Lewis E5 Acids and Bases (Session 1) March 19 - 24 Session one Pre-lab (p.151) due 1st hour discussion of E4 Lab (Parts 1and 2A) Session two Lab: Parts 2B, 3 and 4
Acids
Bronsted: Acids are proton donors. Problem Compounds that do not contain H are acids are acids; e.g. AlCl3.
DEMO
Compounds containing cations other than H+ are acids!
Problem: Some acids do not contain protons Example: Al3+ (aq) = pH 3!
Deodorants and acid loving plant foods contain aluminum salts
Lewis Acids and Bases
Defines acid/base without using the word proton:
Cl-H +
O
Lewis Acids
Electron deficient species ; potential electron pair acceptors.
H ClH
H
O
H H
+
Lewis acids:
H+
Cu2+
Al3+
Acid
Base
Base
Acid
Im deficient!
A BASE DONATES unbonded ELECTRON PAIR/S. An ACID ACCEPTS ELECTRON PAIR/S . Acid
1
Lewis Bases
Electron rich species; electron pair donors.
Lewis Acid-Base Reactions Example
H+ +
O
H H
H
O
H H
+
Acid + Base Ammonia (ammine) hydroxide ion (hydroxo) water__ (aquo)
Complex ion
The acid reacts with the base by bonding to one
or more available electron pairs on the base.
The product is a complex or complex ion
Lewis Acid-Base Reactions
Lewis Acid-Base Reaction
Net Reaction Examples
H+
+
Lewis acid
Lewis base
H2O
Hydronium ion
[H(H2O)]+
Cu2+
Lewis acid
+ 4 H2 O
Lewis base
Tetra aquo copper(II)ion
[Cu(H2O)4]2+
Metal ions are Lewis acids and BOND
DEMO
to electron pairs on water molecules (Lewis bases)!
Metal Aquo Complex Ions
The metal ions in a salt solution bond to water
Part 1. Acidity of Cations
Why do different cations in aqueous solution exhibit different acid strengths? Compare the pH of nitrate salts containing different metal ions.
molecules to forms aquo complex ions.
Example: Cu2+(aq) + SO42- (aq)
[Cu(H2O)4]2+ Hydrated solid: [Cu(H20)4 ]SO4
DEMO
tetra aquo copper(II) sufate
2
Part 1. Acidity of Cations
EXPERIMENT VARIABLE Nitrate salts with different metal ions EXPERIMENT CONSTANT Water to dissolve salts is from same source and of constant pH (neutral or non-acidic).
Metal Ion Acid Strength
DEMO pH of Na+, Mg2+, and Al3+
pH comparison of chloride salts of aluminum, barium, and sodium
Metal Ion Acid Strength
Al3+ > Mg2+ > Na+ Metal Ion Acid Strength: ___________________ Results suggest that metal ion acidity and metal ion charge are linked Results suggest that metal ion acidity and metal ion oxidizing agent strength are linked
Metal Ion Acidity Acid strength
1A 1
Oxidizing agent strength
IIA 4
H
3
VIIIA 2
He Be
IIIA 5
1 s1
Li
2s1 11 3s1 19
B
IVA 6
C
VA 7
N P
VIA 8
O S
VIIA 1 s2 9 10
F
Ne
18
Na K
2s2 12 3s2 20 4s2 38
2 s2 2 p1 2 s2 2 p2 2 s2 2 p3 2 s2 2 p4 2 s2 2 p5 2 s2 2 p6
Mg
IIIB 21 IVB VB 22 VIB VIIB VIIIB ! VIIIB 24 IB 29 IIB 30
13
Al Sc Y Ti
40 23
14
Si
15
16
17
Cl
35
Ar
36
3 s2 3 p1 3 s2 3 p2 3 s2 3 p3 3 s2 3 p4 3 s2 3 p5 3 s2 3 p6
Ca Sr
4s1 37 5s1 55 6s1 87
10 1 10 2 3d1 4 s2 3d2 4 s2 3d3 4 s2 3d5 4 s1 3d5 4 s2 3d6 4 s2 3d7 4 s2 3d8 4 s2 3d 4s 3d 4s 4 s2 4 p14 s2 4 p2 4 s2 4 p3 4 s2 4 p4 4 s2 4 p5 4 s2 4 p6
V
Cr
42
25
Mn Fe
43 44
26
27
Co
45
28
Ni
46
Cu
47
Zn
48
31
Ga
49
32
Ge
50
33
As
51
34
Se
Br
53
Kr
54
Rb Cs Fr
39
5s2 56 6s2 88 7s2
10 1 10 2 4d1 5 s2 4d2 5 s2 4d3 5 s2 4d5 5 s1 4d5 5 s2 4d7 5 s1 4d8 5 s1 4d1 0 4d 5s 4d 5s 5 s2 5 p1 5 s2 5 p2 5 s2 5 p3 5 s2 5 p4 5 s2 5 p5 5 s2 5 p6
Zr
72
41
Nb Mo Tc
73
Ru
76
Rh
77
Pd
78
Ag
79
Cd
80
1
In
Sn
82
Sb
83
52
Te
84
I
Xe
86
Ba Ra
57
La* Hf
Ta
74
W
75
Re
Os
Ir
Pt
Au
10
Hg
10
81
2
Tl
Pb
Bi
Po
85
At
Rn
7s1
5d1 6 s2 5d2 6 s2 5d3 6 s2 5d4 6 s2 5d5 6 s2 5d6 6 s2 5d7 6 s2 5d9 6 s1 5d 6s 5d 6s 6 s2 6 p1 6 s2 6 p2 6 s2 6 p3 6 s2 6 p4 6 s2 6 p5 6 s2 6 p6 8 9 1 0 4 1 0 5 1 0 6 1 0 7 1 0 8 1 0 9 + Element synthesized, Ac# + + + + + + but no official name assigned 6d1 7 s2 6d2 7 s2 6d3 7 s2 6d4 7 s2 6d5 7 s2 6d6 7 s2 6d7 7 s2
Acidity of Metal Ion Model
. H Mx+ .. O .
