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The word “Thermo” means Heat and “Dynamics” means flow. Thus, thermodynamics implies the flow of heat. But, now this term is used in much broader sense not only with heat rather with all forms of energies like chemical, physical, electrical energy, radiant energy, kinetic and potential energy. The study of this branch of science based upon the three important generalizations namely- First, Second and third law of thermodynamics. Though these laws are not theoretical proven but still nothing contrary has been found until now and nothing contrary will ever be known.

Importance of Thermodynamics

Thermodynamics is finding wide applications in all branches of science like chemistry, physics, as well as engineering. It is of great importance in physical chemistry, some of which are as follow:

  • Helps to determining criteria of feasibility of process.
  • Helps to study extent to which process would take place.
  • Helps to deduce important generalizations (like- Raoult’s Law of vapour pressure lowering, expression for elevation in boiling point, depression in freezing point, Van’t Hoff law of dilute solutions, laws of thermochemistry, distribution law, the phase rule etc.)

Drawbacks of Thermodynamics

Though having wide range of application, thermodynamics has certain limitations such as

  • Applicable to the matter in bulk only (i.e. group of atoms, molecules or ions) and not to individual atoms or molecules.
  • Provides no information about the mechanism of the process (i.e. path of the process) but only concerned to the initial and final stages of the process.
  • Provides no information about the rate at which process would proceed.

Terminology of Thermodynamics

System: Specified portion of the universe selected to study the effect of temperature, pressure etc. (i.e. for thermodynamic consideration) is known as System. It is separated from the surroundings either by some real or imaginary boundaries through which system can exchange matter and energy with the surroundings.

  • Isolated System: When neither energy nor matter exchange with surroundings, then system is Isolated System
  • Closed System: If system only exchange energy with surroundings, but not matter
  • Open System: If system exchanges both energy and matter with surroundings

Surroundings: The part of the Universe except the system is called as Surroundings.

Surrounding & Systems

Homogeneous System:  A system is said to be homogeneous system if it consists of only one phase. For example: mixture of miscible liquids (water + alcohol), mixture of gases (carbon dioxide + oxygen)

Heterogeneous System:  A system is said to be heterogeneous system if it consists of two or more phases. For example: mixture of immiscible liquids (water + oil)

Macroscopic Properties: The properties which are associated with large number of particles are known as macroscopic properties. Such as temperature, pressure, refractive index, viscosity, density, color, etc.

State of System: The state of a system defines a condition of a system when the different macroscopic properties have definite values. If any changes occur in any one of the macroscopic properties, the system is said to be in different state. Thus, the first and last state of the system is known as Initial and final state of the system respectively. There are four common macroscopic properties (temperature, pressure, volume and composition) these are enough to define any state of the system, if these properties are fixed; all other properties get fixed automatically.

State Variables: As on changing the macroscopic properties state of the system changes, therefore the macroscopic properties are called as state variables. It is important to note that Temperature and pressure are generally known as independent variables whereas volume is known as dependent variable.

Thermodynamic Equilibrium:  When there is no change in the macroscopic properties with the passage of time, system is said to be in thermodynamic equilibrium.

  • Mechanical Equilibrium: If there is no mechanical work is done by one part of system on another part of the system, the system is said to be in mechanical equilibrium. This is possible only when the pressure is kept constant throughout the system.
  • Thermal Equilibrium: If there is no flow of heat from one part of system to another part of the system, the system is said to be in thermal equilibrium. This is possible only when the temperature is kept constant throughout the system.
  • Chemical Equilibrium: When there is no change in the composition of the system with the passage of time.
Thermodynamic Processes & Equilibrium

Thermodynamic Processes:  The process by which a system changes from initial state to final state is known as thermodynamic process. In chemical thermodynamics following are the commonly studied processes

  • Isothermal Process: When the temperature remains constant throughout the process.
  • Adiabatic Process: When the no heat exchanges occurs between the system and surroundings throughout the process.
  • Isobaric Process: When the pressure remains constant throughout the process.
  • Isochoric Process: When the volume remains constant throughout the process.

Extensive Properties: Those properties which depend upon the amount of the matter in the present in the system. For example- mass, volume, heat capacity etc.

Intensive Properties: Those properties which depend upon the nature of the matter in the present in the system. For example- pH, temperature, pressure etc.

NOTE- If the amount of the matter (substance) is specified than extensive property becomes intensive property. For example- heat capacity is extensive property but specific heat capacity is intensive property similarly as mass and volume is extensive property but density and specific volume is intensive property.

Reversible and Irreversible Processes:  A reversible process is a process which occurs infinitesimally slowly such the changes occurred in the forward process can be retraced back to the initial state so that the system remains in the state of equilibrium throughout the process at all time. On the other hand, a process which does not occur in the above manner is called an irreversible process.

Amrinder Singh

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