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Chemistry: A Molecular Approach , 1 st Ed. Nivaldo Tro. Chapter 18 Electrochemistry. Roy Kennedy Massachusetts Bay Community College Wellesley Hills, MA. 2007, Prentice Hall. Redox Reaction. one or more elements change oxidation number all single displacement, and combustion,

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chapter 18 electrochemistry

Chemistry: A Molecular Approach, 1st Ed.Nivaldo Tro

Chapter 18Electrochemistry

Roy Kennedy

Massachusetts Bay Community College

Wellesley Hills, MA

2007, Prentice Hall

redox reaction
Redox Reaction
  • one or more elements change oxidation number
    • all single displacement, and combustion,
    • some synthesis and decomposition
  • always have both oxidation and reduction
    • split reaction into oxidation half-reaction and a reduction half-reaction
  • aka electron transfer reactions
    • half-reactions include electrons
  • oxidizing agentis reactant molecule that causes oxidation
    • contains element reduced
  • reducing agentis reactant molecule that causes reduction
    • contains the element oxidized

Tro, Chemistry: A Molecular Approach

oxidation reduction
Oxidation & Reduction
  • oxidation is the process that occurs when
    • oxidation number of an element increases
    • element loses electrons
    • compound adds oxygen
    • compound loses hydrogen
    • half-reaction has electrons as products
  • reduction is the process that occurs when
    • oxidation number of an element decreases
    • element gains electrons
    • compound loses oxygen
    • compound gains hydrogen
    • half-reactions have electrons as reactants

Tro, Chemistry: A Molecular Approach

rules for assigning oxidation states
Rules for Assigning Oxidation States
  • rules are in order of priority
  • free elements have an oxidation state = 0
    • Na = 0 and Cl2 = 0 in 2 Na(s) + Cl2(g)
  • monatomic ions have an oxidation state equal to their charge
    • Na = +1 and Cl = -1 in NaCl
  • (a) the sum of the oxidation states of all the atoms in a compound is 0
    • Na = +1 and Cl = -1 in NaCl, (+1) + (-1) = 0

Tro, Chemistry: A Molecular Approach

rules for assigning oxidation states1
Rules for Assigning Oxidation States
  • (b) the sum of the oxidation states of all the atoms in a polyatomic ion equals the charge on the ion
    • N = +5 and O = -2 in NO3–, (+5) + 3(-2) = -1
  • (a) Group I metals have an oxidation state of +1 in all their compounds
    • Na = +1 in NaCl
  • (b) Group II metals have an oxidation state of +2 in all their compounds
    • Mg = +2 in MgCl2

Tro, Chemistry: A Molecular Approach

rules for assigning oxidation states2
Rules for Assigning Oxidation States
  • in their compounds, nonmetals have oxidation states according to the table below
    • nonmetals higher on the table take priority

Tro, Chemistry: A Molecular Approach

oxidation and reduction

oxidation

reduction

Oxidation and Reduction
  • oxidation occurs when an atom’s oxidation state increases during a reaction
  • reduction occurs when an atom’s oxidation state decreases during a reaction

CH4 + 2 O2 → CO2 + 2 H2O

-4 +1 0+4 –2 +1 -2

Tro, Chemistry: A Molecular Approach

oxidation reduction1
Oxidation–Reduction
  • oxidation and reduction must occur simultaneously
    • if an atom loses electrons another atom must take them
  • the reactant that reduces an element in another reactant is called the reducing agent
    • the reducing agent contains the element that is oxidized
  • the reactant that oxidizes an element in another reactant is called the oxidizing agent
    • the oxidizing agent contains the element that is reduced

2 Na(s) + Cl2(g) → 2 Na+Cl–(s)

Na is oxidized, Cl is reduced

Na is the reducing agent, Cl2 is the oxidizing agent

Tro, Chemistry: A Molecular Approach

identify the oxidizing and reducing agents in each of the following
Identify the Oxidizing and Reducing Agents in Each of the Following

