Table of Contents
Chapter 21: Electrochemistry
21.1 – Voltaic Cells
21.2 – Types of Batteries
21.3 – Electrolysis
Table of Contents
Chapter 21: Electrochemistry
21.1 – Voltaic Cells
Basic Assessment Questions
Try it out!
Identify what is oxidized, what is reduced, the
oxidizing agent & the reducing agent.
Zn + NiSO4 → Ni + ZnSO4
Ox: Zn
Red: Ni2+
Ox ag: Ni2+
Red ag: Zn
• Define
1. Redox reactions
2. Oxidation
3. Reduction
4. Half-reactions
Electrochemistry: Basic Concepts
• Electrochemistry is using chemistry to create
electricity
• Electrical current: The
flow of electrons in a
particular direction
• Redox reactions can be
used to produce an
electrical current.
• This is what occurs in a
battery—one form of an
electrochemical cell
Electrochemistry: Basic Concepts
Definitions
• An electrochemical cell is a device that uses
redox reactions to create electricity OR
electric energy to cause chemical reactions
• A voltaic or galvanic cell converts chemical
energy to electrical energy by a spontaneous
redox reaction. It is a type of
electrochemical cell.
Balanced: Zn(s) + CuSO4(aq) → ZnSO4(aq) + Cu(s)
Total ionic:
Zn(s) + Cu2+(aq) + SO42-(aq) → Zn2+(aq) + SO42-(aq) + Cu(s)
Net ionic: Zn(s) + Cu2+(aq) → Zn2+(aq) + Cu(s)
red
ox
Half Reactions:
Zn(s) → Zn2+(aq) + 2e-
Cu2+(aq) + 2e-→ Cu(s)
Electrochemical Cell
Zinc strip
Copper strip
Cannot transfer eSO42-
SO42-
1M Zn2+
1M Cu2+
Zn(s) → Zn2+(aq) + 2e-
Cu2+(aq) + 2e-→ Cu(s)
Electrochemical Cell
Copper wire
(+) charge builds up in one
solution, (-) charge builds
up in the other
Allows etransfer
Electrochemical Cell
Allows ions
to pass from
one side to
the other
Salt bridge
KCl
Ions pass
through
plugs in
the bridge,
but
solutions
do not mix
Electrochemical Cell
e- flow
Cle-
e-
Zn
K+
Cl-
Zn2+
Zn → Zn2+ + 2e-
K+
e- e-
Cl-
K+
Cu
Cu2+
Cu2+ + 2e- → Cu
What is this called? What does it do?
Why can redox reactions create electricity?
e- flow
KCl
Zn
Cu
1M Zn2+
Zn → Zn2+ + 2e-
1M Cu2+
Cu2+ + 2e- → Cu
Voltaic/Galvanic Cell
e- flow
KCl
electrodes
electrodes
half-cells
Zn
1M Zn2+
Cu
1M Cu2+
electrolytes
Electrochemistry: Basic Concepts
Definitions
• Half-cells: Where oxidation and reduction
reactions separately take place (2 parts)
• Electrode: an object in the half-cell that
conducts electrons to or from another
substance
• Electrodes are immersed in electrolytes:
usually a solution of ions
Voltaic/Galvanic Cell
Voltaic
Cell
e- flow
KCl
Zn
oxidation
ANODE
Cu
1M Zn2+
Zn → Zn2+ + 2e-
1M Cu2+ reduction
CATHODE
Cu2+ + 2e- → Cu
Electrochemistry: Basic Concepts
Definitions
• Anode: The electrode where oxidation takes
place
• Cathode: The electrode where reduction
takes place
Electrochemistry: Basic Concepts
Practice
• In the next slide, identify…
• Salt bridge
• What is oxidized
• What is reduced
• Cathode
• Anode
• Direction of e- flow through wire
Voltaic/Galvanic Cell
oxidized
reduced
Energy in a cell
• We need to be able to…
o find out how much energy we can get out of a
cell
o write the balanced equations of cells from
half-reactions (tomorrow)
Electrochemistry: Basic Concepts
Equations of Cells
• Standard Reduction Potentials: lists which
are more likely to take electrons
reduced
Cu2+ + 2e- → Cu
Cu2+ | Cu
• More (+) means it’s more likely to take
electrons
Electrochemistry: Basic Concepts
Copper-Hydrogen cell under standard conditions
H2 (g) + Cu2+(aq) → 2H+(aq) + Cu(s)
(net ionic equation)
Cell notation
H2 | H+ || Cu2+ | Cu
oxidation half-cell
ANODE
reduction half-cell
CATHODE
salt bridge and wire
Electrochemistry: Basic Concepts
Cell Potential
• To find a voltaic cell’s standard potential
(how many volts it can use):
Eºcell = Eºreduction - Eºoxidation
• HINT: Write down everything. Show all
work. When you do it in your head, you get
confused.
