Week 4

Prepared by:Mrs Faraziehan Senusi

PA-A11-7C

David P. White Prentice Hall ©

2003

Autoionization

Strong acids and bases

Weak acids and bases

Chapter 2Acids and Bases

Acid-base titration

Acids and Bases

Lesson Plan

At the end of this topic, the students will be able:

To explain the concept of acids and bases according to Arrhenius, Bronsted and Lewis

To calculate pH, pOH, pKa, pKb and pKw


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Arrhenius Theory

• Acid = substance that contains hydrogen and ionize in water to produce hydrogen ion (H+).

HCl H+ + Cl-

• Base = substance that contains the OH (hydroxyl) group and ionize in water to produce hydroxide ion (OH-)

NaOH Na+ + OH-

Lewis Theory

• A more general definition• Acid = Substance which can accepts a pair

of electrons.• Base = Substance which can donates a pair

of electrons.• Example: BF3 (acid)and NH3 (base)

Bronsted Theory

• Acid = substance capable of donating a proton (H+).• Base = substance that can accept a proton (H+)

acceptor.• For example:

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

Base (B)

Acid (B)

conj. acidacidbase conj. base

For example:

NH3 (aq) + H2O (l) NH4+ (aq) + OH- (aq)

Base (B)

Acid (B)

Bronsted Theory

conj. acidacidbase conj. base

• Water can act both as Bronsted acid and Bronsted base.

• Water is an amphiprotic.

• Amphiprotic = a substance that can act both as Bronsted acid and Bronsted base.

Proton Transfer Equilibria in Water

• Proton transfer between acids and bases is FAST IN BOTH DIRECTIONS.

• So, previous equations give:

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

NH3 (aq) + H2O (l) NH4+ (aq) + OH- (aq)

Conjugate Acids and Conjugate Bases

• When a species donates a proton (so it acts a Bronsted acid), it becomes the conjugate base.

• When a species accepts a proton (so it acts a Bronsted base), it becomes the conjugate acid.

• A Brønsted-Lowry acid-base reaction occurs when an acid and a base react to form their conjugate base and conjugate acid, respectively:


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The Strength of Bronsted Acids and Bronsted Base

The stronger the base, the weaker is its conjugate acid.

Theodore L. Brown , H. Eugene Lemay , Bruce E. Bursten , Catherine J. Murphy ,David P. White, Chemistry the central science.

• The strength of a Bronsted acid is measured by its acidity constant (Ka).

• The strength of a Bronsted base is measured by its basicity constant (Kb).

Acidity constant (or acid ionization constant), Ka:

HX (aq) + H2O (l) H3O+ (aq) + X- (aq)

HX

X OH3

aK

• If Ka is very small (Ka <<1) , [HX] is large with respect to [X-],

• So,

1. PROTON RETENTION by the acid is favoured.

2. Very small fraction of acid is DEPROTONATED.

Basicity constant (or base ionization constant), Kb:

B (aq) + H2O (l) BH+ (aq) + OH- (aq)

B

OH BH

bK

• If Kb is very small (Kb <<1) , [B] is large with respect to [BH+],

• So,

1. Very small fraction of base is PROTONATED.

Autopyrolysis (or Autoionization)

• Since water is amphiprotic, a proton transfer equilibrium exists with an absence of added acids or bases.

• In pure water at 25oC,

[H3O+] = [OH-] = 1.0 x 10-7 M

Kw =1.0 x 10-14

• Autopyrolysis constant:

Kw = [H3O+][OH-]

2H2O(l) H3O+(aq) + OH-(aq)

OHOHO2H

OHHAOHA

AOHOHHA

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2

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Relation between Ka and Kb of conjugate acid-base pair

• The sum of two dissociation reactions is the autoionization of water.

