Physical Properties of Alkalis
Alkalis have the following properties:
- Alkalis have bitter taste and a slippery soapy feel.
- Alkaline solutions have pH values greater than 7. (More about pH value in the next sub-topic)
- Alkaline solutions turn red litmus paper blue.
Chemical Properties of Alkalis
There are four common reactions of alkalis:
- Neutralisation reaction
- Reaction with ammonium salt
- Precipitation of metal hydroxides
- Reaction with metals
Neutralisation Reaction
When an alkali reacts with an acid, the reaction is called a neutralisation reaction.
In a neutralisation reaction, only salt and water are produced.
$$\text{Alkali} + \text{acid} \rightarrow \text{salt} + \text{water}$$
All neutralisation can be summarized as the following:
- Metal oxides: $\text{O}^{2-}+\text{H}^{+} \rightarrow \text{H}_{2}\text{O}$
- Metal hydroxides: $\text{OH}^{-} + \text{H}^{+} \rightarrow \text{H}_{2}\text{O}$
Example of neutralisation reactions:
- Zinc oxide and hydrochloric acid: $\text{ZnO} (\text{s}) + 2 \text{HCl} (\text{aq}) \rightarrow \text{ZnCl}_{2} (\text{aq}) + \text{H}_{2}\text{O} (\text{l})$
- Sodium hydroxide and sulphuric acid: $2\text{NaOH}(\text{aq}) + \text{H}_{2}\text{SO}_{4}(\text{aq}) \rightarrow \text{Na}_{2}\text{SO}_{4} (\text{aq}) + 2 \text{H}_{2}\text{O}(\text{l})$
Reaction With Ammonia Salts
When a mixture of an alkali and ammonium salt is heated, ammonia gas is given off.
$\text{Alkali} + \text{ammonium salt} + \text{heat} \rightarrow \text{ammonia gas}$
The reaction is summarized as:
$$\text{OH}^{-} + \text{NH}_{4}^{+} \rightarrow \text{NH}_{3} + \text{H}_{2}\text{O}$$
Example of such a reaction:
- Ammonium chloride and sodium hydroxide: $\text{NH}_{4}\text{Cl} (\text{s}) + \text{NaOH} (\text{s}) \rightarrow \text{NaCl} (\text{s}) + \text{NH}_{3} (\text{g}) + \text{H}_{2}\text{O} (\text{l})$
- ammonium sulphate and calcium hydroxide: $(\text{NH}_{4})_{2}\text{SO}_{4} (\text{s}) + \text{Ca}(\text{OH}_{2}) (\text{s}) \rightarrow \text{CaSO}_{4} (\text{s}) + 2 \text{NH}_{3} (\text{g}) + 2 \text{H}_{2}\text{O} (\text{l})$
Precipitation of Metal Hydroxides
Solutions of alkalis are used to precipitate insoluble metal hydroxides from solutions of their salts.
$$\text{M}^{n+} (\text{aq}) + \text{nOH}^{-} (\text{aq}) \rightarrow \text{M}(\text{OH})_{\text{n}} (\text{s})$$
Example of such a reaction:
Aqueous sodium hydroxide and aqueous copper(II) sulphate
$$2\text{NaOH}(\text{aq}) + \text{CuSO}_{4} (\text{aq}) \rightarrow \text{Cu}(\text{OH})_{2} (\text{s}) + \text{Na}_{2}\text{SO}_{4} (\text{aq})$$
or: $\text{Cu}^{2+} (\text{aq}) + 2 \text{OH}^{-} (\text{aq}) \rightarrow \text{Cu}(\text{OH})_{2} (\text{aq})$
A blue precipitate of copper(II) hydroxide is produced.
Aqueous sodium hydroxide and aqueous magnesium chloride
$$2 \text{NaOH} (\text{aq}) + \text{MgCl}_{2} (\text{aq}) \rightarrow \text{Mg}(\text{OH})_{2} (\text{s}) + 2 \text{NaCl} (\text{aq})$$
or: $\text{Mg}^{2+} (\text{aq}) + 2 \text{OH}^{-} (\text{aq}) \rightarrow \text{Mg}(\text{OH})_{2} (\text{s})$
A white precipate of magnesium hydroxide is produced.
Reaction with metals
Alkalis usually do not react with metals.
Strong alkalis such as sodium hydroxide react with aluminium to give hydrogen.
$$2 \text{Al} (\text{s}) + 6 \text{NaOH} (\text{aq}) + 6 \text{H}_{2}\text{O} (\text{l}) \rightarrow 2 \text{Na}_{3}\text{Al}(\text{OH})_{6} (\text{s}) + 3\text{H}_{2} (\text{g})$$
