Ions are charged particles which are formed when atoms, or groups of atoms, lose or gain electrons. For the examination, you need to know which ions are produced by acids, and which are produced by alkalis. You will also need to know the ionic equation for neutralisation.
State symbols
State symbols are used in symbol equations:
- (s) means solid
- (l) means liquid (not the same as dissolved in water - see below)
- (g) means gas
- (aq) means aqueous (dissolved in water)
Acids
When acids dissolve in water they produce aqueous hydrogen ions, H+(aq). For example, looking at hydrochloric acid:
HCl(aq) → H+(aq) + Cl–(aq)
Alkalis
When alkalis dissolve in water they produce aqueous hydroxide ions, OH–(aq). For example, looking at sodium hydroxide:
NaOH(aq) → Na+(aq) + OH–(aq)
Ammonia is slightly different. This is the equation for ammonia in solution:
NH3(aq) + H2O(l) → NH4+(aq) + OH–(aq)
Be careful to write OH– and not Oh– or oh–.
Neutralisation reaction
When the H+(aq) ions from an acid react with the OH–(aq) ions from an alkali, a neutralisation reaction occurs to form water. This is the equation for the reaction:
H+(aq) + OH–(aq) → H2O(l)
For example, hydrochloric acid and sodium hydroxide solution react together to form water and sodium chloride solution. The acid contains H+ ions and Cl– ions, and the alkali contains Na+ ions and OH– ions. The H+ ions and OH– ions produce the water, and the Na+ ions and Cl– ions produce the sodium chloride, NaCl(aq).
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