Lecture #4 of 26 - UCI Department of Chemistry

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Lecture #4 of 26

91

Looking forward… our review of Chapter “0”

● Cool applications

● Redox half-reactions

● Balancing electrochemical equations

● History of electrochemistry and Batteries

● IUPAC terminology and Ecell = Ered – Eox

● Thermodynamics and the Nernst equation

● Common reference electrodes

● Standard and Absolute potentials

● Latimer and Pourbaix diagrams

● Calculating Ecell under non-standard state conditions

● Conventions

92

Thus, the potentials for half-cell reactions are actually full-cell potential (difference(s)) versus SHE, or other!

𝐸cell = 𝐸red − 𝐸oxSince the method of half-reactions ultimately results in us taking their difference, we can add an arbitrary constant to all half reactions… By one (somewhat arbitrary) convention, it is often assumed that Eo for the standard hydrogen electrode (SHE) is equal to zero.

Half-Reaction for Hydrogen gas (H2):

H+ + e− → 12𝐻2 𝑔

𝐸H2= 𝐸H2

0 + 𝑅𝑇𝐹ln

𝑝H2H+ 𝐸H2

0 = 0

Half reactions must be referenced to something…

RECALL:

Potential Standard Potential

Rigorously, these two

terms in Q need to be

divided by the

standard-state

condition, and include

activity coefficients

vs. SHE

Given this value,

what is this

experimental redox

potential versus?

93

𝐸cell = 𝐸red − 𝐸oxSince the method of half-reactions ultimately results in us taking their difference, we can add an arbitrary constant to all half reactions… By one (somewhat arbitrary) convention, it is often assumed that Eo for the standard hydrogen electrode (SHE) is equal to zero.

𝐸H2= 𝐸H2

0 + 𝑅𝑇𝐹ln

𝑝H2H+ 𝐸H2

0 = 0

Half reactions must be referenced to something…

* 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

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–

Zn2+ + 2e– Zn

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)

95CRC 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.%20Klasse/KJ1042%20Termodynamikk%20med%20lab/Lab/Oppgave%205%20-%20Standard%20reduksjonspotensial/Rapportfiler/E0.pdf

96

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–

To get the balanced overall reaction?…

Ecell = E0cathode – E0

anode = 1.51 V + 0.76 V = +2.27 V

MnO4– + 8H+ + 5e– Mn2+ + 4H2O

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

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)

Look up half-reactions and standard reduction potentials in an

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

To get the balanced overall reaction?… multiply the anode

reaction by 5 and subtract it from 2 times the cathode reaction:

Zn2+ + 2e– Zn

97

* 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.%20Klasse/KJ1042%20Termodynamikk%20med%20lab/Lab/Oppgave%205%20-%20Standard%20reduksjonspotensial/Rapportfiler/E0.pdf

98

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

Zhu, …, Hamers, Nature Materials, 2013, 12, 836

diamond


99First 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 else combined… It 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 that ammonia in these (and other “double salts”) must 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. Cady’s 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

100

Example: solvated electrons

Q: What is 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)


Zhu, …, Hamers, Nature Materials, 2013, 12, 836

What is this?

diamond


101Absolute potentials can be measured / approximated…very carefully…

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

Prof. Sergio Trasatti

(Università de Milano, Italy)


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


Born–Haber cycle:

e(Hg)

𝜇𝑒𝐻𝑔




Born–Haber cycle:(two ways to solve this)

e(Hg)

𝜇𝑒𝐻𝑔

… but we need an electrode!


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


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

Hg

Pt

Born–Haber cycle:(two ways to solve this)

e(Hg)

105












● Conventions

106

7+ 6+ 4+ 3+ 2+ 0

Oxidation Reduction

1,69 V

Two diagrams of empirical standard potentials…

A Latimer diagram is a summary of the E0 values for an element; it is useful for visualizing the complete redox series for an element and for determining when disproportionation will occur.

from Wiki

Wendell Mitchell Latimer

(1893–1955)http://academictree.org/chemistry/peopleinfo.php?pid=24644

Chemist

Latimer, The oxidation states of the elements and their potentials in aqueous solution, 1938

107

from Wiki

7+ 6+ 4+ 3+ 2+ 0

Oxidation Reduction

(1) Does Mn2+ disproportionate?(2) What is the standard reduction potential of MnO4

– to MnO2?

1,69 V

Total Reaction: 3Mn2+ Mno + 2Mn3+ Eototal = ?

