What does one mean by exothermic and endothermic reactions? Give examples.

Exothermic reactions

Exothermic reactions are the type of reaction in which heat is evolved or the reaction in which heat is released along with the formation of products are called exothermic reactions.

Exothermic Reaction

For example

When carbon burns in oxygen to form carbon dioxide, a lot of heat is produced in this reaction:

C (s)+O2 (g)—-CO2 (g)+HEAT

The burning of carbon in oxygen is an exothermic reaction because heat is evolved in this reaction.

When Methane is burnt in the oxygen of air it forms carbon dioxide and water vapour. A large amount of heat is produced in this reaction.

CH4 (g)+ 2O2 (g)——CO2 (g)+ 2H20 (g)+ Heat Energy

The burning of methane gas is an exothermic reaction because heat is produced in this reaction.

During digestion Starch Carbohydrate it is broken down into a simple carbohydrate glucose. This glucose then undergoes slow combustion by combining with oxygen in the cells of our body to produce energy in a process called respiration. In addition to other functions this energy maintains our body heat. During respiration, glucose combines with oxygen in the cells of our body to form carbon dioxide and water along with the production of energy.

C6H12O6 (aq) + 6O2 (g)———-6CO2 (g) + 6H2O (l) + Energy

Glucose        +    oxygen ——carbon dioxide + water + energy

Respiration is a type of exothermic process because energy is produced during this process.

 

Endothermic reaction

The reactions which occur by the absorption of heat/ energy( either in the form of light or electricity) are called endothermic reactions.

Endothermic Reaction

For example

All the decomposition reactions require energy to take place. So all the decomposition reactions are endothermic reactions

When calcium carbonate is heated it decomposes to form calcium oxide and carbon dioxide.

CaCO3  (s)            + Heat——-   CaO(s)       + CO2

Calcium carbonate + Heat—— Calcium oxide + Carbon dioxide

The decomposition of calcium carbonate is an endothermic reaction because heat energy is absorbed in this reaction.

Photosynthesis is an endothermic reaction because sunlight energy is absorbed during the process of photosynthesis by the green plants.

The electrolysis of water to form hydrogen and oxygen is also an endothermic reaction. This is because electric energy is absorbed during this reaction.

Examples of Endothermic reactions

2HgO (s)           +    Heat——-2Hg (l)    + O2 (g)

Mercuric Oxide +   Heat ——-Mercury + Oxygen

N2  (g) + O2 (g)  + Heat ——- 2NO (g)

 

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