Please Pick Up Electrochemical Cells Problem Set

Please Pick Up

Electrochemical Cells Problem Set

Electrochemical CellsEdward A. Mottel

Department of Chemistry

Rose-Hulman Institute of Technology

04/18/23

Electrochemical Cells

Reading assignment: • Chang: Chapter 19.1-19.2

A physical arrangement designed for electron flow involving• an oxidation reaction• a reduction reaction

04/18/23

Voltaic Cellalso called a Galvanic cell

An electrochemical cell which• spontaneously generates a positive

electrical potential• can be used for useful work• has Ecell > 0 as constructed

Example• A discharging battery

· rechargeable or non-rechargeable• Corrosion of a piece of iron

04/18/23

Electrolytic Cell

An electrochemical cell which• requires an external energy source to force

the cell in a non-spontaneous direction.• has Ecell < 0 as constructed.

Examples• A battery being recharged.• A piece of metal being electroplated.

04/18/23

Electrochemical Cell Structure

CathodeSolution

AnodeSolution

AnodeElectrode

CathodeElectrodeSalt Bridge

04/18/23


CathodeSolution

AnodeSolution

AnodeElectrode


04/18/23


CathodeSolution

AnodeSolution

AnodeElectrode


04/18/23


CathodeSolution

AnodeSolution

AnodeElectrode


0.000 V2.002 V

04/18/23


Half-cell reactions Electrodes Electron flow Ion flow Shorthand notation

04/18/23

Half-Cell Reactions

Each electrochemical cell involves both an oxidation reaction and a reduction reaction.

The oxidation cell and the reduction cell are referred to as half-cells.

Al(s) Al3+(aq) + 3 e–

Cu2+(aq) + 2 e– Cu(s)

04/18/23

Anode Reaction

CathodeSolution

AnodeSolution

AnodeElectrode


2.002 V

Al(s) Al3+(aq) + 3 e–

04/18/23

AnodeThe electrode at which oxidation occurs

CathodeSolution

AnodeSolution

AnodeElectrode


2.002 VAl(s) Al3+(aq) + 3 e–

04/18/23

Anode of a Voltaic Cell is Negative

CathodeSolution

AnodeSolution

AnodeElectrode


2.002 VAl(s) Al3+(aq) + 3 e–

Al

e–

-

because electrons are released

04/18/23

CathodeSolution

AnodeSolution

AnodeElectrode


2.002 VAl(s) Al3+(aq) + 3 e–

Al

e–

-

04/18/23

Cathode Reaction

CathodeSolution

AnodeSolution

AnodeElectrode


2.002 VAl(s) Al3+(aq) + 3 e–

Cu2+(aq) + 2 e– Cu(s)

Al

e–

-

04/18/23

CathodeThe electrode at which reduction occurs

CathodeSolution

AnodeSolution

AnodeElectrode


2.002 VAl(s) Al3+(aq) + 3 e– Cu2+(aq)+ 2 e– Cu(s)

Al

e–

-

04/18/23

Cathode of a Voltaic Cell is Positive

CathodeSolution

AnodeSolution

AnodeElectrode


2.002 VAl(s) Al3+(aq) + 3 e–

Al

e–

Cu2+

e–

because electrons are attracted and consumed

+-

Cu2+(aq)+ 2 e– Cu(s)

04/18/23

CathodeSolution

AnodeSolution

AnodeElectrode


2.002 VAl(s) Al3+(aq) + 3 e–

Al

e–

Cu2+

e–

- +


04/18/23

Electrons are transferredthrough a wire from anode to cathode

CathodeSolution

AnodeSolution

AnodeElectrode


2.002 VAl(s) Al3+(aq) + 3 e–

Al

e–

Cu2+

e–

- +


04/18/23

Electron Current Flowmay be used to perform useful work

CathodeSolution

AnodeSolution

AnodeElectrode


Al(s) Al3+(aq) + 3 e–

Al

e–

Cu2+

e–

- +


Electrical connection is made at the electrodes,the site at which oxidation and reduction occurs.

04/18/23

Keeping It Straight

Electronsare released

In a voltaic cellit is the

negative electrode

Electronsare attracted

and consumed

In a voltaic cellit is the

positive electrode

Anode

Oxidation

Cathode

Reduction

04/18/23

Electrons are transferred through a wire from the anode to the cathode.

Electron Flow

Ion Flow Anions are attracted to the anode and cations

migrate away from anode.

Salt Bridge The salt bridge contains an ionic compound such as

KNO3 or NaCl dissolved in a gel such as agar-agar.

04/18/23

indicate what is happeningto all the charged species

in the anode cell.

List charged species

Show their locationand their motion

Draw a Diagram

04/18/23

Anode Cell

Al

Al3+

e–

NO3–

NO3–

K+

Al

Show the motion ofall the charged species

+

+

+

+

+

NO3–

04/18/23

Ion Flow

Cations are attracted to the cathode and anions migrate away from cathode.

Draw a diagram indicating what is happeningto all the charged species in the cathode cell.

04/18/23

Cathode Cell

Show the motion ofall the charged species

e–

–

–

–

–

––

Cu2+Cu

NO3–

K+

NO3–

K+Identify the main species

04/18/23

Salt Bridge

A salt bridge may be used to physically separate ions in one half-cell from ions in the other half-cell.

Draw a diagram indicating what is happeningto all the charged species in the salt bridge.

