# 0.8 Reactions in aqueous solutions  (Page 9/10)

 Page 9 / 10

## Other reactions in aqueous solutions

There are many types of reactions that can occur in aqueous solutions. In this section we will look at two of them: acid-base reactions and redox reactions. These reactions will be covered in more detail in Grade 11.

## Acid-base reactions

Acid base reactions take place between acids and bases. In general, the products will be water and a salt (i.e. an ionic compound). An example of this type of reaction is:

$\mathrm{NaOH\left(aq\right)}+\mathrm{HCl\left(aq\right)}\to \mathrm{NaCl\left(aq\right)}+{\mathrm{H}}_{2}\mathrm{O \left(l\right)}$

This is an special case of an ion exchange reaction since the sodium in the sodium hydroxide swaps places with the hydrogen in the hydrogen chloride forming sodium chloride. At the same time the hydroxide and the hydrogen combine to form water.

## Redox reactions

Redox reactions involve the exchange of electrons. One ion loses electrons and becomes more positive, while the other ion gains electrons and becomes more negative. To decide if a redox reaction has occurred we look at the charge of the atoms, ions or molecules involved. If one of them has become more positive and the other one has become more negative then a redox reaction has occurred. For example, sodium metal is oxidised to form sodium oxide (and sometimes sodium peroxide as well). The balanced equation for this is:

$4\mathrm{Na}+{\mathrm{O}}_{2}\to 2{\mathrm{Na}}_{2}{O}$

In the above reaction sodium and oxygen are both neutral and so have no charge. In the products however, the sodium atom has a charge of $+1$ and the oxygen atom has a charge of $-2$ . This tells us that the sodium has lost electrons and the oxygen has gained electrons. Since one species has become more positive and one more negative we can conclude that a redox reaction has occurred. We could also say that electrons have been transferred from one species to the other. (In this case the electrons were transferred from the sodium to the oxygen).

## Demonstration: oxidation of sodium metal

You will need a bunsen burner, a small piece of sodium metal and a metal spatula. Light the bunsen burner. Place the sodium metal on the spatula. Place the sodium in the flame. When the reaction finishes, you should observe a white powder on the spatula. This is a mixture of sodium oxide ( ${\mathrm{Na}}_{2}\mathrm{O}$ ) and sodium peroxide ( ${\mathrm{Na}}_{2}{\mathrm{O}}_{2}$ ).

Sodium metal is very reactive. Sodium metal reacts vigourously with water and should never be placed in water. Be very careful when handling sodium metal.

## Aim:

To use experiments to determine what type of reaction occurs.

## Apparatus:

Soluble salts (e.g. potassium nitrate, ammonium chloride, sodium carbonate, silver nitrate, sodium bromide); hydrochloric acid ( $\mathrm{HCl}$ ); sodium hydroxide( $\mathrm{NaOH}$ ); bromothymol blue; zinc metal; copper (II) sulphate; beakers; test-tubes

## Method:

• For each of the salts, dissolve a small amount in water and observe what happens.
• Try dissolving pairs of salts (e.g. potassium nitrate and sodium carbonate) in water and observe what happens.
• Dissolve some sodium carbonate in hydrochloric acid and observe what happens.
• Carefully measure out $20{\mathrm{cm}}^{3}$ of sodium hydroxide into a beaker.
• Add some bromothymol blue to the sodium hydroxide
• Carefully add a few drops of hydrochloric acid to the sodium hydroxide and swirl. Repeat until you notice the colour change.
• Place the zinc metal into the copper sulphate solution and observe what happens.

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