Lecture #2 of 17 - UCI Department of Chemistryardo/echem/UCI-CHEM248-2019S_lectur… · EXAMPLE:...

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Lecture #2 of 17

53

Looking forward… our review of Chapter “0”

● Cool applications

● Redox half-reactions

● Balancing electrochemical equations

● History of electrochemistry

● IUPAC terminology and Ecell = Ered – Eox

● Nernst equation and Common reference electrodes

● Standard and Absolute potentials

● Latimer and Pourbaix diagrams

● Calculating Ecell under non-standard state conditions

● Conventions

54A Short History Lesson…

Electrochemistry associated with Luigi Galvani who discovered

“animal electricity,” while trying to Frankenstein frogs legs (1791)

Luigi Galvani(1737–1798)

from Wiki

Physician, Physicist, Philosopher

55Voltaic pileInvented by Alessandro Volta (1800) but the elements of the pile

(galvanic cells) were named after Galvani.

(salt water)

What are the combined half-reactions?

At the Tempio Voltiano (the Volta Temple)

near Volta's home in Como, Italy.

http://en.wikipedia.org/wiki/Voltaic_pile

Alessandro Volta(1745–1827)

from Wiki

Volta presenting his "Voltaic

Pile" to Napoleon and his

http://en.wikipedia.org/wiki/Alessandro_Volta

Physicist

http://en.wikipedia.org/wiki/Voltaic_pilehttp://en.wikipedia.org/wiki/Alessandro_Volta

56Galvanic Cells

Physically separating the

half-reactions allows the

electrons to go over a

long distance, from the

anode to the cathode via

a (solid) conductor: basis

for conversion of chemical

energy into electricity =

“Electrochemistry” (2019)!

Salt bridge is an ionic conduit to

prevent buildup of charge in both

compartments and also to prevent

bulk mixing of the two solutions

Every non-equilibrium

cell is a galvanic cell (in

one direction, i.e. the

spontaneous direction)

57Electrolysis of waterVolta’s results were shared with

the scientific community and then,

boom, many people demonstrated

electrolysis the same year, and

later, electroplating!

William Cruickshank(17??–1810(1))

Johann Wilhelm Ritter(1776–1810)

Sir Anthony Carlisle(1768–1840)

William Nicholson(1753–1815)

http://en.wikipedia.org/wiki/Johann_Wilhelm_Ritter

Chemist

Chemist Surgeon

Chemist

http://en.wikipedia.org/wiki/Johann_Wilhelm_Ritter

58Daniell (galvanic) Cell (1836)

Half-reactions are

physically separated!

anode

oxidation

cathode

reduction

Zn (s) Zn2+ (aq) + 2e– Cu2+ (aq) + 2e– Cu (s)

NET REACTION: Zn (s) + Cu2+ (aq) Zn2+ (aq) + Cu (s)

John Frederic Daniell(1790–1845)

from Wiki

No more H2 from the

(primary) battery!

Chemist, Meteorologist

59Voltage Produced by Galvanic Cells

The difference in electric

potential between the anode

and the cathode is called:

Cell potential

Cell voltage

emf (electromotive force)

Cell DiagramZn (s) + Cu2+ (aq) Cu (s) + Zn2+ (aq)

[Cu2+] = 1 M and [Zn2+] = 1 M

Zn (s) | Zn2+ (aq, 1 M) || Cu2+ (aq, 1 M) | Cu (s)

anode cathodesalt bridge

This should be –0.76 V!(we will discuss this soon)

60EXAMPLE: What is the (standard) potential of a galvanic cell

consisting of a Cd electrode in a 1.0 M Cd(NO3)2 solution and a

Cr electrode in a 1.0 M Cr(NO3)3 solution?

Cd2+ (aq) + 2e– Cd (s) E0 = -0.40 V

Cr3+ (aq) + 3e– Cr (s) E0 = -0.74 V

Cd2+ will get reduced to Cd

Cr will get oxidized to Cr3+

… thus, it is reducing

2e– + Cd2+ (1 M) Cd (s)

Cr (s) Cr3+ (1 M) + 3e–Anode (oxidation):

Cathode (reduction):

2Cr (s) + 3Cd2+ (1 M) 3Cd (s) + 2Cr3+ (1 M)

x 2

x 3

E0 = Ecathode – Eanodecell0 0

E0 = -0.40 V – (-0.74 V) cell

E0 = +0.34 V (positive = spontaneous, since )cell

More negative of the two

Which half-reaction is reducing?

… if your answer is negative then you switched the anode/cathode in the galvanic cell

61Electrochemistry:

conventions… oh, conventions!