3 H2S + 2 NO3– + 2 H+® 3S + 2 NO + 4 H2O

MnO2 + 4 HBr ® MnBr2 + Br2 + 2 H2O

Tro, Chemistry: A Molecular Approach

identify the oxidizing and reducing agents in each of the following1

oxidation

reduction

oxidation

reduction

Identify the Oxidizing and Reducing Agents in Each of the Following

red ag

ox ag

3 H2S + 2 NO3– + 2 H+® 3S + 2 NO + 4 H2O

MnO2 + 4 HBr ® MnBr2 + Br2 + 2 H2O

+1 -2 +5 -2 +1 0 +2 -2 +1 -2

ox ag

red ag

+4 -2 +1 -1 +2 -1 0 +1 -2

Tro, Chemistry: A Molecular Approach

balancing redox reactions
Balancing Redox Reactions
  • assign oxidation numbers
    • determine element oxidized and element reduced
  • write ox. & red. half-reactions, including electrons
    • ox. electrons on right, red. electrons on left of arrow
  • balance half-reactions by mass
    • first balance elements other than H and O
    • add H2O where need O
    • add H+1 where need H
    • neutralize H+ with OH- in base
  • balance half-reactions by charge
    • balance charge by adjusting electrons
  • balance electrons between half-reactions
  • add half-reactions
  • check

Tro, Chemistry: A Molecular Approach

ex 18 3 balance the equation i aq mno 4 aq i 2 aq mno 2 s in basic solution
Ex 18.3 – Balance the equation:I(aq) + MnO4(aq) I2(aq) + MnO2(s)in basic solution

Tro, Chemistry: A Molecular Approach

ex 18 3 balance the equation i aq mno 4 aq i 2 aq mno 2 s in basic solution1
Ex 18.3 – Balance the equation:I(aq) + MnO4(aq) I2(aq) + MnO2(s)in basic solution

Tro, Chemistry: A Molecular Approach

ex 18 3 balance the equation i aq mno 4 aq i 2 aq mno 2 s in basic solution2
Ex 18.3 – Balance the equation:I(aq) + MnO4(aq) I2(aq) + MnO2(s)in basic solution

Tro, Chemistry: A Molecular Approach

ex 18 3 balance the equation i aq mno 4 aq i 2 aq mno 2 s in basic solution3
Ex 18.3 – Balance the equation:I(aq) + MnO4(aq) I2(aq) + MnO2(s)in basic solution

Tro, Chemistry: A Molecular Approach

practice balance the equation h 2 o 2 ki h 2 so 4 k 2 so 4 i 2 h 2 o
Practice - Balance the Equation H2O2 + KI + H2SO4® K2SO4 + I2 + H2O

Tro, Chemistry: A Molecular Approach

practice balance the equation h 2 o 2 ki h 2 so 4 k 2 so 4 i 2 h 2 o1
Practice - Balance the Equation H2O2 + KI + H2SO4® K2SO4 + I2 + H2O

+1 -1 +1 -1 +1 +6 -2 +1 +6 -2 0 +1 -2

oxidation

reduction

ox: 2 I-1® I2 + 2e-1

red: H2O2 + 2e-1 + 2 H+®2 H2O

tot 2 I-1 + H2O2 + 2 H+® I2 + 2 H2O

1 H2O2 + 2 KI + H2SO4® K2SO4 + 1 I2 + 2 H2O

Tro, Chemistry: A Molecular Approach

practice balance the equation clo 3 1 cl 1 cl 2 in acid
Practice - Balance the EquationClO3-1 + Cl-1 ®Cl2 (in acid)

Tro, Chemistry: A Molecular Approach

practice balance the equation clo 3 1 cl 1 cl 2 in acid1
Practice - Balance the EquationClO3-1 + Cl-1 ® Cl2 (in acid)

+5 -2 -1 0

oxidation

reduction

ox: 2 Cl-1® Cl2 + 2 e-1 } x5

red: 2 ClO3-1 + 10 e-1 + 12 H+ ®Cl2 + 6 H2O} x1

tot 10 Cl-1 + 2ClO3-1 + 12 H+® 6 Cl2 + 6 H2O

1 ClO3-1 + 5 Cl-1 + 6 H+1 ® 3 Cl2+ 3 H2O

Tro, Chemistry: A Molecular Approach

electrical current
Electrical Current
  • when we talk about the current of a liquid in a stream, we are discussing the amount of water that passes by in a given period of time
  • when we discuss electric current, we are discussing the amount of electric charge that passes a point in a given period of time
    • whether as electrons flowing through a wire or ions flowing through a solution

