18.5 Other types of reactions | Reactions in aqueous solution

18.5 Other types of reactions (ESAFT)

We will look at two types of reactions that occur in aqueous solutions. These are ion-exchange reactions and redox reactions. Ion exchange reactions include precipitation reactions, gas forming reactions and acid-base reactions. Redox reactions are electron transfer reactions. It is important to remember the difference between these two types of reactions. In ion exchange reactions ions are exchanged, in electron transfer reactions electrons are transferred. These terms will be explained further in the following sections.

Ion exchange reactions (ESAFU)

Ion exchange reactions can be represented by:

\[\text{AB (aq)} + \text{CD (aq)} \rightarrow \text{AD} + \text{CB}\]

Either AD or CB may be a solid or a gas. When a solid forms this is known as a precipitation reaction. If a gas is formed then this may be called a gas forming reaction. Acid-base reactions are a special class of ion exchange reactions and we will look at them separately.

The formation of a precipitate or a gas helps to make the reaction happen. We say that the reaction is driven by the formation of a precipitate or a gas. All chemical reactions will only take place if there is something to make them happen. For some reactions this happens easily and for others it is harder to make the reaction occur.

Ion exchange reaction: A type of reaction where the positive ions exchange their respective negative ions due to a driving force.

Ion exchange reactions are used in ion exchange chromatography. Ion exchange chromatography is used to purify water and as a means of softening water. Often when chemists talk about ion exchange, they mean ion exchange chromatography.

We have already looked at precipitation reactions.

Gas forming reactions

These reactions are similar to precipitation reactions with the exception that instead of a precipitate forming, a gas is formed instead. An example of a gas forming reaction is sodium carbonate in hydrochloric acid. The balanced equation for this reaction is:

\[\text{Na}_{2}\text{CO}_{3}\text{(s)} + 2\text{HCl (aq)} \rightarrow 2\text{CO}_{2}\text{(g)} + 2\text{NaCl (aq)} + \text{H}_{2}\text{O (l)}\]

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:

\[\text{NaOH (aq)} + \text{HCl (aq)} \rightarrow \text{NaCl (aq)} + \text{H}_{2}\text{O (l)}\]

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 (ESAFV)

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:

\[4\text{Na} + \text{O}_{2} \rightarrow 2\text{Na}_{2}\text{O}\]

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).

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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 ($\text{Na}_{2}\text{O}$) and sodium peroxide ($\text{Na}_{2}\text{O}_{2}$).

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

Reaction types

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 ($\text{HCl}$); sodium hydroxide ($\text{NaOH}$); 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 $\text{20}$ $\text{cm$^{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.

Results

Answer the following questions:

What did you observe when you dissolved each of the salts in water?
What did you observe when you dissolved pairs of salts in the water?
What did you observe when you dissolved sodium carbonate in hydrochloric acid?
Why do you think we used bromothymol blue when mixing the hydrochloric acid and the sodium hydroxide? Think about the kind of reaction that occurred.
What did you observe when you placed the zinc metal into the copper sulphate?
Classify each reaction as either precipitation, gas forming, acid-base or redox.
What makes each reaction happen (i.e. what is the driving force)? Is it the formation of a precipitate or something else?
What criteria would you use to determine what kind of reaction occurs?
Try to write balanced chemical equations for each reaction

Conclusion

We can see how we can classify reactions by performing experiments.

In the experiment above, you should have seen how each reaction type differs from the others. For example, a gas forming reaction leads to bubbles in the solution, a precipitation reaction leads to a precipitate forming, an acid-base reaction can be seen by adding a suitable indicator and a redox reaction can be seen by one metal disappearing and a deposit forming in the solution.

18.4 Precipitation reactions

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