Acid-base reactions  (Page 3/5)

1. Hydrochloric acid reacts with sodium hydroxide to form sodium chloride (the salt) and water. Sodium chloride is made up of Na ${}^{+}$ cations from the base (NaOH) and Cl ${}^{-}$ anions from the acid (HCl). $\mathrm{HCl}+\mathrm{NaOH}\to {\mathrm{H}}_2\mathrm{O}+\mathrm{NaCl}$
2. Hydrogen bromide reacts with potassium hydroxide to form potassium bromide (the salt) and water. Potassium bromide is made up of K ${}^{+}$ cations from the base (KOH) and Br ${}^{-}$ anions from the acid (HBr). $\mathrm{HBr}+\mathrm{KOH}\to {\mathrm{H}}_2\mathrm{O}+\mathrm{KBr}$
3. Hydrochloric acid reacts with sodium hydrocarbonate to form sodium chloride (the salt) and hydrogen carbonate. Sodium chloride is made up of Na ${}^{+}$ cations from the base (NaHCO ${}_3$ ) and Cl ${}^{-}$ anions from the acid (HCl). $\mathrm{HCl}+{\mathrm{NaHCO}}_3\to {\mathrm{H}}_2{\mathrm{CO}}_3+\mathrm{NaCl}$

You should notice that in the first two examples, the base contained OH ${}^{-}$ ions, and therefore the products were a salt and water . NaCl (table salt) and KBr are both salts. In the third example, NaHCO ${}_3$ also acts as a base, despite not having OH ${}^{-}$ ions. A salt is still formed as one of the products, but no water is produced.

It is important to realise how important these neutralisation reactions are. Below are some examples:

• Domestic uses Calcium oxide (CaO) is put on soil that is too acid. Powdered limestone (CaCO ${}_3$ ) can also be used but its action is much slower and less effective. These substances can also be used on a larger scale in farming and also in rivers.
• Biological uses Acids in the stomach (e.g. hydrochloric acid) play an important role in helping to digest food. However, when a person has a stomach ulcer, or when there is too much acid in the stomach, these acids can cause a lot of pain. Antacids are taken to neutralise the acids so that they don't burn as much. Antacids are bases which neutralise the acid. Examples of antacids are aluminium hydroxide, magnesium hydroxide ('milk of magnesia') and sodium bicarbonate ('bicarbonate of soda'). Antacids can also be used to relieve heartburn.
• Industrial uses Alkaline calcium hydroxide (limewater) can be used to absorb harmful SO ${}_2$ gas that is released from power stations and from the burning of fossil fuels.