Acids and bases 3
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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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Acids and Bases (3)

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Acids and bases 3

Acids and Bases (3)


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


So what have you learnt

So, what have you learnt?


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