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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?