Cathode – electrode where catholyte species are reduced

Anode – electrode where anolyte species are oxidized

62Electrochemistry:




Negative/Positive electrode – cathode or anode?… it depends!

For the discharging (galvanic) battery, label the anode and cathode.

http://autoshop101.com/


63Electrochemistry:




Negative/Positive electrode – cathode or anode?… it depends!

For the charging (electrolytic) battery, label the anode and cathode.


e– e–

anodecathode

This is a generator

(AKA power supply)


64Electrochemistry:


Positive electrode – positively charged; immersed in the posolyte

Negative electrode – negatively charged; immersed in the negolyte

… Sheesh!

… Take-home message: For batteries, don’t call electrodes

anodes and cathodes (but naming convention used by most is for

discharge)


anode

e– e–

cathode

cathodeanode

e– e–

… I’m not kidding!


Zn Cu

Zn(s) Zn2+(aq) Cu2+(aq) Cu(s)

low impedance to measure current

Zn | Zn2+(aq) || Cu2+(aq) | Cu

I cellJohn Frederic Daniell

(1790–1845)from Wiki

Chemist, Meteorologist

The Daniell Cell (1836)

Zn Cu




I cell

This only works for

less than one second

and then stops due to

Kirchhoff’s current law,

which states that all

current at each location

must sum to zero…

… capacitive charging



Zn Cu

A i+

Bi

I cell

Now it works =

“electrochemistry”!

Salt bridge

contains an

inert, redox

inactive salt

solution

(electrolyte)




Zn Cu

A i+

Bi


I cell



* Name this cell type

* Identify anode

* Identify cathode

* Name the

electrode signs


Zn Cu

A i+

Bi


I cell



* Name this cell type

* Identify anode

* Identify cathode

* Name the

electrode signs

* primary galvanic cell

(A/–) (C/+)


Zn Cu

A i+

Bi


I cell




(A/–) (C/+)

This will eventually fully

discharge and reach

equilibrium (ΔG = Ecell = 0)

Then, either direction of

polarization bias results

in electrolytic function

(i.e. charging)!

71PROBLEM TIME!

Youtry!

(a) What is the standard Ecell for agalvanic cell based on zinc and silver?

(b) If we wanted to electrolyticallycharge the cell from part a (beforeany reactions took place), whatpotential would we have to apply?

72PROBLEM TIME!

Youtry!

(a) What is the standard Ecell for agalvanic cell based on zinc and silver?

(b) If we wanted to electrolyticallycharge the cell from part a (beforeany reactions took place), whatpotential would we have to apply?

(a) Ecell = +0.80 V –(-0.76 V) = 1.56 V

(b) Ebias < –1.56 V

73

• Coulomb (in units of C = A·s) is the unit of charge (96,485 C are in a mole of singly charged species = Faraday constant, F ≈ 96,500 C/mol ≈ 105 C/mol)

• Electricity is the flow of current (A = C/s) and is negative (cathodic) or positive (anodic) depending on the direction and sign of the current-carrying species (e.g. e–, H+)

• (Electrode) (electric) potential (V or E; in units of V = J/C) is written as a reductionThis relates to Gibbs free energy as ΔG = –RT ln K = –nFEcell (electrical work per mole), and…

… partial molar Gibbs free energy is the electrochemical potential ( , in units of J/mol)

Chemical potential ( , in units of J/mol)

Galvani/Inner (electric) potential (ϕ, in units of V)

… and in summary, ϕ

Also, standard state is a solvent, a solid, and a species at unit activity (~1 M solutes, ~1 bar gases)

Also, E = IR (Ohm’s law) when resistance is constant

• Galvanic cells produce power (in units of W = A x V = C/s x J/C = J/s) by spontaneous redox reactions

• Electrolytic cells require a power input to drive redox reactions; thus, the reactions are thermodynamically unfavorable

• Batteries have anodes and cathodes, but these names change depending on if the battery is being discharged (galvanic) or charged (electrolytic), and so the terminology of negative electrode and positive electrode is preferred

International Union of Pure and Applied

Chemistry (IUPAC)

(Sources: B&F, M3LC course textbook,and http://goldbook.iupac.org/)

(Accepted) Nomenclature and Terminology that you know, but may have forgotten

74














Chemistry (IUPAC)


integrate, over timedifferentiate, with respect to time


75














Chemistry (IUPAC)


E0(Cu2+/0) = +0.34 V vs. SHE

E0(Cu0/2+) = –0.34 V vs. SHE

… is incorrect!

You can subtract redox

potentials but do not change the

sign of the potential and then

call it an oxidation potential!


