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 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 Sodium hydroxide Sodium sulfate Sulfuric acid Water
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)
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) • Alkalis react with solutions of metal ions • Precipitation reaction: Used as a test to identify metal ions in metal salts
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 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 • 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