Arrhenius Theory

Acid = substance that ionize in water to produce

hydrogen ion (H+).
HCl  H+ + Cl-

CHAPTER 4 Base =substance that ionize in water to produce

hydroxide ion (OH-)
NaOH  Na+ + OH-
ACIDS AND BASES

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Brønsted-Lowry Theory Brønsted-Lowry Theory

Concept: acid-base reactions involve the transfer of H+ For example:
Acid (B)
ions from one substance to another
Acid =substance capable of donating a proton (H+).
Base = substance that can accept a proton (H+) NH3 (aq) + H2O (l)  NH4+ (aq) + OH- (aq)
acceptor.
Base (B)
For example: Base (B)

HF (g) + H2O (l)  H3O+ (aq) + F- (aq)

Acid (B)
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 Proton Transfer Equilibria in
Brønsted-Lowry Theory Water
Water can act both as Brønsted acid and Brønsted Proton transfer between acids and bases is FAST IN
base. BOTH DIRECTIONS.
Water is an amphiprotic. So, previous equations give:
Amphiprotic = a substance that can act both as HF (g) + H2O (l) H3O+ (aq) + F- (aq)
Brønsted acid and Brønsted base.
NH3 (aq) + H2O (l) NH4+ (aq) + OH- (aq)

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Conjugate Acids and The Strength of

Conjugate Bases Brønsted-
When a species donates a proton (so it acts a Brønsted
Lowry Acids
acid), it becomes the conjugate base. and Bases
When a species accepts a proton (so it acts a Brønsted The stronger the acid, the
base), it becomes the conjugate acid. weaker is its conjugate base.
The stronger the base, the
weaker is its conjugate acid.

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The Strength of Brønsted- The Strength of Brønsted-
Lowry Acids and Bases Lowry Acids and Bases
Strong acid completely transfer its proton to water, no Acidity constant (or acid ionization constant), Ka:
undissociated molecules
HX (aq) + H2O (l) H3O+ (aq) + X- (aq)
Weak acid partially dissociates in aq solution  exist as
acid molecules + constituent ions.
Negligible acidity (CH4) contains hydrogen but has no
Ka 
H O X 
3
 

acidic behaviour in water. HX 

The strength of a Brønsted acid is measured by its
acidity constant (Ka). The larger the value Ka, the stronger the acid
The strength of a Brønsted base is measured by its
basicity constant (Kb).

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The Strength of Brønsted- The Strength of Brønsted-

Lowry Acids and Bases Lowry Acids and Bases
If Ka is very small (Ka <<1) , [HX] is large with respect to Basicity constant (or base ionization constant), Kb:
[X-],
So, B (aq) + H2O (l) BH+ (aq) + OH- (aq)
1. PROTON RETENTION by the acid is favoured.
2. Very small fraction of acid is DEPROTONATED.
Kb 
BH OH 
 

B

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The Strength of Brønsted- Autopyrolysis (or
Lowry Acids and Bases Autoionization)
If Kb is very small (Kb <<1) , [B] is large with respect to Since water is amphiprotic, a proton transfer
[BH+], equilibrium exists with an absence of added acids or
So, bases.

Very small fraction of base is PROTONATED.

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Autopyrolysis (or Autopyrolysis (or

Autoionization) Autoionization)
Autopyrolysis constant: pK= -log K
 
K c  H 3O  OH   pKa + pKb = pKw
pH/OH= -log[H3O+]/[OH-]
In pure water at 25oC, pH + pOH = 14.00

  
K w  H 3O  OH   1.0 10 14

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Strong and Weak Acids and

Bases
An acid or base is classified as either weak or
strong depending on the size of its acidity constant.
Strong acid: The proton transfer equilibrium lies
strongly in favour of donation of a proton to water.
pKa < 0, Ka > 1

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Strong and Weak Acids and Strong and Weak Acids and
Bases Bases
Weak acid: The proton transfer equilibrium lies in Strong base: species that is mostly protonated in
favour of ionized acid. water. pKb < 0, Kb > 1
pKa > 0, Ka < 1
Weak base: species that is only partially protonated
in water. pKb > 0, Kb < 1

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Percent Ionization Percent Ionization

Measure the strength of an acid, strong acid has The conjugate base of any strong acid is a weak
greater percent ionization. base.
The conjugate base of any weak acid is a strong
base.

Example: 0.035M solution of HNO2 contains 3.7 x The conjugate acid of any strong base is a weak
10-3 M H+(aq). acid.
The conjugate acid of any weak base is a strong
acid.

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 Characteristic of Brønsted-
Polyprotic Acids Lowry Acid
Polyprotic acid = a species that loses protons in Acidic proton is the donatable proton.
succession, and successive deprotonations are There are 3 types of acidic proton:
progressively less favourable. 1. Aqua Acid = the acidic proton is on a water
This is because the additional electrostatic work molecule coordinated to a central metal ion.
must be done to remove the positively charged For example:
proton. [Fe(H2O)6]3+(aq) + H2O(l)  Fe(H2O)5(OH)]2+(aq) + H3O+(aq)

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Characteristic of Brønsted- Characteristic of Brønsted-

Lowry Acid Lowry Acid
The successive stages in the deprotonation of an
2. Hydroxoacid = the acidic proton is on a hydroxyl aqua acid:
group without a neighbouring oxo group (=O) AQUA ACID HYDROXOACID OXOACID
For example: Te(OH)6 The strength of aqua acids increase with increasing
3. Oxoacid = the acidic proton is on a hydroxyl group positive charge of the central metal ion and with
with an oxo group (=O) attached to the same atom. decreasing ionic radius.
For example: H2SO4

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Lewis Theory Lewis Theory
A more general definition A Brønsted-Lowry base is a Lewis base, but Lewis
base can donate its electron pair to other than H+
Acid – electron-pair acceptor
Base – electron-pair donor.
Example: BF3 (acid)and NH3 (base)

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