Physical & Chemical Properties of Alkalis

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.

$$2 \text{Al} (\text{s}) + 6 \text{NaOH} (\text{aq}) + 6 \text{H}_{2}\text{O} (\text{l}) \rightarrow 2 \text{Na}_{3}\text{Al}(\text{OH})_{6} (\text{s}) + 3\text{H}_{2} (\text{g})$$

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