Acids and Alkalis When an acid reacts with an alkali the general equation is: Acid + Alkali Salt + Water
1. Hydrochloric acid + sodium hydroxide Sodium hydroxide is an alkali. It neutralises hydrochloric acid solution to produce sodium chloride (common salt) and water Hydrochloric acid+Sodium hydroxide Sodium chloride+Water HCl + NaOH NaCl + H 2 O
2. Sulphuric acid + Calcium hydroxide Sulphuric acid + Calcium hydroxide Calcium sulphate + water H 2 SO 4 + Ca(OH) 2 CaSO4 + H 2 O
Spectator ions Sometimes when we write equations involving ions, some ions appear on both sides of the equation. These ions are spectators i.e. they are present in the water but don’t actually take part in the reaction.
Look at this example for the reaction between hydrochloric acid and sodium hydroxide. H + Cl - (aq) + Na + OH - (aq) Na + Cl - (aq) + H 2 O (l) The ions in the red type appear on both sides of the reaction, without being changed and so are spectator ions. We can rewrite the equation so that it only contains the ions which take part in the reaction.
Acidic solutions have excess H + ions. During neutralisation, these ions react to produce water molecules. H + (aq) + OH - (aq) H 2 O (l) The excess H + ions in acid and OH - ions in alkali react to form water molecules This shows that neutralisation is a reaction of H + ions to form water.