CHAPTER 17 ELECTROCHEMISTRY

Oxidation and Reduction (Redox) Electrons are transferred Spontaneous redox rxns can transfer energy Electrons (electricity) Heat Non-spontaneous redox rxns can be made to happen with electricity

Oxidation and Reduction G ain E lectrons = R eduction An old memory device for oxidation and reduction goes like this… LEO says GER L ose E lectrons = O xidation

Oxidation and Reduction R eduction I s G ain Another memory device (my favorite!) for oxidation and reduction goes like this… O xidation I s L oss “OIL RIG”

Steps for acidic solution 1.Write two separate equations for oxidation and reduction –include any compound containing the element involved –does not need to include everything

Steps for acidic solution 2.For each half-reaction: –balance all elements but H and O –balance O using H 2 O –balance H using H + –balance charge using electrons (e - )

Steps for acidic solution If needed, multiply one or both half reactions by an integer so that number of electrons is equal in both half reactions

Steps for acidic solution 4.Add the half-reactions together and simplify 5.Check to be sure all is balanced.

A SPONTANEOUS OXIDATION-REDUCTION REACTION.

Figure 17.1 A Method to Separate the Oxidizing and Reducing Agents of a Redox Reaction

Electrochemistry

Electrochemistry Terminology #1  Oxidation  Oxidation – A process in which an element attains a more positive oxidation state Na(s)  Na + + e -  Reduction  Reduction – A process in which an element attains a more negative oxidation state Cl 2 + 2e -  2Cl -

Electrochemistry Terminology #3 Oxidizing agent  Oxidizing agent The substance that is reduced is the oxidizing agent Reducing agent  Reducing agent The substance that is oxidized is the reducing agent

Electrochemistry Terminology #4 Anode  Anode The electrode where oxidation occurs Cathode  Cathode The electrode where reduction occurs Memory device: Reduction at the Cathode

Figure 17.3 a-b The Electrode Compartment in Which Oxidation occurs is called the Anode; the Electrode Compartment in which Reduction Occurs is Called the Cathode Cell Potential (E cell ) = electromotive force (emf)

17–16 Figure 17.4 Digital Voltmeters Draw only a Negligible Current and are Convenient to Use

Copyright © Houghton Mifflin Company. All rights reserved. 17–17 Reaction in a Galvanic Cell Zn Zn e- and 2H + + 2e- H 2

17–18 A Galvanic Cell involving the Half-Reactions

Table of Reduction Potentials Measured against the Standard Hydrogen Electrode

Zn - Cu Galvanic Cell Cu e -  Cu E = +0.34V The less positive, or more negative reduction potential becomes the oxidation… Zn  Zn e - E = +0.76V Zn + Cu 2+  Zn 2+ + Cu E 0 = V

Copyright © Houghton Mifflin Company. All rights reserved. 17–21 Let’s Describe a Galvanic Cell Fe e - Fe E o = V MnO e - + 8H + Mn H 2 0 E o = 1.51 V Fe Fe e - E o = 0.44 V EoEo = 1.95V cell

17–22 Now you try! Describe completely the galvanic cell based on the following half-reactions under standard conditions. Ag + + e - Ag E o = 0.80 V Fe 3+ + e - Fe 2 + E o = 0.77 V

Measuring Standard Electrode Potential Potentials are measured against a hydrogen ion reduction reaction, which is arbitrarily assigned a potential of zero volts.

Galvanic (Electrochemical) Cells Spontaneous redox processes have: A positive cell potential, E 0 A negative free energy change, (-  G)

Zn - Cu Galvanic Cell Zn e -  Zn E = -0.76V Cu e -  Cu E = +0.34V From a table of reduction potentials:

Line Notation Zn(s) | Zn 2+ (aq) || Cu 2+ (aq) | Cu(s) An abbreviated representation of an electrochemical cell Anodesolution Anodematerial Cathodesolution Cathodematerial ||||

Calculating  G 0 for a Cell  G 0 = -nFE 0 n = moles of electrons in balanced redox equation F = Faraday constant = 96,485 coulombs/mol e - E 0 Zn + Cu 2+  Zn 2+ + Cu E 0 = V

The Nernst Equation Standard potentials assume a concentration of 1 M. The Nernst equation allows us to calculate potential when the two cells are not 1.0 M. R = 8.31 J/(mol  K) T = Temperature in K n = moles of electrons in balanced redox equation F = Faraday constant = 96,485 coulombs/mol e -

Nernst Equation Simplified At 25  C (298 K) the Nernst Equation is simplified this way:

Equilibrium Constants and Cell Potential equilibrium At equilibrium, forward and reverse reactions occur at equal rates, therefore: The battery is “dead” 2. The cell potential, E, is zero volts Modifying the Nernst Equation (at 25  C):

E 0 Zn + Cu 2+  Zn 2+ + Cu E 0 = V Calculating an Equilibrium Constant from a Cell Potential

Concentration Cell Step 1: Determine which side undergoes oxidation, and which side undergoes reduction. Both sides have the same components but at different concentrations. ???

Concentration Cell Both sides have the same components but at different concentrations. The 1.0 M Zn 2+ must decrease in concentration, and the 0.10 M Zn 2+ must increase in concentration Zn 2+ (1.0M) + 2e -  Zn (reduction) Zn  Zn 2+ (0.10M) + 2e - (oxidation) ??? Cathode Anode Zn 2+ (1.0M)  Zn 2+ (0.10M)

Concentration Cell Step 2: Calculate cell potential using the Nernst Equation (assuming 25  C). Both sides have the same components but at different concentrations. ??? Cathode Anode Zn 2+ (1.0M)  Zn 2+ (0.10M) Concentration Cell

Nernst Calculations Zn 2+ (1.0M)  Zn 2+ (0.10M)

Electrolytic Processes A negative -E 0 cell potential, ( -E 0 ) +  G A positive free energy change, ( +  G ) NOT Electrolytic processes are NOT spontaneous. They have:

Electrolysis of Water In acidic solution Anode rxn: Cathode rxn: V V V

Electroplating of Silver Anode reaction: Ag  Ag + + e - Electroplating requirements: 1. Solution of the plating metal 3. Cathode with the object to be plated 2. Anode made of the plating metal 4. Source of current Cathode reaction: Ag + + e -  Ag

Solving an Electroplating Problem Q: How many seconds will it take to plate out 5.0 grams of silver from a solution of AgNO 3 using a 20.0 Ampere current? 5.0 g Ag + + e -  Ag 1 mol Ag g 1 mol e - 1 mol Ag C 1 mol e - 1 s 20.0 C = 2.2 x 10 2 s

17–40 Figure 17.9 Concentration Cell

Copyright © Houghton Mifflin Company. All rights reserved. 17–41 Figure Concentration Cell

Copyright © Houghton Mifflin Company. All rights reserved. 17–42 Figure Schematic Diagram of the Cell Described in Sample Exercise 17.7