lecture 8 - stanford universitydionne.stanford.edu/matsci202_2011/lecture8_ppt.pdflecture 8 ch i l r...
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![Page 1: Lecture 8 - Stanford Universitydionne.stanford.edu/MatSci202_2011/Lecture8_ppt.pdfLecture 8 Ch i l R ti I d P t E hChemical Reactions: Ion and Proton Exchange Suggested reading: Chapter](https://reader033.vdocuments.site/reader033/viewer/2022060418/5f1591326e6a1f4fff5666c2/html5/thumbnails/1.jpg)
Lecture 8Lecture 8
Ch i l R ti I d P t E hChemical Reactions: Ion and Proton Exchange
Suggested reading: Chapter 3 & 4 1-4 3Suggested reading: Chapter 3 & 4.1 4.3
Office hours this week: W d (10 12) || Th (2 4) || F i (UG l 9 10 )Wed (10-12) || Thurs (2-4) || Fri (UG only, 9am-10am)
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Roadmap of the course
Th i i Quantum chemistry:
H d i & The origin of the
elements
Hydrogenic & multielectron atoms
Quantum
Periodic table trends (radii, ionization
energies
Quantum chemistry of molecules: MO theoryenergies,
electronegativities)
Quantum chemistry
Molecular energies, bond strength, molecular shapes
c e st y of solids:
band theory
Solid bonding
and and stability
The rest of this course: reaction chemistry (acid-base, redox, complexes, nano, bio)
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Cation exchange in ionic crystals
CdSe Ag2Se
d2 d A dCd2+ ionic radius = 109 pmSe2- ionic radius = 184 pm
Ag+ ionic radius = 128 pmSe2- ionic radius = 184 pm
The exchange reaction is completely kinetically hindered at ambient temperature and pressure in the bulk.
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Tunable fluorescence
Can ion exchange occur in nanocrystals?
Shape controldiameter2 nm 7 nm
Size control
100 nm
50 nm
~ 5 nm
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Journal Presentation by Ryan, Scott, Denys
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Cd2+
Nanocrystal ion exchange (from B. Sadtler)
CdSe
Ag+
3 nm40 nm 3 nm 40 nm
NC exchange videoNC exchange video
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Partial Ag+ exchange produces striped CdS-Ag2S nanorods
Ag+ cation exchange leads to alternating CdS and Ag2S regions along
g2
alternating CdS and Ag2S regions along the nanorod
Small Ag2S regions nucleate over
Increasing Ag+/Cd2+ ratio
nanorod surface and grow into the CdSlattice
10 nm20 nm 20 nm
R. Robinson. B. Sadtler, D. Demchenko, C. Erdonmez, L.-W. Wang, A. P. Alivisatos. Science 2007, 317, 355.
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Bright-field TEM Energy filtered TEM
Morphology of CdS-Cu2S nanorods
Bright field TEM Energy filtered TEM
20Cd regions
20 nm 20 nmg
Cu regions
Hi-Res TEM
Cu2S nucleates at the ends and the exchange reaction proceeds into the nanorod
4 nm
nanorod
Epitaxial connection at the CdS-Cu2S interfaceinterface
B. Sadtler, D. O. Demchenko, H. Zheng, S. M. Hughes, M. Merkle, U. Dahmen, L.W.-Wang, A.P. Alivisatos. JACS 2009, 131, 5285
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Comparison of morphologies produced by Ag+ and Cu+ exchange
Ag+/Cd2+ = 0.2 Ag+/Cd2+ = 0.8
Ag+ cation exchange: non-selective Ag2S nucleation followed by partial phase segregation
As Ag2S regions grow into the nanorod, ripening occurs to
Increasing Ag+/Cd2+ ratio
reduce elastic strain
Ag /Cd2 ratio
Cu+ cation exchange: selective Cu2S nucleation of low energy interfaces
Cu2S nucleates at one or both ends producing a stable configuration
Increasing Ag+/Cd2+ ratio
20 nm 2 nm
configuration
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Difficult to directly exchange two divalent cations (e g Cd2+ and Pb2+)
Sequential Cation Exchange
Cu+(MeOH)
d2
Pb2+(TBP)
( )
Difficult to directly exchange two divalent cations (e.g. Cd and Pb )
PbSCu2SCdS
Cd2+(MeOH) Cu+(TBP)
100 nm 100 nm 100 nm
y (a
u)
nce
(au) PbS
Cu2SCdS
PbS
Cu2S
Inte
nsit
y
Abs
orba
n CdS Cu2S
CdS
400 800 1200 1600 2000 30 40 50 60 70
Diffraction angle (2)Wavelength (nm)
J.M. Luther, H. Zheng, B. Sadtler, A. P. Alivisatos, JACS 2009, 131, 16851
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Exchange in ionic nanocrystals can lead to entirely new materials & devices that cannot be synthesized through y g
direct techniques.
