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
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.
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
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
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