Reduction: Mn2+ Mno Eo = +1.18 V

Oxidation: Mn2+ Mn3+ Eo = +1.51 V



Disproportionation – spontaneous and simultaneous reduction and oxidation of a molecule (the opposite is comproportionation (AKA: symproportionation))

NO. Eo = Ered – Eox = 1.18 – 1.51 = –0.33 V

108

from Wiki

7+ 6+ 4+ 3+ 2+ 0

Oxidation Reduction

(1) Does Mn2+ disproportionate?(2) What is the standard reduction potential of MnO4

– to MnO2?ΔGo = -nFEo = -3FEo

ΔGo = -nFEo1 + -nFEo

2 = -F((1 x 0.56 V) + (2 x 2.26 V)) = -F(5.08 V)Set them equal to each other, and thus, 3Eo = 5.08 and Eo = 1.69 V



… for #1, you can work with Eo only (not ΔGo), because the reaction is always balanced/equal in the number of electrons

1,69 V

Disproportionation – spontaneous and simultaneous reduction and oxidation of a molecule (the opposite is comproportionation (AKA: symproportionation))

NO. Eo = Ered – Eox = 1.18 – 1.51 = –0.33 V

109

from Wiki

7+ 6+ 4+ 3+ 2+ 0

Oxidation Reduction



Recall from before… ???

???

… anyway, why are these bottom E0

values not on the Latimer diagram?

… because they are at basic/alkaline standard state with ~1 M OH–!

1,69 V

1,69 V

110

from Wiki

7+ 6+ 4+ 3+ 2+ 0

Oxidation Reduction



Recall from before… ???

???

What would this E0

value be when at acidic standard state?

111Two diagrams of empirical standard potentials…


???

1,69 V

from Wiki

7+ 6+ 4+ 3+ 2+ 0

Oxidation Reduction

???

𝐸 = 𝐸𝑎𝑐𝑖𝑑0 −

0.05916 V

𝑛log

𝑀𝑛𝑂21 𝐻2𝑂

2

𝑀𝑛𝑂42− 1

𝐻+ 4

𝐸 = 𝐸𝑎𝑐𝑖𝑑0 − 1.65648 V = 0.60 V

What would this E0


= 𝐸𝑎𝑐𝑖𝑑0 −

0.05916 V

2log

1 1

1 1 10−14 4 = 𝐸𝑎𝑐𝑖𝑑0 − 0.02958 V 56

1,69 V

112

from Wiki

7+ 6+ 4+ 3+ 2+ 0

Oxidation Reduction



???

???

𝐸 = 𝐸𝑎𝑐𝑖𝑑0 − 1.65648 V = 0.60 V

𝐸𝑆𝐻𝐸0 = 2,25648 V

SWEET!

… but then why did the CRC not list this? …

What would this E0


𝐸 = 𝐸𝑎𝑐𝑖𝑑0 −

0.05916 V

𝑛log

𝑀𝑛𝑂21 𝐻2𝑂

2

𝑀𝑛𝑂42− 1

𝐻+ 4= 𝐸𝑎𝑐𝑖𝑑

0 −0.05916 V

2log

1 1

1 1 10−14 4 = 𝐸𝑎𝑐𝑖𝑑0 − 0.02958 V 56

113

Marcel Pourbaix

(1904–1998)http://corrosion-doctors.org/Biographies/PourbaixBio.htm

… Second one (not truly standard potentials)…

Chemist

1,69 V

from Wiki

7+ 6+ 4+ 3+ 2+ 0

Oxidation

???

Pourbaix, Atlas of electrochemical equilibria in aqueous solutions, 1974

… but then why did the CRC not list this? …

… because in acid, the reaction does not occur!

A Pourbaix diagram is a map of the predominant equilibrium species of an aqueous electrochemical system; it is useful for identifying which materials/species are present/stable

… mostly based on thermochemical data

from Wiki

114

from Wiki

RHE

E(O2,H+/H2O)

Marcel Pourbaix



Anyway, … standard state is here, at ~1 M H+ (pH = 0) → SHE… but if written under alkaline conditions, ~1 M OH– is standard state (pH 14)

Chemist


Why don’t I like this? … Even though EVERYONE plots it this way



H+ transfer

e– transfer

1e–,1H+ reaction(slope = –60 mV)

2e–,3H+ reaction(slope = –90 mV)…



… you get the idea

115

RHE

E(O2,H+/H2O)

Marcel Pourbaix



Chemist




(1) What is the electrocatalyst for O2 evolution through water oxidation?(2) At what pH values is a solid electrocatalyst for H2 evolution stable?

MnO2

pH 7.5 – 13

from Wiki

116












● Conventions

117

How else could we write this? … “60 mV/2 log”! (at room temp.)

𝐸 = 𝐸0 −𝑅𝑇

𝑛𝐹ln𝑄Nernst Equation:

118

{Facile

Recall• Ecell does not require “n”• ΔG requires “n” (-nFEcell)

𝐸 = 𝐸0 −𝑅𝑇

𝑛𝐹ln𝑄Nernst Equation:

Pt(s)|Hg(l) Pt(s)|Hg2Cl2(s)+ Cl–(aq)

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Cu SCE

Cu2+(aq) Cu(s)

– –

Common Inert Electrodes: Platinum, Carbon, Gold

Common Reactive Electrodes: Copper, Zinc, Cadmium, Lead, Silver

high impedance to measure potential

NOT The Daniell Cell

Lecture #4 of 26 - UCI Department of Chemistry

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