7.1 Acids and Bases

8/8/2019 7.1 Acids and Bases

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What are acids?

According to Arrhenius,acid is a substance

which ionises or dissociates in water toproduce hydrogen ions, H+.

For examples,hydrochloride acids, HCI.

Obtain by dissolving pure hydrogen chloride gasin water.

HCI (g) HCI (aq)


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Hydrogen chloride molecule reacts

with water and ionises to produce

hydrogen ions, H+

.HCI (aq) H+ (aq) + CI- (aq)

Hydrogen ions combines with watermolecules to form the hydroxonium

ion, H3O+.

H+ (aq) + H20 (l) H3O+ (aq)


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Overall equation for ionisation of hydrogen

chloride :

HCI (aq) + H2O (l) H3O+(aq) + CI- (aq)

+ +

H CI

O

H H

O

H H H CI


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Other examples

a) Nitric acid (HNO3)

b) Sulphuric acid (H2SO4)c) Phosphoric acid (H3PO4)

Acids can classify into monoprotic acid or

polyprotic acid based on basicity. Basicity of an acid is the number of ionisable

hydrogen atoms per acid molecule.


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Acids are classified into mineral acids and

organic acids.

Mineral Acid

Name Formula

Carbonic acid H2CO3

Hydrochloric

acid

HCI

Nitric Acid HNO3

Sulphuric acid H2SO4

Phosphoric

acid

H3PO4

Organic Acid

Name Formula

Methanoicacid

HCOOH

Ethanoic acid CH3COOH

Ascorbic acid C6H8O5

Citric Acid C6H8O7

Lactic Acid C3H6O3


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What are bases?

Arrhenius definition of a base :

- Base is a substance which ionises in water to

produce hydroxide ions, OH-

. Consists ofmetal hydroxides (OH-)and metal

oxides (O2-).

Some bases can dissolve in water. The solublebases are known as alkali.

Alkali is a base that is soluble in water and

ionises to produce hydroxide ions.


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Examples are sodium hydroxide

(NaOH) and ammonia (NH3).

Sodium hydroxide, NaOH

NaOH (aq) Na+(aq)+ OH- (aq)

Ammonia, NH3

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

+

+

N

H H H

O

H H NH H H

HO

H


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Bases also react with acid to form salt and

water only.

For example,sodium hydroxide react withhydrochloric acid to form sodium chloride

and water.

NaOH (aq) + HCl (aq) NaCI (aq)+ HCl (l)


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Base is divided into soluble base and insoluble

base.

Soluble base (alkali)

Name Formula

Ammonia NH3

Sodium

hydroxide

NaOH

Potassium

hydroxide

KOH

Calcium oxide CaO

Barium oxide BaO

Insoluble base

Name Formula

Magnesiumoxide

MgO

Aluminium oxide Al2O3

Zinc oxide ZnO

Copper (II) oxide CuO

Copper (II)

hydroxide

Cu (OH)2


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Uses of Acids

Carbonic acid is used to manufacture soft

drinks.

Vinegar contains ethanoic acid and used asfood preservative.

Methonoic acid is used as electrolyte in car

batteries. Yogurt contains lactic acid.


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Uses of Bases and Alkalis

Soaps and detergents contain

sodium hydroxide.

Toothpaste contains magnesium

hydroxide.

Gastric pills contain aluminium

hydroxide or magnesium

hydroxide. Ammonia solution is used to make

chemical fertilizers.


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Role of Water and Properties of

Alkalis

Dry base does not show alkaline properties.

In dry form, hydroxide ions,OH- are not free

to move. When dry base is dissolved in water, it will

dissociate in water to form hydroxide ions,

OH-

which are free to move.


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Properties of Acid

Physical properties

- Tastes sour

- Corrosive- Turn blue litmus paper to red

- pH below 7

- Conduct electricity


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Chemical Properties

a)React with reactive metals to form

salt and hydrogen gas.

b)React with carbonate to form salt,

water and carbon dioxide.

c)React with base to form salt and

water.

CaCO3 (s) + 2HCl (aq) CaCI2 (aq)+ H20 (l)

+ CO2(g)

Zn (s) + 2HCl (aq) ZnCI2 (aq)+ H2 (g)

NaOH (aq) + HCl (aq) NaCI (aq)+ HCl (l)


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Properties ofBases

Physical properties

- Tastes bitter and feels smooth

- Corrosive- Turn red litmus paper to blue

- pH above 7

- Conduct electrcity


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Chemical Properties

a)React with ammonium salts to form

salt, water and ammonia gas whenheated.

b)React with metal ions to form

insoluble metal hydroxide.

b)React with acid to form salt and

water.

2OH-

(aq) + Cu2+

(aq)

Cu(OH)2

NaOH (aq) + NH4Cl (s) NaCI (aq)+ H2O

(l) + NH3 (g)

NaOH (aq) + HCl (aq)

NaCI (aq)+ HCl (l)

7.1 Acids and Bases

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