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Unformatted text preview: ous redox reactions take place in a voltaic cell.
aka galvanic cell. • Nonspontaneous redox reactions can be forced to occur in an
electrolytic cell by the addition of electrical energy. 11 Voltaic Cell
Oxidation and reduction reactions kept separate: half-cells.
Electric circuit: e- flow via wire, ion flow via solution & salt bridge
Electrodes (anode & cathode): conductive solid allowing transfer of e-. Resulting voltage is called
electromotive force, emf, in
units of volts
cations released Anode
Zn(s) Cathode cations attracted,
Cu(s) Cell Potential & Notation • The difference in potential energy between anode and cathode in
a voltaic cell is called the cell potential or emf (in Vs). • The cell potential under standard conditions is called the
standard emf, E°.
25°C, 1 bar for gases, 1 M concentration for solutions
sum of the cell potentials for the half-reactions: E° = E°red2 + E°ox1 • Electrode(anode) | electrolyte || electrolyte | Electrode(cathode)
oxidation half-cell on left, reduction half-cell on the right
single “|” denote the phase barrier
− if multiple electrolytes in same phase, a comma is used rather than |
− often use an inert electrode double line “||” denote salt bridge
Some times simply write as Red1,Ox1|| Red2,Ox2
Redox couple: Ox1/Red1 and Ox2/Red2 Write the cell notation & sketch the voltaic cell for
Fe(s) + MnO4−(aq) → Mn2+(aq) + Fe2+(aq)
Fe(s) | Fe2+(aq) || MnO4−(aq), Mn2+(aq), H+(aq) | Pt(s) Redox couples & Half-reactions Standard Reduction Potential • A half-reaction with a strong tendency to be reduced
has a large positive half-cell potential
when two half-cells are connected, the electrons will
flow toward that half-reaction. • We cannot measure the absolute tendency of a halfreaction, we can only measure it relative to another
half-reaction, i.e. relative potential/emf. • We select as a standard half-reaction the reduction of
H+ to H2 under standard conditions, which we assign
an absolute potential = 0 V.
standard hydrogen electrode, SHE 2H+(aq,1M)+2e- → H2(g,1bar)
→ 2H+(aq,1M)+2e- Eo re
d= 0 Zn(s) → Zn2+(aq,1M) +2e- Eoox = 0.76V
Zn2+(aq,1M) +2e- → Zn(s) Eored = –0.76V Standard Reduction Potential Half-Cell Potentials • • The SHE reduction potential is defined to be exactly 0 V.
half-reactions with a stronger tendency toward reduction than the SHE
have a positive value for E°red
half-reactions with a stronger tendency toward oxidation than the SHE
have a negative value for E°red It is convenient to always write the half-cell potential based on its
Red1 + Ox2
Half reactions: Red1
Ox2 + ν e- Ox1 + Red2 Cell potentials refer to reduction potentials. Ox1 + ν eRed2 Red1,Ox1|| Red2,Ox2
Spontaneous only if Eo>0 EoL = Eored1 = -Eoox1
EoR = Eored2
Eo = Eored2 + Eoox1 = Eored,R – Eored,L = EoR – EoL Predicting whether a
metal dissolves in acid Exception: HNO3...
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