• Express strength of acid in terms of the strength of its conjugate base

Ka.Kb = Kw • Two reactions(c.a–base pairs) added to give

a third reaction, the equilibrium constant for the third reaction is the product of the equilibrium constants for the two added reactions

• pK= -log K• pOH = - log[OH-]• pH = - log[H+]• pKa + pKb = pKw• pH + pOH = 14.00

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• Weak acid: The proton transfer equilibrium lies in

favour of ionized acid.

pKa > 0, Ka < 1

• Strong base: species that is mostly protonated in water.

pKb < 0, Kb > 1

• Weak base: species that is only partially protonated in water.

pKb > 0, Kb < 1

Strong and Weak Acids and Bases

Example Strong Acid & Strong Base


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?pH

10 1.0H

10 1.0 10 1.0 0 f

10 1.0 10 1.0 10 1.0 -

0 0 10 1.0 i

(aq) Cl (aq) H (aq) HCl

3-

3-3-

3-3-3-

3-

-

a)

b) ?

a) Calculate the pH of a 0.50 M HF solution at 25oC, Ka =7.1 x 10-4

b) Calculate the pH of a 0.050 M HF solution at 25oC, Ka =7.1 x 10-4

• Require x to be less than 5%. approximation is valid if expression is equal to or less than 5%

a

acbbx

2

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Example Weak Acid

• Write for Ka = _______ = 7.1 x 10-4 •

• Write for ionization constant expression,Ka =

• Then, x must be small compared to 0.50, we make approximation of

• Calculate for pH,

pH = ?b) Try calculate the pH of a 0.050 M HF ??? ~ Quadratic equation

a)


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Percent Ionization

• Measure the strength of an acid

• The stronger the acid, the greater the percent ionization

%100ionconcentrat acid initial

mequilibriuat ion concentrat acid ionized

• Calculate the pH of a 0.40 M ammonia solution. Kb =1.8 x 10-5.

Example Weak Base

Kb = ___________ = 1.8 x 10-5

x = ?

Check % of approximation = ≤ 5% ?

pOH = ? pH = ?

• The conjugate base of any strong acid is a weak base.

• The conjugate base of any weak acid is a strong base.

• The conjugate acid of any strong base is a weak acid.

• The conjugate acid of any weak base is a strong acid.

Polyprotic Acids• Polyprotic acid = a species that loses

protons in succession, and successive deprotonations are progressively less favourable.

• This is because the additional electrostatic work must be done to remove the positively charged proton.

• These substances may yield more than one hydrogen ion per molecule.

• For example, for carbonic acid, H2CO3,

• Note that the conjugate base in the first ionization stage becomes the acid in the second ionization stage.


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Characteristic of Bronsted Acid

• Acidic proton is the donatable proton.• There are 3 types of acidic proton:

1. Aqua Acid = the acidic proton is on a water molecule coordinated to a central metal ion.

• For example:

[Fe(H2O)6]3+(aq) + H2O(l) Fe(H2O)5(OH)]2+(aq) + H3O+(aq)

2. Hydroxoacid = the acidic proton is on a hydroxyl group without a neighbouring oxo group (=O)

• For example: Te(OH)6

3. Oxoacid = the acidic proton is on a hydroxyl group with an oxo group (=O) attached to the same atom.

• For example: H2SO4

• The successive stages in the deprotonation of an aqua acid:

AQUA ACID HYDROXOACID OXOACID

• The strength of aqua acids increase with increasing positive charge of the central metal ion and with decreasing ionic radius.


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• Oxalic acid (H2C2O4) is a poisonous substance used chiefly as a bleaching and cleansing agent (for example, to remove bathtub rings).

• Calculate the concentrations of all the species present at equilibrium in a 0.10 M solution.

Ka1 = 6.5 x10-2 & Ka2 = 6.1 x10-5

Example Polyprotic Acids

Ka1 = ___________ = 6.5 x10-2

x = ?

Check % of approximation = ≤ 5% ? If not, ~ quadratic equation

At equilibrium,

First stage of ionization:

Second stage of ionization:

Ka2 = ___________ = 6.1 x10-5

y = ?

Check % of approximation = ≤ 5% ? If not, ~ quadratic equation

At equilibrium,

Week 4

Technology

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