Engineering Chemistry S Datta

THERMODYNAMICS 69
If, as a result of a series of changes, the system returns to its initial state, its thermodynamic
parameters also return to their original values, such a process is called a cyclic process.
If a change takes place at a constant temperature maintained throughout, it is called an
isothermal process.
If a change takes place in a thermally insulated system which does not permit heat
exchange with the surrounding, the process is called an adiabatic process and the temperature
of the system may increase or decrease.
If a process is carried out at constant pressure, the process is called an isobaric process,
which is accompanied by volume changes.
If a change takes place in which the volume of the system remains constant the process
is called an isochoric process. A process is said to be reversible if the energy change in each step
of the process can be reversed in direction by an infinitesimal change in any of the variables
acting on the system. A process can be made reversible by performing the change very slowly
with no friction and no finite temperature differences.
An irreversible process is such that the system and the surrounding after undergoing
changes cannot get back to their initial state and tend to proceed to a definite direction but
cannot proceed to the reverse direction. All the natural processes are irreversible, example,
expansion of gases from high pressure to low pressure, heat flowing from a hotter to a colder
body, etc. Irreversible processes are all spontaneous processes.
Equilibrium state. A system is said to be in equilibrium when its composition is fixed,
temperature is uniform and also same with the surroundings and there is no unbalanced force
the system and also between the system and the surroundings. Hence, a system in equilibrium
state has definite temperature, pressure and composition.
State function is a thermodynamic property, which depends only on the state of the
system but not on the paths followed for the change. Internal energy change (∆E), enthalpy
change (∆H), free energy change (∆G) and entropy change (∆S) are such functions.
Internal energy. Every system within itself has a quantity of energy which is called the
internal or intrinsic energy. This energy is a function of the temperature, chemical composition,
pressure and volume of the system. The magnitude of internal energy of a system is determined
by the kinetic, rotational, vibrational movements of the molecules of the system. It is an extensive
property. Five moles of a substance in a specified state has five times the internal energy
possessed by a single molecule in a similar state. The absolute value of the internal energy of
a system cannot be ascertained. When a system changes from a thermodynamic state 1 to a
thermodynamic state 2, the change in the internal energy ∆E = E 2
– E 1
is independent of the
path of transformation but depends on the initial and final conditions of the system.
If a system undergoes a series of changes and returns to its initial state i.e., if a cyclic
process is completed, internal energy is found to return to its original value, hence the sum of
the changes of internal energy for a cyclic process i.e., ∑∆E = 0. So internal energy (E) is a state
function. Otherwise, it can be stated that when a system changes from initial state (1) to a
z
final state (2) and comes back to (1) (Fig. 4.1), net change in internal energy is zero i.e., dE = 0
(cyclic integration of E is 0), so dE is path independent and E is a state function.