76














Chemistry (IUPAC)


Based on our current sign convention, it is best

to only write reduction potentials; however, if

we lived in an oxidation-potential-centric world,

we could write them all (i.e. everything) as

oxidation potentials; simply put, it is best to

not mix the conventions and so stick with

reduction potentials

E0(Cu2+/0) = +0.34 V vs. SHE

E0(Cu0/2+) = –0.34 V vs. SHE

… is incorrect!

You can subtract redox

potentials but do not change the

sign of the potential and then

call it an oxidation potential!


77














Chemistry (IUPAC)



78














Chemistry (IUPAC)


Cannot be

measured

independently!


79














Chemistry (IUPAC)



80














Chemistry (IUPAC)



81














Chemistry (IUPAC)

opposites



82International Union of Pure and Applied

Chemistry (IUPAC)












• A battery has an anode/anolyte and a cathode/catholyte, but these descriptors change depending on whether the battery is being discharged (galvanic) or charged (electrolytic); negative electrode/negolyte and positive electrode/posolyte are better


83

Electrochemical potential of species i in phase is an energy(J/mol),

, where

G (Gibbs free energy (J))

ni (amount of species i (mol))

(chemical potential (J/mol))

zi (valency of species i)

F 5 (Faraday constant (C/mol)

(Galvani/inner electric potential (V))

ai (activity of species i)

For an uncharged species .

Parsons, Pure & Appl. Chem., 1973, 37, 501

IUPAC Gold (http://goldbook.iupac.org)

For “clarity,” a brief (more rigorous) “review” of thermodynamics…

84Half reactions, at non-unity activity, obey the Nernst equation…

Take and use the relation ,

What is Q?

, because

ap is the activity of product p

ar is the activity of reactant r

vi is the stoichiometric number of i

i is the activity coefficient of i

ci is the concentration of i

ci0 is the standard state concentration of i



But first… what is Q, again? … the reaction quotient!

, because









But first… what is Q, again? … the reaction quotient!

, because fundamentally

which we never have!









Reaction quotient as

product and quotient of

species’ activities

from Wiki

Physicist

Walther Hermann Nernst

(1864 1941)

Nobel Prize (Chemistry, 1920)






from Wiki

Physicist


(1864 1941)




Memorize ~60 mV per order in log10, but do not forget n and that this is at 25 °C!

from Wiki

Physicist


(1864 1941)





91

standard (SHE)

Rigorously, each needs

to be divided by the

standard-state condition

92

Thus, the potentials for half-cell reactions are actually

full-cell potential (difference(s)) versus SHE, or other!

standard (SHE)

93

standard (SHE)

* Normal hydrogen electrode (NHE)is an empirical SHE ([H+] = 1; not standard state)* Standard hydrogen electrode (SHE)is a hypothetical, perfect NHE (a = 1; not empirical)* Reversible hydrogen electrode (RHE)is the SHE but the same regardless of pH* And generally, formal potentials(E0’) take into consideration non-idealities and changesin ionic strengths so that the reaction quotient only has concentrations, and not activities

Ramette, J. Chem. Educ., 1987, 64, 885

94

Potential Standard Potential

* Normal hydrogen electrode (NHE)is an empirical SHE ([H+] = 1; not standard state)* Standard hydrogen electrode (SHE)is a hypothetical, perfect NHE (a = 1; not empirical)* Reversible hydrogen electrode (RHE)is the SHE but the same regardless of pH* And generally, formal potentials(E0’) take into consideration non-idealities and changesin ionic strengths so that the reaction quotient only has concentrations, and not activities

standard (SHE)

Ramette, J. Chem. Educ., 1987, 64, 885

95


standard (SHE)

Given this value, what

is this experimental

redox potential versus?

96

vs. SHE


standard (SHE)

Given this value, what

is this experimental

redox potential versus?

97

Look up half-reactions and standard reduction potentials in an

Electrochemical Series table (CRC, B&F Appendix C, WWW):

Anode: E0anode = -0.76 V

Cathode: E0cathode = +1.51 V

Note: Although strictly correct, do not use “–E0” as the “E0 for oxidation”

Note: Be careful to choose the correct half-reaction with MnO4–

Zn Zn2+ + 2e–

MnO4– + 8H+ + 5e– Mn2+ + 4H2O

EXAMPLE: Write a balanced chemical equation and

calculate the standard cell potential for the galvanic cell:

Zn(s) | Zn2+ (1 M) || MnO4– (1 M), Mn2+ (1 M), H+ (1 M) | Pt(s)

98CRC Handbook of Chemistry and Physics, 92nd Edition

http://folk.ntnu.no/andersty/2.%20Klasse/KJ1042%20Termodynamikk%20med%20lab/Lab/

Oppgave%205%20-%20Standard%20reduksjonspotensial/Rapportfiler/E0.pdf

* All values versus SHE

MnO4– + 8H+ + 5e–

Mn2+ + 4H2O

Zn Zn2+ + 2e–

http://folk.ntnu.no/andersty/2. Klasse/KJ1042 Termodynamikk med lab/Lab/Oppgave 5 - Standard reduksjonspotensial/Rapportfiler/E0.pdf

99





To get the balanced overall reaction… ?

