NEET Chemistry Thermodynamics / रासायनिक उष्मागतिकी Basic Concept

Basic Terms of Thermodynamics

Thermodynamics

Thermodynamics (thermo means heat and dynamics means motion) is the branch of science which deals with the study of different forms of energy and the quantitative relationships between them.

The complete study of thermodynamics is based upon three generalizations celled first, second and third laws of thermodynamics. These laws have been arrived purely on the basis of human experience and there is no theoretical proof for any of these laws.

(1) System, surroundings and Boundary : A specified part of the universe which is under observation is called the system and the remaining portion of the universe which is not a part of the system is called the surroundings.

The system and the surroundings are separated by real or imaginary boundaries. The boundary also defines the limits of the system. The system and the surroundings can interact across the boundary.

(2) Types of systems

(i) Isolated system : This type of system has no interaction with its surroundings. The boundary is sealed and insulated. Neither matter nor energy can be exchanged with surrounding. A substance contained in an ideal thermos flask is an example of an isolated system.

(ii) Closed system : This type of system can exchange energy in the form of heat, work or radiations but not matter with its surroundings. The boundary between system and surroundings is sealed but not insulated. For example, liquid in contact with vapour in a sealed tube forms a closed system. Another example of closed system is pressure cooker.

(iii) Open system : This type of system can exchange matter as well as energy with its surroundings. The boundary is not sealed and not insulated. Sodium reacting with water in an open beaker is an example of open system.

(iv) Homogeneous system: A system is said to be homogeneous when it is completely uniform throughout. A homogeneous system is made of one phase only. Examples: a pure single solid, liquid or gas, mixture of gases and a true solution.

(v) Heterogeneous system: A system is said to be heterogeneous when it is not uniform throughout, i.e., it consist of two or more phases. Examples : ice in contact with water, two or more immiscible liquids, insoluble solid in contact with a liquid, a liquid in contact with vapour, etc.

(vi) Macroscopic system : A macroscopic system is one in which there are a large number of particles (may be molecules, atoms, ions etc. )

Note : All physical and chemical processes taking place in open in our daily life are open systems because these are continuously exchanging matter and energy with the surroundings.

(3) Macroscopic Properties of the System : Thermodynamics deals with matter in terms of bulk (large number of chemical species) behaviour. The properties of the system which arise from the bulk behaviour of matter are called macroscopic properties. The common examples of macroscopic properties are pressure, volume, temperature, surface tension, viscosity, density, refractive index, etc.

The macroscopic properties can be sub - divided into two types

(i) Intensive properties : The properties which do not depend upon the quantity of matter present in the system or size of the system are called intensive properties.  Pressure, temperature, density, specific heat, surface tension, refractive index, viscosity, melting point, boiling point, volume per mole, concentration etc. are the examples of intensive properties of the system.

(ii) Extensive properties : The properties whose magnitude depends upon the quantity of matter present in the system are called extensive properties. Total mass, volume, internal energy, enthalpy, entropy etc. are the well known examples of extensive properties. These properties are additive in nature.

 Note : Any extensive property if expressed as per mole or per gram becomes an intensive property. For example, mass and volume are extensive properties, but density and specific volume, i.e. the mass per unit volume and volume per unit mass respectively are intensive properties. Similarly, heat capacity is an extensive property but specific heat is an intensive property.

(4) State of a system and State Variables : The state of a system means the condition of existence of the system when its macroscopic properties have definite values. If any of the macroscopic properties of the system changes, the state of the system is also said to change. Thus, the state of the system is fixed by its macroscopic properties.

Macroscopic properties which determine the state of a system are referred to as state variables or state functions or thermodynamic parameters. The change in the state properties depends only upon the initial and final states of the system, but is independent of the manner in which the change has been brought about. In other words, the state properties do not depend upon a path followed.

Following are the state functions that are commonly used to describe the state of the system

(5) Thermodynamic equilibrium : “A system is said to have attained a state of thermodynamic equilibrium when it shows no further tendency to change its property with time”.

The criterion for thermodynamic equilibrium requires that the following three types of equilibrium exist simultaneously in a system.

(i) Chemical Equilibrium : A system in which the composition of the system remains fixed and definite.

(ii) Mechanical Equilibrium : No chemical work is done between different parts of the system or between the system and surrounding. It can be achieved by keeping pressure constant.

(iii) Thermal Equilibrium : Temperature remains constant i.e. no flow of heat between system and surrounding.

(6) Thermodynamic process : When the thermodynamic system changes from one state to another, the operation is called a process. The various types of the processes are

(i) Isothermal process : The process is termed isothermal if temperature remains fixed, i.e., operation is done at constant temperature. This can be achieved by placing the system in a constant temperature bath, i.e., thermostat. For an isothermal process dT = 0, i.e., heat is exchanged with the surroundings and the system is not thermally isolated.

(ii) Adiabatic process : If a process is carried out under such condition that no exchange of heat takes place between the system and surroundings, the process is termed adiabatic. The system is thermally isolated, i.e., dQ = 0. This can be done by keeping the system in an insulated container, i.e., thermos flask. In adiabatic process, the temperature of the system varies.

(iii) Isobaric process: The process is known as isobaric in which the pressure remains constant throughout the change i.e., dP = 0.

(iv) Isochoric process: The process is termed as isochoric in which volume remains constant throughout the change, i.e., dV = 0.

(v) Cyclic process: When a system undergoes a number of different processes and finally return to its initial state, it is termed cyclic process. For a cyclic process dE = 0 and dH = 0.

(vi) Reversible process: A process which occurs infinitesimally slowly, i.e. opposing force is infinitesimally smaller than driving force and when infinitesimal increase in the opposing force can reverse the process, it is said to be reversible process.

(vii) Irreversible process: When the process occurs from initial to final state in single step in finite time and cannot be reversed, it is termed an irreversible process. Amount of entropy increases in irreversible process.

Irreversible processes are spontaneous in nature. All natural processes are irreversible in nature

Difference between reversible and irreversible process