Post on 07-Mar-2018
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
CHEM 511 Chapter 4 page 4 of 12
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.
CHEM 511 Chapter 4 page 6 of 12
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-)
CHEM 511 Chapter 4 page 7 of 12
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.
CHEM 511 Chapter 4 page 8 of 12
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
CHEM 511 Chapter 4 page 9 of 12
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
CHEM 511 Chapter 4 page 12 of 12
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.