Acid ppt

1
Acid-Base Equilibria The Nature of Acids and
Bases
What makes an Acid an Acid?

• An acid possess a sour taste

• An acid dissolves active metals like magnesium

• An acid causes certain vegetable dyes to turn
• characteristic colors

What makes a Base a Base?

• A bases possess a bitter taste

• A base feels slippery to the touch

• A base causes certain vegetable dues to turn a
• characteristic color

2
Acid-Base Equilibria The Nature of Acids and
Bases
The Arrhenius Definition of an Acid and a Base
An acid is a substance that produces H ions in
water solutions
A base is a substance that produces OH- ions in a
water solution
If a solution contains more OH- ions than H ions
we say that the solution is basic
If a solution contains more H ions than OH- ions
we say that the solution is acidic
3
Acid-Base Equilibria The Proton in Water
H2O
When HCl dissolves in water we write
HCl(g) ? H(aq) Cl-(aq)
H

H
?
O

H
hydronium ion (H3O)
4
Acid-Base Equilibria The Proton in Water
H2O
When HCl dissolves in water we write
HCl(g) ? H(aq) Cl-(aq)
5
Acid-Base Equilibria The Proton in Water
H2O
When HCl dissolves in water we write
HCl(g) ? H(aq) Cl-(aq)
Acidic solutions are formed by a chemical
reaction in which and acid transfers a proton
(H) to water.
HCl(aq) H2O(aq) ?? H3O(aq) Cl-(aq)
6
Acid-Base Equilibria The Proton in Water
The Bronsted-Lowry Concept of Acids and Bases
An acid may be defined as a substance that is
capable of donating protons and a base may be
defined as substance than accepts protons.

H
H
_






Cl
H

?
N
H
H
N
H
Cl



H
H
Bronsted base
Bronsted acid
7
Acid-Base Equilibria The Proton in Water
The Bronsted-Lowry Concept of Acids and Bases
Describing the relationship between the Arrhenius
and Bronsted definitions of acids and bases,
NH3(aq) H2O(aq) ?? NH4(aq) OH-(aq)
The Bronsted definition says that H2O is a an
acid because it donated a proton and NH3 is a
base because it accepted the proton
The Arrhenius definition says that this solution
is basic
Water is an acid
8
Acid-Base Equilibria The Proton in Water
The Bronsted-Lowry Concept of Acids and Bases
HC2H3O2(aq) H2O(aq) ?? H3O(aq) C2H3O2(aq)
The Bronsted definition says that acetic acid is
a an acid because it donates a proton to water
and and water is a base because it accepts the
proton from the acetic acid
Water is also a base
9
Acid-Base Equilibria The Proton in Water
The Bronsted-Lowry Concept of Acids and Bases
HC2H3O2(aq) H2O(aq) ?? H3O(aq) C2H3O2(aq)
Bronsted base
Conjugate acid
Conjugate base
Bronsted acid
NH3(aq) H2O(aq) ?? NH4(aq) OH-(aq)
Conjugate base
Conjugate acid
Bronsted acid
Bronsted base
The stronger the acid, the weaker its conjugate
base the weaker the acid, the stronger its
conjugate base.
10
Acid-Base Equilibria The Proton in Water
The Bronsted-Lowry Concept of Acids and Bases
Of course all this Bronsted-Lowry stuff raises
a number of questions.

• If water can be an acid and a base, can it act
  as a proton donor and acceptor
• to itself?

• What makes one acid or base strong and another
  acid or base weak?

11
Acid-Base Equilibria The Proton in Water
The Relative Strengths of Acids and Bases

• What makes one acid or base strong and another
  acid or base weak?

12
Acid-Base Equilibria The Proton in Water
The Dissociation of Water and the pH Scale

• If water can be an acid and a base, can it act
  as a proton donor and acceptor
• to itself?