Tro, Chemistry: A Molecular Approach

redox reactions current
Redox Reactions & Current
  • redox reactions involve the transfer of electrons from one substance to another
  • therefore, redox reactions have the potential to generate an electric current
  • in order to use that current, we need to separate the place where oxidation is occurring from the place that reduction is occurring

Tro, Chemistry: A Molecular Approach

electric current flowing directly between atoms
Electric Current Flowing Directly Between Atoms

Tro, Chemistry: A Molecular Approach

electric current flowing indirectly between atoms
Electric Current Flowing Indirectly Between Atoms

Tro, Chemistry: A Molecular Approach

electrochemical cells
Electrochemical Cells
  • electrochemistryis the study of redox reactions that produce or require an electric current
  • the conversion between chemical energy and electrical energy is carried out in an electrochemical cell
  • spontaneous redox reactions take place in a voltaic cell
    • aka galvanic cells
  • nonspontaneous redox reactions can be made to occur in an electrolytic cell by the addition of electrical energy

Tro, Chemistry: A Molecular Approach

electrochemical cells1
Electrochemical Cells
  • oxidation and reduction reactions kept separate
    • half-cells
  • electron flow through a wire along with ion flow through a solution constitutes an electric circuit
  • requires a conductive solid (metal or graphite) electrode to allow the transfer of electrons
    • through external circuit
  • ion exchange between the two halves of the system
    • electrolyte

Tro, Chemistry: A Molecular Approach

electrodes
Electrodes
  • Anode
    • electrode where oxidation occurs
    • anions attracted to it
    • connected to positive end of battery in electrolytic cell
    • loses weight in electrolytic cell
  • Cathode
    • electrode where reduction occurs
    • cations attracted to it
    • connected to negative end of battery in electrolytic cell
    • gains weight in electrolytic cell
      • electrode where plating takes place in electroplating

Tro, Chemistry: A Molecular Approach

voltaic cell
Voltaic Cell

the salt bridge is required to complete the circuit and maintain charge balance

Tro, Chemistry: A Molecular Approach

current and voltage
Current and Voltage
  • the number of electrons that flow through the system per second is the current
    • unit = Ampere
    • 1 A of current = 1 Coulomb of charge flowing by each second
    • 1 A = 6.242 x 1018 electrons/second
    • Electrode surface area dictates the number of electrons that can flow
  • the difference in potential energy between the reactants and products is the potential difference
    • unit = Volt
    • 1 V of force = 1 J of energy/Coulomb of charge
    • the voltage needed to drive electrons through the external circuit
    • amount of force pushing the electrons through the wire is called the electromotive force, emf

Tro, Chemistry: A Molecular Approach

cell potential
Cell Potential
  • the difference in potential energy between the anode the cathode in a voltaic cell is called the cell potential
  • the cell potential depends on the relative ease with which the oxidizing agent is reduced at the cathode and the reducing agent is oxidized at the anode
  • the cell potential under standard conditions is called the standard emf, E°cell
    • 25°C, 1 atm for gases, 1 M concentration of solution
    • sum of the cell potentials for the half-reactions

Tro, Chemistry: A Molecular Approach

cell notation
Cell Notation
  • shorthand description of Voltaic cell
  • electrode | electrolyte || electrolyte | electrode
  • oxidation half-cell on left, reduction half-cell on the right
  • single | = phase barrier
    • if multiple electrolytes in same phase, a comma is used rather than |
    • often use an inert electrode
  • double line || = salt bridge

Tro, Chemistry: A Molecular Approach

standard reduction potential
Standard Reduction Potential
  • a half-reaction with a strong tendency to occur has a large + half-cell potential
  • when two half-cells are connected, the electrons will flow so that the half-reaction with the stronger tendency will occur
  • we cannot measure the absolute tendency of a half-reaction, we can only measure it relative to another half-reaction
  • we select as a standard half-reaction the reduction of H+ to H2 under standard conditions, which we assign a potential difference = 0 v
    • standard hydrogen electrode, SHE