CdS PbS
ITO
nanocrystal photovoltaicsvertically-aligned nanorod array
hole carrier
electron carrier200
nm h+
electron carrier
~ 2 e‐
bottom electrode
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Exchange in ionic nanocrystals can lead to entirely new materials & devices that cannot be synthesized through y g
direct techniques.
Jungwon Park; Haimei Zheng; Young-wook Jun; A. Paul Alivisatos; J. Am. Chem. Soc. 2009, 131, 13943-13945.
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How can we rationalize ion exchange?
The thermodynamic driving force for exchange between two cations can be controlled by the solvent and surfactant system
• In the CdSe−Ag2Se pair, the forward exchange from CdSe to Ag2Se is thermodynamically driven by the preferential solvation of Cd2+ ions relative to Ag+ in methanol (MeOH). g ( )
• The reverse exchange from Ag2Se to CdSe is favored by the addition of Cd2+, along with tributylphosphine (TBP). , g y p p ( )
• These exchange reactions can be qualitatively understood in terms of hard−soft acid−base theory:y
• The monovalent Ag+ cation is softer than the divalent Cd2+ cation. Therefore, MeOH, a hard base, preferentially binds Cd2+ cations. Therefore, MeOH, a hard base, preferentially binds Cd cations. Similarly, the soft base, TBP, binds strongly to Ag+ cations.
J.M. Luther, H. Zheng, B. Sadtler, A. P. Alivisatos, JACS 2009, 131, 16851
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A brief history of Acids and BasesGreece (BCE)
•Acids: “sour-tasting” substances:•The early word for acids, “oxein”, which mutated into the Latin word for vinegar, acetum, which became anglicized to “ id”“acid”• Bases could counteract acids and felt “soapy.”•The early word for base, “alkaline” is derived the Arabic word for “roasting:” the first bases were obtained from soaps, made by
A h i (1859 1927)
g p , yroasting ashes and treating them with water and lime.
• Dissertation at University of Uppsala: proposed that chemical reactions in solution were reactions between ions
Arrhenius (1859-1927)
ions• Acids dissociate in aqueous solution to form hydrogen
ions (H+) and bases form hydroxide (OH−) ions• Awarded non since laude approbatur (equivalent to a “D”)
L i h 1903 N b l P i• Later went on to win the 1903 Nobel Prize
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Bronsted & Lowry (1923)
Bronsted Acid: A proton donorBronsted Acid: A proton donor
HF(aq) + H2O(l) H3O+(aq)+F-(aq)
Bronsted Base: a proton acceptor
H2O(l) + NH3(aq)NH4+(aq)+OH-(aq)
Water is amphiprotic: can act as both a Bronsted acid and a Bronsted base
H3O+ (hydronium ion): • Participates extensively in hydrogen bonding. p y y g g
Better representation is H9O4+ (right).
• Mass spec suggests a cage of H2O molecules can condense around one H3O+ ion in a regular condense around one H O ion in a regular pentagonal dodecahedral arrangement, resulting in H+(H2O)21.
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Proton Transfer & Equilibrium
The central feature of Bronsted acid-base chemistry in aqueous solution is that of rapid attainment of equilibrium in the proton
transfer reactiontransfer reaction
Proton transfer between acids and bases is fast in both directionsdirections.
HF(aq) + H2O(l) H3O+(aq)+F-(aq)
H2O(l) + NH3(aq) NH4+(aq)+OH-(aq)
Acid1 + Base2 Acid2 + Base1
Conjugate base of acid 1
Conjugate acid of base 1base2
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The strengths of Bronsted AcidsThe strength of a Bronsted acid is measured by it’s acidity constant (or The strength of a Bronsted acid is measured by it s acidity constant (or
acidity ionization constant), and the strength of a Bronsted base is measured by it’s basicity constant.
HX(aq) + H2O(l) H3O+(aq)+X-(aq)
][]][[ 3
HXXOHKa
Acidity constant:
f [ ] l h [ ] b hIf Ka<<1, [HX] is large with respect to [X-] proton retention by the acid is favored
41053 xK 105.3 xKa
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The strengths of Bronsted BasesThe strength of a Bronsted acid is measured by it’s acidity constant (or The strength of a Bronsted acid is measured by it s acidity constant (or
acidity ionization constant), and the strength of a Bronsted base is measured by it’s basicity constant.