Water is capable of auto-ionizing.
H2O (l) ? H (aq) OH- (aq)
The reaction occurs to a very small extent about
1 in 108 molecules is ionized at any given moment
Protons transfer from one molecule to another at
a rate of about 1000 times per second
13
Acid-Base Equilibria The Proton in Water
The Dissociation of Water and the pH Scale
If this is true H2O (l) ? H (aq) OH- (aq)
H OH-
K
Than this is true
H2O
And since water is a liquid and its concentration
is therefore constant, this expression may be
written as
H OH- where Kw is the ion product
constant and is equal to 1.0 x 10-14
Kw
Note that H OH- 1.0 x 10-7 M, water is
therefore said to be neutral
However, in most solutions these concentrations
vary. If
If H gt OH- , solution is acidic
If H gt OH- , solution is basic
14
Acid-Base Equilibria The Proton in Water
The Bronsted-Lowry Concept of Acids and Bases
Sample exercise Indicate whether each of the
following solutions is neutral, acidic, or
basic (a) H 2 x 10-5 M, (b) OH- 0.010
M, ( c) OH- 1.0 x 10-7 M
Sample exercise Calculate the concentration of
H (aq) in (a) a solution in which the OH- is
0.020M, (b) a solution in which the OH- 2.5 x
10-6 M. Indicate whether the solution is acidic
or basic
15
Acid-Base Equilibria The Proton in Water
The Dissociation of Water and the pH Scale
16
Acid-Base Equilibria The Proton in Water
The Dissociation of Water and the pH Scale
Because the concentration of H ions is often
quite small, it can be conveniently expressed in
terms of pH -log H
For example, solution with a H 2. 5 x 105
has a pH of
pH -log 2. 5 x 10-5 4.6
Likewise a solution with a pH of 3.8 has a H
concentration of
Antilog -3.8 1.58 x 104 M
17
Acid-Base Equilibria The Proton in Water
The Dissociation of Water and the pH Scale
In a sample of lemon juice, H 3.8 x 10-4 M.
What is the pH. A commonly available window
cleaner has a H 5.3 x 10-9 M
In a sample of freshly pressed apple juice has a
pH of 3.76. Calculate the H
Now, you try it!
18
Acid-Base Equilibria The Proton in Water
The Dissociation of Water and the pH Scale
Because the concentration of OH- ions is often
quite small, it can be conveniently expressed in
terms of p)H -log OH-
Now lets think about this, if the H OH-,
then the pH pOH 7
Sooooo if Kw H OH- and Kw 1.0 x 10-14,
then -log Kw 14 pH pOH
Sample exercise What is the pH of a solution
with a pOH of 2.5? Is the solution acidic or
basic?
19
Acid-Base Equilibria The Proton in Water
Measuring the pH Using Indicators
20
Acid-Base Equilibria The Differences Between
Strong and Weak Acids
HX H X-
HX HX H
X-
Initial Equilibrium
Initial
Equilibrium
21
Acid-Base Equilibria The Differences Between
Strong and Weak Acids
22
Acid-Base Equilibria The Differences Between
Strong and Weak Acids
Dealing with a Strong Acid
What is the pH of 0.010 M solution of HCl?
Dealing with a weak acid that is only partially
ionizable
HX-
Since HX (aq) ? H(aq) X-(aq), then Ka
HX
The smaller the value of the acid dissociation
constant Ka, the weaker the acid
What is the Ka of a 0.10 M solution of formic
acid (HCHO2) which has a pH 2.38?
HCHO2 ? H CHO2
I
C
E
23
Acid-Base Equilibria The Differences Between
Strong and Weak Acids
What is the concentration of H ions in a 0.10 M
solution of HC2H3O2 (Ka 1.8 x 10-5)
HC2H3O2 ? H C2H3O2
I
C
E
What is the pH of the solution? What is the
percent ionization of this solution?
24
Acid-Base Equilibria The Differences Between
Strong and Weak Acids
What is the pH and percent ionization of a 0.20 M
solution of HCN? Ka 4.9 x 10 -10
I
C
E
25
Acid-Base Equilibria Dealing with Polyprotic
Acids
Substances that are capable of furnishing more
than one proton to water are called polyprotic
acids.
H2SO3(aq) ? H(aq) HSO3-(aq) K a1
1.7 x 10-2
HSO3-(aq) ? H(aq) SO32-(aq) K a2
6.4 x 10-8
Because Ka1 is so much larger than subsequent
dissociation constants for most polyprotic acids,
almost all the H (aq) in the solution come from
the first ionization reaction.
26
Acid-Base Equilibria Dealing with Polyprotic
Acids
27
Acid-Base Equilibria Dealing with Polyprotic
Acids
What is the pH of 0.0037 M solution of carbonic
acid (H2CO3)
H2CO3 ? H HCO3-
I
C
E
HCO3 - ? H CO32-
I
C
E
28
Acid-Base Equilibria Strong Bases
The most common soluble strong Bases are the
hydroxides of group IA and heavier group 2A metals
What is the pH of a 0.010 M solution of Ba(OH)2?
29
Acid-Base Equilibria Dealing with Weak Bases
The base dissociation constant Kb refers to the
equilibrium in which a base reacts with H2O to
from the conjugate acid and OH-
Weak base H2O ? conjugate acid OH-
NH3 (aq) H2O (l) ? NH4(aq) OH-(aq)
NH4 OH-
Kb
NH3
Calculate the OH- in a 0.15 M solution of NH3.
NH3 H2O ? NH4 OH-
I
C
E
30
Acid-Base Equilibria Classes of Weak Acids
Amines
Anions of Weak Acids
31
Acid-Base Equilibria Anions of Weak Acids
HC2H3O2(aq) H2O(aq) ?? H3O(aq) C2H3O2- (aq)
Bronsted base
Conjugate acid
Conjugate base
Bronsted acid
A second class of weak base is composed of the
anions of weak acids Anions of weak acids can be
incorporated into salts
NaC2H3O2 ?? Na(aq) C2H3O2- (aq)
C2H3O2- H2O ? HC2H3O2 OH- Kb 5.6 x 1010
32
Acid-Base Equilibria Anions of Weak Acids
Calculate the pH of a 0.01 M solution of sodium
hypochlorite (NaClO)
H2O ? OH-
I
C
E
33
Acid-Base Equilibria Anions of Weak Acids
Now its you turn the Kb for BrO- is 5.0 x
10-6. Calculate the pH of a 0.050 M solution of
NaBrO
34
Acid-Base Equilibria Relationship Between Ka and
Kb
NH4(aq) ? NH3(aq) H (aq)
NH3(aq) H2O? NH4(aq) OH- (aq)
NH4OH-
HNH3
Kb
Ka
NH3
NH4
NH4(aq) ? NH3(aq) H (aq)
NH3(aq) H2O(l)? NH4(aq) OH- (aq)
H2O ? H(aq) OH-(aq)
When two reactions are added to give a third
reaction, the equilibrium constant for the third
reaction reaction is given by the product of the
equilibrium constants for the two added reactions
Ka x Kb Kw
pKa pKb pKw
35
Acid-Base Equilibria Relationship Between Ka and
Kb
Calculate the (a) base-dissociation constant, Kb,
for the fluoride ion, is the pKa of HF 3.17
pKa -log Ka
3.17 -log Ka
Antilog -3.17 6.76 x 10-4
Since
Ka x Kb Kw
(6.76 x 10-4)x Kb 1.0 x 10-14
Kb 1.0 x 10-14/ 6.76 x 10-4 1.5 x 10-11
36
Acid-Base Equilibria Relationship Between Ka and
Kb
Calculate the pKb for carbonic acid (Ka 4.3 x
10-7)
Now its your turn
37
Acid-Base Equilibria Acid-Base Properties of
Salt Solutions