Tro, Chemistry: A Molecular Approach

half cell potentials
Half-Cell Potentials
  • SHE reduction potential is defined to be exactly 0 v
  • half-reactions with a stronger tendency toward reduction than the SHE have a + value for E°red
  • half-reactions with a stronger tendency toward oxidation than the SHE have a  value for E°red
  • E°cell = E°oxidation + E°reduction
    • E°oxidation = E°reduction
    • when adding E°values for the half-cells, do not multiply the half-cell E° values, even if you need to multiply the half-reactions to balance the equation

Tro, Chemistry: A Molecular Approach

slide39

Ex 18.4 – Calculate Ecell for the reaction at 25CAl(s) + NO3−(aq) + 4 H+(aq)  Al3+(aq) + NO(g) + 2 H2O(l)

Tro, Chemistry: A Molecular Approach

slide40
Ex 18.4a – Predict if the following reaction is spontaneous under standard conditionsFe(s)+ Mg2+(aq)  Fe2+(aq) + Mg(s)

Tro, Chemistry: A Molecular Approach

slide42

Practice - Sketch and Label the Voltaic CellFe(s) ½ Fe2+(aq) ½½ Pb2+(aq) ½ Pb(s) , Write the Half-Reactions and Overall Reaction, and Determine the Cell Potential under Standard Conditions.

Tro, Chemistry: A Molecular Approach

slide43

ox: Fe(s)  Fe2+(aq) + 2 e−E = +0.45 V

red: Pb2+(aq) + 2 e−  Pb(s)E = −0.13 V

tot: Pb2+(aq) + Fe(s)  Fe2+(aq) + Pb(s)E = +0.32 V

Tro, Chemistry: A Molecular Approach

predicting whether a metal will dissolve in an acid
Predicting Whether a Metal Will Dissolve in an Acid
  • acids dissolve in metals if the reduction of the metal ion is easier than the reduction of H+(aq)
  • metals whose ion reduction reaction lies below H+ reduction on the table will dissolve in acid

Tro, Chemistry: A Molecular Approach

e cell d g and k
E°cell,DG° and K
  • for a spontaneous reaction
    • one the proceeds in the forward direction with the chemicals in their standard states
    • DG° < 1 (negative)
    • E° > 1 (positive)
    • K > 1
  • DG° = −RTlnK = −nFE°cell
    • n is the number of electrons
    • F = Faraday’s Constant = 96,485 C/mol e−

Tro, Chemistry: A Molecular Approach

example 18 6 calculate d g for the reaction i 2 s 2 br aq br 2 l 2 i aq

E°ox, E°red

E°cell

DG°

Example 18.6- Calculate DG° for the reactionI2(s) + 2 Br−(aq) →Br2(l) + 2 I−(aq)

Given:

Find:

I2(s) + 2 Br−(aq) →Br2(l) + 2 I−(aq)

DG°, (J)

Concept Plan:

Relationships:

ox: 2 Br−(aq) → Br2(l) + 2 e− E° = −1.09 v

Solve:

red: I2(l) + 2 e− → 2 I−(aq)E° = +0.54 v

tot: I2(l) + 2Br−(aq)→ 2I−(aq) + Br2(l)E° = −0.55 v

Answer:

since DG° is +, the reaction is not spontaneous in the forward direction under standard conditions

Tro, Chemistry: A Molecular Approach

example 18 7 calculate k at 25 c for the reaction cu s 2 h aq h 2 g cu 2 aq

E°ox, E°red

E°cell

K

Example 18.7- Calculate K at 25°C for the reactionCu(s) + 2 H+(aq) →H2(g) + Cu2+(aq)

Given:

Find:

Cu(s) + 2 H+(aq) →H2(g) + Cu2+(aq)

K

Concept Plan:

Relationships:

ox: Cu(s) → Cu2+(aq) + 2 e− E° = −0.34 v

Solve:

red: 2 H+(aq) + 2 e− → H2(aq)E° = +0.00 v

tot: Cu(s) + 2H+(aq)→ Cu2+(aq) + H2(g)E° = −0.34 v

Answer:

since K < 1, the position of equilibrium lies far to the left under standard conditions

Tro, Chemistry: A Molecular Approach

nonstandard conditions the nernst equation
Nonstandard Conditions - the Nernst Equation
  • DG = DG° + RT ln Q
  • E = E° - (0.0592/n) log Q at 25°C
  • when Q = K, E = 0
  • use to calculate E when concentrations not 1 M

