Chapter 18 Electrochemistry Chemistry: A Molecular Approach, 1st Ed. Nivaldo Tro Chapter 18 Electrochemistry 2007, Prentice Hall

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 agent is reactant molecule that causes oxidation contains element reduced reducing agent is reactant molecule that causes reduction contains the element oxidized Tro, Chemistry: A Molecular Approach

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 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 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 States in their compounds, nonmetals have oxidation states according to the table below nonmetals higher on the table take priority Nonmetal Oxidation State Example F -1 CF4 H +1 CH4 O -2 CO2 Group 7A CCl4 Group 6A CS2 Group 5A -3 NH3 Tro, Chemistry: A Molecular Approach

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 oxidation reduction Tro, Chemistry: A Molecular Approach

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 3 H2S + 2 NO3– + 2 H+ ® 3 S + 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 Following red ag ox ag 3 H2S + 2 NO3– + 2 H+ ® 3 S + 2 NO + 4 H2O MnO2 + 4 HBr ® MnBr2 + Br2 + 2 H2O +1 -2 +5 -2 +1 0 +2 -2 +1 -2 reduction oxidation ox ag red ag +4 -2 +1 -1 +2 -1 0 +1 -2 reduction oxidation Tro, Chemistry: A Molecular Approach

Common Oxidizing Agents

Common Reducing Agents

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

Assign Oxidation States Ex 18.3 – Balance the equation: I(aq) + MnO4(aq)  I2(aq) + MnO2(s) in basic solution Assign Oxidation States Separate into half-reactions ox: I(aq)  I2(aq) red: MnO4(aq)  MnO2(s) Assign Oxidation States I(aq) + MnO4(aq)  I2(aq) + MnO2(s) Separate into half-reactions ox: red: Tro, Chemistry: A Molecular Approach

Balance half-reactions by mass Ex 18.3 – Balance the equation: I(aq) + MnO4(aq)  I2(aq) + MnO2(s) in basic solution Balance half-reactions by mass then O by adding H2O ox: 2 I(aq)  I2(aq) red: MnO4(aq)  MnO2(s) + 2 H2O(l) Balance half-reactions by mass ox: 2 I(aq)  I2(aq) red: MnO4(aq)  MnO2(s) Balance half-reactions by mass ox: I(aq)  I2(aq) red: MnO4(aq)  MnO2(s) Balance half-reactions by mass in base, neutralize the H+ with OH- ox: 2 I(aq)  I2(aq) red: 4 H+(aq) + MnO4(aq)  MnO2(s) + 2 H2O(l) 4 H+(aq) + 4 OH(aq) + MnO4(aq)  MnO2(s) + 2 H2O(l) + 4 OH(aq) 4 H2O(aq) + MnO4(aq)  MnO2(s) + 2 H2O(l) + 4 OH(aq) MnO4(aq) + 2 H2O(l)  MnO2(s) + 4 OH(aq) Balance half-reactions by mass then H by adding H+ ox: 2 I(aq)  I2(aq) red: 4 H+(aq) + MnO4(aq)  MnO2(s) + 2 H2O(l) Tro, Chemistry: A Molecular Approach

Balance Half-reactions by charge ox: 2 I(aq)  I2(aq) + 2 e Ex 18.3 – Balance the equation: I(aq) + MnO4(aq)  I2(aq) + MnO2(s) in basic solution Balance Half-reactions by charge ox: 2 I(aq)  I2(aq) + 2 e red: MnO4(aq) + 2 H2O(l) + 3 e  MnO2(s) + 4 OH(aq) Balance electrons between half-reactions ox: 2 I(aq)  I2(aq) + 2 e } x3 red: MnO4(aq) + 2 H2O(l) + 3 e  MnO2(s) + 4 OH(aq) }x2 ox: 6 I(aq)  3 I2(aq) + 6 e red: 2 MnO4(aq) + 4 H2O(l) + 6 e  2 MnO2(s) + 8 OH(aq) Tro, Chemistry: A Molecular Approach

Add the Half-reactions ox: 6 I(aq)  3 I2(aq) + 6 e Ex 18.3 – Balance the equation: I(aq) + MnO4(aq)  I2(aq) + MnO2(s) in basic solution Add the Half-reactions ox: 6 I(aq)  3 I2(aq) + 6 e red: 2 MnO4(aq) + 4 H2O(l) + 6 e  2 MnO2(s) + 8 OH(aq) tot: 6 I(aq)+ 2 MnO4(aq) + 4 H2O(l)  3 I2(aq)+ 2 MnO2(s) + 8 OH(aq) Check Reactant Count Element Product 6 I 2 Mn 12 O 8 H 2 charge Tro, Chemistry: A Molecular Approach

Practice - Balance the Equation H2O2 + KI + H2SO4 ® K2SO4 + I2 + H2O Tro, Chemistry: A Molecular Approach