• Anions of weak acids, HX, are basic and will
  react with H2O to produce OH-

X- (aq) H2O (l) ? HX(aq) OH-(aq)

• Anions of strong acids, such as NO3-, exhibit no
  basicitiy, these ions do not react
• with water and consequently do not influence the
  pH

• Anions of polyprotic acids, such as HCO3-, that
  still have ionizable protons
• are capable of acting as either proton donors or
  acceptors depending upon the
• magnitudes of the Ka or Kb

This last one requires a bit of an explanation
38
Acid-Base Equilibria Acid-Base Properties of
Salt Solutions

• Anions of polyprotic acids, such as HCO3-, that
  still have ionizable protons
• are capable of acting as either proton donors or
  acceptors depending upon the
• magnitudes of the Ka or Kb

Predict whether the salt Na2HPO4 will form an
acidic or basic solution on dissolving in water.
Na2HPO4 ? 2Na (aq) HPO4-
HPO4- acting like an acid
HPO4- (aq) H2O ? H3O PO43-(aq)
K3 4.2 x 10-13
HPO4- acting like an base
HPO4- (aq) H2O ? H2PO42-(aq) OH-(aq)
So HPO- is the conjugate base of H2PO4-. Since
the K2 of H2PO4- 6.2 x 10-8 then
Kw
1.0 x 10-14
1.6 x 10-7
Kb


6.2 x 10-8
Ka
Since Kb is larger than Ka, HPO4- will act like a
base
39
Acid-Base Equilibria Acid-Base Properties of
Salt Solutions

• Salt derived from a strong base and a strong acid
  will have a pH of 7

• Salt derived from a strong base and a weak acid
  will have a pH above 7

• Salt derived from a weak acid and a weak base
  depends upon whether
• the dissolved ion acts as an acid or a base as
  determined by the size of
• the Ka or Kb

40
Acid-Base Equilibria Acid-Base Character and
Chemical Structure
A substance HX will transfer a proton only if the
H X bond, is already polarized in the
following way
In ionic compounds such as NaH, the H atom
possess a negative charge and behaves like a
proton acceptor.
Very strong bonds in an HX are less easily
ionizable than weak bonds
41
Acid-Base Equilibria Acid-Base Character and
Chemical Structure
42
(No Transcript)
43
(No Transcript)