Second law of thermodynamics

Second law of thermodynamics

Aug 26, 2022, 16:45 IST

Advantage of Second law of thermodynamics

1. Second law helps us to determine the direction in which energy can be transferred

2. It helps us to predict whether a given process or a chemical reaction can occur spontaneously.

3. It helps us to know the equilibrium conditions of a chemical reaction.

4. It helps us to calculate the maximum fraction of heat that can be converted to work in a given process.

Second law of thermodynamics

It is impossible to use a cyclic process to extract heat from a reservoir and to convert into work without transforming at the same time a certain amount of heat from a hotter to a colder part of the body or It is impossible to convert heat into work without compensation.

Definition of Entropy

If any process is carried out reversibly, so that dqrev is the heat absorbed by the system the process at constant temperature (T), then the entropy change (dS) is given by

In the gaseous state and at low pressures, the molecules are free to move about unrestricted. If the pressure is increased, the same number of molecules now occupy a smaller volume. The chances of finding a molecule within a given volume are greater under these conditions than when it was at lower pressure. Hence, the molecules are more ordered (or less randomly distributed) at higher pressure. Added to this, the force of intermolecular attraction increases as the pressure increases and this too has a restricting effect on the molecules.

In the liquid state, the molecules are much closer together than in the gaseous state, because the volume containing the same number of molecules is much smaller. The molecules in the liquid state are more highly ordered than a gas at high pressure. Also in the liquid state, the forces of intermolecular attraction are greater, causing a still more highly ordered state.

In the solid state, the atoms, molecules or ions that form the crystal are usually held in fixed crystal lattices, which can only vibrate and rotate. Hence in a solid, the atoms, molecules or ions have little or no choice as to where they shall be i.e., they are highly ordered or have a very small freedom.

This gradual decrease in the measure of disorder of the molecules is identified with entropy changes, being high for a gas and low for a solid.

Second law of thermodynamics derivation