Galvanic Cell ConceptGalvanic Cell ConceptSeparating the oxidation and reduction half-reactions Separating the oxidation and reduction half-reactions will make it so the energy in the electron transfer can will make it so the energy in the electron transfer can be harnessed.be harnessed.A salt bridge or porous disk is needed to connect the A salt bridge or porous disk is needed to connect the half-reactions so ions can flow and electrons don’t half-reactions so ions can flow and electrons don’t build up on one side of the reaction (one beaker)build up on one side of the reaction (one beaker)

Galvanic Cell DefinitionGalvanic Cell DefinitionDevice which chemical energy Device which chemical energy is changed to electrical energy.is changed to electrical energy.Oxidation occurs at the Oxidation occurs at the ANODEANODEReduction occurs at the Reduction occurs at the CATHODE (cat gets fat = CATHODE (cat gets fat = cathode gains electrons a.k.a. cathode gains electrons a.k.a. reduction)reduction)– An ox and a red cat An ox and a red cat

((ananode/ode/oxoxidation, idation, redreduction/uction/catcathode)hode)

ElectrodesElectrodes

If there is an element (not ion) in either If there is an element (not ion) in either half-reaction, it is what that particular half-reaction, it is what that particular electrode is made of electrode is made of – ***comes up later!***comes up later!

When all reactants/products are in When all reactants/products are in solution (aq) Pt or graphite can be used solution (aq) Pt or graphite can be used as the electrode.as the electrode.

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Cell Potential/Electromotive ForceCell Potential/Electromotive Force(EMF) Represented by (EMF) Represented by E°E°cellcell

Unit = volt (V) = 1 joule/coulombUnit = volt (V) = 1 joule/coulombMeasured with a voltmeter (not completely Measured with a voltmeter (not completely accurate b/c of heat). accurate b/c of heat). A potentiometer is used instead where the A potentiometer is used instead where the maximum cell potential can be measured.maximum cell potential can be measured.

Standard Reduction PotentialsStandard Reduction PotentialsIf we can find the potential for each half-If we can find the potential for each half-reaction (Table 18.1 pg. 829), we can reaction (Table 18.1 pg. 829), we can determine the cell potential (determine the cell potential (E°E°cellcell))Half-reaction manipulations (DO NOT Half-reaction manipulations (DO NOT MANIPULATE VOLTAGE):MANIPULATE VOLTAGE):– One must be reversed (oxidation)…can reverse E One must be reversed (oxidation)…can reverse E

so you have -E = -voltage…so you have -E = -voltage…– Electrons lost must = electrons gained, so Electrons lost must = electrons gained, so

multiplication of reaction may be needed (DO NOT multiplication of reaction may be needed (DO NOT MULTIPLY VOLTAGE BY THIS NUMBER!)MULTIPLY VOLTAGE BY THIS NUMBER!)

EQUATION (MEMORIZE)EQUATION (MEMORIZE)E°E°cellcell = = E°E°(cathode) - (cathode) - E°E°(anode)(anode)

Table 18.1Table 18.1Better oxidizing agents: easily reduced, LEFT Better oxidizing agents: easily reduced, LEFT side rxn. = largest, most positive standard side rxn. = largest, most positive standard reduction potentialreduction potentialBetter reducing agents: easily oxidized, RIGHT Better reducing agents: easily oxidized, RIGHT side = most negative standard reduction side = most negative standard reduction potential (aka most positive standard oxidizing potential (aka most positive standard oxidizing potential)potential)Example: Which is best reducing agent CuExample: Which is best reducing agent Cu++, F, F--, , HH--, H, H22O, IO, I22, K (find in reactants), K (find in reactants)Answer: Answer: K (-2.92)K (-2.92) > H > H-- (-2.23) > Cu (-2.23) > Cu++ (0.16) > I (0.16) > I22 (1.20) > H(1.20) > H22O (1.23) > FO (1.23) > F-- (2.92) (2.92)

Galvanic Cell ExampleGalvanic Cell ExampleCalculate the emf values (Calculate the emf values (EE°°cellcell) for the following) for the following

MgMg(s)(s) + 2H + 2H++(aq)(aq) -> Mg -> Mg2+2+

(aq)(aq) +H +H2(g)2(g)

Answer: Answer: EE°°cellcell = +2.37V = +2.37V

In Galvanic Cells…In Galvanic Cells…

When When E°E°cellcell is positive, the reaction will run is positive, the reaction will run spontaneously. (last slide)spontaneously. (last slide)If negative, it will run in the opposite If negative, it will run in the opposite direction (will NOT run spontaneously as direction (will NOT run spontaneously as written).written).