Tro, Chemistry: A Molecular Approach

e at nonstandard conditions
E at Nonstandard Conditions

Tro, Chemistry: A Molecular Approach

slide50

E°ox, E°red

E°cell

Ecell

Example 18.8- Calculate Ecellat 25°C for the reaction3 Cu(s) + 2 MnO4−(aq) + 8 H+(aq) →2 MnO2(s) + Cu2+(aq) + 4 H2O(l)

Given:

Find:

3 Cu(s) + 2 MnO4−(aq) + 8 H+(aq) →2 MnO2(s) + Cu2+(aq) + 4 H2O(l)

[Cu2+] = 0.010 M, [MnO4−] = 2.0 M, [H+] = 1.0 M

Ecell

Concept Plan:

Relationships:

Solve:

ox: Cu(s) → Cu2+(aq) + 2 e− }x3E° = −0.34 v

red: MnO4−(aq) + 4 H+(aq) + 3 e− → MnO2(s) + 2H2O(l) }x2 E° = +1.68 v

tot: 3 Cu(s) + 2 MnO4−(aq) + 8 H+(aq) → 2 MnO2(s) + Cu2+(aq) + 4 H2O(l))E° = +1.34 v

Check:

units are correct, Ecell > E°cell as expected because [MnO4−] > 1 M and [Cu2+] < 1 M

Tro, Chemistry: A Molecular Approach

concentration cells
Concentration Cells
  • it is possible to get a spontaneous reaction when the oxidation and reduction reactions are the same, as long as the electrolyte concentrations are different
  • the difference in energy is due to the entropic difference in the solutions
    • the more concentrated solution has lower entropy than the less concentrated
  • electrons will flow from the electrode in the less concentrated solution to the electrode in the more concentrated solution
    • oxidation of the electrode in the less concentrated solution will increase the ion concentration in the solution – the less concentrated solution has the anode
    • reduction of the solution ions at the electrode in the more concentrated solution reduces the ion concentration – the more concentrated solution has the cathode

Tro, Chemistry: A Molecular Approach

concentration cell

when the cell concentrations are equal there is no difference in energy between the half-cells and no electrons flow

when the cell concentrations are different, electrons flow from the side with the less concentrated solution (anode) to the side with the more concentrated solution (cathode)

Concentration Cell

Cu(s) Cu2+(aq) (0.010 M)  Cu2+(aq) (2.0 M) Cu(s)

Tro, Chemistry: A Molecular Approach

leclanche acidic dry cell
LeClanche’ Acidic Dry Cell
  • electrolyte in paste form
    • ZnCl2 + NH4Cl
      • or MgBr2
  • anode = Zn (or Mg)

Zn(s) ®Zn2+(aq) + 2 e-

  • cathode = graphite rod
  • MnO2 is reduced

2 MnO2(s) + 2 NH4+(aq) + 2 H2O(l) + 2 e-

® 2 NH4OH(aq) + 2 Mn(O)OH(s)

  • cell voltage = 1.5 v
  • expensive, nonrechargeable, heavy, easily corroded

Tro, Chemistry: A Molecular Approach

alkaline dry cell
Alkaline Dry Cell
  • same basic cell as acidic dry cell, except electrolyte is alkaline KOH paste
  • anode = Zn (or Mg)

Zn(s) ®Zn2+(aq) + 2 e-

  • cathode = brass rod
  • MnO2 is reduced

2 MnO2(s) + 2 NH4+(aq) + 2 H2O(l) + 2 e-

® 2 NH4OH(aq) + 2 Mn(O)OH(s)

  • cell voltage = 1.54 v
  • longer shelf life than acidic dry cells and rechargeable, little corrosion of zinc

Tro, Chemistry: A Molecular Approach

lead storage battery
Lead Storage Battery
  • 6 cells in series
  • electrolyte = 30% H2SO4
  • anode = Pb

Pb(s) + SO42-(aq) ®PbSO4(s) + 2 e-

  • cathode = Pb coated with PbO2
  • PbO2 is reduced

PbO2(s) + 4 H+(aq) + SO42-(aq) + 2 e-

® PbSO4(s) + 2 H2O(l)

