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Acids and Alkalis

Neutralisation - Ionic Equations.

If we take the reaction between
hydrochloric acid and sodium hydroxide from the previous page,

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

We can write it in terms of ions,
since both the acid and the alkali form ions in water.

H+(aq) + Cl-(aq)    +   Na+(aq) +  OH-(aq)      arrow   Na+(aq) +    Cl-(aq)    +   H2O(l)

Chloride ions (Cl-(aq)), and sodium ions (Na+(aq))
appear on both sides of the equation.
They are spectator ions, they are not changed by the reaction,
and so they may be left out of the equation.

This leaves the equation

hydrogen ion  +   hydroxide ion   arrow   water
H+(aq)            +         OH-(aq)   arrow   H2O(l)

Compare this reaction with the ionisation of water.

This is the reaction that always occurs
when an acid + alkali arrow salt + water.
The hydrogen ion of the acid + the hydroxide ion of the alkali
 combine to form water,
 leaving the metal from the alkali and the non-metal from the acid
to form a salt solution.

How much acid is needed to neutralise an alkali? (next page).

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