According to second law of thermodynamics:
“All spontaneous processes are thermodynamically irreversible”.
Or
“Without the help of an external agency, a spontaneous process cannot be reversed”.
The change of heat into mechanical work is done by Carnot heat engine. In real engines, it is not possible to convert heat energy completely into work because heat is lost in friction, as radiation etc. Hence, Cannot calculated efficiency and reversible work by ideal engine.
In this engine, at standard conditions, source of heat (at T2K) and after work, to store rest of heat,a sink (at T2K) is required. As a source of heat, working substance, ideal gas is taken in such a cylinder whose walls are insulators but bottom is conducting. A frictionless piston is required in insulators. The Carnot cycle when acting as a heat engine consists of the following four steps: two isothermal and two adiabatic processes. Four stages are A, B, C and D, In whole Carnot cycle, at temperature T1, q1 heat is absorbed from the source and w work is done and rest part q2 is given in the sink at temperature T2. The work done by Carnot engine is given by the following formula,
w = q1 – q2 = q1(T2 -T1)/T1
In this way, in a cyclic process, heat absorbed by the source cannot be converted to equivalent amount of work without any loss of heat. The efficiency of Carnot Engine can be calculated by this. “That absorbed heat which can be converted to work is called efficiency of Carnot engine.
n = w/q1 = (T2 -T1)/T1.
(T2 – T1)/T1 < 1
Hence, efficiency of Carnot engine is always less than one. This proves second law of thermodynamics, according to it, “heat can neither be converted to equivalent amount of work”.
The efficiency of Carnot engine explains the entropy concept of second law of thermodynamics.
Free energy (∆G) gives a criteria for spontaneity at constant pressure and temperature,
(i) If ∆G is negative (<0), the process is spontaneous.
(ii) If ∆G is positive (>0), the process is non spontaneous.
