Acids and Bases (3)

Bases and Alkalis

Bases are the oxides or hydroxides of metals. Contains either oxide ions (O2-) or hydroxide ions (OH-)

Base Formula Ions presentSodium oxide Na2O

Zinc oxide ZnO

Copper(II) oxide CuO

Magnesium hydroxide Mg(OH)2

Aluminium hydroxide Al(OH)3

Na+ ; O2-

Zn2+ ; O2-

Cu2+ ; O2-

Mg2+ ; OH-

Al3+ ; OH-

The Definition of a Base

An base is a substance that reacts with an acid to give a salt and

water only.

Base + Acid Salt + Water

Alkalis: Special Class of Bases

Soluble bases are called alkalis.

All alkalis are bases, but not all bases are alkalis.

Most bases are insoluble in water.

The Definition of an Alkali

An alkali is a substance that produces hydroxide ions, OH- (aq)

in water.

Laboratory Alkalis

Sodium hydroxide, NaOH Aqueous ammonia, NH3

Calcium hydroxide, Ca(OH)2

Properties of Alkalis

Bitter taste and soapy feel. Hazardous

Concentrated solutions: Corrosive & burn skin (Caustic)

Dilute solutions: Irritants Acids change the colour of indicators.

Acids turn red litmus blue – a simple test for alkali.

Chemical Reactions of Alkalis (1)

Alkalis react with acids to form a salt and water only.

Neutralisation reaction: The hydrogen ions (from the acid) and the hydroxide ions (from the alkali) react to form water.

Chemical Reactions of Alkalis (1)

For example,

+ +

+ +

The ionic equation for any neutralisation reaction:

H+ (aq) + OH- (aq) H2O (l)

Sodium hydroxide

Hydrochloric acid

Sodium chloride

Water

WaterSodium

hydroxide Sulfuric acid Sodium sulfate

Which one is not a neutralisation reaction?

Reaction (1)

Reaction (2)

Manganese (II) oxide

Hydrochloric acid

Manganese (II) chloride

Water+ +

Manganese (IV) oxide

Hydrochloric acid

Manganese (II) chloride

Water+ + Chlorine+

MnO (s) + 2HCl (aq) MnCl2 (aq) + H2O (l)

MnO2 (s) + 4HCl (aq) MnCl2 (aq) + 2H2O (l) + Cl2 (g)

Chemical Reactions of Alkalis (2)

Alkalis heated with ammonium salts give off ammonia gas.

Alkali + Ammonium salt Ammonia + Water + Salt

Ammonia gas is recognized by its characteristic pungent smell.

Moist red litmus paper turns blue

Chemical Reactions of Alkalis (3)

Alkalis react with solutions of metal ions Precipitation reaction:

Used as a test to identify metal ions in metal salts

Strong Alkalis

When strong alkalis are added to water, they become OH-(aq) ions in solutions.

E.g. NaOH(s) Na+(aq) + OH-(aq) KOH(s) K+(aq) + OH-(aq)

water

water

Weak Alkalis

When weak alkalis are added to water, only a small fraction of the molecules form OH-(aq) ions.

Most of the molecules remain unchanged.

E.g. NH3 (g) + H2O (l) NH4+ (aq) + OH- (aq)

Uses of Alkalis

Alkalis, like acids, are common in our daily lives. To neutralise acids Dissolve grease

Uses of Bases and Alkalis

Ammonia solution:- In window cleaning solutions- In fertilisers

Calcium oxide:- In neutralising acidic soil- To make iron, concrete and cement

Magnesium hydroxide:- In toothpaste to neutralise acid on teeth- In antacids to relieve indigestion

Sodium hydroxide:- In making soaps and detergents- In industrial-cleaning detergents

So, what have you learnt?