You give my answer but not practically theorically if someone don't know whether the heat will evolved or absorbed then how he will get to know whether the reaction  will be exothermic or endothermic?

 

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Exothermic Reaction

The chemical reactions which proceed with the evolution of heat energy are called as exothermic reactions.

Carbon burns in oxygen to form carbon dioxide and heat energy is produced.

C + O₂ → CO₂ + Heat

Water added to quick lime (calcium oxide), slaked lime (calcium hydroxide) is produced with lot of heat energy.

CaO + H₂O → Ca(OH)₂ + Heat

Endothermic Reaction

The chemical reactions which proceed with the absorption of heat energy are called as endothermic reactions. Sometimes, the beaker in which the reaction mixture is present becomes cold.

Formation of carbon disulphide

C + 2S + Heat → CS₂

When nitrogen and oxygen are heated together to a particular temperature of about 3000^oC, nitric oxide gas is formed.

N₂ + O₂ + Heat → 2NO

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