  • cell voltage = 2.09 v
  • rechargeable, heavy

Tro, Chemistry: A Molecular Approach

nicad battery
NiCad Battery
  • electrolyte is concentrated KOH solution
  • anode = Cd

Cd(s) + 2 OH-1(aq) ®Cd(OH)2(s) + 2 e-1 E0 = 0.81 v

  • cathode = Ni coated with NiO2
  • NiO2 is reduced

NiO2(s) + 2 H2O(l) + 2 e-1 ® Ni(OH)2(s) + 2OH-1 E0 = 0.49 v

  • cell voltage = 1.30 v
  • rechargeable, long life, light – however recharging incorrectly can lead to battery breakdown

Tro, Chemistry: A Molecular Approach

ni mh battery
Ni-MH Battery
  • electrolyte is concentrated KOH solution
  • anode = metal alloy with dissolved hydrogen
    • oxidation of H from H0 to H+1

M∙H(s) + OH-1(aq) ®M(s) + H2O(l) + e-1 E° = 0.89 v

  • cathode = Ni coated with NiO2
  • NiO2 is reduced

NiO2(s) + 2 H2O(l) + 2 e-1 ® Ni(OH)2(s) + 2OH-1 E0 = 0.49 v

  • cell voltage = 1.30 v
  • rechargeable, long life, light, more environmentally friendly than NiCad, greater energy density than NiCad

Tro, Chemistry: A Molecular Approach

lithium ion battery
Lithium Ion Battery
  • electrolyte is concentrated KOH solution
  • anode = graphite impregnated with Li ions
  • cathode = Li - transition metal oxide
    • reduction of transition metal
  • work on Li ion migration from anode to cathode causing a corresponding migration of electrons from anode to cathode
  • rechargeable, long life, very light, more environmentally friendly, greater energy density

Tro, Chemistry: A Molecular Approach

fuel cells
Fuel Cells
  • like batteries in which reactants are constantly being added
    • so it never runs down!
  • Anode and Cathode both Pt coated metal
  • Electrolyte is OH– solution
  • Anode Reaction:

2 H2 + 4 OH–

→ 4 H2O(l) + 4 e-

  • Cathode Reaction:

O2 + 4 H2O + 4 e-

→ 4 OH–

Tro, Chemistry: A Molecular Approach

electrolytic cell
Electrolytic Cell
  • uses electrical energy to overcome the energy barrier and cause a non-spontaneous reaction
    • must be DC source
  • the + terminal of the battery = anode
  • the - terminal of the battery = cathode
  • cations attracted to the cathode, anions to the anode
  • cations pick up electrons from the cathode and are reduced, anions release electrons to the anode and are oxidized
  • some electrolysis reactions require more voltage than Etot, called the overvoltage

Tro, Chemistry: A Molecular Approach

electroplating
electroplating
  • In electroplating, the work piece is the cathode.
    • Cations are reduced at cathode and plate to the surface of the work piece.
    • The anode is made of the plate metal. The anode oxidizes and replaces the metal cations in the solution

Tro, Chemistry: A Molecular Approach

electrochemical cells2
Electrochemical Cells
  • in all electrochemical cells, oxidation occurs at the anode, reduction occurs at the cathode
  • in voltaic cells,
    • anode is the source of electrons and has a (−) charge
    • cathode draws electrons and has a (+) charge
  • in electrolytic cells
    • electrons are drawn off the anode, so it must have a place to release the electrons, the + terminal of the battery
    • electrons are forced toward the anode, so it must have a source of electrons, the − terminal of the battery

Tro, Chemistry: A Molecular Approach

electrolysis
Electrolysis
  • electrolysis is the process of using electricity to break a compound apart
  • electrolysis is done in an electrolytic cell
  • electrolytic cells can be used to separate elements from their compounds
    • generate H2 from water for fuel cells
    • recover metals from their ores

Tro, Chemistry: A Molecular Approach

electrolysis of water
Electrolysis of Water

Tro, Chemistry: A Molecular Approach

electrolysis of pure compounds
Electrolysis of Pure Compounds
  • must be in molten (liquid) state
  • electrodes normally graphite
  • cations are reduced at the cathode to metal element
  • anions oxidized at anode to nonmetal element

