cell voltages
DESCRIPTION
Cell Voltages. To compare cells compare voltages of cells in their standard state. Standard States For solids and liquids: the state of the pure solid or liquid at 1 atm and at a specified temperature For gases: the gaseous phase at 1 atm and at a specified temperature - PowerPoint PPT PresentationTRANSCRIPT
Cell Voltages
To compare cells compare voltages of cells in their standard state.
Standard StatesFor solids and liquids: the state of the pure solid or liquid at 1
atm and at a specified temperatureFor gases: the gaseous phase at 1 atm and at a specified
temperatureFor solutions : Concentrations of 1 mol/L under a pressure of
1 atm and a specified temperature.
(temperature typically specified at 298.15 K)
Eo is the standard cell voltage or potential of a standard cell - when all reactants and products are in their standard state.
Gro = - n F Eo
Eo = - Gro
n F
Eo > 0 ; spontaneous cell reaction under standard conditions
A standard Cr3+(aq)|Cr(s) and a standard Co2+(aq)|Co(s) half cell are connected to make a galvanic cell. The voltage of the cell equals 0.464 V at 25oC. Write an equation to represent the reaction taking place in the cell and calculate its Go
r.
Cr(s) Cr3+(aq) + 3e- anode
Co2+(aq) + 2e- Co(s) cathode
Overall cell reaction
2Cr(s) + 3 Co2+(aq) 2Cr3+(aq) + 3 Co(s) Eo = 0.464 V
Gor= - n F Eo
= - (6 moles) (9.64853 x 104 coulomb/mole) (0.464 V)
= - 2.69 x 105 J or - 269 kJ
Standard Reduction PotentialsUnder standard conditions:
Eo = Eo (right half cell) - Eo (left half cell)
For a galvanic cell:Eo = Eo (cathode) - Eo (anode)where the Eo are the standard reduction potentials of the
electrodes.
For Eo > 0; spontaneous cell reaction
To determine which of two half cells will be the anode and which the cathode compare standard reduction potentials
Cr3+(aq) + 3e- Cr(s) Eo(Cr3+|Cr) = - 0.744 V
Co2+(aq) + 2e- Co(s) Eo(Co2+|Co) = - 0.28 V
Eo(Co2+|Co) > Eo(Cr3+|Cr)
Define the following half reaction to be the reference2 H+(aq, 1 M) + 2e- -> H2 (g, P = 1 atm) Eo = 0 V
All standard reduction potentials are determined relative to this reference.
If the standard reduction potential of a half reaction is > 0 => greater tendency to be reduced relative to H+(aq, 1 M)
If standard reduction potential of a half reaction < 0 => lower tendency to be reduced relative to H+(aq, 1 M)
In general, the more positive the standard reduction potential, the greater the electron-pulling power of the reduction half reaction, and therefore the more oxidizing the species
The more negative the standard reduction potential, the greater the electron-donating power of the oxidation half reaction, and therefore the more reducing the species
Variation of standard reduction potentials. The most negative values occur in the s block and the most positive values occur close to fluorine.
The standard potential of an electrode can be determined by setting up a standard cell in which one electrode has a known standard potential and measuring the resulting cell voltage.
For example, the standard potential of a zinc electrode is -0.76 V, and the standard emf of the cell
Zn(s) | Zn2+ (aq) || Sn4+ (aq), Sn2+ (aq) | Pt (s) is + 0.91 V
Eo = Eo (cathode) - Eo (anode)+ 0.91 V = Eo (Sn4+ (aq), Sn2+ (aq) ) - Eo (Zn(s) | Zn2+ (aq) )
Eo (Sn4+ (aq), Sn2+ (aq) ) = + 0.91 V + Eo (Zn(s) | Zn2+ (aq) ) = 0.91V - 0.76 V = + 0.15 V
Using standard reduction potentials
Strong oxidizing agents - have large positive standard reduction potentials
Examples: F2, MnO4-, H2O2
O2 (in acidic medium) is a fairly strong oxidizing agent.
Strong reducing agents - have large negative standard reduction potentials
Examples: Na, Li
Electrochemical series: list of relative strengths of oxidizing and reducing agents.
The strongest oxidizing agents are at the top of the table; the strongest reducing agents are at the bottom
M(s) + 2H+(aq) M2+(aq) + H2(g)
spontaneous if Eo(M2+|M) < 0
Disproportionation: a single species is both reduced and oxidized.
Must be able to both give up and accept electrons
Half reaction in which the species is reduced must have a larger reduction potential than the half reaction in which it is oxidized.
Is Fe2+(aq) in its standard state unstable with respect to disproportionation at 25oC?
Fe3+(aq) + e- Fe2+(aq) Eo = 0.771 V
Fe2+(aq) + 2e- Fe(s) Eo = -0.477 V
Overall disproportionation reaction
3 Fe2+(aq) 2 Fe3+(aq) + Fe(s) Eo = -0.477 - 0.771 = -1.218 V
No disproportionation
Effect of Concentration on EGr = Gr
o + RT ln Q
Gr = - n F EGr
o = - n F Eo
- n F E = - n F Eo + RT ln Q
E = Eo - (RT/ n F ) ln Q Nernst Equation
relates cell voltage with concentrations of reactants and products (through Q)
At 25.00oC (298.15 K), R T / F = 0.025693 V
E = Eo - (0.025693 / n ) ln Q
The reduction potential of a non-standard half cell is:
E = Eo - (RT/ nhc F ) ln Qhc
For Zn2+ (aq) + 2 e- -> Zn(s)
E = Eo - (RT/ 2 F ) ln (1 / [Zn2+(aq)]
Ion-Selective ElectrodespH or concentration of ions can be measured by using an
electrode that responds selectively to only one species of ion.
In a pH meter, one electrode is sensitive to the H3O+(aq) concentration, and the other electrode serves as a reference.
A calomel electrode has a reduction half reaction Hg2Cl2 (s) + 2 e- -> 2 Hg(l) + 2 Cl- (aq) Eo = +0.27 V
When combined with the H+(aq)/H2(g) electrode, the overall cell reaction is:Hg2Cl2 (s) + H2 (g) -> 2 H+ (aq) + 2 Hg(l) + 2 Cl- (aq)
Q = [H+(aq)]2 [Cl- (aq)]2 / PH2
If PH2 is held at 1 atm then Q = [H+(aq)]2 [Cl- (aq)]2
E = Eo - (RT/ n F ) ln [H+(aq)]2 [Cl- (aq)]2
The [Cl- (aq)] is held constant since the calomel electrode consists of a saturated solution of KCl.
E depends only on [H+(aq)].
Other electrodes are selectively sensitive to ions such as Ca2+, NH4
+, Na+, S2-.