Acids, bases & salts

The characteristic properties of acids and bases

Acids

Acids are proton donors.

• They have a pH of less than 7
• Oxides or hydroxides of nonmetals are acidic
• Aqueous solutions of acids contain H+ ions

Acid reactions:

Metal + acid -> salt + hydrogen

Metal oxide + acid -> salt + water (neutralisation)

Metal carbonate + acid -> salt + water + carbon dioxide

Metal hydroxide + acid -> salt + water (neutralisation)

Weak and strong acids:

• Weak acid — partially dissociates in water (e.g. ethanoic acid): CH3COOH (pH 4-6)
• Strong acid — fully dissociates with water (means that the hydrogen ions are broken off fully) e.g. HCl (pH 1-3)

Identification of acids:

• Turns damp blue litmus paper red
• Turns thymolphthalein colourless
• Turns methyl orange red

Bases

Bases are proton acceptors.

• They have a pH above 7
• Alkalis are soluble bases (all alkalis are bases but not all bases are alkalis)
• Oxides or hydroxides of metals are basic
• Aqueous solutions of alkalis contain OH- ions

Base reactions:

Base + acid -> salt + water

Base + ammonium salt -> ammonia + water + salt (decomposition reaction)

E.g. NaOH + (NH4)2SO4 -> NH3 + H2O + Na2SO4

Weak and strong bases:

• Weak bases — partially dissociates in water (pH 8-10)
• Strong bases — fully dissociates with water (pH 11-14)

Identification of bases:

• Turns damp red litmus paper blue
• Thymolphthalein turns blue
• Methyl orange turns yellow

Neutralisation

Neutralisation reaction between acids and alkalis, ionic equation:

H+ (aq) + OH- (aq) -> H2O (l)

Oxides

• Metallic oxide — a base, e.g. CuO and CaO
• Non-metallic oxide — an acid, e.g. SO2 and CO2
• Amphoteric oxide — oxides that react with both base and acid to produce salt and water, e.g. zinc oxide and aluminium oxide
• Neutral oxides — oxides that do not react with either acid nor base, e.g. carbon monoxide and nitric oxide

Preparation of salts

Solubility rules of salts

Methods of preparing salts

• Insoluble salt prep — soluble + soluble (precipitation)
• Soluble salt prep — metal + acid (for salts not containing K, NH4, Na)
• Soluble salt prep — acid + alkali (titration) for salts containing K, NH4, Na

Insoluble salt prep (precipitation)

1. 2 aqueous solutions of soluble substances mixed and reacted
2. Filtrate filtered out
3. Rinse with distilled water
4. Dry residue in oven

Soluble salt prep (metal / insoluble base + acid)

1. React insoluble base and acid
2. Filter out excess solid
3. Heat solution in evaporating dish over a water bath
4. Stop when crystals start to appear around the edge
5. Leave solution at room temp for crystals to form

Soluble salt prep (acid + alkali by titration)

1. Prepare acid in a burette
2. Prepare a known amount of alkali in a beaker
3. Add a suitable indicator into the alkali
4. Slowly drip the acid into the alkali until it reaches its end point (when the indicator changes colour)
5. Record the volume of titrant used
6. Repeat experiment without the indicator
7. Solution is evaporated and cooled to form crystals