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Chapter: 11th 12th std standard Class Organic Inorganic Physical Chemistry Higher secondary school College Notes

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Second law of thermodynamics

"It is impossible to construct an engine which operated in a complete cycle will absorb heat from a single body and convert it completely to work without leaving some changes in the working system". This is called as the Kelvin - Planck statement of II law of thermodynamics.


SECOND LAW OF THERMODYNAMICS


Second law of thermodynamics can be stated in many ways:

i)     "It is impossible to construct an engine which operated in a complete cycle will absorb heat from a single body and convert it completely to work without leaving some changes in the working system". This is called as the Kelvin - Planck statement of II law of thermodynamics.

ii) "It is impossible to transfer heat from a cold body to a hot body by a machine without doing some work". This is called as the clausius statement of II law of thermodynamics.

iii) 'A process accompanied by increase in entropy tends to be spontaneous". This statement is called as the entropy statement of II law of thermodynamics. Entropy is a measure of randomness or disorder of the molecules of a system and it is a thermodynamic state function. A system always spontaneously changes from ordered to a disordered state. Therefore entropy of a spontaneous process is constantly increasing.

iv) "Efficiency of a machine can never be cent percent".

v) The heat Efficiency of any machine is given by the value of ratio of output to input energies. Output can be in the form of any measurable energy or temperature change while input can be in the form of heat energy or fuel amount which can be converted to heat energy.

 

thus % efficiency = (output/ input) x 100

The machine can be a heat engine also. Consider a heat engine which has an initial temperature T1 and final temperature as T2, then if T1>T2 then when some amount of heat is being converted into work, T2 is the lowered temperature.

The efficiency 'h' is given by,

% efficiency = (T1 -T2 / T1) x 100

According to II law of thermodynamics it is impossible to have a machine or heat engine which converts the input energy completely into output energy or output work without any amount of heat or energy being absorbed by the machine.

 

Hence, % efficiency can never be achieved as cent percent.

% efficiency = 1 -(T2 / T1) x 100

By II law, T2<T1 % efficiency less than 100. 9.3


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