electrochemistry. 17.1/17.2 galvanic cells and standard reduction potentials day 1

ElectrochemistryElectrochemistry

17.1/17.2 GALVANIC CELLS AND 17.1/17.2 GALVANIC CELLS AND STANDARD REDUCTION POTENTIALSSTANDARD REDUCTION POTENTIALS

Day 1

Electrochemistry Terminology Electrochemistry Terminology #1#1

OxidationOxidation – A process in which an element attains a more positive oxidation state

Na(s) Na+ + e-

ReductionReduction – A process in which an element attains a more negative oxidation state

Cl2 + 2e- 2Cl-


GGainain EElectronslectrons == RReductioneduction

An old memory device for An old memory device for oxidation and reduction goes oxidation and reduction goes like this…like this…

LEOLEO sayssays GERGERLEOLEO sayssays GERGER

LLoseose EElectronslectrons = = OOxidationxidation


Oxidizing agentOxidizing agentThe substance that is reduced

is the oxidizing agent Reducing agentReducing agent

The substance that is oxidized is the reducing agent


Anode Anode The electrode

where oxidation occurs

CathodeCathodeThe electrode

where reduction occurs

Memory Memory device:device:

RedReductionuctionat the at the

CatCathodehode

Table of Table of Reduction Reduction PotentialsPotentials

Measured Measured against against the the StandardStandardHydrogenHydrogenElectrodeElectrode

Always written as REDUCTION potentials

Measuring Measuring Standard Standard Electrode Electrode PotentialPotential

Potentials are measured against a Potentials are measured against a hydrogen ion reduction reaction, which hydrogen ion reduction reaction, which is arbitrarily assigned a potential of is arbitrarily assigned a potential of zero zero voltsvolts..

Relative Strength of Oxidizing Relative Strength of Oxidizing Reducing AgentsReducing Agents

1. The more + the reduction The more + the reduction potential, the stronger the potential, the stronger the Oxidizing Agent, (easier to Oxidizing Agent, (easier to reduce)reduce)

– Compare the reduction potential of Compare the reduction potential of each substance each substance

– Whichever has the more positive Whichever has the more positive reduction potential is the better reduction potential is the better oxidizing agentoxidizing agent

Galvanic (Electrochemical) Galvanic (Electrochemical) CellsCells

Galvanic (Electrochemical) Galvanic (Electrochemical) CellsCells

Spontaneous redox processes have:

• A positive cell potential, E0

cell• A negative free energy change,

(G is negative)

CalculatioCalculation of Eºn of Eºcellcell

Zn2+ + 2e- Zn Eº = -0.76VCu2+ + 2e- Cu Eº = +0.34V

From a From a table of table of reduction reduction potentials:potentials:

Calculation of EºCalculation of Eºcellcell

EºEºcellcell = Eº = Eºredred+ Eº+ Eºoxidoxid

• Both values are written as reduction potentials in the table

• Determine what is oxidized• Flip the equation• Flip the sign• NeverNever multiply the potential by multiply the potential by

and integer, it is an and integer, it is an intensive intensive propertyproperty

Zn - Cu Zn - Cu Galvanic Galvanic

CellCell

Cu2+ + 2e- Cu Eº (red) = +0.34V

The less positive, The less positive, or more negative or more negative reduction reduction potential potential becomes the becomes the oxidation.oxidation.

Zn Zn2+ + 2e- Eº(oxid) = +0.76V

Cu2 + + Zn + Zn2+ + Cu Eºcell = +

1.10 V

Line Line NotationNotation

Zn(Zn(ss) | Zn) | Zn2+2+((aqaq) || Cu) || Cu2+2+((aqaq) | Cu() | Cu(ss))

An abbreviated An abbreviated representation representation of an of an electrochemical electrochemical cellcell

AnodeAnodesolutionsolution

AnodeAnodematerialmaterial

CathodeCathodesolutionsolution

CathodeCathodematerialmaterial|| ||||||

Complete description of a Complete description of a galvanic cellgalvanic cell

1. The cell potential Eºcell2. Drawing of the cell* 3. Direction of electron flow

a. Designation of anode and cathodeb. Composition of the electrode (Pt

or Graphite if no metals are in the half reactions)

c. All ions in the solution from the half reactions

*line notation is used if indicated

17.3 CELL POTENTIAL, 17.3 CELL POTENTIAL, ELECTRICAL WORK AND FREE ELECTRICAL WORK AND FREE ENERGYENERGY

Day 2

Cell Potential and WorkCell Potential and Work

CJ

VEcell charge

systemthebydonework max

Galvanic Cell: Ecell >0 and *work <0

qw

E maxcell

*The amount of work a cell can perform (by the system on the surroundings)

cellqEw max

Charge and Chemical Charge and Chemical ReactionsReactions

Charge (q): number of coulombs transferred during a redox reaction

nFq

n=number of moles of electrons transferredF= Faraday Constant (96485 C/mole e-)

Charge and Chemical Charge and Chemical ReactionsReactions

Combine

nFq

cellnFEw max

Need balanced reaction and cell potential

cellmax qEw

Calculating Calculating GG00 for a Cell for a Cell

GG00 = -nFE = -nFE00

nn = moles of electrons in balanced redox equation= moles of electrons in balanced redox equation

