Page 120 - Chemistry_Form_2

Chemistry
for Secondary Schools

4. Fill the burette with dilute hydrochloric acid and record the initial volume, V 1 .
5. Run the acid into the alkali (base), a few drops at a time, until the indicator
just changes colour. Swirl the components of the flask after every addition
of the acid.
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6. Record the new level of the remaining acid in the burette as V 2 . Calculate
the volume of the acid used in the reaction.

7. In a clean beaker, mix the same volume of the base used with the same
volume of the acid obtained from the titration. Do not add any indicator.
8. Transfer 10 cm of the resulting solution into an evaporating dish and
3
carefully heat the solution until crystals start to form.

9. Stop heating and allow the remaining liquid to evaporate, leaving behind
a white solid.

Questions

1. Why is the phenolphthalein indicator important in an acid-base experiment?

2. What is the balanced chemical equation representing the reaction in the
experiment? Give the name of the white solids obtained in Step 9.
3. What is the ionic equation of the reaction in the experiment?

Applications of neutralisation reactions

Neutralisation reactions have many applications in daily life. Some of the
applications are explained as follows:

Treating insect stings and bites
Insect bites or stings, and plants with stinging hair often cause small and red
lumps on the skin which may be painful and itchy. Insects such as bees, have
stings that inject an acidic liquid in the blood through the skin. The stings can
be neutralised by rubbing baking soda on the affected area. Ant bites and nettle
(a plant with stinging hair) stings contain methanoic acid which is neutralised by
using baking soda or other alkaline substances such as cucumber and avocado.
Wasp stings are alkaline and can be neutralised with vinegar which contains
acetic acid. Figure 5.9 shows different sources of stings or bites.

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