Chapter 4 Modern Theories of Acid and bases.pdf

Modern Theories of Acid and base

1. CHE 101G GENERAL CHEMISTRY (For Genetics Engineering and Biotechnology) 3
Hours/Week, 3.0 Credits

1. Electronic Structure: Quantum theory, atomic spectrum of hydrogen and the Bohr model,
Quantum numbers, Concept of Energy levels and atomic orbital, Electronic configuration,
Chemical bonding and molecular structure.
2. The periodic Table: Development of the periodic table, Electron arrangements and the
periodic table, Summarized chemical properties of s-block, p-block and d-block elements.
3. Introduction to Thermodynamics: The Basic Concepts: Systems and surroundings, State and
state functions, Equilibrium states and reversibility, Energy, Heat and work. The First Law:
Statement and formulation, Derivation of expression for expansion work and its application at
different conditions, Heat capacity. Relation between Cp & Cv.
4. Acids and Bases: Theories and Modern definition of acids and bases, Dissociation
constant, strength, pH, Buffer solution etc.
5. Gaseous state: Measurement on gases, the ideal gas law, Volumes of gases involved in
reactions, Gas mixtures, Partial pressure, Kinetic theory of gases, Real gases.
6. Introduction to Chemical Kinetics: Rate laws, rate constant, equilibrium constant, order
of reaction etc.
7. Organic Chemistry: Introduction, Classification, Nomenclatures, preparations and
Properties (Physical & Chemical) of (i) Aliphatic and aromatic hydrocarbons, (ii) Aldehydes and
ketones, (iii) Carboxylic acids and (iv) Alcohols and phenols (v) Carbohydrates (mono- and
disaccharides).

3. I.Theories and modern Definition of acids and bases
II. Dissociation constant
III. Strength
IV. pH
V. Buffer solution etc.
Chapter 4

Acids and Bases:

4. Concepts of Acid and Bases
Acids:
The term acid was first used in the seventeenth century; it comes from the
Latin root ac-, meaning “sharp”, as in acetum, vinegar. Some early
writers suggested that acidic molecules might have sharp corners or
spine-like projections that irritate the tongue or skin.


Acids have long been recognized as a distinctive class of compounds
whose aqueous solutions exhibit the following properties:

1)A characteristic sour taste (think of lemon juice!);
2)ability to change the color of litmus from blue to red;
3)react with certain metals to produce gaseous H2;
4)react with bases to form a salt and water.

5. I. Theories and modern definition of acids and bases
The name base has long been associated with a class of compounds whose
aqueous solutions are characterized by:
a bitter taste;
a “soapy” feeling when applied to the skin;
ability to restore the original blue color of litmus
that has been turned red by acids;
ability to react with acids to form salts.
Bases:

6. Arrhenius Theory (Acid):

By 1890 the Swedish chemist Svante Arrhenius (1859-1927) was able
to formulate the first useful theory of acids:
"an acidic substance is one whose molecular unit contains at least
one hydrogen atom that can dissociate, or ionize, when dissolved
in water, producing a hydrated hydrogen ion and an anion."
H
3
CCOOH → H
+
(aq) + H
3
CCOO
–
(aq) acetic acid
H
2
SO
4
→ H
+
(aq) + HSO
4
–
(aq)
sulfuric acid
HCl → H
+
(aq) + Cl
–
(aq) hydrochloric acid
Strictly speaking, an “Arrhenius acid” must contain hydrogen.
However, there are substances that do not themselves contain hydrogen, but
still yield hydrogen ions when dissolved in water; the hydrogen ions come
from the water itself, by reaction with the substance. A more useful
operational definition of an acid is therefore the following:
An acid is a substance that yields an excess of hydrogen ions when
dissolved in water.