Tro, Chemistry: A Molecular Approach

electrolysis of nacl l
Electrolysis of NaCl(l)

Tro, Chemistry: A Molecular Approach

mixtures of ions
Mixtures of Ions
  • when more than one cation is present, the cation that is easiest to reduce will be reduced first at the cathode
    • least negative or most positive E°red
  • when more than one anion is present, the anion that is easiest to oxidize will be oxidized first at the anode
    • least negative or most positive E°ox

Tro, Chemistry: A Molecular Approach

electrolysis of aqueous solutions
Electrolysis of Aqueous Solutions
  • Complicated by more than one possible oxidation and reduction
  • possible cathode reactions
    • reduction of cation to metal
    • reduction of water to H2
      • 2 H2O + 2 e-1 ®H2 + 2 OH-1 E° = -0.83 v @ stand. cond.

E° = -0.41 v @ pH 7

  • possible anode reactions
    • oxidation of anion to element
    • oxidation of H2O to O2
      • 2 H2O ® O2 + 4e-1 + 4H+1 E° = -1.23 v @ stand. cond.

E° = -0.82 v @ pH 7

    • oxidation of electrode
      • particularly Cu
      • graphite doesn’t oxidize
  • half-reactions that lead to least negative Etot will occur
    • unless overvoltage changes the conditions

Tro, Chemistry: A Molecular Approach

electrolysis of nai aq with inert electrodes
Electrolysis of NaI(aq) with Inert Electrodes

possible oxidations

2 I-1® I2 + 2 e-1 E° = −0.54 v

2 H2O ® O2 + 4e-1 + 4H+1 E° = −0.82 v

possible oxidations

2 I-1® I2 + 2 e-1 E° = −0.54 v

2 H2O ® O2 + 4e-1 + 4H+1 E° = −0.82 v

possible reductions

Na+1 + 1e-1® Na0 E° = −2.71 v

2 H2O + 2 e-1 ®H2 + 2 OH-1 E° = −0.41 v

possible reductions

Na+1 + 1e-1® Na0 E° = −2.71 v

2 H2O + 2 e-1 ®H2 + 2 OH-1 E° = −0.41 v

overall reaction

2 I−(aq) + 2 H2O(l)® I2(aq)+H2(g) + 2 OH-1(aq)

Tro, Chemistry: A Molecular Approach

faraday s law
Faraday’s Law
  • the amount of metal deposited during electrolysis is directly proportional to the charge on the cation, the current, and the length of time the cell runs
    • charge that flows through the cell = current x time

Tro, Chemistry: A Molecular Approach

slide73

t(s), amp

charge (C)

mol e−

mol Au

g Au

Example 18.10- Calculate the mass of Au that can be plated in 25 min using 5.5 A for the half-reaction Au3+(aq) + 3 e− → Au(s)

Given:

Find:

3 mol e− : 1 mol Au, current = 5.5 amps, time = 25 min

mass Au, g

Concept Plan:

Relationships:

Solve:

Check:

units are correct, answer is reasonable since 10 A running for 1 hr ~ 1/3 mol e−

Tro, Chemistry: A Molecular Approach

corrosion
Corrosion
  • corrosion is the spontaneous oxidation of a metal by chemicals in the environment
  • since many materials we use are active metals, corrosion can be a very big problem

Tro, Chemistry: A Molecular Approach

rusting
Rusting
  • rust is hydrated iron(III) oxide
  • moisture must be present
    • water is a reactant
    • required for flow between cathode and anode
  • electrolytes promote rusting
    • enhances current flow
  • acids promote rusting
    • lower pH = lower E°red

Tro, Chemistry: A Molecular Approach

preventing corrosion
Preventing Corrosion
  • one way to reduce or slow corrosion is to coat the metal surface to keep it from contacting corrosive chemicals in the environment
    • paint
    • some metals, like Al, form an oxide that strongly attaches to the metal surface, preventing the rest from corroding
  • another method to protect one metal is to attach it to a more reactive metal that is cheap
    • sacrificial electrode
      • galvanized nails

Tro, Chemistry: A Molecular Approach

sacrificial anode
Sacrificial Anode

Tro, Chemistry: A Molecular Approach