macgrew hill ppt for electrochemistry module 1

1. Voltaic Cells
Lesson 1

2. Focus Question
How do voltaic cells harness energy from
chemical reactions?

3. New Vocabulary
salt bridge
electrochemical cell
voltaic cell
half-cell
anode
cathode
reduction potential
standard hydrogen electrode

4. Review Vocabulary
oxidation: the complete or partial loss of electrons
from a reacting substance; increases an atom’s or
ion’s oxidation number
reduction: the complete or partial gain of electrons
by the atoms of a substance; decreases the atom’s
or ion’s oxidation number

5. • Electrochemistry is the study of the redox
processes by which chemical energy is converted to
electrical energy and vice versa.
• Redox reactions involve a transfer of electrons from
the species that is oxidized to the species that is
reduced.
Redox in Electrochemistry

6. • A salt bridge is a pathway to allow the passage of
ions from one side to another, so that ions do not
build up around the electrodes.
• An electrochemical cell is an apparatus that uses a
redox reaction to produce electrical energy or uses
electrical energy to cause a chemical reaction.
Redox in Electrochemistry

7. • A voltaic cell is a type of electrochemical cell that
converts chemical energy to electrical energy by a
spontaneous redox reaction.
Redox in Electrochemistry

8. • An electrochemical cell consists of two parts called
half-cells, in which the separate oxidation and
reduction reactions take place.
• The electrode where oxidation takes place is called
the anode.
• The cathode is the electrode where reduction occurs.
• Electric potential energy is a measure of the amount
of current that can be generated from a voltaic cell to
do work.
Chemistry of Voltaic Cells

9. • Electric charge can flow between two points only
when a difference in electric potential energy exists
between the two points.
• A volt is a unit used to measure cell potential—the
force from the difference in electric potential energy
between two electrodes.
Chemistry of Voltaic Cells

10. • The tendency of a
substance to gain
electrons is its reduction
potential.
• When two half-reactions
are coupled, the voltage
generated corresponds
to the difference in
potential between the
half-reactions.
Calculating Electrochemical Cell Potentials

11. • The standard hydrogen electrode consists of a
small sheet of platinum immersed in a hydrochloric
acid solution that has a hydrogen ion concentration
of 1 M. Hydrogen gas (H2), at a pressure of 1 atm, is
bubbled in and the temperature is maintained at
25°C.
• The standard hydrogen electrode is the standard
against which all other reduction potentials are
measured.
Calculating Electrochemical Cell Potentials

12. Calculating Electrochemical Cell Potentials
• Formula for Cell Potential
• The standard potential of a cell is the standard
potential of the reduction half-cell minus the standard
potential of the oxidation half-cell
𝐸cell
0
= 𝐸reduction
0
− 𝐸oxidation
0

13. Use with Example Problem 1.
Problem
The following reduction half-reactions
represent the half-cells of a voltaic cell.
I2(s) + 2e– → 2I–(aq)
Fe2+(aq) + 2e– → Fe(s)
Determine the overall cell reaction and the
standard cell potential. Describe the cell
using cell notation.
Response
ANALYZE THE PROBLEM
You are given the half-cell equations and can find
standard reduction potentials in Table 1. The half-
reaction with the lower reduction potential will
be an oxidation. With this information, you can
write the overall cell reaction, calculate the
standard cell potential, and describe the cell in
cell notation.
KNOWN UNKNOWN
Standard reduction
potentials for the half-cells
overall cell
reaction = ?
E 0
cell
= E 0
reduction
− E 0
oxidation
E 0
cell
= ?
cell notation = ?
SOLVE FOR THE UNKNOWN
Find the standard reduction potentials of each
half-reaction in Table 1.
I2(s) + 2e–→2I–(aq) E 0
I2|I– = +0.536 V
Fe2+(aq) + 2e–→Fe(s) E 0
Fe2+|Fe
= –0.447 V
The reduction of iodine has the higher reduction
potential, so this half-reaction proceeds in the
forward direction as a reduction. The iron half-
reaction proceeds in the reverse direction as an
oxidation.
Calculate a Cell Potential

14. EVALUATE THE ANSWER
The calculated potential is reasonable given
the potentials of the half-cells. E0 is reported
to the correct number of significant figures.
SOLVE FOR THE UNKNOWN (continued)
• Rewrite the iron half-reaction in the correct
direction.
I2(s) + 2e–→2I–(aq) (reduction half-cell reaction)
Fe2+(aq)+2e–→Fe(s) (oxidation half-cell reaction)
• Add the two equations.
I2(s) + Fe(s) → Fe2+(aq) + 2I–(aq)
Calculate the standard cell potential.
• State the formula for cell potential.
E0
cell
= E 0
reduction
– E 0
oxidation
• Substitute E0
I2|I– and E0
Fe2+|Fe
in the generic
equation.
E0
cell
= E 0
I2|I– – E0
Fe2+|Fe
• Substitute E 0
I2|I– = +0.536 V and
E 0
Fe2+|Fe
= –0.447 V.
E0
cell
= +0.536 V – (–0.447 V)
E0
cell
= +0.983V
Describe the cell using cell notation.
• First, write the oxidation half-reaction
using cell notation: reactant then product.
Fe | Fe2+
• Next, write the reduction half-reaction to
the right. Separate the half-cells by a
double vertical line.
Fe | Fe2+ || I2 | I–
Cell notation: Fe | Fe2+ || I2 | I–
Calculate a Cell Potential

15. • Cell potentials can be used to determine if a
proposed reaction under standard conditions will
be spontaneous.
• If the calculated potential is positive, the reaction is
spontaneous.
• If the calculated potential is negative, the reaction
is not spontaneous.
Use Standard Reduction Potentials

16. Quiz
standard hydrogen electrode
D
salt bridge
C
half-cell
B
voltaic cell
A
Which is a pathway to allow the passage of ions
from one side of a redox process to the other?
1.
CORRECT

17. Quiz
electrochemical cell
D
standard hydrogen
electrode
C
half-cell
B
salt bridge
A
Which is an apparatus that uses a redox reaction
to produce electrical energy or uses electrical
energy to cause a chemical reaction?
2.
CORRECT

18. Quiz
oxidation cell
D
half-cell
C
salt cell
B
voltaic cell
A
Which is a type of electrochemical cell that converts
chemical energy to electrical energy by a spontaneous
redox reaction?
3.
CORRECT

19. Quiz
cathodes
D
half-cells
C
anodes
B
voltaic cells
A
Which is the name of the two parts of an electrochemical
cell, in which the separate oxidation and reduction
reactions take place?
4.
CORRECT

20. Quiz
reaction potential
D
oxidation potential
C
electrochemical
potential
B
reduction potential
A
Which is the tendency of a substance to gain
electrons?
5.
CORRECT