From the first law of thermodynamics, it follows that when one form of energy disappears, an equivalent amount of another form of energy makes its appearance. Similarly, every chemical individual i.e. an element or a compound is associated with a certain fixed amount of chemical energy also known as internal or intrinsic energy. Chemical energy may be converted into heat energy or vice versa. Thermochemistry is a specific application of the first law of thermodynamics or law of conservation of energy.
A branch of chemistry which deals with heat energy changes taking place in the course of a chemical reaction is called as thermochemistry. In other words, thermochemistry is the quantitative study of the enthalpy changes which accompany chemical changes.
Thermochemistry is of great practical importance. It provides data from which the relative, enthalpy of chemical compounds can be deducted. Hence, thermochemistry is a basis for the study of chemical bonding.
Exothermic reaction : The reaction in which heat is evolved is called as exothermic reaction. "The reactants lose heat, and change into products e.g. • - ...
N2+3H2-» 2NH3 " AH = -100.4 kj
Here. 1 mole of nitrogen reacts with 3 moles of hydrogen and forms * 2 moles of ammonia. During the reaction, 100.4 kj ace lost by the
reactants and they change into products. Some examples of exothermic reactions are:
(1) C + 02-» COz , AH = - 395.4 kj
(2) NaOH + HCl-> NaCl+H20 AH = -57.3 kj
(3) Zn + CuS04—ZnS04 + Cu AH =-225.9kJ
The reaction in which heat is absorbed is
called as endothermic reaction. Some examples of endothermic reactions are :
(1) C+2S-> CS2 AH = + 87.86 kj
(2) 2C + H2-> C2H2 AH = + 225.9 kj
(3) H2+I2-> 2HI AH = + 51.88 kj
(4) 302 203 AH = 288.696 kj
(5) N2 + 02 2NO AH = 205.016 kj
Let us consider a general equation :
A + B-» C +D
Let HA/ Hb, Hc and Hp be the (heat contents) enthalpies of the substances A, B, C and D respectively. If the reaction is carried out at constant atmospheric pressure, we have
AH = (HC+HD)-(HA+HB) AH = I H(products) - £ H(reactants) Change in enthalpy = Sum of the enthalpy of products
- Sum of the enthalpy of reactants
H may be zero, negative or positive.
(i) When AH = 0, no heat is evolved or absorbed. Here enthalpy of products is equal to enthalpy of reactants.
(ii) When AH is negative, enthalpy of products is less than that of reactants. Therefore, change in enthalpy is given out in the form of heat, i.e. heat is given out. Such reactions are called exothermic reactions. Hence, the reactions which are accompanied by evolution of heat are known as exothermic reactions.
(iii) When AH is positive, enthalpy of products is greater than that of reactants. An equivalent amount of heat is absorbed by the system from the surroundings. Such reactions are called as endothermic reactions. Hence, the reactions which are accompanied by absorption of heat are known as endothermic reactions.
In thermodynamic sense, heat evolved is shown with a -ve sign as the system has lost heat. When heat is absorbed it is shown with a +ve sigh as the system has gained heat.
Table 2.4: Distinction between Exc Exothermic reaction
thermic and Endothermic reactions Endothermic reaction
1. Heat is evolved.
1. Heat is absorbed.
2. Forward reaction is spontaneous.
2. Backward reaction may be spontaneous.
3. Products are more stable.
3. Reactants are more stable.
4. Enthalpy of the products is less than that of the reactants.
4. Enthalpy of the products is more than that of the reactants.
5. ziH is negative.
5. AH is positive.
6. e.g. C + 02 —» C02; AH = - 395.4 kj
6. e.g. N2 + 02-> 2NO; AH = 205 kj