(a) What do you understand by exothermic and endothermic reactions?(b) Give one example of an exothermic reaction and one of an endothermic reaction.(c) Which of the following are endothermic reactions and which are exothermic reactions?    (i) Burning of natural gas   (ii) Photosynthesis   (iii) Electrolysis of water    (iv) Respiration   (v) Decomposition of calcium carbonate


(a)Exothermic reaction: "Exo" meaning releases and "thermic" means heat. Therefore, an exothermic reaction is a chemical reaction that releases energy through light or heat.

It is expressed in a chemical equation:
Reactants → products + heat energy.

Endothermic reaction: An endothermic reaction is a chemical reaction in which energy is absorbed in the form of light or heat.
It is expressed in a chemical equation:
Reactants + Heat energy  Products.

(b) Example of exothermic reaction: when carbon is burned in presence of oxygen, a lot of heat is released in this process.
$C\ ( s) \ +\ O_{2} \ ( g)\rightarrow \ CO_{2}( g) \ +\ Heat$

Example of endothermic reaction: when nitrogen is burned in oxygen a lot of heat is required for this reaction to take place.
$N_{2} \ ( g) \ +\ O_{2} \ ( g) \ +\ Heat\ \rightarrow \ 2NO\ ( g)$

(c)  (i) Burning of natural gas: Exothermic reaction
 (ii) Photosynthesis: Endothermic reaction
 (iii) Electrolysis of water: Endothermic reaction
 (iv) Respiration: Exothermic reaction
 (v) Decomposition of calcium carbonate: Endothermic reaction

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Updated on: 10-Oct-2022

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