Acid, bases and salts

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Zn + 2HCl ------------------- ZnCl2 + H2

Fe + H2SO4 ---------------- FeSO4 + H2

Method 2 (Acid + Base)

Useful for making salts of less reactive metals, e.g. lead, copper.

e.g.

CuO + H2SO4 ---------------- CuSO4 + H2O

MgO + 2HCl ------------------ MgCl2 + H2O

Add excess base to acid.

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Method 3 (Acid + Carbonate)

Useful particularly for making salts of more reactive metals, e.g. calcium, sodium.

e.g.

CaCO3 + 2HCl ------------- CaCl2 + H2O + CO2.

Na2CO3 + H2SO4 ------------ Na2SO4 + H2O + CO2.

Method 4 (Acid + Alkali)

This is useful for making salts of reactive metals, and ammonium salts. It is different from methods 1-3, as both reactants are in solution. This means neutralisation must be achieved, by adding exactly the right amount of acid to neutralise the alkali. This can be worked out by titration

e.g.

NaOH + HCl -------------- NaCl + H2O

2NH4OH + H2SO4 ---------------------- (NH4)2SO4 + 2H2O

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Making Insoluble Salts

This involves mixing solutions of two soluble salts that between them contain the ions that make up the insoluble salt. It is made by two methods.

PRECIPITATION

BaCl2(aq) + MgSO4(aq)  BaSO4(s) + MgCl2(aq)

DIRECT COMBINATION

Fe + S ---heat---- FeS

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Precipitation reaction

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Types of Salts

Normal Salts:

Normal salts are formed when all the replaceable hydrogen ions in the acid have been completely replaced by metallic ions.

HCl(aq) + NaOH(aq)  NaCl(aq) + H2O(l)

H2SO4(aq) + ZnO(aq)  ZnSO4(aq) + H2O(l)

Normal salts are neutral to litmus paper.

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Acid salts:

Acid salts are formed when replaceable hydrogen ions in acids are only partially replaced by a metal. Acid salts are produced only by acids containing more then one replaceable hydrogen ion. Therefore an acid with two replaceable ions e.g. H2SO4 will form only one acid salt, while acid with three replaceable hydrogen ions e.g. H3PO4 will form two different acid salts.

H2SO4(aq) + KOH(aq)  KHSO4(aq) + H2O(l)

H3PO4(aq) + NaOH  NaH2PO4(aq) + H2O(l)

H3PO4(aq) + 2NaOH(aq)  Na2HPO4(aq) + 2H2O(l)

An acid salt will turn blue litmus red. In the presence of excess metallic ions an acid salt will be converted into a normal salt as its replaceable hydrogen ions become replaced.