Electrochemistry: Basic Concepts
Examples: Find the cell potential of a copper-zinc
cell.
Cu2+(aq) + Zn(s) → Cu(s) + Zn2+(aq)
Step 1: Cell notation: Zn | Zn2+ || Cu2+ | Cu
Step 2: ID ox and red
ox
red
Step 3: Find reduction potentials
Zn2+|Zn -0.762 V
Cu2+ | Cu +0.342 V
Step 4: Equation Eºcell = Eºred - Eºox
Electrochemistry: Basic Concepts
Examples: Find the cell potential of a copper-zinc
cell.
Step 4: Equation Eºcell = Eºred - Eºox
= EºCu2+|Cu - EºZn2+|Zn
= 0.342 V – (-0.762 V)
= +1.104 V
The standard reduction potential of the copperzinc cell is +1.104 V
Additional Assessment Questions
Try it out!
Calculate the cell potential.
2Ag+(aq) + Co(s) → Co2+(aq) + 2Ag(s)
+ 1.08V
Electrochemistry: Basic Concepts
Potential Difference
• A spontaneous reaction: proceeds naturally.
Voltaic cells are always spontaneous.
• A non- spontaneous reaction: requires an
outside influence to proceed. (eg. forcing the
electrons in the opposite direction)
• In a Electrochemical cell…
• when Eº = (+), it is spontaneous
• when Eº = (-), it is not spontaneous
Electrochemistry: Basic Concepts
Spontaneity in a Cell
1. Eºcell = + 1.322 V
spontaneous
2. Eºcell = + 0.214 V
spontaneous
3. Eºcell = - 0.071 V
not spontaneous
4. Eºcell = + 0.421 V
spontaneous
5. Eºcell = - 1.125 V
not spontaneous
Additional Assessment Questions
Question 2b
Calculate the cell potential to determine if each
of these redox reactions is spontaneous.
1.
2.
3.
4.
Mn2+ + 2Br- → Br2 + Mn
Fe2+ + Sn2+ → Fe3+ + Sn
Ni2+ + Mg → Mg2+ + Ni
Pb2+ + Cu+ → Pb + Cu2+
-2.251 V, nonspontaneous
-0.908 V, nonspontaneous
+2.115 V, spontaneous
-0.279 V, nonspontaneous
Practice problems
• Voltaic Cells handout
• Due tomorrow at the end of class
• Finished? Read ch 21 – it really will help!
• I will not be covering 21.2 or 21.3 in class, but there
are WebAssign questions on it - you’re going to
have to read it!
Part 1
Oxidation half: Cu → Cu2+ + 2eReduction half: Ag+ + e- → Ag
Activity
• At your tables, use whatever you have in your
backpacks to make model of an electrochemical
cell
• Be able to explain to another group what each
object represents in the “electrochemical cell” and
what it does in the “cell”
• You have 3 minutes
Electrochemistry: Basic Concepts
Sometimes, you must find a balanced
reaction of cells from only reduction halfreactions.
