Galvanic Cells What will happen if a piece of Zn metal is

immersed in a CuSO4 solution? A spontaneous redox reaction occurs:

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

Spontaneous reaction: a reaction that doesn’t need to be driven by an outside source of energy

Galvanic Cells The previous reaction occurred when the

Zn metal was in direct contact with the Cu2+ ions.

Redox reactions can also occur when reactants are indirectly in contact with each other in a galvanic (voltaic) cell.

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

NaNO3completes the electrical circuit

Galvanic CellsZn (s) + Cu2+ (aq) Zn2+ (aq) + Cu (s)

Oxidation half cell

Reductionhalf cell

electrodeelectrode

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 occurslocated in the reduction half-cell

the “positive” electrodeelectrons move toward (are gained at) the cathode

Galvanic CellsConsider 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 CellsZn (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.

Why do electrons flow spontaneously from one electrode to the other?

Electrons flow spontaneously 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 Eo

cell 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 Eo

red 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 Eo

red 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 CellsExample: 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 CellsExample: What is the Eo

cell for the following reaction?

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

Galvanic Cells

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

Br2 (l) or I2 (s)