FF = = Faraday constant = 96,485 coulombs/mol eFaraday constant = 96,485 coulombs/mol e--

Zn + Cu2+ Zn2+ + Cu EE00 = + 1.10 V

17.4 DEPENDENCE OF CELL 17.4 DEPENDENCE OF CELL POTENTIAL ON CONCENTRATIONPOTENTIAL ON CONCENTRATION

Day 3

The Nernst EquationThe Nernst EquationStandard potentials assume a Standard potentials assume a concentration of 1 M. The Nernst concentration of 1 M. The Nernst equation allows us to calculate potential equation allows us to calculate potential when the two cells are not 1.0 M.when the two cells are not 1.0 M.

RR = 8.31 J/(mol= 8.31 J/(molK) K)

TT = Temperature in K = Temperature in K

nn = moles of electrons in balanced redox equation = moles of electrons in balanced redox equation

FF = Faraday constant = 96,485 coulombs/mol e = Faraday constant = 96,485 coulombs/mol e--

Nernst Equation SimplifiedNernst Equation Simplified

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

Equilibrium Constants and Cell Equilibrium Constants and Cell PotentialPotential

At equilibriumequilibrium, forward and reverse reactions occur at equal rates, therefore:1.1. The battery is “dead”

2. The cell potential, E, is zero volts

Modifying the Nernst Equation (at 25 C):

Zn + Cu2+ Zn2+ + Cu EE00 = + 1.10 V

Calculating an Equilibrium Calculating an Equilibrium Constant from a Cell PotentialConstant from a Cell Potential

ConcentratioConcentration Celln Cell

Step 1: Determine which side undergoes oxidation, and which side undergoes reduction.

Both sides have the

same components

but at different

concentrations.

??????


Both sides have the

same components

but at different

concentrations.

The 1.0 M Zn2+ must decrease in concentration, and the 0.10 M Zn2+ must increase in concentration

Zn2+ (1.0M) + 2e- Zn (reduction) Zn Zn2+ (0.10M) + 2e-

(oxidation)

??????

CathodeCathodeAnodeAnode

Zn2+ (1.0M) Zn2+

(0.10M)

Concentration CellConcentration Cell

Step 2: Calculate cell potential using the Nernst Equation (assuming 25 C).

Both sides have the same

components but at different

concentrations.

??????

CathodeCathodeAnodeAnode

Zn2+ (1.0M) Zn2+

(0.10M)


Nernst CalculationsNernst Calculations

Zn2+ (1.0M) Zn2+

(0.10M)

17.7 ELECTROLYSIS17.7 ELECTROLYSISDay 3

ElectrolytElectrolytic ic

ProcesseProcessess

A negative cell potential, (--EE00)

A positive free energy change, (++GG)

Electrolytic processes are NOTNOT spontaneous. They have:

Electrolysis of Electrolysis of WaterWater

In acidic solutionIn acidic solution

Anode rxn:Anode rxn:

Cathode rxn:Cathode rxn:-1.23 V-1.23 V

-0.83 V-0.83 V

-2.06 V-2.06 V

Electroplating Electroplating of Silverof Silver

Anode reactionAnode reaction::

Ag Ag Ag Ag++ + e + e--

Electroplating requirementsElectroplating requirements::1. Solution of the plating metal1. Solution of the plating metal

3. Cathode with the object to be plated3. Cathode with the object to be plated2. Anode made of the plating metal2. Anode made of the plating metal

4. Source of current4. Source of current

Cathode reactionCathode reaction::

AgAg++ + e + e-- Ag Ag

Solving an Electroplating Solving an Electroplating ProblemProblem

Q: How many seconds will it take to Q: How many seconds will it take to plate out 5.0 grams of silver from a plate out 5.0 grams of silver from a solution of AgNOsolution of AgNO33 using a 20.0 Ampere using a 20.0 Ampere current?current?

5.0 g

AgAg++ + e + e- - Ag Ag

1 mol Ag

107.87 g

1 mol e-

1 mol Ag

96 485 C

1 mol e-

1 s

20.0 C

= 2.2 x 10= 2.2 x 1022 s s

Uses of electrolysisUses of electrolysis

• Electrolysis of water– Produces H2 and O2 gas – 2H2O H2 + O2

•Production of aluminium, lithium, sodium, potassium, magnesium, calcium

•Production of chlorine and sodium hydroxide

Other usesOther uses

• Seperating mixtures of ions. – More positive reduction potential

means the reaction proceeds forward. – Ion with highest reduction potential

will plate out first or at the lowest voltage.

Consider Ag, Cu, and Zn ions, Consider Ag, Cu, and Zn ions, which will plate out first?which will plate out first?Ag+ + e- Ag E=0.80 VCu2+ + 2e- Cu E= 0.35 VZn2+ + 2e- Zn E = -0.76 V

Silver, then copper, then zinc

electrochemistry. 17.1/17.2 galvanic cells and standard reduction potentials day 1

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