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Acids and Bases (3). Bases and Alkalis. Bases are the oxides or hydroxides of metals. Contains either oxide ions (O 2- ) or hydroxide ions (OH - ). Na + ; O 2-. Zn 2+ ; O 2-. Cu 2+ ; O 2-. Mg 2+ ; OH -. Al 3+ ; OH -. The Definition of a Base.

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bases and alkalis
Bases and Alkalis
  • Bases are the oxides or hydroxides of metals.
    • Contains either oxide ions (O2-) or hydroxide ions (OH-)

Na+ ; O2-

Zn2+ ; O2-

Cu2+ ; O2-

Mg2+ ; OH-

Al3+ ; OH-

the definition of a base
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
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
The Definition of an Alkali

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

laboratory alkalis
Laboratory Alkalis
  • Sodium hydroxide, NaOH
  • Aqueous ammonia, NH3
  • Calcium hydroxide, Ca(OH)2
properties of alkalis
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
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 11
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

Sodium hydroxide

Sodium sulfate

Sulfuric acid

Water

which one is not a neutralisation reaction

Manganese (II) oxide

Hydrochloric acid

Manganese (II) chloride

+

+

Water

Manganese (IV) oxide

Hydrochloric acid

Manganese (II) chloride

+

+

+

Water

Chlorine

Which one is not a neutralisation reaction?
  • Reaction (1)
  • Reaction (2)

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

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

chemical reactions of alkalis 2

Moist red litmus paper turns blue

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.

chemical reactions of alkalis 3
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

water

water

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)

weak alkalis
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
Uses of Alkalis
  • Alkalis, like acids, are common in our daily lives.
    • To neutralise acids
    • Dissolve grease
uses of bases and alkalis
Uses of Bases and Alkalis
  • Ammonia solution:
  • In window cleaning solutions
  • In fertilisers
  • 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
  • Calcium oxide:
  • In neutralising acidic soil
  • To make iron, concrete and cement
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