Transcript EXAMPLE: Separate Iron and Magnesium?

Chapter 6 Problems

6-29, 6-31, 6-39, 6.41, 6-42, 6-48,
Outline

Equilibrium of Acids and Bases

Bronsted-Lowry Acids/Bases






Define strong
Define weak
pH of pure water at 25oC
Define Ka and Kb
Relationship b/w Ka and Kb
Chapter 8 – Activity

Relationship with K
Acids and Bases & Equilibrium
Section 6-7
Strong Bronsted-Lowry Acid

A strong Bronsted-Lowry Acid is one
that donates all of its acidic protons to
water molecules in aqueous solution.
(Water is base – electron donor or the
proton acceptor).
Strong Bronsted-Lowry Base

Accepts protons from water molecules
to form an amount of hydroxide ion,
OH-, equivalent to the amount of base
added.
Question

Can you think of a salt that when dissolved
in water is not an acid nor a base?
Weak Bronsted-Lowry acid

One that DOES not donate all of its
acidic protons to water molecules in
aqueous solution.

Example?
Weak Bronsted-Lowry base


Does NOT accept an amount of protons
equivalent to the amount of base
added, so the hydroxide ion in a weak
base solution is not equivalent to the
concentration of base added.
example:
Common Classes of Weak
Acids and Bases
Weak Acids
Weak Bases
Equilibrium and Water
Question:
Calculate the Concentration of
H+ and OH- in Pure water at 250C.
EXAMPLE: Calculate the Concentration
of H+ and OH- in Pure water at 250C.
H2O
Kw =
KW =
H+ + OH-
EXAMPLE: Calculate the Concentration
of H+ and OH- in Pure water at 250C.
H2O
Kw =
KW =
H+ + OH-
Example
What is the concentration of OH- in a
solution of water that is 1.0 x 10-3 M in
[H+] (@ 25 oC)?
“From now on,
+
Kw = [H ][OH ]
assume the

to
1.0 x 10-14 = [1 x 10-3][OH-]temperature
be 25oC unless
1.0 x 10-11 = [OH-]
otherwise
stated.”
pH
~ -3 -----> ~ +16
pH + pOH = - log Kw = pKw = 14.00
Weak Acids and Bases
HA
Ka
HA + H2O(l)
H+ + A-
H3O+ + A-
Weak Acids and Bases
B + H2O
Kb
BH+ + OH-
Relation Between Ka and Kb
Relation between Ka and Kb

Consider Ammonia and its conjugate
acid.
NH3 + H2O
NH4 + H2O
+
Kb
Ka
NH4+ + OHNH3 + H3O+
Example
The Ka for acetic acid is 1.75 x 10-5. Find Kb
for its conjugate base.
Example
Calculate the hydroxide ion concentration in a
0.0100 M sodium hypochlorite solution.
OCl- + H2O  HOCl + OH[ HOCl][OH  ]
Kb 
[OCl  ]

The acid dissociation constant = 3.0 x 10-8
1st Insurance Problem
Challenge on page 120
Chapter 8
Activity
Write out the equilibrium constant for
the following expression
Fe3+ + SCN-
D Fe(SCN)2+
[ Fe( SCN ) 2 ]
K
[ Fe3 ][SCN  ]
Q: What happens to K when we add, say KNO3 ?
Keq
K decreases when an inert salt is added!!! Why?
8-1 Effect of Ionic Strength on
Solubility of Salts

Consider a saturated solution of Hg2(IO3)2
in ‘pure water’. Calculate the concentration
of mercurous ions.
Hg2(IO3)2(s) D Hg22+ + 2IO3I
C
E
some
-x
some-x
+x
+x
+2x
+2x
Ksp=1.3x10-18

K sp  [ Hg22 ][IO3 ]2  1.3 1018
2
18
Ksp  [ x][2x]  1.310
[ x]  6.9 107
A seemingly strange effect is observed when a salt such as KNO3 is
added. As more KNO3 is added to the solution, more solid dissolves until
[Hg22+] increases to 1.0 x 10-6 M. Why?
Increased solubility

Why?

LeChatelier’s Principle?

Complex Ion?

?
The potassium hydrogen
tartrate example
OH
O
K+-O
OH
O
OH
potassium hydrogen tartrate
Alright, what do we mean by
Ionic strength?
Ionic strength is dependent on the
number of ions in solution and their
charge.
Ionic strength (m) = ½ (c1z12+ c2z22 + …)

Or Ionic strength (m) = ½ S cizi2
Examples

Calculate the ionic strength of (a) 0.1 M solution of
KNO3 and (b) a 0.1 M solution of Na2SO4 (c) a
mixture containing 0.1 M KNO3 and 0.1 M Na2SO4.
(m) = ½ (c1z12+ c2z22 + …)
Alright, that’s great but how does
it affect the equilibrium constant?

Activity = Ac = [C]gc

AND
A A
[C ] g [ D] g
K

b
A A
[ A] g [ B] g
c
C
a
A
d
D
b
B
c
c
C
a a
A
d
d
D
b
B
Relationship between activity
and ionic strength
Debye-Huckel Equation
0.51z x m
2
 log g x 
1  3.3 x m
m = ionic strength of solution
g = activity coefficient
Z = Charge on the species x
 = effective diameter of ion
(nm)
2 comments
(1) What happens to g when m approaches zero?
(2) Most singly charged ions have an effective radius of about 0.3 nm
Anyway … we generally don’t need to calculate g – can get it from a table
Activity coefficients are
related to the hydrated
radius of atoms in
molecules
Relationship between m and g
Back to our original problem

Consider a saturated solution of Hg2(IO3)2
in ‘pure water’. Calculate the concentration
of mercurous ions.
Hg2(IO3)2(s) D Hg22+ + 2IO3-
Ksp=1.3x10-18
Back to our original problem

Consider a saturated solution of Hg2(IO3)2
in ‘pure water’. Calculate the concentration
of mercurous ions.
Hg2(IO3)2(s) D Hg22+ + 2IO3-
Ksp=1.3x10-18
In 0.1 M KNO3 - how much Hg22+ will be dissolved?
Back to our original problem

Consider a saturated solution of Hg2(IO3)2
in ‘pure water’. Calculate the concentration
of mercurous ions.
Hg2(IO3)2(s) D Hg22+ + 2IO3-
Ksp=1.3x10-18