“Two or more reactants (elements or compounds) combine and formed a single product is known as a combination reaction.“
eg 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) + Heat
(Quick lime) (Slaked lime)
In this reaction, calcium oxide and water combine to form a single product.
Let us discuss some more examples of combination reactions.
(i) Burning of coal
C(s) + O2 (g) → CO2 (g)
(ii) Formation of water from H2 (g) and O2 (g)
2H2 (g) + O2 (g) → 2H2O(l)
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.
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.
where heat is given out along with the formation of a single product.