Announcements
First Exam December 21---Christmas Break begins December 22. Silberberg Chapter 17-18, Skoog 2-7, 11-13
--Error Analysis (I will give error propagation equations)--Mean, median, SD, RSD, CL and what they mean--Equilibrium Concepts and Problems (17 and 18)--pH and pOH and inverse (Ka and Kb)--ppm, ppt, ppb, % w/w, %w/v to molarity to moles to gramsdescribe making up solutions (Chapter 11 Skoog problem)--LeChatliers principle--Hydrolysis Problem --Bronsted-Lowry, and Lewis Acid Base
10-15 Conceptual Multiple choice questions that have little if any math
3-5 multiple piece questions that demonstrate problem solving
What happens when we add ionic salts formed
from acid-base reactions to water?
Hydrolysis Reactions (salt + water)
Ionic Salt + Water -----> ????? pH
There are 4-Possible Acid-Base Neutralization Reactions
1. Strong acid + Strong base -----> Ionic salt + H2O
2. Strong acid + Weak base -----> Ionic salt + H2O
3. Weak acid + Strong base -----> Ionic salt + H2O
4. Weak acid + Weak base -----> Ionic salt + H2O
HCl (aq) + NaOH (aq) -----> Na+ + Cl- + H2O
HBr (aq) + NH3 (aq) -----> NH4+ + Br- + H2O
2HCOOH (aq) + Ca(OH)2 (aq) Ca+2 + 2HCOO- + 2H2O
CH3COOH (aq) + NH3 (aq) NH4+ + CH3COO- + H2O
N
A
B
?
1. Salts That Yield Neutral Solutions
Salts contain an alkali metal or alkaline earth metal ions and the conjugate base of common strong acids (e.g. Cl-, Br-, I- and NO3
-) give neutral solutions when added to water.
The anion of a strong acid (Cl-, I-, Br-) is a much weaker base than
water and the cation of from the strong acid is a much weaker acid
than water!
Why?
NaCl(s) + H2O Na+ + Cl + H2O
NaCl(s) + H2O HCl + OH
2. Salt That Yield Acidic Solutions:
(1) the anion of a strong acid and the cation of a weak base (the cation acts
as a weak acid); (2) highly charged metal cations; (3) cations of strong
bases and anions of polyprotic acids with an ionizable proton.
NH4Cl(s) NH4+ (aq) + Cl-(aq)
NH4+(aq) + H2O(l) NH3(aq) + H3O
+ (aq)Case 1
Fe(NO3)3(s) + 6H2O(l) Fe(H2O)63+(aq) + 3NO3
-(aq)
Fe(H2O)63+(aq) + H2O(l) Fe(H2O)5OH
2+(aq) + H3O+(aq)
Case 2
NaH2PO4(s) Na+(aq) + H2PO4
- (aq)
H2PO4-(aq) + H2O(l) HPO4
2-(aq) + H3O+(aq)
Case 3
3. Salts That Yield Basic Solutions:
Derived from a strong base and a weak acid. The anion of a weak acid and the cation of a strong base (the anion acts as a weak base)
CH3COONa (s) Na+ (aq) + CH3COO
- (aq)H2O
CH3COO- (aq) + H2O (l) CH3COOH (aq) + OH
- (aq)
hydrolyzible
4. Solutions in which both the cation and the anion hydrolyze:
Both cation and anion react with water (hydrolyze)!
which won wins (exerts its greatest affect on the pH is determined by the Ka vs Kb of the ions!
Example: NH4HS
HS(aq) + H2O H2S(aq) + OH(aq)
NH+4 (aq) + H2O NH3(aq) + H3O+(aq)
Ka(NH4+) = 5.7 x 10-10 Kb(HS
-) = 1.0 x 10-7
Since Kb > Ka, the solution is basic.
Salts from Neutralization Impact Can be Acidic or Basic
Predicting the relative acidity of salt solutions
PROBLEM:
Predict whether aqueous solutions of the following compounds are acidic, basic, or neutral (write an equation for the reaction of the appropriate ion with water to explain pH effect).
PLAN: Consider the acid-base nature of the anions and cations. Strong acid-strong base combinations produce a neutral solution; strong acid-weak base, acidic; weak acid-strong base, basic.
(a) potassium perchlorate, KClO4 (b) sodium benzoate, C6H5COONa
(c) chromium trichloride, CrCl3 (d) sodium hydrogen sulfate, NaHSO4
(a) potassium perchlorate, KClO4 (b) sodium benzoate, C6H5COONa
(c) chromium trichloride, CrCl3 (d) sodium hydrogen sulfate, NaHSO4
SOLUTION:
(a) The ions are K+ and ClO4-, which come from a strong base
(KOH) and a strong acid (HClO4). The salt solution will be neutral.
(b) Na+ comes from the strong base NaOH while C6H5COO- is
the anion of a weak organic acid. The salt solution will be basic.
(c) Cr3+ is a small cation with a large + charge, so its hydrated form
will react with water to produce H3O+. Cl- comes from the strong acid
HCl. The salt solution will be acidic.
(d) Na+ comes from a strong base. HSO4- can react with water
to form H3O+. The salt solution will be acidic.
