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Home » Class 11 » Chemistry » Thermodynamics » Exothermic And Endothermic Reaction

Exothermic And Endothermic Reaction

Last Updated on February 16, 2023 By Mrs Shilpi Nagpal

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 = Hp -HR

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. HR < Hp.

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.

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Filed Under: Chemistry, Class 11, Thermodynamics Tagged With: endothermic reaction, enthalpy change, example of endothermic reaction, example of exothermic reaction, exothermic reaction, thermochemistry, ΔH is negative for exothermic reaction, ΔH is positive for endothermic reaction

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  1. YoUkNoWwHo says

    February 1, 2021 at 11:17 am

    THANK YOU FOR THE NOTES

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