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 ClO3-1 + Cl-1 ® Cl2 (in acid) Tro, Chemistry: A Molecular Approach

Practice - Balance the Equation ClO3-1 + 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 + 2 ClO3-1 + 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 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 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 Tro, Chemistry: A Molecular Approach

Electric Current Flowing Indirectly Between Atoms Tro, Chemistry: A Molecular Approach

Electrochemical Cells electrochemistry is 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 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 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 the salt bridge is required to complete the circuit and maintain charge balance Tro, Chemistry: A Molecular Approach

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 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 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

Fe(s) | Fe2+(aq) || MnO4(aq), Mn2+(aq), H+(aq) | Pt(s) Tro, Chemistry: A Molecular Approach

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

Tro, Chemistry: A Molecular Approach

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

Tro, Chemistry: A Molecular Approach

Separate the reaction into the oxidation and reduction half-reactions Ex 18.4 – Calculate Ecell for the reaction at 25C Al(s) + NO3−(aq) + 4 H+(aq)  Al3+(aq) + NO(g) + 2 H2O(l) Separate the reaction into the oxidation and reduction half-reactions ox: Al(s)  Al3+(aq) + 3 e− red: NO3−(aq) + 4 H+(aq) + 3 e−  NO(g) + 2 H2O(l) find the E for each half-reaction and sum to get Ecell Eox = −Ered = +1.66 v Ered = +0.96 v Ecell = (+1.66 v) + (+0.96 v) = +2.62 v Tro, Chemistry: A Molecular Approach

Separate the reaction into the oxidation and reduction half-reactions Ex 18.4a – Predict if the following reaction is spontaneous under standard conditions Fe(s) + Mg2+(aq)  Fe2+(aq) + Mg(s) Separate the reaction into the oxidation and reduction half-reactions ox: Fe(s)  Fe2+(aq) + 2 e− red: Mg2+(aq) + 2 e−  Mg(s) look up the relative positions of the reduction half-reactions red: Fe2+(aq) + 2 e−  Fe(s) since Mg2+ reduction is below Fe2+ reduction, the reaction is NOT spontaneous as written Tro, Chemistry: A Molecular Approach

the reaction is spontaneous in the reverse direction Mg(s) + Fe2+(aq)  Mg2+(aq) + Fe(s) ox: Mg(s)  Mg2+(aq) + 2 e− red: Fe2+(aq) + 2 e−  Fe(s) sketch the cell and label the parts – oxidation occurs at the anode; electrons flow from anode to cathode Tro, Chemistry: A Molecular Approach

Practice - Sketch and Label the Voltaic Cell Fe(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

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 metals whose ion reduction reaction lies below H+ reduction on the table will dissolve in acid Tro, Chemistry: A Molecular Approach

E°cell, DG° and K for a spontaneous reaction DG° = −RTlnK = −nFE°cell proceeds in the forward direction with the chemicals in their standard states DG° < 0 (negative) E° > 0 (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 DG° for the reaction I2(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: E°ox, E°red E°cell DG° Solve: ox: 2 Br−(aq) → Br2(l) + 2 e− E° = −1.09 v 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) → H2(g) + Cu2+(aq) Given: Find: Cu(s) + 2 H+(aq) → H2(g) + Cu2+(aq) K Concept Plan: Relationships: E°ox, E°red E°cell K Solve: ox: Cu(s) → Cu2+(aq) + 2 e− E° = −0.34 v 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 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 Tro, Chemistry: A Molecular Approach

Example 18.8- Calculate Ecell at 25°C for the reaction 3 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) + 3Cu2+(aq) +4 H2O(l) [Cu2+] = 0.010 M, [MnO4−] = 2.0 M, [H+] = 1.0 M Ecell Concept Plan: Relationships: E°ox, E°red E°cell Ecell Solve: ox: Cu(s) → Cu2+(aq) + 2 e− }x3 E° = −0.34 v red: MnO4−(aq) + 4 H+(aq) + 3 e− → MnO2(s) + 2 H2O(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 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 different, electrons flow from the side with the less concentrated solution (anode) to the side with the more concentrated solution (cathode) when the cell concentrations are equal there is no difference in energy between the half-cells and no electrons flow Cu(s) Cu2+(aq) (0.010 M)  Cu2+(aq) (2.0 M) Cu(s) Tro, Chemistry: A Molecular Approach

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 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

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

Tro, Chemistry: A Molecular Approach

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 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

Tro, Chemistry: A Molecular Approach

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 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 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 Tro, Chemistry: A Molecular Approach

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) Tro, Chemistry: A Molecular Approach

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 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 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 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

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: t(s), amp charge (C) mol e− mol Au g Au 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 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 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

Tro, Chemistry: A Molecular Approach

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 Tro, Chemistry: A Molecular Approach