CHM 511 Chapter 7 page 1 of 14 Chapter 7 - USC …faculty.uscupstate.edu/cbender/Web page folder...
Transcript of CHM 511 Chapter 7 page 1 of 14 Chapter 7 - USC …faculty.uscupstate.edu/cbender/Web page folder...
CHM 511 Chapter 7 page 1 of 14
Chapter 7 Acids, bases, and ions in aqueous solution
Properties of water
Water can crystallize in 13 different polymorphs at various temperatures and pressures. (see phase
diagrams)
Typical phase diagram for water: Phase diagram showing polymorphs of ice:
At atmospheric pressure, ice crystallizes in a wurtzite structure with oxygen atoms in both Zn and S
positions. What is the geometry of Zn and S in wurtzite?
Upon melting, the lattice collapses,
and the cavities get occupied by water
molecules. What affect does this have
on the density? At what temperature
is water most dense?
CHM 511 Chapter 7 page 2 of 14
Hydrogen bond strength is ~25 kJ/mol, but these are constantly being formed and broken. Water
clusters may be formed, such as (H2O)10, which has ice-like structure. This is not the property
responsible for the “effect” of homeopathy.
For H+ species in water, other clusters are formed.
Note the movement of H+ in water:
Recall hydrated diameter from the Debye-Hückel Equation...do these numbers make sense?
Ion
Conductance
(siemans)
Hydrated
diameter (nm)
H+ 350 0.90
Na+ 51 0.45
K+ 74 0.30
OH- 192 0.35
Cl- 76 0.30
NO3- 71 0.30
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Classifications of acids/bases
Arrhenius acid: any substance that increases the H+ concentration when dissolved in water
Brønsted-Lowry acid: any substance that acts as a proton donor
Brønsted-Lowry base: any substance that acts as a proton acceptor
Amphiprotic materials?
Proton transfer in water
Conjugate acid/base pairs
HA + :B A- + HB+
acid base conj. base conj. acid
Note: The conjugate acid is an acid and the conjugate base is a base
Furthermore: the stronger the acid (HA), the weaker the conjugate base's strength (A-)
Acid Strength
Measured by acid ionization (aka acid dissociation or acidity constant)
Examples of strong acids?
Examples of weak acids?
Base Strength
Measured by base dissociation constant (aka basicity constant)
For amphiprotic species, there's an autoprotolysis constant
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Polyprotic acids -Acids with more than one donatable hydrogen ion
Will have multiple Ka values
Ka1 H3A + H2O H2A- + H3O
+
Ka2 H2A- + H2 O HA2- + H3O
+
Ka3 HA2- + H2O A3- + H3O+
Ka1 > Ka2 > Ka 3 always. Why?
Note that some formulas, not all hydrogens are acidic!
H3PO4 H3PO3 H3PO2
phosphoric acid phosphorous acid phosphinic acid
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Types of Brønsted acids
Hydrogen halides/hydrogen chalcogenides
HX pKa H2X pKa
HF 1.4 H2O 14.0
HCl -9.3 H2S 7.04
HBr -11.7 H2Se 3.9
HI -12.4 H2Te 2.6
Acidity trends?
Aqua Acid: when an acidic proton is from a coordinated water molecule
Cations and anions are each surrounded by multiple water molecules. Metals are typically
surrounded by 6 waters, anions are more difficult to study, but monatomic anions (e.g., Cl-) have 6
waters. Other anions: studies are few and/or inconclusive.
Hydroxoacid: the acidic proton is on an -OH group without an oxo (X=O) group
Oxoacid: the acidic proton is on an -OH group adjacent to an oxo group
For comparison
HOAc 1.8 10-5
HNO2 4.5 10-4
H2C2O4 5.9 10-2
Al3+(aq) 1.4 10-5
Cr3+(aq) 1.6 10-4
Zn2+(aq) 2.5 10-10
Fe2+(aq) 3.2 10-10
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Aqua acid characteristics
central atoms have low oxidation states
typical of s- and d-block elements
metals on left of p-block (Al, In, Ga)
Danger!!! Can't always look at aqua acids as an ionic model
Good correlation for alkali and alkaline earth metals
OK correlation for some d-block metals (Fe2+, Zn2+ , Sc3+, Cr3+)
Poor correlation for heavy d-metals and early p-block elements (Hg2+, Sn2+, Tl3+—suggests
that there may be covalency to M-O bond)
Oxoacid characteristics
central atom has a high oxidation number or
intermediate oxidation state of p-block element
Bell’s Rules (aka Pauling's Rules)
For an oxoacid: OpE(OH)q, pK ~ 8-5p
For each successive pKa value (if polyprotic) increase pKa by 5
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Polyoxo compound formation
Condensation polymers can form
Cation formation: typical of metals. See Baes and Mesmer diagrams for Al
0.1 m Al3+ 1 10-5 m Al3+
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Saturated with Al(OH)3
As pH is increased, H+ gets removed until you form Al(OH)3 which precipitates as a gelatinous
mass. Further increasing the pH causes Al(OH)3 to redissolve (1,4 = Al(OH)4-)
Anion formation
Common for early d-block ions or oxides in high oxidation states and non-metal oxides
See distribution diagram for Si
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Anhydrous Oxides
Acidic oxides either (a) combine with water to release H+ or (b) react with hydroxide
Basic oxides either (a) transfer a proton in water or (b) react with an acid
In general
basic oxides are ionic compounds
acidic oxides are covalent compounds
Oxides or hydroxides that react with both acids and bases are amphoteric
Amphoteric oxides are at the boundary of acidic and basic oxides
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Amphoterism for d-block elements
Most +3/+4 oxidation states for 1st period transition metal oxides
higher oxidation states give acidic oxides
Skip section on solubilities (covered in CHM 321)
Skip section on lattice energy and hydration energy (covered in chapter 6)
Skip section on common ion effect (covered in CHM 321)
Skip section on Coordination Complexes (and intro)—covered earlier this semester
Stability constants of coordination complexes (this was also covered in CHM 321)
Complexes form by reaction of Lewis acid/base pairs.
