Combination Reaction

Activity

1. Take a small amount of calcium oxide or quick lime in a beaker.
2. Slowly add water to this.
3. Touch the beaker as shown in Fig. 1.3.
4. Do you feel any change in temperature?

Figure

Formation of slaked lime by the reaction of calcium oxide with water

Calcium oxide reacts vigorously with water to produce slaked lime (calcium hydroxide) releasing a large amount of heat.

CaO(s) + H₂O(l) → Ca(OH)₂(aq)

(Quick lime) (Slaked lime)

In this reaction, calcium oxide and water combine to form a single product, calcium hydroxide. Such a reaction in which a single product is formed from two or more reactants is known as a combination reaction.

A solution of slaked lime produced by the reaction 1.13 is used for white washing walls. Calcium hydroxide reacts slowly with the carbon dioxide in air to form a thin layer of calcium carbonate on the walls. Calcium carbonate is formed after two to three days of white washing and gives a shiny finish to the walls. It is interesting to note that the chemical formula for marble is also CaCO₃.

Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s) + H₂O(l) (1.14)

Calcium Calcium

Hydroxide carbonate

Let us discuss some more examples of combination reactions.

(i) Burning of coal

C(s) + O₂(g) → CO₂(g) (1.15)

(ii) Formation of water from H2(g) and O2(g)

2H₂(g) + O₂(g) → 2H₂O(l)

In simple language we can say that when two or more substances (elements or compounds) combine to form a single product, the reactions are called combination reactions.

In Activity 1.4, we also observed that a large amount of heat is evolved. This makes the reaction mixture warm. Reactions in which heat is released along with the formation of products are called exothermic

chemical reactions.

Other examples of exothermic reactions are –

(i) Burning of natural gas

CH₄(g) + 2O₂ (g) → CO₂ (g) + 2H₂O (g)

(ii) Do you know that respiration is an exothermic process?

We all know that we need energy to stay alive. We get this energy from the food we eat. During digestion, food is broken down into simpler substances. For example, rice, potatoes and bread contain carbohydrates. These carbohydrates are broken down to form glucose. This glucose combines with oxygen in the cells of our body and provides energy. The special name of this reaction is respiration, the process of which you will study in Chapter 6.

C₆H₁₂O₆(aq) + 6O₂(aq) → 6CO₂(aq) + 6H₂O(l) + energy (Glucose).

(iii) The decomposition of vegetable matter into compost is also an example of an exothermic reaction.

Identify the type of the reaction taking place in Activity 1.1, where heat is given out along with the formation of a single product.