# Acid-base reactions  (Page 3/5)

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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.

## Interesting fact

Bee stings are acidic and have a pH between 5 and 5.5. They can be soothed by using substances such as calomine lotion, which is a mild alkali based on zinc oxide. Bicarbonate of soda can also be used. Both alkalis help to neutralise the acidic bee sting and relieve some of the itchiness!
Acid-base titrations

The neutralisation reaction between an acid and a base can be very useful. If an acidic solution of known concentration (a standard solution) is added to an alkaline solution until the solution is exactly neutralised (i.e. it has neither acidic nor basic properties), it is possible to calculate the exact concentration of the unknown solution. It is possible to do this because, at the exact point where the solution is neutralised, chemically equivalent amounts of acid and base have reacted with each other. This type of calculation is called volumetric analysis . The process where an acid solution and a basic solution are added to each other for this purpose, is called a titration , and the point of neutralisation is called the end point of the reaction. So how exactly can a titration be carried out to determine an unknown concentration? Look at the following steps to help you to understand the process.

Step 1:

A measured volume of the solution with unknown concentration is put into a flask.

Step 2:

A suitable indicator is added to this solution (bromothymol blue and phenolpthalein are common indicators).

Step 3:

A volume of the standard solution is put into a burette (a measuring device) and is slowly added to the solution in the flask, drop by drop.

Step 4:

At some point, adding one more drop will change the colour of the unknown solution. For example, if the solution is basic and bromothymol blue is being used as the indicator in the titration, the bromothymol blue would originally have coloured the solution blue. At the end point of the reaction, adding one more drop of acid will change the colour of the basic solution from blue to yellow. Yellow shows that the solution is now acidic.

Step 5:

Record the volume of standard solution that has been added up to this point.

Step 6:

Use the information you have gathered to calculate the exact concentration of the unknown solution. A worked example is shown below.

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