Lewis acid: a chemical that is an _____________ acceptor
Lewis base: a chemical that is an _____________ donor
When a metal acquires multiple ligands, it does so in a stepwise fashion, i.e., stepwise stability
constants (i.e. stepwise formation constants).
Ni(OH2)62+ + NH3 Ni(OH2)5(NH3)
2+ + H2O log Kf1 = 2.79
Ni(OH2)5(NH3)2+ + NH3 Ni(OH2)4(NH3)2
2+ + H2O log Kf2 = 2.26
Ni(OH2)4(NH3)22+ + NH3 Ni(OH2)3(NH3)3
2+ + H2O log Kf3 = 1.69
Ni(OH2)3(NH3)32+ + NH3 Ni(OH2)2(NH3)4
2+ + H2O log Kf4 = 1.25
Ni(OH2)2(NH3)42+ + NH3 Ni(OH2)(NH3)5
2+ + H2O log Kf5 = 0.74
Ni(OH2)(NH3)52+ + NH3 Ni(NH3)6
2+ + H2O log Kf6 = 0.03
Trend?
Circles mean amphoteric oxides in all oxidation states;
boxes mean acidic oxides in the highest oxidation states,
with amphoteric oxides in lower oxidation states.
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A β-value will comprise multiple values of Kf: nz
n2
zn
n][L])[M(OH
][MLβ
Note that as anionic ligands attach to cations, the charges begin to cancel, resulting in a negative
enthalpy change...but the entropy change is positive. This latter value more than makes up for the
former, if the complex is stable.
Another noticeable effect is the chelate effect: metal ions form stronger complexes with
polydentate ligands than with monodentate ligands.
Consider the following reactions:
EX. Cd2+(aq) + 2en Cd(en)2
2+ + 4H2O K = 2 1010
Cd2+(aq) + 4CH3NH2 Cd(CH3NH2)4
2+ + 4H2O K = 3 106
Consider enthalpy issues: What bond is breaking? What bond is forming? Is it the same in both
reactions? Inductive effects?
Consider the entropy issues: number of species in reactants and products? other effects?
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Complexes made of monodentate ligands are not generally stable with s-block elements
s-block elements will form complexes with polydentate ligands like crown ethers and cryptands
Sulfur and nitrogen variants of crown ethers...
Crown ethers and cryptands can be used to isolate alkalide ions (M-)
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EX. Dissolve Na in ethylenediamine, add 2.2.2 crypt (or 2.2.1 crypt) and you get a salt. Will this
species be diamagnetic or paramagnetic?
Also interesting is the dissolution of alkali metals in liquid ammonia
video of Na in NH3 http://www.youtube.com/watch?v=JefumJFatsw
video of concentrated Li in NH3 http://www.youtube.com/watch?feature=fvwp&NR=1&v=Qx-gVTaRAq4
Factors affecting stability of monodentate ligands
Size: Assuming the same charge, the smaller the cation, the more stable the complex with L.
Charge: the higher the charge, the more stable the complex with L.
Polarizability also plays a factor.
Hard and soft acids and bases (HSAB)
Hard acids bond in order: I- < Br- < Cl- < F- and R2S << R2O and R3P << R3N
Soft acids bond in order: F- < Cl- < Br - < I- and R2O << R2 S and R3N << R3P
See diagram next page!!
CHM 511 Chapter 7 page 14 of 14
Note: Hg2+ binds strongly to I- and weakly F -
Note: Al3+ bonds strongly to F- and weakly to Br-