is changed
to electrical energy
• Examples – batteries and fuel
cells
• Name comes from the work
of Volta and Galvani
• Uses a spontaneous redox
reaction to produce a current
that can be used to do
electrical work
• Electrons flow from one terminal
to the other when the terminals
are connected by an external
circuit
2Ag1+ + Cu(s)
e-
Ag
2Ag(s) + Cu 2+
e-
Cu
• Electrons are
transferred from the
ANODE to the
CATHODE using an
external metal
conductor
• Anode
• Oxidation occurs here
• Electrons leave the
cell here
Ag+
Cu2+
• Electrodes are submerged in an electrolyte
• A salt solution that contains ions
• Electrolyte may be involved in the reaction
or the ions may be used to carry the charge
• Cathode
• Reduction occurs here
• Electrons are
accepted by the
species being
reduced and enter
the cell here
e-
e-
To complete the
circuit, a salt bridge is
used
salt bridge
• A salt bridge allows ion migration in solution but
prevents extensive mixing of electrolytes
• It neutralizes charge buildups in electrode compartments
• So, it completes the circuit!
• It can be a simple porous disk or a gel saturated
with a non-interfering, strong electrolyte like KCl
NO3NO3- is released
to Cu side as Cu
is converted to Cu2+
KNO3
K+
K+ is released
as Ag+ is
converted to Ag
Salt Bridge
Porous Disk
Galvanic Cells
• AN OX
• Oxidation occurs at the anode
• RED CAT
• Reduction occurs at the cathode
• FAT CAT
• The electrons in a galvanic
(voltaic) cell always flow From
the Anode To the CATode
• Recall that a galvanic cell
consists of an oxidizing
agent in one compartment
that “pulls” electrons
through a wire from a
reducing agent in the other
compartment
• The “pull” or driving force on
the electrons is called the
CELL POTENTIAL or
electromotive force (emf)
• If pull occurs
spontaneously, cell is a
good battery!
• A VOLTMETER is used to measure cell potential
• The unit of electrical potential is the volt (V)
• Defined as 1 joule of work per coulomb of charge transferred
• The cell potential (always positive for a galvanic cell) and the
balanced cell reaction is written somewhere on the diagram
• The direction of electron flow is given
• Obtained by inspecting the half-reactions and using the direction that
gives a positive E0cell
• The anode and cathode are designated
• The nature of each electrode and the ions present in each
compartment are labeled
• A chemically inert conductor such as Pt is required if none of the
substances participating in the half-reaction is a conducting solid
• Example – Fe2+ and Fe3+
• Reaction in a galvanic cell is always an oxidationreduction reaction that can be broken down into two
half-reactions
• Each half-reaction has a cell potential
• We can obtain the overall cell potential by summing the halfcell potentials!
• A cell will always run spontaneously in the direction that
produces a POSITIVE cell potential
• Each potential is measured against an ultimate
reference electrode called the STANDARD
HYDROGEN ELECTRODE (SHE)
• SHE consists of a piece of inert
Platinum that is bathed by
hydrogen gas into a 1M HCl
solution at 1 atm
• SHE is assigned a potential of
ZERO volts
Eo = 0.000 V
• All other standard potentials
are then reported relative to
SHE
H2
Pt
plate
1 M HCl
Reduction Half-Reaction
Standard Hydrogen Electrode
• Half-reaction cell potentials are listed in a convenient table!
• The values in the table correspond to REDUCTION halfreactions with all solutions at 1 M, all gases at 1 atm, and 25°C
(298K) for all
2+ + 2 e- → Cu
Cu
Symbol for
standard
E0 = -0.34 V versus SHE
conditions!
SO42- + 4 H+ + 2 e- → H2SO3 + H2O
E0 = 0.20 V versus SHE
• Elements that have the MOST POSITIVE reduction potentials are
easily REDUCED
• In general, non-metals
• Elements that have the LEAST POSITIVE reduction potentials are
easily OXIDIZED
• In general, metals
• Table can also be used to tell the strength of various oxidizing
and reducing agents
• Another form of the activity series
• Metals having LESS POSITIVE reduction potentials are MORE active
and will replace metals with more positive potentials
E o, V
Half reaction
F2 (g) + 2e-
2F- (aq)
2.87
O2 (g) + 4H+ + 4e-
2H2O (l)
1.23
Ag+ + e-
Ag (s)
0.80
Cu2+ + 2e-
Cu0
0.34
2H+ + 2e-
H2 (g)
0.000
Fe2+ + 2e-
Fe (s)
-0.44
Zn2+ + 2e-
Zn (s)
Al3+ + 3e-
Al (s)
-1.66
Li+ + e-
Li (s)
-3.05
-0.76
• You know that both an oxidation and a reduction must occur
together
• So, one of your half reactions must be reversed!
• The “correct” (spontaneous or galvanic) direction for a reaction
is the one where Ecell is a positive value
• The half reaction with the most positive E value will proceed as
a reduction!
• The other will be reversed – oxidation!
1. Decide which element is oxidized or reduced using the table of
reduction potentials
•
THE MORE POSITIVE REDUCTION POTENTIAL GETS TO BE REDUCED
2. Write both equations AS IS from the chart with their voltages
3. REVERSE the equation that will be OXIDIZED and change the sign
of the voltage!
•
This is now E0oxidation
4. Balance the two half-reactions using integers
• Number of electrons lost must equal number gained
• DO NOT MULTIPLY VOLTAGE VALUES because cell potential is an intensive
property!
5. Add the two half reactions and the voltages together
0
0
0
Ecell
= Eoxidation
+ Ereduction
• For our copper - zinc cell at standard conditions:
E o red
Cu2+ + 2eCu (s)
Zn (s)
Zn2+ + 2e-
+ 0.34 V
+ 0.76 V
• The least positive one reverses, so the “correct” reaction at standard
conditions is:
Cu2+ + Zn (s)
Cu (s) + Zn2+
Ecell = 0.34 v + 0.76 v = 1.10 V
• Consider a galvanic cell based on the reaction:
Al3+ aq + Mg s → Al s + Mg 2+ (aq)
• Give the balanced cell reaction and calculate E0 for the cell
• Rather than drawing an entire cell, a type of shorthand can be
used – line notation
• Line notation can be thought of as an “Ion Sandwich” in
alphabetical order
Anode metal | Anode ion || Cathode ion | Cathode metal
• “|” indicates phase boundary (solid → solution or gas or solution or gas → solid)
• “||” indicates salt bridge
Mg s
Mg 2+ aq
Al3+ aq
Al (s)
• Phase (AND concentration if not 1M) specified in parentheses
• ZnSO4 (aq, 0.5M)
• A comma should be used to separate 2 components in the same
phase
Pt s Fe2+ aq , Fe3+ (aq) Ag + aq Ag (s)
• Calculate the cell voltage for the following reaction. Draw a
diagram of the galvanic cell for the reaction and label
completely
• See previous slide for requirements of a complete diagram
Fe3+ (aq) + Cu (s) → Cu2+ (aq) + Fe2+ (aq)