C2aL13  Making Salts from Bases

Key Words

Base - a metal hydroxide or metal oxide

Excess - too much of a substance.

Salt - an ionic compound formed as a result of the reaction of a metal and a non-metal

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Grade E

Bases are metal oxides or hydroxides.  Most bases are insoluble in water.  Hydroxides can dissolve in water to form an alkaline solution.  Oxides do not dissolve.

If we add an acid to a base, there is a reaction to form a salt and water.

Base + Acid ® salt + water

magnesium oxide + hydrochloric acid ® magnesium chloride + water

MgO (s) + 2HCl (aq) ® MgCl2 (aq) + H2O (l)

This reaction is useful in making salts of low reactivity metals.

Grade C

Here is a reaction between another base and an acid to give a salt:

zinc oxide + sulphuric acid ® zinc sulphate + water

ZnO (s) + H2SO4 (aq) ®  ZnSO4 (aq) + H2O (l)

To make the salt, we add an excess of zinc oxide to ensure that all the acid is used up.  We then filter the zinc oxide from the zinc sulphate.

To get the zinc sulphate crystals, we need to evaporate the water.

 

When we name a salt, the first part comes from the metal, and the second from the acid.

  • If the salt contains just two elements, the second part of the name ends in "-ide", e.g. potassium bromide, KBr.

  • If the salt has more than two elements, its name ends in "-ate", e.g. sodium nitrate, NaNO3.

When we write the formula, we must make sure that the charges on each ion balance out to make a neutral salt.

Metal Ions Non Metal Ions
Na+, K+, Li + Cl-, F-, Br-
Ca2+, Mg2+ SO42-
Cu2+, Fe2+ NO3-
Al3+, Fe3+ PO43-

Iron can have two kinds of ions:

  • Fe2+, iron (II) or Ferrous;

  • Fe3+, iron (III) or Ferric.

Make sure that the ionic charges balance, e.g:

Mg2+ + 2Cl- ® MgCl2

Grade A

There are no notes at this level