We will go through steps to follow and
examples
Electrochemistry: Basic Concepts
Equations of Cells
• Standard Reduction Potentials: lists which
are more likely to take electrons
reduced
Cu2+ + 2e- → Cu
Cu2+ | Cu
• When comparing standard reduction
potentials in a cell, the ½ reaction that is…
• more (+) is reduction
• more (-) is oxidation
Electrochemistry: Basic Concepts
1) Identify which reaction is oxidation and which is
reduction, 2) write the balanced equation
Ag+(aq) + e- → Ag(s) EºAg+|Ag = +0.800 V reduction
Ni2+ + 2e- → Ni(s)
EºNi2+|Ni = -0.257 V
Ni(s)
→ Ni2+ + 2e2 Ag+(aq) + 2 e- → 2 Ag(s)
Ni(s) + 2Ag+(aq) → Ni2+ + 2Ag(s)
oxidation
Electrochemistry: Basic Concepts
1) Identify which reaction is oxidation and which is
reduction, 2) write the balanced equation
Magnesium in a solution of Mg2+
Lead in a solution of Pb2+
Mg2+(aq) + 2e- → Mg(s)
Pb2+(aq) + 2e- → Pb(s)
Electrochemistry: Basic Concepts
1) Identify which reaction is oxidation and which is
reduction, 2) write the balanced equation
Mg2+(aq) + 2e- → Mg(s)
EºMg2+|Mg = -2.372 V
ox
Pb2+(aq) + 2e- → Pb(s)
EºPb2+|Pb = -0.126 V
red
Mg(s)
→ Mg2+(aq) + 2ePb2+(aq) + 2e- → Pb(s)
Mg(s) + Pb2+ → Mg2+(aq) + Pb(s)
Table of Contents
Chapter 21: Electrochemistry
21.2 – Types of Batteries
21.3 – Electrolysis
In class we will cover only what you need
to know for the test. Please use your book
to answer questions for WebAssign.
Electrochemistry: Basic Concepts
21.2: Batteries
• Alessandro Volta:
• If one cell generates a current, several
cells should make a larger current
• He piled several cells together to make the
first battery
• Batteries are one or more electrochemical
cells in a single package that generates
electrical current
Electrochemistry: Basic Concepts
21.3: Electrolysis
• Electrolysis is using electric energy to bring
about a chemical reaction.
• Electrolytic cell: An electrochemical cell in
which electrolysis occurs.
• Electrolysis forces a current in the reverse
direction (a nonspontaneous reaction) by
passing an electric current through it
(recharging)
Electrochemistry: Basic Concepts
Voltaic Cell
e-
e-
Electrolytic Cell
e-
Voltage
source
e-
An electrolytic cell is just the opposite of a voltaic cell
Electrochemistry: Basic Concepts
Electroplating
• Reduction of silver ions onto cheaper metals
forms silverplate.
Click box to view movie clip.
Practice problems
• Finish Ch 21 Practice Problems (due at the end of
class)
• Begin WebAssign (due Monday, 11 pm)
• You will need your book (21.2 & 21.3) for some
questions on WebAssign. Now is a great time to
look those up.
• Review for test is Monday!
• Test on Ch 10, 20, 21 is Tuesday!
End of Ch 21
Electrochemistry: Basic Concepts
The Electrolysis Process
Click box to view movie clip.
Electrochemistry: Basic Concepts
Electrolytic Cell
Basic Assessment Questions
Distinguish between a voltaic cell and an
electrolytic cell.
spontaneous redox reaction;
nonspontaneous redox reaction by electrolysis
Electrochemistry: Basic Concepts
Electroplating
• Reduction of silver ions onto cheaper metals
forms silverplate.
Click box to view movie clip.
Table of Contents
Chapter 21: Electrochemistry
21.2 – Types of Batteries
In class we will cover batteries only briefly.
You will not be tested on this. Don’t take
notes. Please use your book to answer
questions for WebAssign.
