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 • 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 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 Tro, Chemistry: A Molecular Approach

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–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+® 3S + 2 NO + 4 H2O MnO2 + 4 HBr ® MnBr2 + Br2 + 2 H2O Tro, Chemistry: A Molecular Approach

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

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

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 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 EquationClO3-1 + Cl-1 ®Cl2 (in acid) Tro, Chemistry: A Molecular Approach

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 • 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 • 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 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

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

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

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

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 • 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,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

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

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 • 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

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

Chapter 18 Electrochemistry