Predicting the relative acidity of salt solutions
0.1M solutions of the following are acid/ base/ neutral or amphoteric?
HCl
NaCl
NaCN
HCN
Na2S
Na3PO4
NH4Cl
Predicting the behavior of acid-base salts
Kw relates Ka of and acid and Kb of the conjugate base
Determine the pH of a 0.100 M solution of the salt NaF if
the Ka of HF is given as 6.80 X 10-4 .
pOH=5.92 pH=8.08
Kw relates Ka of and acid and Kb of the conjugate base
Determine the pH of a 0.100 M solution of the salt NaF if
the Ka of HF is given as 6.80 X 10-4 .
0.1M - 0 0
x x
(0.1-x) ! 0.1 x
-x
x
F(aq) + H2O HF(aq) + OH(aq)
I
C
E
[B]initialKb
> 100 =0.100 M
6.80 104 > 100
Determining the pH of a solution of A-
Sodium acetate (CH3COONa, abbreviated NaAc) has
applications in photographic development and textile dyeing. What is the pH of a 0.25 M aqueous solution of NaAc? The Ka of
acetic acid (HAc) (conjugate acid of Ac) is 1.8 x 10-5.
PLAN:
Use Ka to find Kb and recall pH + pOH = 14.
Write balanced equation knowing that sodium salts are completely soluble in water so [Ac-] = 0.25 M.
Be organized set up ICE table and fill in the unknowns
Write the equilibrium expression--looking for pH.
What is the pH of a 0.25 M aqueous solution of NaAc given Ka
of acetic acid (HAc) (conjugate acid of Ac) is 1.8 x 10-5.
Kb = [HAc][OH-]
[Ac-]=
Kw
Ka
SOLUTION:
initial 0.25 - 0 0
change -x +x +x-
equilibrium -0.25 - x x x
Ac-(aq) + H2O(l) HAc(aq) + OH-(aq)
concentration
= 5.6 x 10-10Kb = 1.0 x 10-14
1.8 x 10-5
Kw
Ka=
Kb =
[HAc][OH-]
[Ac-]
[Ac-] = 0.25 M - x ! 0.25 M (since Kb is small)
5.6 x 10-10 ! x2/0.25 M
[OH-] = x ! 1.2 x 10-5 M
[H3O+] = Kw/[OH
-]
[H3O+] = Kw/[OH
-] = 1.0 x 10-14/1.2 x 10-5= 8.3 x 10-10 M
pH = -log (8.3 x 10-10 M) = 9.1
Check assumption: [Ac-]in / Kb > 100 = .25/ 5.6 x 10-10 > 100
Molecular Structure and Its Connection to the Strength of an Acid
Acidity Depends On Molecular Structure and Strength
The strength of an acid (base) depends on its
tendency to ionizie or donate a proton (accept
a proton) !
Two properties generally dictate acid
strength: electronegativity of and bond
strength.
Which property dominates depends on the
compounds and whether we move across the
periodic table or down a group!
Molecular Structure and Strength
2-families of compounds:
1. Non-metal hydrides
2. Oxo-acids ZOH
Binary acids like
HF, HCl, HBr
Strengths of Non-metal hydrides
Electronegativity increases,
acidity increases
Bond strength decreases,
acidity increases
Non-metal Hydrides
6A(16)
H2O
H2S
H2Se
H2Te
7A(17)
HF
HCl
HBr
HI
Electronegativity increases, acidity increases
Bond strength decreases,
acidity increases
Non-metal hydrides: two factors determine acid strength, namely,the electronegativity of the central non-metal
atom E and the strength
of the E-H bond
Covalent acidic compounds made up of a hydrogen atom bonded to an oxygen atom which is bound to a central atom labeled Z.
IIA IVA VA VIA VIIA
H3BO3 H2CO3 HNO3
H2SiO4 H3PO4 H2SO4 HClO3
H3AsO4 H2SeO4 HBrO3
H6TeO6 HIO3
Z O H Z O- + H+!- !+
Oxo-acids are Acid Ternary Compounds
electronegative central atom
For oxoacids with different central atoms that are from the same group of the periodic table and that have the same oxidation number, acid strength increases with increasing electronegativity.
Relative Strength of Oxo-Acids
Oxoacids of Chlorine:
Relative Strength of Oxo-acids
H O I H O Br H O Cl> >!+ !" !+ !" !+ !"
Acid Strength: HIO < HOBr < HOCl
H O Cl
O
O
O HClO3 > HClO2 > HClO
Electronegativity Increases...Acidity Increases
Indicate whether each is a strong or weak acid, name each acid and its sodium salt, and predict the relative strengths of the following groups of oxoacids:
a) HClO, HBrO, and HIO.
b) HNO3 and HNO2.
c) H3PO3 and H3PO4.
Sample Exercise
Lewis Acid-Base Definitions
A Lewis Acid is an electron-pair acceptor. It is also a Bronsted-Lowry acid. These are generally cations and neutral molecules with vacant valence orbitals, such as H+, Al3+, Cu2+, BF3.
A Lewis Base is an electron-pair donor. It is also a Bronsted-base. These are generally anions and
