Process and its types

A transition from one equilibrium state to another is called a process. Processes are of different types.

i. Isothermal process : It is the process in which the temperature of the system remains constant throughout the transformation. In such a process heat flows from the system to the surroundings, and vice versa, so as to keep the temperature constant. For a given temperature the internal energy $(U)$ of the system remains constant. Thus, $\Delta T = 0$ and $\Delta U = 0$.

ii. Isobaric process : In an isobaric process the pressure remains constant during the transformation. In the laboratory, chemical reactions are carried out in open containers at constant atmospheric pressure, or $\Delta P = 0$.

iii. Isochoric process : It is a process during which the volume of the system remains constant during the transformation. A chemical reaction carried out in a closed container is isochoric. For an isochoric process, $\Delta V = 0$.

iv. Adiabatic process : A process in which there is no exchange of heat between the system and the surroundings is an adiabatic process ($Q = 0$). In an adiabatic process the system is completely insulated from the surroundings. For an exothermic process the heat released rises the temperature of the system; if the process is endothermic, the temperature falls. This results in either an increase or a decrease of internal energy.

v. Reversible process : Consider a gas enclosed in a cylinder fitted with a movable piston. Let the external pressure $P_{ext}$ on the outer surface of the piston be set equal to the pressure $P$ of the gas. Neither expansion nor compression of the gas occurs. A system is then said to be in mechanical equilibrium with its surroundings.

Consider that $P_{ext}$ is reduced by an infinitesimal amount: now $P_{ext}$ is infinitesimally smaller than $P$, and the piston moves out slowly, allowing the gas to expand. If instead $P_{ext}$ is slightly increased, so that it becomes infinitesimally greater than $P$, the piston moves inward with a compression of the gas. …