13.0 Law of Thermodynamics

  Thermodynamics and Thermochemistry

It is a study of heat change.

(a) Lavoisier and Laplace's Law

This law is based on the first law of thermodynamics.

The heat released in the preparation of a compound from its constituent elements is same as the amount of heat required to decompose that compound into its elements.

For example,

$$C\left( s \right) + {O_2}\left( g \right) \to C{O_2}\left( g \right)\quad \quad \quad \Delta H = - 94.3\,Kcal$$

$$C{O_2}\left( g \right) \to C\left( s \right) + {O_2}\left( g \right)\quad \quad \quad \Delta H = + 94.3\;Kcal$$

(b) Hess's Law of Constant Heat Summation

When a chemical reaction gets completed directly in one step or indirectly in two or more steps, the total energy in the reaction remains same i.e, the change does not depend on the path of a chemical reaction.

$$Q = {q_1} + {q_2} + {q_3}$$

Example: Combustion of carbon

$$C\left( s \right) + {O_2}\left( g \right) \to C{O_2}\left( g \right) + 94\;Kcal\quad \quad \Delta H = - 94\;Kcal$$

Step 1: $$C\left( s \right) + \frac{1}{2}{O_2}\left( g \right) \to CO\left( g \right) + 16.4\;Kcal\quad \quad ....(a)$$

Step 2: $$CO\left( s \right) + \frac{1}{2}{O_2}\left( g \right) \to C{O_2}\left( g \right) + 67.6\;Kcal\quad \quad ...(b)$$

On summing up equation (a) and (b), $\Delta H = - 94\;Kcal$

Application - (a) Determination of heat of formation, the heat of combustion, the heat of transition, the heat of reaction and bond energy.