04/18/23

Salt Bridge

NO3

–NO3

–

K+

K+

K+

Al3+ K+ NO3

–

NO3

–

04/18/23

Shorthand Line Notation

Al(s) | Al3+ (1.00 M) | | Cu2+ (1.00 M) | Cu(s)

Why is a graphite or a platinum electrode needed?

anode | anode solution | | cathode solution | cathode

H2(g, 1 atm), Pt(s) | H+ (1 M) | | Cl– (1 M) | Cl2(g, 1 atm), C(gr)

04/18/23

Types of Electrochemical Cells

Concentration Cell Standard Redox Cell Non-standard (Combination) Redox Cell

04/18/23

Concentration Cell

The oxidation and reduction reactions are identically reverse of each other.

The observed cell potential is due solely to differences in concentrations of the solutions involved.

Low potentials generated (mV)

04/18/23

Concentration Cell

Example:

Zn(s) | Zn2+ (0.23 M) | | Zn2+ (1.00 M) | Zn(s)

Zn(s) Zn2+(0.23 M) + 2 e–

Zn2+ (1.00 M) + 2 e– Zn(s)

Write the oxidation and reduction half-cellreactions taking place in this cell.

04/18/23

Concentration Cell

QZn M

Zn M

[ (? )]

[ (? )]

2

2

Zn

Znanode

cathode

[ ]

[ ]

2

2

Zn M

Zn M

[ ( . )]

[ ( . )]

2

2

023

1 00

Example:

Zn(s) | Zn2+ (0.23 M) | | Zn2+ (1.00 M) | Zn(s)

Write the Q term for this cell.

04/18/23

Concentration Cell

Example:

Zn(s) | Zn2+ (0.23 M) | | Zn2+ (1.00 M) | Zn(s)

Zn(s) Zn2+(0.23 M) + 2 e–

Zn2+ (1.00 M) + 2 e– Zn(s)

Determine the standard cell potential for this cell.

04/18/23

Concentration Cell

E°cell = 0.00 V Low potentials generated (mV)

04/18/23

Standard Redox Cell

The oxidation and reduction reactions are different.

Concentrations of solutions are 1 M and reactant gas pressures are 1 atm.

The observed cell potential is due to the differences in the activity of the reactants.

04/18/23

Standard Redox Cell

Example:

Ni(s) | Ni2+ (1.00 M) | | Ag+ (1.00 M) | Ag(s)

Ni(s) Ni2+(1.00 M) + 2 e–


Ag+ (1.00 M) + e– Ag(s)


04/18/23

Standard Redox Cell

Example:• Ni(s) | Ni2+ (1.00 M) | | Ag+ (1.00 M) | Ag(s)

QNi

Ag

[ ( . M )]

[ ( .

2

M )]2100

100

Why is this called a standard redox cell?

04/18/23

Standard Redox Cell

Example:

Ni(s) | Ni2+ (1.00 M) | | Ag+ (1.00 M) | Ag(s)

Ni(s) Ni2+(1.00 M) + 2 e–


Ag+ (1.00 M) + e– Ag(s)

04/18/23

Standard Redox Cell

E°cell 0.00 V Potentials (voltage) generated can be quite

high

04/18/23

The oxidation and reduction reactions are different.

The solution concentrations are not 1 M. Gas pressures are not 1 atm.

Non-standard (Combination) Redox Cell

04/18/23

Example:Mn(s) | Mn2+ (1.00 M) | | Pb2+ (0.23 M) | Pb(s)


Mn(s) Mn2+(1.00 M) + 2 e–

Pb2+ (0.23 M) + 2 e– Pb(s)



04/18/23


Example:

Mn(s) | Mn2+ (1.00 M) | | Pb2+ (0.23 M) | Pb(s)

QMn

Pb

[ ( . M )]

[ ( . M )]

2

2

100

0 23

04/18/23

Example:Mn(s) | Mn2+ (1.00 M) | | Pb2+ (0.23 M) | Pb(s)

Mn(s) Mn2+(1.00 M) + 2 e–

Pb2+ (0.23 M) + 2 e– Pb(s)

Why is this called a non-standard redox cell?



04/18/23

The majority of the observed cell potential is due to the differences in the activity of the reactants, modified slightly by non-standard conditions.

E°cell 0.00 V Potentials generated can be quite high (V)


04/18/23

Electrode Materials

Inert electrodes can or must be used in some instances.• The reactant or product is a gas or liquid.• The reactant and product of a half-cell are

soluble.• The product is being plated out onto an

inert electrode.

04/18/23

Inert ElectrodesExamples

H2(g, 30 atm), C(gr) | KOH (0.789 M) | |

KOH (0.789 M) | O2(g, 20 atm), C(gr)

Pt(s) | Cr2+ (1.00 M), Cr3+ (1.00 M) | | Cu2+ (1.00 M) | Au(s)

Co(s) | Co2+ (0.789 M) | | Hg2+ (0.50 M) | Hg(l), Pt(s)

04/18/23

Pt(s) | Cr2+(1.0 M), Cr3+(1.0 M) | | Cu2+ (1.0 M) | Au(s)

Draw a beaker diagram for this cell. Identify what is being oxidized and what is

being reduced. Indicate the flow of all cations, anions and

electrons in your diagram. What is the standard cell potential? What is the Q term?

04/18/23

04/18/23

Documents

Please Pick Up Electrochemical Cells Problem Set