Galvanic Cells

When Zn metal is immersed in an aqueous solution of CuSO4, a spontaneous redox reaction occurs:

This same redox reaction can also occur when reactants are indirectly in contact with each other in a galvanic (voltaic) cell.

Zn (s) + Cu2+ (aq) Zn2+ (aq) + Cu (s)

Galvanic Cells

Galvanic (voltaic) cell: A device in which a spontaneous redox

reaction occurs as electrons are transferred from the reductant to the oxidant through an external circuit

used to perform electrical work using the energy released during a spontaneous redox reaction.

Galvanic Cells

In a galvanic cell, the two half reactions occur in separate compartments called half-cells. 1 half-cell contains the oxidation half

reaction 1 half-cell contains the reduction half

reaction

Each half cell contains: electrode electrolyte solution

Galvanic Cells

The two half cells are connected by external circuit (wire) between the

electrodes

salt bridge between the electrolyte solutionsionic solution that will not react with other components in the galvanic cell

NaNO3

completes the electrical circuit

Galvanic Cells

Zn (s) + Cu2+ (aq) Zn2+ (aq) + Cu (s)

Oxidation half cell

Reductionhalf cell

electrode

electrode

Galvanic Cells

Two types of electrodes: anode:

the electrode at which oxidation occurs

located in the oxidation half-cellthe “negative” electrode

electrons are released here cathode:

the electrode at which reduction occurs

located in the reduction half-cellthe “positive” electrode

electrons move toward (are gained at) the cathode

Galvanic Cells

Consider the following reaction:Zn (s) + Ni2+ (aq) Zn2+ (aq) + Ni (s)

Which metal will be the anode?

Which metal will be the cathode?

Galvanic Cells

In some galvanic cells, one (or both) of the half reactions does not involve a metal:

Cr2O72- (aq) + 14 H+ (aq) + 6 I- (aq)

2 Cr3+ (aq) + 3 I2 (s) + 7 H2O (l)

In these cases, an unreactive metal conductor is used as the electrode platinum foil

Galvanic Cells

Zn (s) + 2 H+ (aq) Zn2+ (aq) + H2 (g)

Oxidation half-reaction:Zn (s) Zn2+ (aq) + 2 e-

Reduction half-reaction:2 H+ (aq) + 2 e- H2 (g)

In this case a standard hydrogen electrode is used as the cathode.

Cell EMF

The redox reactions occurring in a galvanic cell are spontaneous.

Electrons flow spontaneously from one electrode to the other because there is a difference in potential energy between the anode and the cathode.

Galvanic Cells

Anode higher

potential energy

Cathode lower

potential energy

Galvanic Cells

The difference in electrical potential between the anode and the cathode is called the cell potential or cell voltage (Ecell)

measured in volts

Standard cell potential (Eocell):

the cell potential measured under standard conditions25oC 1M concentrations of reactants and products in solution

or 1 atm pressure for gases

Galvanic Cells

Eocell depends on the half-cells or half-

reactions present

Standard potentials have been assigned to each individual half-cell

By convention, the standard reduction potential (Eo

red) for each half cell is used and tabulated

Galvanic Cells

Standard reduction potential: potential of a reduction half-reaction

under standard conditions

measured relative to the reduction of H+ to H2 under standard conditions:

2H+ (aq, 1M) + 2 e- H2 (g, 1 atm) Eored = 0 V

Galvanic Cells

As Eored becomes increasingly positive, the

driving force for reduction increases. Reduction becomes more spontaneous Reaction occurs at cathode

F2 (g) + 2e- 2 F- (aq) Eored = +2.87 V

Ag+ (aq) + e- Ag (s) Eored = + 0.80 V

Which reaction is more spontaneous as written?

Which reaction will tend to occur at the cathode if the two reactions were combined in a galvanic cell?

Galvanic Cells

As Eored becomes increasingly negative,

the driving force for oxidation increases.

Li+ (aq) + e- Li (s) Eored = -3.05

The negative reduction potential indicates that the reverse (oxidation) half-reaction is spontaneous.

The reaction that occurs at the anode is:

Li (s) Li+ (aq) + e-

Galvanic Cells

Example: Given the following standard reduction potentials, which of the metals will be most easily oxidized?

Ag+ (aq) + e- Ag (s) Eored = 0.80 V

Zn2+ (aq) + 2 e- Zn (s) Eored = -0.76 V

Na+ (aq) + e- Na (s) Eored = -2.71 V

Galvanic Cells

Standard cell potential

Eocell = Eo

red (cathode) - Eored (anode)

reduction oxidation

Galvanic Cells

Example: What is the Eocell for the following

reaction?Zn (s) + Cu2+ (aq) Zn2+ (aq) + Cu (s)

Galvanic Cells

Example: Given the following reduction half-reactions, identify the metal at the anode, the balanced reaction for the galvanic cell, and the Eo

cell.

Al3+ (aq) + 3 e- Al (s) Eored = -1.66 V

Fe2+ (aq) + 2 e- Fe (s) Eored = -0.440 V

Galvanic Cells

Galvanic Cells

Oxidizing Agent (oxidant): the substance that causes another to

be oxidized the substance that is reduced the substance that gains electrons

The strongest oxidizing agent is the substance that has the greatest tendency to be reduced. The most positive Eo

red

Galvanic Cells

Example: Use the reduction potentials given in Appendix E to determine which of the following is the stronger oxidizing agent:

Br2 (l) or I2 (s)