Cell Potential, Electrical Work, Cell Potential, Electrical Work, and Free Energyand Free Energy

Spontaneous IF: Spontaneous IF: – Positive cell potentialPositive cell potential– Negative Gibbs Free Energy (we will learn Negative Gibbs Free Energy (we will learn

more about Gibbs Free Energy in a later more about Gibbs Free Energy in a later chapter)chapter)

Line NotationLine NotationNot required for AP ExamNot required for AP ExamA A doubledouble vertical line separates the anode on vertical line separates the anode on left and cathode on rightleft and cathode on right– Represents a salt bridge or porous diskRepresents a salt bridge or porous disk

A A singlesingle vertical line separates different phases vertical line separates different phasesEx: Ex: anode Cd -> Cdanode Cd -> Cd2+2+ + 2e + 2e--

cathode Hgcathode Hg2+2+ + 2e + 2e-- -> Hg -> HgLine notation: CdLine notation: Cd(s)(s) I Cd I Cd2+2+

(aq)(aq) II Hg II Hg2+2+(aq)(aq) I Hg I Hg

Galvanic Cell: Complete DescriptionGalvanic Cell: Complete DescriptionIn half-reaction descriptions…FOUR items needed:In half-reaction descriptions…FOUR items needed:1.1. The cell potential - positive when The cell potential - positive when E°E°cellcell = = E°E°(cathode) (cathode)

- - E°E°(anode) and the balanced cell reaction(anode) and the balanced cell reaction2.2. The direction of electron flow, obtained by inspecting The direction of electron flow, obtained by inspecting

the half-reactions and using the direction that gives a the half-reactions and using the direction that gives a positive positive E°E°cellcell..

3.3. Designation of the anode and cathode.Designation of the anode and cathode.4.4. The nature of each electrode and the ions present in The nature of each electrode and the ions present in

each compartment. A chemically inert conductor is each compartment. A chemically inert conductor is required if none of the substances participating in required if none of the substances participating in the half-reaction is a conducting solid.the half-reaction is a conducting solid.

Example: Complete DescriptionExample: Complete DescriptionDescribe a galvanic cell based on the two half-Describe a galvanic cell based on the two half-reactions below.reactions below.

CuCu2+2+ + 2e + 2e-- -> Cu -> Cu E° = 0.34 VE° = 0.34 VCrCr22OO77

2-2- + 14H + 14H++ + 6e + 6e-- -> 2Cr -> 2Cr3+3+ + 7H + 7H22O E° = 1.33VO E° = 1.33V1.1. Balanced cell rxn: 3CuBalanced cell rxn: 3Cu(s)(s) + Cr + Cr22OO77

2-2-(aq)(aq) + 14H + 14H++

(aq)(aq) -> -> 3Cu3Cu2+2+

(aq)(aq) + 2Cr + 2Cr3+3+(aq)(aq) + 7H + 7H22OO(l)(l)

EE°°cell cell = 0.99V (needs to be positive)= 0.99V (needs to be positive)2.2. 1.33 (cathode) -0.34 (anode) means Cu needs to 1.33 (cathode) -0.34 (anode) means Cu needs to

be reversed. Cu will be giving off ebe reversed. Cu will be giving off e-- which will which will travel from Cu (anode) to cathode (platinum travel from Cu (anode) to cathode (platinum electrode).electrode).

Continued…Continued…3. Anode (copper metal electrode), cathode 3. Anode (copper metal electrode), cathode

(platinum electrode)(platinum electrode)4. The copper metal electrode (anode) will be 4. The copper metal electrode (anode) will be

in the Cu/Cuin the Cu/Cu2+2+ compartment while the compartment while the platinum electrode (cathode) will be in the platinum electrode (cathode) will be in the CrCr22OO77

2-2-/Cr/Cr3+3+ compartment compartment

Changing ConcentrationChanging ConcentrationIn chemical reactions…In chemical reactions…Standard conditions = 1Standard conditions = 1MM for all for allWhen reactants are >1When reactants are >1MM, it will increase , it will increase product concentration and will increase the cell product concentration and will increase the cell potentialpotentialWhen products are <1When products are <1MM, it will decrease the , it will decrease the product concentration (oppose forward product concentration (oppose forward reaction), decreases cell potentialreaction), decreases cell potentialMgMg(s)(s) + 2H + 2H++

(aq)(aq) -> Mg -> Mg2+2+(aq)(aq) +H +H2(g)2(g)

A galvanic cell that has the same component A galvanic cell that has the same component on each side but at different concentrationson each side but at different concentrationsCauses a cell potentialCauses a cell potentialVoltages are typically smallVoltages are typically smallEx: a cell has on its left side a 0.20 Ex: a cell has on its left side a 0.20 MM Cu Cu2+2+ solution and a 0.050 solution and a 0.050 MM Cu Cu2+2+ solution on the solution on the right sideright side

Concentration CellConcentration Cell