Acids and Bases (3)

 Bases are the oxides or hydroxides of metals.  Contains either oxide ions (O 2- ) or hydroxide ions (OH - ) BaseFormulaIons present Sodium oxideNa 2 O Zinc oxideZnO Copper(II) oxideCuO Magnesium hydroxideMg(OH) 2 Aluminium hydroxideAl(OH) 3 Na + ; O 2- Zn 2+ ; O 2- Cu 2+ ; O 2- Mg 2+ ; OH - Al 3+ ; OH -

An base is a substance that reacts with an acid to give a salt and water only. Base + Acid  Salt + Water

 Soluble bases are called alkalis.  All alkalis are bases, but not all bases are alkalis.  Most bases are insoluble in water.

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

 Sodium hydroxide, NaOH  Aqueous ammonia, NH 3  Calcium hydroxide, Ca(OH) 2

 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.

 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.

For example, +  + The ionic equation for any neutralisation reaction: H + (aq) + OH - (aq)  H 2 O (l) Sodium hydroxide Hydrochloric acid Sodium chloride Water Sodium hydroxide Sulfuric acid Sodium sulfate

 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)  MnCl 2 (aq) + H 2 O (l) MnO 2 (s) + 4HCl (aq)  MnCl 2 (aq) + 2H 2 O (l) + Cl 2 (g)

 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

 Alkalis react with solutions of metal ions  Precipitation reaction: Used as a test to identify metal ions in metal salts

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

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. NH 3 (g) + H 2 O (l) NH 4 + (aq) + OH - (aq)

 Alkalis, like acids, are common in our daily lives.  To neutralise acids  Dissolve grease

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?