Acid–alkali reactions

Acid–alkali reactions

Activity 10.7.1

Exploring neutralisation

We are learning how to:

• describe chemical reactions involving acids

• define neutralisation.

For this experiment you need to work in two groups: one with an acid and the other without.

Here is what you need:

• A dilute acid

• A dilute alkali

• Liquid indicator (phenolphthalein)

• Measuring cylinder

SAFETY

• Droppers

• Test tube

• Straw.

Observe the safety icon on the reagent bottles. Remember to observe safety rules when working with hazardous materials. Avoid spillage.

Here is what you should do:

Follow these instructions carefully and correctly.

1. Measure 1 cm3 of alkali and pour it into the test tube.

2. Use a dropper to place one drop of indicator into the alkali.

3. Observe and record the colour. For the groups with the acid:

4. Using a second dropper, apply acid to the alkali in the test tube and shake it gently after each application.

5. Keep adding until a colour difference appears.

6. Use a third dropper to add drops of alkali to observe colour changes.

7. Add acid and alkali alternately until you think you have found a midpoint between acid and alkali.

8. Explain what you experienced as you changed from adding acid to alkali and back.

For the group with no acid:

9. Get one student to gently blow exhaled air through the straw into the alkali with the indicator.

10. They should keep exhaling until the colour changes.

11. Use the second dropper to add drops of alkali to observe colour changes.

12. Then the student should exhale into this combination.

13. Repeat until you think you have found a midpoint between exhaled air and alkali.

Alkalis are bases that are soluble in water. Acids and alkalis have opposite chemical properties. When an alkali and acid combine, there is a point where they both cancel out the effect of each other and this is called neutralisation. A neutral substance has a pH of 7, the midpoint of acids and alkalis.

Since acids and alkalis chemically react with each other, a new product is formed.

Check your understanding

1. Describe how to explore the neutralisation of an acid by an alkali.

Fun fact

Indigestion is caused by a build-up of stomach acid. Indigestion tablets are alkaline and so they neutralise the acid.

Key term neutralisation the effect of acid and alkali cancelling each other out

Ammonium compounds

Ammonia gas

We are learning how to:

• make and test for ammonia gas

• use neutralisation reactions to make ammonium compounds.

Ammonia is a pungent gas that must be handled with great care. It will dissolve in the moisture on the eyeballs or in the nose and produce an alkaline solution that will cause irritation.

When ammonia dissolves in water a weak alkali is formed. You have already tested aqueous ammonia with acid–alkali indicators.

This solution is often called ammonium hydroxide but the term ‘aqueous ammonia’ is a more accurate description. Although the chemical formula is often given as NH4OH, it is better represented as NH3∙H2O.

Ammonia gas can be made by heating any ammonium compound with an alkali. The combination of compounds most often used is shown in the following equation: ammonium chloride + calcium hydroxide → ammonia + calcium chloride + water

Activity 10.10.1

Making ammonia gas

Here is what you need:

• Ammonium chloride

• Calcium hydroxide

• Boiling tube

• Delivery tube

• Stand and clamp

Here is what you should do:

• Tweezers

• Red litmus paper

• Test tube

• 250 cm3 beaker.

1. Mix equal amounts of solid ammonium chloride and calcium hydroxide and place the mixture in a boiling tube. Only use a small sample.

2. Support the boiling tube horizontally using a stand and clamp.

Fun fact

The maximum concentration of ammonia it is possible to have in water has a density of 0.880 g/cm3 It is often referred to as ‘880 ammonia’.

3. Place a bung and delivery tube as shown in Fig 10.10.2.

4. Gently heat the mixture until you are aware of a pungent smell.

5. Place an inverted test tube over the delivery tube and collect a test tube of ammonia gas.

6. Hold a piece of red litmus paper with tweezers, dampen it and place it into the bottom of the test tube. What colour does it turn?

7. Two-thirds fill a beaker with water.

8. Keeping the test tube inverted, push the open end into the water in the beaker and observe what happens.

9. Explain your observations.

Ammonia is the only common gas that is alkaline, so turning damp litmus paper from red to blue is a test for the presence of this gas.

Ammonia is less dense than air and is very soluble in water.

Ammonium compounds

Ammonia forms salts by neutralising acids. These salts are ammonium compounds.

Ammonium nitrate and ammonium sulfate are made in large quantities and used by farmers to increase the fertility of their soils.

Ammonium compounds provide plants with extra nitrogen so they grow well and produce large crops.

Check your understanding

1. Would dry ammonia gas turn a piece of dry red litmus paper blue? Explain your answer.

2. Phosphoric acid is another strong acid.

a) What is the name of the compound formed when ammonia neutralises phosphoric acid?

b) Write a word equation for this reaction.

Key terms

pungent having a sharp smell acid–alkali indicator chemical that turns different colours in acidic and alkaline conditions density/dense mass per unit volume