CHEM 511 Chapter 4 page 1 of 12

Chapter 4

Acids and Bases Read pages 116-125 on your own (covered in previous classes)

Brønsted-Lowry acid: proton donor

Brønsted-Lowry base: proton acceptor

Typical strong acids? Weak acids?

Typical strong bases? Weak bases?

Amphiprotic materials?

Distribution diagrams

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A bit about water, first...

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.

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?

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

(siemens)

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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Characteristics of Brønsted acids

Aqua Acid: when an acidic proton is on a coordinated water molecule

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

The oxidation state can dictate which acid is formed

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 of 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

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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Oxoacid characteristics

central atom has a high oxidation number or

intermediate oxidation state of p-block element

Substituted oxoacids

Groups on the central atom can affect acidity

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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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

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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Polyoxo compound formation

Condensation polymers can form

Cation formation: typical of metals. Below are 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-)

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Anion formation

Common for early d-block ions or oxides in high oxidation states and non-metal oxides

Below are distribution diagrams for Si

Phosphorus can form condensation polymers with chains and

rings.

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Lewis acidity and basicity A Lewis acid is: an electron acceptor

A Lewis base is: an electron donor

Common examples of Lewis acids

1. Metal and ligand

2. atom with an incomplete octet

3. atoms with ability to rearrange their octet

4. atoms that can expand their octet

Group 3A/13 Lewis acids Dimerization of AlCl3

Note this reaction: BX3 + N(CH3)3 X3B-N(CH3)3

Stability of product complex decreases with more electronegative X, BF3 < BCl3 < BBr3

Is this the expected trend with respect to electronegativity of the halogen?

Group 4A/14 Not much for C with respect to inorganic chemistry (when considering Lewis acidity)

Si (and others) can expand its octet, leading to reactive species

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Group 5A/15 (pnictogens)

Notable Lewis acid is SbF5

Hard and soft acids and bases

Hard acids bond in order: I- < Br- < Cl- < F- and R3P << R3N and R2S << R2O

Soft acids bond in order: F- < Cl- < Br - < I- and R2O << R2 S and R3N << R3P

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

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

Chemical consequences of

hard/soft acids/bases?

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Solvent Leveling What is the strongest acid observed in water?

What is the strongest base in water?

Leveling Effect: Inability of a solvent to differentiate among the relative strengths of acids

stronger than the solvent's conjugate acid.

In all cases, Ka × Kb = Ksolv

and pKa + pKb = pKsolv

Any acid is leveled if pKa < 0 (i.e., Ka > 1) and any base is leveled if pKb < 0

Thus, for bases with pKa > pKsolv the base will act like solv-

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Non-Aqueous solvents Ammonia What reactions have we observed with NH3 previously?

In NH3, weak acids may act as strong acids:

Precipitation reactions can occur, though lead to differences from water:

KCl(aq) + AgNO3(aq)

AgCl(am) + KNO3(am)

Nitrogen inversion in NH3

Hydrogen Fluoride Both highly reactive AND toxic! Can’t store in glass containers due to reaction with silica—use

Teflon, instead.

Autoprotolysis pKHF ~11, but the reaction is a bit different than you might expect due to strong

hydrogen bonding

Only very strong acids can protonate HF...

...otherwise, a weak acid may GET protonated by HF

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Sulfuric Acid Autoionization is (surprisingly?) high: pKH2SO4 ~3.6

...though other reactions also occur:

H2SO4 H2O + SO3

H2O + H2SO4 H3O+ + HSO4

-

SO3 + H2SO4 H2S2O7

H2S2O7 + H2SO4 H3SO4+ + HS2O7

-

Viscosity is a result of hydrogen bonding. Ionic conductivity is very high: how does HSO4- and

H3SO4+ move through solution so fast since the solution is so viscous?

Boric acid in H2SO4 becomes a powerful acid...

H3BO3 + 6H2SO4 [B(HSO4)4]- + 3H3O

+ + 2HSO4- K(H2SO4) = 0.4

To make sure water is not present in H2SO4, add SO3 to “scavenge” any water molecules. This

gives a chemical called “oleum” or “fuming sulfuric acid”.

Superacids Superacids are non-aqueous systems that are many times more acidic (Brønsted) than

concentrated aqueous solutions of H2SO4. Created by mixing a strong Brønsted acid with a

strong Lewis acid.