Zn Zn2+ + 2e–

MnO4– + 8H+ + 5e– Mn2+ + 4H2O






100





To get the balanced overall reaction, multiply the anode

reaction by 5 and add to 2 times the cathode reaction, giving:

Ecell = E0cathode – E

0anode = 1.51 V + 0.76 V = +2.27 V

Zn Zn2+ + 2e–

MnO4– + 8H+ + 5e– Mn2+ + 4H2O

2MnO4– (aq) + 16H+ (aq) + 5Zn (s) 5Zn2+ (aq) + 2Mn2+ (aq) + 8H2O (l)






101

* All values versus SHE

CRC Handbook of Chemistry and Physics, 92nd Edition

http://folk.ntnu.no/andersty/2.%20Klasse/KJ1042%20Termodynamikk%20med%20lab/Lab/

Oppgave%205%20-%20Standard%20reduksjonspotensial/Rapportfiler/E0.pdf

Q: What processes occur in electrochemistry? … one more …

(remember this?)

* How negative can Eo be?

http://folk.ntnu.no/andersty/2. Klasse/KJ1042 Termodynamikk med lab/Lab/Oppgave 5 - Standard reduksjonspotensial/Rapportfiler/E0.pdf

102

Example: solvated electrons

Q: What processes occur in electrochemistry? … one more …

A: Winter, 2017: Those involving the motion/transport of charge –

carried by entities other than unsolvated electrons and holes –

through phase(s), or the transfer of charge across interface(s).

Prof. Robert Hamers (Univ. of Wisconsin)

http://hamers.chem.wisc.edu/people

Hamers, Nature Materials, 2013, 12, 836

diamond


103First description of conductivity using solvated electrons

Cady, J. Phys. Chem., 1897, 1, 707

During the first part of the twentieth century, E. C. Franklin and C. A. Kraus probably did more to elucidate the chemistry of

liquid ammonia solutions than everybody is perhaps little known that their work was prompted by the

research and insight of H. P. Cady, carried out while he was an undergraduate! Whilst working on cobalt ammine

complexes, Cady proposed function in a manner akin to water in salts

with water of crystallization. He suggested further that liquid ammonia would probably be found to resemble water in its

physical and chemical properties thus adding a second to our list of ionizing solvents. undergraduate work, carried

out without supervision, published in 1897, was perhaps the first physical chemistry study of liquid ammonia solutions.

Zurek, Edwards, & Hoffman, Angew. Chem. Int. Ed., 2009, 48, 8198

104

Example: solvated electrons

Q: What is electrochemistry? … one more …

A: Any process involving the motion/transport of charge – carried

by entities other than unsolvated electrons and holes – through

phase(s), or the transfer of charge across interface(s).

Prof. Robert Hamers (Univ. of Wisconsin)


Hamers, Nature Materials, 2013, 12, 836

What is this?

diamond


105Absolute potentials can be measured / approximated…

very carefully…

Trasatti, Pure & Appl. Chem., 1986, 58, 955

Prof. Sergio Trasatti

(Università de Milano, Italy)


very carefully…


FOR YOUR REFERENCE


very carefully…

Trasatti, Electroanal. Chem. Interfac. Electrochem., 1974, 52, 313


Born–Haber cycle:

e(Hg)

FOR YOUR REFERENCE


very carefully…



Born–Haber cycle:

e(Hg)

… but we need an electrode!

FOR YOUR REFERENCE


very carefully…



Born–Haber cycle:

e(Hg)


FOR YOUR REFERENCE


very carefully…


Trasatti, Pure & Appl. Chem., 1986, 58, 955… but we need an electrode!

Born–Haber cycle:

e(Hg)

FOR YOUR REFERENCE


very carefully…

Farrell and McTigue, Electroanal. Chem. Interfac. Electrochem., 1982, 139, 37



Born–Haber cycle:

e(Hg)


Hg

Pt

FOR YOUR REFERENCE

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