7. Arhenius base:
Arrhenius Base is one which yields hydroxide ions when dissolved in
water
Sodium hydroxide is an Arrhenius base because it contains hydroxide ions.
NaOH(s) → Na
+
(aq) + OH
–
(aq)
Na
2
O
(s)
+ H
2
O → [2 NaOH] → 2 Na
+
(aq) + 2 OH
–
(aq)
NH
3
+ H
2
O → NH
4
+
(aq) + OH
–
(aq)
We can therefore define a base as follows:
Sodium hydroxide is an Arrhenius base because it contains hydroxide ions. However,
other substances which do not contain hydroxide ions can nevertheless
produce them by reaction with water, and are therefore also classified as bases.

However,
A base is a substance that yields an excess of hydroxide ions when
dissolved in water.

8. Brønsted – Lowry concept of acids and bases
The older Arrhenius theory unable to explain why NH
3, which contains no
OH
–
ions, is a base and not an acid, why a solution of FeCl
3 is acidic, or why a
solution of Na
2S is alkaline.
In 1923 the Danish chemist J.N. Brønsted and English chemist T.M. Lowry, proposed that
An acid is a proton donor; a base is a proton acceptor.
HCl acid donates its proton to the acceptor (base) H
2
O.
There are no OH
–
ions in NH
3
, it is clearly not an Arrhenius base. It is,
however, a Brønsted base:
In this case, the water molecule acts as the acid, donating a proton to the base NH
3 to
create the ammonium ion NH
4
+.

9. Conjugate acid-base pairs
A reaction of an acid with a base is thus a proton exchange reaction; transfer of
a proton from an acid to a base must necessarily create a new pair of species that
can, at least in principle, constitute an acid-base pair of their own.
 NH
4
+
is the conjugate acid to the base NH
3
, because NH
3
gained a hydrogen ion to form
NH
4
+
, the conjugate acid.

 OH
-
is the conjugate base to the acid H
2
O, because H
2
O donates a hydrogen ion to form
OH
-
, the conjugate base.
Conjugate acid-base pair
)aq(
4
)g(
3
NHNH

)aq()l(
2OHOH



Conjugate acid-base pair
(Base) (Acid)
(Conjugate acid) (Conjugate base)
Note: The stronger the acid or base, the weaker the conjugate. The weaker
the acid or base, the stronger the conjugate.

10. 1)What is the conjugate base of HF?
Answer... F
-

Consider... HF = H
+
+ F
-

2)What is the conjugate base of benzoic acid, C
6
H
5
COOH?
Answer...
C
6
H
5
COO
-

Consider... HC
6
H
5
COO
-
-> H
+
+ C
6
H
5
COO
-

3)What is the conjugate acid of benzoate, C
6
H
5
COO
-
?
Answer... C
6
H
5
COOH
Consider... Recall the previous question.
4)What is the conjugate base of ammonium ion NH
4
+
?
Answer... NH
3

Consider... NH
4
+
= NH
3
+ H
+


CH
3COOH (aq) + H
2O (l) H
3O
+
(aq) + CH
3COO
-
(aq)
acid
1 base
2 acid
2 base
1
acid
1 and base
1 are a conjugate acid-base pair
acid2 and base2 are a conjugate acid-base pair

11. The Brønsted acid-base theory has been used throughout the history of acid and
base chemistry. However, this theory is very restrictive and focuses primarily
on acids and bases acting as proton donors and acceptors. Sometimes
conditions arise where the theory doesn't necessarily fit, such as in solids and
gases.
Lewis concept of acids and bases
In 1923, G.N. Lewis proposed an alternate theory to describe acids and
bases. His theory gave a generalized explanation of acids and bases
based on structure and bonding.
Lewis Acid: a species that accepts an electron pair and will have vacant
orbitals
Lewis Base: a species that donates an electron pair and will have lone-pair
electrons

13. The strength of acids and bases
Strong acids: Completely dissociates into ions in aqueous solution
Strong bases: Completely dissociates into ions in aqueous solution

14. The strength of acids and bases
Weak acids: Partially dissociates into ions in aqueous solution
In contrast to HCl, HF is a weak acid, one that does not completely ionize in
solution.

