14.1 Types of reactions  (Page 3/4)

Chlorine water frees bromine gas from the bromide and colours the carbon tetrachloride a reddish brown.

Chlorine water frees iodine gas from an iodide and colours the carbon tetrachloride purple.

Precipitation reactions and ions in solution

1. Silver nitrate ( ${\mathrm{AgNO}}_3$ ) reacts with potassium chloride ( $\mathrm{KCl}$ ) and a white precipitate is formed.
   1. Write a balanced equation for the reaction that takes place.
   2. What is the name of the insoluble salt that forms?
   3. Which of the salts in this reaction are soluble?
2. Barium chloride reacts with sulphuric acid to produce barium sulphate and hydrochloric acid.
   1. Write a balanced equation for the reaction that takes place.
   2. Does a precipitate form during the reaction?
   3. Describe a test that could be used to test for the presence of barium sulphate in the products.
3. A test tube contains a clear, colourless salt solution. A few drops of silver nitrate solution are added to the solution and a pale yellow precipitate forms. Which one of the following salts was dissolved in the original solution?
   1. $\mathrm{NaI}$
   2. $\mathrm{KCl}$
   3. ${\mathrm{K}}_2{\mathrm{CO}}_3$
   4. ${\mathrm{Na}}_2{\mathrm{SO}}_4$
   (IEB Paper 2, 2005)
   

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:

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:

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 ( ${\mathrm{Na}}_2{\mathrm{O}}_2$ ).