+
Acidity of Metal Ion Model
Equation describing acidity of metal ions: [X(H2O)6 ]2+ + H2O [X(H2O)5(OH)]+ + [H-H2O]+ Electron deficient proton/s in the bonded water molecule (of the metal aquo complex ion) are donated(bond) to an external water molecule When protons bond to water molecules, H3 O+ ions are formed producing a pH drop.
H +
Metal ion acid strength is related to its ability to
attract and bond to electrons (oxidizing agent strength)
When a metal ion bonds to a water molecule, the
water molecules polarity increases and the protons in the water molecule become more electron deficient
3
Q. Complete a balanced equation to show formation
of hydronium ions:
Q. Complete balanced a equation to show formation
of hydronium ions:
[Al(H2O)6]3+ + HOH [Al(H2O)5(OH)]2+ + [H3O]+
[Al(H2O)6]3+ + 2 HOH [Al(H2O)4(OH)2]+ + 2 [H3O]+
Part 2. Complexation Reactions The reaction of a Lewis acid with a Lewis base (NH3 , OH-, ). Data Analysis What kinds of observations allow you to know that a complexation reaction is occurring? Are Lewis acid-base reaction results predictable from a) metal ion acid strength? b) the position of the metal ions element in the Periodic Table?
Part 2. Complexation Reactions
Lewis acids [Cu(H2 O)4 ]2+ [Ag(H2O)2]+ [Zn(H2O)4]2+ Assigned team metal ions from Part 1 Lewis bases OH- (NaOH) NH3
Lewis acids bond to the best available base. - an acid does not react (bond) to any base.
Wanna give me a try?
If a better base is available a Lewis acid will react (exchange partners)!
Acid-Base
+
Base
no reaction
Acid-Base
+
better Base
Reaction
4
Q. The acid exchanging base partners is H+. Which is the BEST base, Cl- or HOH?
H ClH
Lewis Acids bond to the best Lewis Base
Replacement of water molecules in [Cu (H2 O)4 ]2+ with ammonia molecules.
Example:
Cl-H +
O
H
O
H H
+
[Cu (H2O)4]2++ 4 NH3 [Cu(NH3)4]2+ + 4 H2O
Acid
Base
Base
Acid
DEMO
H+ bonds to water (best base) rather than Cl- . The bonded base-acid product = complex ion.
Tetra aquo Cu(II) ion Tetra ammine Cu(II) ion
Complexation Reactions
Colored
aquo transition metal complex ions alter color upon bonding to a different Lewis base.
[Cu (H2O)4]2++ 4 NH3 [Cu(NH3)4]2+ + 4 H2O
[Cu(H2O)4]2+
[Cu(NH3)4]2+
Complex Ion Charge
The
Complex Ion Charge
charge on a metal complex ion = the metal ion charge if the bonded Lewis base is uncharged (e.g., H2O or NH3)
[Cu(H2O)4]2+
The
[Cu(Cl)4]2-
[Cu(H2O)4]2+
[Cu(NH3)4]2+
charge on a metal complex ion will NOT = the metal ion charge if the bonded Lewis base is charged (e.g., Cl- or OH- )
5
Complex Ion Charge
Complex or Complex Ion?
If the Lewis base is charged (e.g., OH- ) and the
reaction stoichiometry with the Lewis acid results in a product with no charge, a complex has formed the complex will be insoluble and precipitate
+ OH-
Q. Complete the equation below: [Cu(Cl)4]2- + 4 H2O [Cu (H2O)4]2+ + 4 Cl- ___________________ [Cu(H2O)4]2+ [Cu (H2O)2(OH)2]
Reactions with NH3
If a precipitate forms upon addition of NH3(aq), the metal aquo complex ion is reacting with the SMALL AMOUNT of OH- ions present in NH3(aq): NH3(aq) + HOH(l)
DEMO Addition of NH3 to Cu2+ resulting in a copper hydroxide ppt. [Cu(H2O)4] 2+ + 2 OH[Cu (H2 O)2(OH)2] + 2 H2 O
Lewis Acid-Base Reactions If a metal ion reacts with a charged Lewis
base such as OH-, the product may be a soluble complex ion or an uncharged insoluble complex depending on reaction stoichiometry.
DEMO
NH4+ (aq...