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.
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
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.
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.