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?


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


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


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


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


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


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+


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


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


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.


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?


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-)


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!!

http://www.youtube.com/watch?v=JefumJFatsw

http://www.youtube.com/watch?feature=fvwp&NR=1&v=Qx-gVTaRAq4


Note: Hg2+ binds strongly to I- and weakly F -

Note: Al3+ bonds strongly to F- and weakly to Br-

chm 511 chapter 7 page 1 of 14 chapter 7 - usc …faculty.uscupstate.edu/cbender/web page folder...

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