BSC. -II PHYSICAL CHEMISTRY THERMODYNAMICS-II

It does not give information concerning feasibility of a thermodynamic process. NOT EXPLAINED BY FIRST LAW

It tells that work obtained is equal to heat absorbed but in actual heat absorbed can not be completely converted into work. Limitations motivating second law

5 The second law of thermodynamics It will arouse changes while the heat transfers from low temp.substance to high temp. one.

All spontaneous processes are thermodynamically irreversible..

The complete conversion of heat into work is impossible without leaving its effects somewhere. SOME AMOUNT OF HEAT MOVES TO SURROUNDING

Without the use of any external agency,heat can not by itself pass from a colder body to a hotter body. HOT BODY COLD BODY HEAT TRANSFER EXTERNAL AGENCY

Sadi Carnot devised a technique, called Carnot Cycle. The Carnot Cycle consists of 4 different operations: 1.Isothermal expansion 2.Adiabetic expansion 3.Isothermal compression 4.Adiabetic compression Sadi Carnot devised a technique, called Carnot Cycle. The Carnot Cycle consists of 4 different operations: 1.Isothermal expansion 2.Adiabetic expansion 3.Isothermal compression 4.Adiabetic compression

ISOTHERMAL(T 2 ) ISOTHERMAL(T 1 ) ADIABETIC I II III IV PRESSURE VOLUME A(V 1 ) B(V 2 ) C(V 3 ) D(V 4 ) State 1 T 2,V 1 State 1 T 2,V 1 Pressure –volume diagram (indicator diagram)

State 1 T 2,V 1 State 2 T 2,V 1 State 4 T 1 State 3 T 1 isothermal adiabatic isothermal q=0, W 4 =C v  T q=0, w 2 = - C V  T w 1 = -RT 2 ln (V2/V1) q 2 = - w W 3 = RT 1 ln (V4/V3) -q 1 = w 3

The net work done by the system (w)=(-w 1 )+(-w 2 )+w 3 +w 4 w=RT2lnV2/V 1 - CV(T2-T1) + RT1lnV4/V3 + CV(T2-T1) w=RT 2 lnV 2 /V 1 +RT 1 ln V 4 /V 3 Since V 1 and V 4 lie on one adiabetic curve and V 3 and V 2 lie on Another, it follows that ( V 4 /V 1 ) =T 2 /T 1 Comparing both the equations, we get:

V 4 /V 3 = V 1 /V 2 Putting these values in equation 1, we get : The net work done by the system,w = R(T 2 -T 1 ) ln V 2 /V 1 The heat absorbed by the system during isothermal expansion is equal to amount of work done during isothermal expansion. Therefore -w 1 = q 2 =RT 2l lnV 2 /V

State 1 T hot State 2 T hot State 4 T cold State 3 T cold isothermal adiabatic isothermal q=0, w=C V  T q=0, w=-C V  T w= -nRT hot ln(V2/V1) q=-w w= -nRT cold ln(V4/V3) q=-w

114 The second law of thermodynamics It will arouse changes while the heat transfers from low temp.substance to high temp. one.

121 The second law of thermodynamics It will arouse changes while the heat transfers from low temp.substance to high temp. one.