Acids and Bases (3). Bases are the oxides or hydroxides of metals. Contains either oxide ions (O...
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Transcript of Acids and Bases (3). Bases are the oxides or hydroxides of metals. Contains either oxide ions (O...
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Acids and Bases (3)
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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-
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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
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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.
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The Definition of an Alkali
An alkali is a substance that produces hydroxide ions, OH- (aq)
in water.
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Laboratory Alkalis
Sodium hydroxide, NaOH Aqueous ammonia, NH3
Calcium hydroxide, Ca(OH)2
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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.
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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.
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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
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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)
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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
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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
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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
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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)
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Uses of Alkalis
Alkalis, like acids, are common in our daily lives. To neutralise acids Dissolve grease
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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
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So, what have you learnt?