Exothermic And Endothermic Reaction

Exothermic reactions

These are those reactions which are accompanied by the evolution of heat.

The quantity of heat produced is shown along with the products with a plus sign.

For Ex:

C ( s) + O₂ (g) ——> CO₂ ( g) + 395.3 kJ

H₂ (g) + ½ O₂ (g) ——–> H₂O ( l ) + 285.8 KJ

N₂ ( g ) + 3 H₂ ( g ) ——–> 2 NH₃ ( g ) + 92.4 KJ

CH₄ ( g ) + 2 O₂ (g) ———–> CO₂ ( g ) + 2 H₂O ( g ) + 890.4 KJ

Endothermic reactions

Those reactions in which heat is absorbed are called endothermic reactions.

The heat absorbed can be written with a plus sign along with the reactants, it is usually shown along with products with a minus sign.

For Ex:

N₂ ( g ) + O₂ ( g ) ——–> 2NO ( g ) -180.7 KJ

C ( s ) + H₂O ( g ) ———–> CO ( g ) + H₂ ( g ) -131.4 KJ

C ( s ) + 2 S ( s ) ———> CS₂ ( g ) -92 KJ

H₂ ( g ) + I₂ (g ) ——> 2 HI ( g ) -52.5 KJ

The enthalpy change accompanying a reaction is given by

ΔH = Heat content of products – Heat content of reactants

ΔH = H_p -H_R

1) A reaction is exothermic i.e. heat is given out in a reaction if the total heat content of the reactant is more than that of the product.

Then according to equation, ΔH will be negative.

C ( s) + O₂ (g) ——> CO₂ ( g)  ΔH= – 393.5 KJ mol^-1

H₂ (g) + ½ O₂ (g) ——–> H₂O ( l ) ΔH= − 285.8 KJ mol^-1

N₂ ( g ) + 3 H₂ ( g ) ——–> 2 NH₃ ( g ) ΔH= −92.4 KJ mol^-1

CH₄ ( g ) + 2 O₂ (g) ———–> CO₂ ( g ) + 2 H₂O ( g ) ΔH = -890.4 KJ mol^-1

ΔH is negative for exothermic reaction.

2) A reaction is endothermic i.e. heat is absorbed in a reaction if the total heat content of reactants is less than that of products i.e. H_R < H_p.

Then according to equation ΔH , will be positive.

Hence ΔH is positive for endothermic reaction.

N₂ ( g ) + O₂ ( g ) ——–> 2NO ( g )  ΔH = 180.7 KJ mol^-1

C ( s ) + H₂O ( g ) ———–> CO ( g ) + H₂ ( g ) ΔH = 131.4 KJ mol^-1

C ( s ) + 2 S ( s ) ———> CS₂ ( g ) ΔH=92 KJ mol^-1

H₂ ( g ) + I₂ (g ) ——> 2 HI ( g )  ΔH=52.5 KJ mol^-1

The branch of chemistry which deals with the heat changes accompanying chemical reaction is called thermochemistry.