13.2 Acid-base reactions (ESBQY)

The reaction between an acid and a base is known as a neutralisation reaction. Often when an acid and base react a salt and water will be formed. We will look at a few examples of acid-base reactions.

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In chemistry the word salt does not mean the white substance that you sprinkle on your food (this white substance is a salt, but not the only salt). A salt (to chemists) is a product of an acid-base reaction and is made up of the cation from the base and the anion from the acid.


Hydrochloric acid reacts with sodium hydroxide to form sodium chloride (a salt) and water. Sodium chloride is made up of ( extNa^+) cations from the base (( extNaOH)) and ( extCl^-) anions from the acid (( extHCl)).

< extHCl (aq) + extNaOH (aq) ightarrow extH_2 extO (l) + extNaCl (aq)>

Hydrogen bromide reacts with potassium hydroxide to form potassium bromide (a salt) and water. Potassium bromide is made up of ( extK^+) cations from the base (( extKOH)) and ( extBr^-) anions from the acid (( extHBr)).

< extHBr (aq) + extKOH (aq) ightarrow extH_2 extO (l) + extKBr (aq)>

Hydrochloric acid reacts with ammonia to form ammonium chloride (a salt). Ammonium chloride is made up of ( extNH_4^+) cations from the base (( extNH_3)) and ( extCl^-) anions from the acid (( extHCl)).

< extHCl (aq) + extNH_3 ext(aq) ightarrow extNH_4 extCl (aq)>

You should notice that in the first two examples, the base contained ( extOH^-) ions, and therefore the products were a salt and water. ( extNaCl) (table salt) and ( extKBr) are both salts. In the third example, ( extNH_3) also acts as a base, despite not having ( extOH^-) ions. A salt is still formed as the only product, but no water is produced.

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

Domestic uses

Calcium oxide (( extCaO)) is a base (all metal oxides are bases) that is put on soil that is too acidic. Powdered limestone (( extCaCO_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 in rivers.

Limestone (white stone or calcium carbonate) is used in pit latrines (or long drops). The limestone is a base that helps to neutralise the acidic waste.

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

Basic calcium hydroxide (limewater) can be used to absorb harmful acidic ( extSO_2) gas that is released from power stations and from the burning of fossil fuels.


Bee stings are acidic and have a pH between ( ext5) and ( ext5,5). They can be soothed by using substances such as bicarbonate of soda and milk of magnesia. Both bases help to neutralise the acidic bee sting and relieve some of the itchiness!


Acid-base reactions

Aim

To investigate acid-base reactions.

Apparatus and materials

Volumetric flask conical flasks sodium hydroxide solution hydrochloric acid solution pipette indicator

Method

*

Use the pipette to add ( ext20) ( extml) of the sodium hydroxide solution to a volumetric flask. Fill up to the mark with water and shake well.

*

Measure ( ext20) ( extml) of the sodium hydroxide solution into a conical flask. Add a few drops of indicator.

Slowly add ( ext10) ( extml) of hydrochloric acid. If there is a colour change stop. If not add another ( ext5) ( extml). Continue adding ( ext5) ( extml) increments until you notice a colour change.

Observations

The solution changes colour after a set amount of hydrochloric acid is added.

In the above experiment you used an indicator to see when the acid had neutralised the base. Indicators are chemical compounds that change colour depending on whether they are in an acid or in a base.

A recommended experiment for informal assessment on discovering natural indicators is included. Learners can test a variety of colourful plants to see what happens to each plant when mixed with an acid or a base. The basic idea is for learners to extract the colour of the plant by boiling the plant matter and then draining the liquid. For substances such as the curry powder, learners can dissolve this in water and for the tea they can brew a cup of tea and then remove the teabag before testing. The resulting liquid can then be tested to see if it is an indicator. An alternative to mixing the acid or base into the liquid is to soak strips of paper in the liquid and then place a drop of the acid or base onto the paper. The experiment below also covers some other substances such as baking powder, vanilla essence and onions. Baking powder fizzes in acids but not in bases. Onions and vanilla essence lose their characteristic smell when in a basic solution.

It is important that learners do not place their faces or noses directly over or into the beaker when smelling the onions and vanilla essence. They must hold the beaker in one hand and use the other hand to waft (i.e. wave their hand back and forth) the smell towards their face.

Acids and bases are corrosive and can cause serious burns so must be handled with care.

Indicators

Aim

To determine which plants and foods can act as indicators.

Apparatus and materials

Possible indicators: Red cabbage, beetroot, berries (e.g. mulberries), curry powder, red grapes, onions, tea (rooibos or regular), baking powder, vanilla essence acids (e.g. vinegar, hydrochloric acid), bases (e.g. ammonia (in many household cleaners)) to test Beakerns

Method

Take a small amount of your first possible indicator (do not use the onions, vanilla essence and baking powder). Boil the substance up until the water has changed colour.

Filter the resulting solution into a beaker being careful not to get any plant matter into the beaker. (You can also pour the water through a colander or sieve.)

Pour half the resulting coloured solution into a second beaker.

Place one beaker onto an A4 sheet of paper labelled “acids”. Place the other beaker onto a sheet of paper labelled “bases”.

Repeat with all your other possible indicators (except the onions, vanilla essence and baking powder).

Into all the beakers on the acid sheet carefully pour ( ext5) ( extml) of acid. Record your observations.

Into all the beakers on the base sheet carefully pour ( ext5) ( extml) of base. Record your observations.

If you have more than one acid or base then you will need to repeat the above steps to get fresh indicator samples for your second acid or base. Or you can use less of the resulting coloured solution for each acid and base you want to test.

Observe the smell of the onions and vanilla essence. Place a small piece of onion into a beaker. This is for testing with the acid. Pour ( ext5) ( extml) of acid. Wave your hand over the top of the beaker to blow the air towards your nose. What do you notice about the smell of the onions? Repeat with the vanilla essence.

Place a small piece of onion into a beaker. This is for testing with the base. Pour ( ext5) ( extml) of base. Wave your hand over the top of the beaker to blow the air towards your nose. What do you notice about the smell of the onions? Repeat with the vanilla essence.

Finally place a teaspoon of baking powder into a beaker. Carefully pour ( ext5) ( extml) of acid into the beaker. Record your observations. Repeat using the base.

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Observations

Beetroot

Berries

Curry powder

Tea

Red grapes

Onions

Vanilla essence

Baking powder

Substance

Colour

Results with acid

Results with base

Red cabbage

You should note that some of the substances change colour in the presence of either an acid or a base. The baking powder fizzes when it is in the acid solution, but no reaction is noted when it is in the base solution. Vanilla essence and onions should lose their characteristic smell when in the base.