15. When a weak acid dissociates into ions in aqueous solution, an equilibrium
is set up

To make calculation easier, the dissociation can be written as
The acid dissociation constant for the weak acid HA is
The relative strength of acids can be expressed as K
a values
Dissociation constant:

16. Base dissociation constant:

When a weak base dissociates into ions in aqueous solution, an equilibrium is set
up

The weaker the base, the less it dissociates, the more the equilibrium lies to the
left
The relative strength of bases can be expressed as K
b values

17. Dissociation constant values and relative acid strength:
Weak acid: K
a < 10
-3
Moderate acid: K
a ~ 1 to 10
-3
Strong acid: K
a > 1

18. pH
The Danish biochemist Soren Sorenson proposed the term pH to refer to the
"potential of hydrogen ion.“ (French term pouvoir hydrogène ).
Therefore, The pH is the negative logarithm of the molarity of H.
The pOH is the negative logarithm of the molarity of OH
-

pH = – log
10
[H
+
] [H
+
] = Antilog (-pH)
pOH = – log
10
[OH
-
]
He defined the "p" as the negative of the logarithm, -log, of [H+].

20. Buffer Solution
“Solutions which resists changes in pH when small amount of acid or bases
are added”
What is a buffer composed of ?
Buffer consist of a weak conjugate acid-base pair, either

1)a weak acid and its conjugate base, or
2)a weak base and its conjugate acid
 Acetic acid (weak organic acid, CH
3
COOH) and a salt containing its conjugate
base, the acetate anion (CH
3
COO
-
), such as sodium acetate (CH
3
COONa)
CH
3
COOH + CH
3
COONa
 Ammonia (weak base, NH
3
) and a salt containing its conjugate acid, the
ammonium cation, such as Ammonium Hydroxide (NH
4
OH)
NH
3
+ NH
4
OH

24. Preparing a Buffer Solution with a Specific pH:
Henderson–Hasselbalch equation
The Henderson–Hasselbalch equation describes the derivation of pH as a measure
of acidity in biological and chemical systems. The equation is also useful for
estimating the pH of a buffer solution and finding the equilibrium pH in acid-base
reactions (it is widely used to calculate the isoelectric point of proteins).
  
 
 
 
 
 

]Salt[
Acid
H or,

A
HA
H or,

HA
A H
AHHA
a
a
a
(aq)(aq)
(aq)
K
K
K

















Where K
a is the dissociation constant of acid.

25.  
 
 
 
 
 
 
 
 
 Acid
Salt
logH,
Salt
Acid
log logH log,
Salt
Acid
log logH log,

]Salt[
Acid
H
a
a
a
a







pKpor
Kor
Kor
K
This is the Henderson–Hasselbalch equation
a
HpKp
If the molar concentrations of the acid and its conjugate base are
approximately equal, that is, [acid] ≈ [conjugate base], then

27. Q.
Q.

28. Q.
Q.

29. Q. Explain how your buffer solution responds to the addition of
small amounts of acid and base.

33. https://books.google.com.bd/books?id=_CsrDAAAQBAJ&printsec=frontcover&dq=Gaurav+Madan&hl=e
n&sa=X&ved=0ahUKEwjS4YbLy7faAhVBo48KHcxmASEQ6AEIJzAA#v=onepage&q=Gaurav%20Mad
an&f=false

36. Q. In order for a species to act as a Brønsted base, an atom in the species must
possess a lone pair of electrons. Explain why this is so.
Answer: A Brønsted-Lowry base must be able to form a bond to a proton. Because a
proton has no electrons, a base must contain an available electron pair that can easily
be donated to form a new bond. These include lone pairs or electron pairs in π-
bonds. The symbol B: is used for a general Brønsted-Lowry base.