Electrochemistry: Basic Concepts
Batteries
• Alessandro Volta:
• If one cell generates a current, several
cells should make a larger current
• He piled several cells together to make the
first battery
• Batteries are one or more electrochemical
cells in a single package that generates
electrical current
Electrochemistry: Basic Concepts
Electroplating
• The object to be plated is made the cathode.
• At the pure silver
anode, oxidation of
silver metal to
silver ions replaces
the silver ions
removed from the
solution by plating
at the cathode.
Electrochemistry: Basic Concepts
•
•
•
•
Electrolytic Cleaning
Electrolysis can be used to clean objects by
pulling ionic dirt away from them.
The electrolysis cell for this cleaning process
includes a cathode that is the object itself, a
stainless steel anode, and an alkaline
electrolyte.
When an electric current is run through the
cell, the chloride ions are drawn out.
Hydrogen gas forms and bubbles out, helping
to loosen corrosion products.
Basic Assessment Questions
Question 1
To electroplate an iron fork with silver, which of the
following must be true?
a. The silver electrode must have more mass than the
fork
b. The iron fork must act as the anode of the cell
c. Electrical current must be applied to the iron fork
d. Iron ions must be present in the cell solution
Electrochemistry: Basic Concepts
Batteries Perform Work
Electrochemistry: Basic Concepts
Modern Batteries
Electrochemistry: Basic Concepts
Modern Batteries
• Although the term battery usually refers to
a series of galvanic cells connected together,
some batteries have only one such cell.
• Other batteries may have a dozen or more
cells.
Electrochemistry: Basic Concepts
Dry Cells
A dry cell is an electrochemical cell in which
the electrolyte (electrode) is a moist paste
Electrochemistry: Basic Concepts
Carbon-Zinc Dry Cell
• A standard D battery is shown both whole
and cut in half to reveal the structure of the
carbon-zinc dry cell.
• Beneath the outside paper
cover of the battery is a
cylinder casing made of
zinc.
• The zinc serves as the
anode and will be oxidized
in the redox reaction.
Electrochemistry: Basic Concepts
Carbon-Zinc Dry Cell
• The carbon rod in the center of the cylinder—
surrounded by a moist, black paste of
manganese (IV)
oxide (MnO2) and
carbon black—acts
as a cathode.
• Ammonium chloride
(NH4Cl) and zinc
chloride (ZnCl2) serve
as electrolytes.
Electrochemistry: Basic Concepts
Carbon-Zinc Dry Cell
• Alkaline batteries contain potassium
hydroxide (KOH) in place of the ammonium
chloride electrolyte,
and they maintain a
high voltage for a
longer period of time.
Electrochemistry: Basic Concepts
Dry Cell
• Primary batteries contain redox reactions that
are not easily reversed. They must be
discarded when the reaction is complete.
• Secondary batteries contain redox reactions
that are easily reversed. They can be
recharged.
• Storage batteries
• Nickel-cadmium rechargeable batteries
Electrochemistry: Basic Concepts
Lead Storage Batteries
• Anode = grids of
porous lead
• Cathode = grids of
lead (IV) oxide
• Produces water and
lead (II) sulfate
(PbSO4)
Electrochemistry: Basic Concepts
Lead Storage Batteries
This battery is
rechargeable because
one can reverse the
reaction
Also called lead-acid
batteries because the
electrolyte is a solution
of sulfuric acid
Often used for car batteries
Electrochemistry: Basic Concepts
Aqueous Lithium Battery
Lightweight batteries
that store a large
amount of energy for
their size.
Lithium:
-lightest known metal
-Lowest SRP
-Oxidation of Li
Electrochemistry: Basic Concepts
Aqueous Lithium Battery
Primary & secondary
batteries
About 3V
Long lasting
Watches, computers,
cameras, etc.
Electrochemistry: Basic Concepts
Fuel Cell
• Burning fuel is an oxidation-reduction reaction
• A fuel cell is a voltaic cell in which the
oxidation of a fuel is used to produce electric
energy.
• Burns fuel in a controlled way while
converting chemical energy to electric
energy, not heat energy
• Cell runs as long as there is fuel