B(aq) + H2O(l) HB+(aq)+OH-(aq)
[B]]][OH[HB
bKBasicity constant:
f [ ] [ ] l ll f f l lIf Kb<<1, [HB+] << [B] only a small fraction of B molecules are protenated
5108.1 xKb
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Ionization Constant for Water
2H2O(l) H3O+(aq)+OH-(aq)
-141 00 10]][OHO[H KA l i -143 1.00x10]][OHO[H
wKAutoprotolysis constant:
pH=-log[H3O+] [H3O+]=10-pH
Also, KaKb=Kw for an acid and it’s conjugate base (or vice-versa), a b w j g b ( )
Recall from thermodynamics: aA+bBcC+dD
dc[D][C])]products([
miQActivity quotient
ba[B][A])]reactants([
n
j
Qy qQ=K in equilibrium:
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Polyprotic AcidsPolyprotic acid: can donate more than one protonPolyprotic acid: can donate more than one proton
H2S
H2S(aq)+H2O(l) H3O+(aq)+HS-(aq)
HS ( ) H O(l) H O+( ) S2 ( )
S][H]][HSO[H
2
31
aK
]][SO[H 23
A l ti id l t i i d i
HS-(aq)+H2O(l) H3O+(aq)+S2-(aq)][HS
]][SO[H-
32 aK
A polyprotic acid loses protons in succession, and successive deprotenations are progressively less favorable (Ka2<<Ka1)
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Polyprotic Acids & Distribution DiagramsThe concentration of each solute at a given pH can be The concentration of each solute at a given pH can be
calculated from the pKa values. Then, we can plot the fraction of solute present for a given pH.
If pH<pK high If pH<pKa1, high hydronium ion concentrations
If pH>pKa3, low hydronium ion concentrations
Distribution diagram for Phosphoric acid
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What governs the strength of acids and bases?
Considering enthaply changes accompanying the proton transfer!
Δpg: Proton gain enthalpy: if large and negative, gas phase is a strong base
Proton gain can be thought of as three key steps:
1. Electron loss from A = - Δeg
2. Electron gain by H = - I(H)3 Combination of H and A = - B(H-A)3. Combination of H and A = - B(H-A)
Δpg = - Δeg - I(H) - B(H-A)
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Periodic Table Trends
Dominant factor in proton affinity i d l ffi i across a period: electron affinity
of A
• Increases from left to right, lowering the proton affinity of
A-A
• Gas phase acidity of HA i h l i i
Acidity of the hydrides of the
increases as the electronegativityof A increases
y yelements
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Periodic Table Trends
Dominant factor in proton paffinity down a group: decrease in H-A bond dissociation enthalpydissociation enthalpy
• Lowers the proton affinity f A hof A-, increasing the gas phase acidity of HA.
A idit f th h d id f th Acidity of the hydrides of the elements
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Acids and bases in solution
A-(aq)+H+(aq)HA(aq)
BasesAcids
HA(aq)+H2O(l)H3O+(aq)+A-(aq) A-(aq)+H2O(l)HA(aq)+OH-(aq)( q) 2 ( ) 3 ( q) ( q)
is exothermic if the effective proton affinity of A-(aq) is lower than that
of H O(l) : less than 1130 kJ/mol
is exothermic if the effective proton affinity of A-(aq) is higher than
that of OH-(aq) : 1188 kJ/molof H2O(l) : less than 1130 kJ/mol
HA(aq) will be strongly acidic
that of OH (aq) : 1188 kJ/mol
A- will be strongly basic( q) g y
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Usually cannot ignore entropy in solution!
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Simple cycle for HCl
½ Cl2(g)Cl (g):
Dissociation energy (106 kJ/mol)
H(g)H+(g):
Ionization energy (1312 kJ/mol)Ionization energy (1312 kJ/mol)
½ H2(g)H(g):
Dissociation energy (218 kJ/mol)
H+(aq)½ H (g): Gibb’s energy of H (aq)½ H2(g): Gibb s energy of formation
(g)(aq): Energy of solvation
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The Born Equation: “Hydrations” of Ions
The energy of solvation of an ion (ΔsolvG = ΔGm) :
Energy involved in transferring the anion from a vacuum into a solvent of
relative permittivity εr
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The Born Equation: Derivation
Starting from the energy u of an electric field:
Using the expression for the electric field E at a distance r from an ion of Using the expression for the electric field E at a distance r from an ion of charge ze and radius ri, we have:
Therefore:
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Highly charged anions are stabilized in polar solvents!
• A large, negative value of ΔsolvGfavors the formation of ions in solution compared with the gas phase
• The interaction of the charged ion with the polar solvent
l l bili h molecules stabilizes the conjugate base A- relative to the parent acid HA
• The acidity of HA is enhanced by the polar solvent
=Z2/r