gcsescience.com                                          28                                          gcsescience.com

Acids and Alkalis

What is an Ionic Equation?

If a reaction involves substances that form ions in water, we can write
the equation showing only the ions. This is called an ionic equation.

What is the Ionic Equation for Neutralisation?

In the reaction between hydrochloric acid and sodium hydroxide
(see the previous page)

HCl_(aq)       +          NaOH_(aq)              NaCl_(aq)      +       H₂O_(l)

We can write it in terms of ions, since both
hydrochloric acid and sodium hydroxide form ions in water.

H⁺_(aq) + Cl^-_(aq) +  Na⁺_(aq) +  OH^-_(aq)      Na⁺_(aq) +  Cl^-_(aq)  +  H₂O_(l)

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

This leaves the ionic equation

hydrogen ion     +     hydroxide ion      water
         H⁺_(aq)            +         OH^-_(aq)            H₂O_(l)

(compare this reaction with the ionisation of water).

This is the reaction that always occurs when
an acid + alkali    salt + water.
It is called the ionic equation for neutralisation.

The hydrogen ion of the acid + the hydroxide ion of the alkali
combine to form a water molecule,
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?
(see the next page).

       Links        Acids and Alkalis         Revision Questions       

gcsescience.com    The Periodic Table    Index    Neutralisation Quiz    gcsescience.com