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ReviewOxidation reduction reactions involve a
transfer of electrons.OIL- RIGOxidation Involves LossReduction Involves Gain
Solid lead(II) sulfide reacts with oxygen in the air at high temperatures to form lead(II) oxide and sulfur dioxide. Which substance is a reductant (reducing agent) and which is an oxidant (oxidizing agent)?
A. PbS, reductant; O2, oxidant
B. PbS, reductant; SO2, oxidant
C. Pb2+, reductant; S2- oxidant
D. PbS, reductant; no oxidant
E. PbS, oxidant; SO2, reductant
ApplicationsMoving electrons is electric current.8H++MnO4
-+ 5Fe+2 +5e- Mn+2 + 5Fe+3 +4H2O
Helps to break the reactions into half reactions.
8H++MnO4-+5e- Mn+2 +4H2O
5(Fe+2 Fe+3 + e- ) In the same mixture it happens without
doing useful work, but if separate
H+
MnO4-
Fe+2
Connected this way the reaction startsStops immediately because charge builds
up.
e-e- e-
e-e-
Cell PotentialOxidizing agent pulls the electron.Reducing agent pushes the electron. The push or pull (“driving force”) is called
the cell potential Ecell
Also called the electromotive force (emf) Unit is the volt(V) = 1 joule of work/coulomb of chargeMeasured with a voltmeter
1 M HCl
H+
Cl-
H2 in
Standard Hydrogen ElectrodeThis is the reference
all other oxidations are compared to
Eº = 0 º indicates standard
states of 25ºC, 1 atm, 1 M solutions.
Cell PotentialZn(s) + Cu+2 (aq) Zn+2(aq) + Cu(s)The total cell potential is the sum of the
potential at each electrode.
Eºcell = EºZn Zn+2 + EºCu+2 Cu
We can look up reduction potentials in a table.
One of the reactions must be reversed, so change it sign.
Cell Potential Determine the cell potential for a galvanic
cell based on the redox reaction. Cu(s) + Fe+3(aq) Cu+2(aq) + Fe+2(aq)
Fe+3(aq) + e- Fe+2(aq) Eº = 0.77 V
Cu+2(aq)+2e- Cu(s) Eº = 0.34 V
Cu(s) Cu+2(aq)+2e- Eº = -0.34 V
2Fe+3(aq) + 2e- 2Fe+2(aq) Eº = 0.77 V
Reduction potentialMore negative Eº
– more easily electron is added– More easily reduced– Better oxidizing agent
More positive Eº – more easily electron is lost– More easily oxidized– Better reducing agent
Line Notation solidAqueousAqueoussolidAnode on the leftCathode on the rightSingle line different phases.Double line porous disk or salt bridge. If all the substances on one side are
aqueous, a platinum electrode is indicated.
Under standard conditions, which of the following is the net reaction that occurs in the cell?
Cd|Cd2+ || Cu2+|Cu
a. Cu2+ + Cd → Cu + Cd2+
b. Cu + Cd → Cu2+ + Cd2+
c. Cu2+ + Cd2+ → Cu + Cd
d. Cu + Cd 2+ → Cd + Cu2+
Galvanic Cell The reaction always runs
spontaneously in the direction that produced a positive cell potential.
Four things for a complete description.
1) Cell Potential
2) Direction of flow
3) Designation of anode and cathode
4) Nature of all the components- electrodes and ions
Potential, Work and Gemf = potential (V) = work (J) / Charge(C)E = work done by system / chargeE = -w/qCharge is measured in coulombs. -w = q E Faraday = 96,485 C/mol e-
q = nF = moles of e- x charge/mole e-
w = -qE = -nFE = G
Potential, Work and G Gº = -nFEº if Eº > 0, then Gº < 0 spontaneous if Eº< 0, then Gº > 0 nonspontaneous In fact, reverse is spontaneous.Calculate Gº for the following reaction:Cu+2(aq)+ Fe(s) Cu(s)+ Fe+2(aq)
Fe+2(aq) + e-Fe(s) Eº = 0.44 V
Cu+2(aq)+2e- Cu(s) Eº = 0.34 V
Cell Potential and Concentration
Qualitatively - Can predict direction of change in E from LeChâtelier.
2Al(s) + 3Mn+2(aq) 2Al+3(aq) + 3Mn(s)Predict if Ecell will be greater or less than Eºcell if [Al+3] = 1.5 M and [Mn+2] = 1.0 M
if [Al+3] = 1.0 M and [Mn+2] = 1.5M if [Al+3] = 1.5 M and [Mn+2] = 1.5 M
The Nernst EquationG = Gº +RTln(Q) -nFE = -nFEº + RTln(Q)
E = Eº - RTln(Q)
nF2Al(s) + 3Mn+2(aq) 2Al+3(aq) + 3Mn(s)
Eº = 0.48 V Always have to figure out n by balancing. If concentration can gives voltage, then
from voltage we can tell concentration.
The Nernst Equation As reactions proceed concentrations of
products increase and reactants decrease. Reach equilibrium where Q = K and
Ecell = 0 0 = Eº - RTln(K)
nFEº = RTln(K)
nF
nF Eº = ln(K) RT
CorrosionRusting - spontaneous oxidation.Most structural metals have reduction
potentials that are less positive than O2 .
Fe Fe+2 +2e- Eº= 0.44 V
O2 + 2H2O + 4e- 4OH-Eº= 0.40 V
Fe+2 + O2 + H2O Fe2O3 + H+ Reactions happens in two places.
Water
Rust
Iron Dissolves-
Fe Fe+2
e-
Salt speeds up process by increasing conductivity
O2 + 2H2O +4e- 4OH-
Fe2+ + O2 + 2H2O Fe2O3 + 8 H+
Fe2+
Preventing CorrosionCoating to keep out air and water.Galvanizing - Putting on a zinc coatHas a lower reduction potential, so it is
more easily oxidized.Alloying with metals that form oxide
coats.Cathodic Protection - Attaching large
pieces of an active metal like magnesium that get oxidized instead.
Running a galvanic cell backwards.Put a voltage bigger than the potential
and reverse the direction of the redox reaction.
Used for electroplating.
Electrolysis
Calculating platingHave to count charge.Measure current I (in amperes)1 amp = 1 coulomb of charge per secondq = I x tq/nF = moles of metalMass of plated metalHow long must 5.00 amp current be
applied to produce 15.5 g of Ag from Ag+
Calculating plating1. Current x time = charge
2. Charge ∕Faraday = mole of e-
3. Mol of e- to mole of element or compound
4. Mole to grams of compound
Or the reverse if you want time to plate
Calculate the mass of copper which can be deposited by the passage of 12.0 A for 25.0 min through a solution of copper(II) sulfate.
How long would it take to plate 5.00 g Fe from an aqueous solution of Fe(NO3)3 at a current of 2.00 A?
Other usesElectrolysis of water.Separating mixtures of ions.More positive reduction potential means
the reaction proceeds forward. We want the reverse.Most negative reduction potential is
easiest to plate out of solution.
RedoxKnow the table
2. Recognized by change in oxidation state.
3. “Added acid”
4. Use the reduction potential table on the front cover.
5. Redox can replace. (single replacement)
6. Combination Oxidizing agent of one element will react with the reducing agent of the same element to produce the free element.
I- + IO3- + H+ I2 + H2O
7. Decomposition.a) peroxides to oxidesb) Chlorates to chloridesc) Electrolysis into elements.d) carbonates to oxides