Lecture - 2 ( Thermodynamics equilibrium and properties of system)

Thermodynamic equilibrium 

A system is said to be in Thermodynamic Equilibrium if it is in phase, thermal, mechanical and chemical equilibrium at same time. If any one of the equilibrium condition disturb then the system cannot comes under Thermodynamic Equilibrium. Let us understand these four equilibrium.

THERMAL EQUILIBRIUM - 

The system is said to be in thermal equilibrium if it has equality of temperature with surrounding. For example a dead body is the perfect example of thermal equilibrium with surrounding.
Normal human body temperature is approximately 37 degree Celsius whereas temperature of surrounding keeps on changing, let us consider surrounding temperature is 25 degree Celsius. Therefore a living human body is a not under thermal equilibrium with surrounding because metabolism of the body is continuously working or producing heat. The metabolism of a dead body shuts down therefore after sometime dead body comes under same temperature with surrounding.

PHASE EQUILIBRIUM - 

When mass of each phase remains constant with time then the system is in phase Equilibrium. For example -Let us consider a closed container that is partially filled with liquid. Now what happened is evaporation takes place inside the container. The air above the liquid can absorb the water vapour molecules upto a certain limit and this limit is known as saturation limit. This closed container is our system and remember I am considering atmospheric pressure and temperature conditions which means pressure and temperature are not changing. When saturation limit occurs then the mass of liquid present in container will remain constant and similarly mass of vapour present in container will also remain constant. At saturation limit condition there is no more tendency of phase change, so at this point our system is in phase equilibrium.

CHEMICAL EQUILIBRIUM - 

Chemical equilibrium is the state when both reactants and products are present in concentrations and have no further tendency to change with time, so that there is no observable change in properties of system. Generally it occurs when rate of forward reaction is equal to rate of backward reaction. In simple language we can say chemical reaction potential should not be present.

Mechanical Equilibrium -

A system is in mechanical equilibrium when net forces acting on that system will be zero. It is basically equality of forces. It can also be defined as, the the pressure acting at any point of the system does not change with time. 

Properties of system 

Any characteristic of the system is called as property of the ssystem.The properties can be classified as Intensive & extensive properties.

Intensive (intrinsic) properties - 

Those properties which are independent of mass of system under consideration. For example - pressure, temperature, density, velocity. 
Remember all specific properties are intensive properties. Example - specific volume, specific entropy, specific enthalpy.

Extensive (extrinsic) properties - 

Those properties which depends on mass of system under consideration are known as extensive properties. Examples - volume, energy, enthalpy, entropy.

Important points with respect to properties 

Property of a system is defined at a particular instant so it is a point function or a state function. Properties does not depend on past history, here past history refers to the path through which a system passes during any process. Properties are exact differential. 
Remember work, heat are not properties of the system because they are not defined for a particular instant, they are not point function they are path function and they depend on the past history, that is the path through which a system passes during any process. Therefore these path functions are inexact Differentials also. 
Don't worry we will discuss about work and heat in detail in my upcoming lectures of thermodynamics.

For video elaboration you guys can check this out Mechanical education hub 

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