BSC II PHYSICAL CHEMISTRY THERMODYNAMICSII LIMITATION OF FIRST
BSC. -II PHYSICAL CHEMISTRY THERMODYNAMICSII
LIMITATION OF FIRST LAW OF THERMODYNAMICS • It does not give information concerning feasibility of a thermodynamic process. why NOT EXPLAINED BY FIRST
• It tells that work obtained is equal to heat absorbed but in actual heat absorbed can not be completely converted into work. e s ng i t va i t o m s n o i t ta i m Li law c d n o
The second law of thermodynamics It will arouse changes while the heat transfers from low temp. substance to high temp. one. 5
DIFFERENT STATEMENTS OF SECOND LAW 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 EXTERNAL hotter body. AGENCY HEAT TRANSFE R COLD BODY HOT BODY
Sadi Carnot devised a technique, called Carnot Cycle. The Carnot Cycle consists of 4 different operations: 1. 2. 3. 4. Isothermal expansion Adiabetic expansion Isothermal compression Adiabetic compression
Pressure –volume diagram (indicator diagram) A(V 1) ISO TH MA L(T 2 )B(V I II IC D(V 4) ISO TH 2) ADIABET PRESSURE IC ADIABET IV ER ER MA L (T 1) C(V 3) VOLUME State 1 T 2, V 1
CARNOT CYCLE w 1= -RT 2 ln (V 2/V 1) q 2 = - w State 1 T 2, V 1 q=0, W 4=Cv T isothermal adiabatic State 4 T 1 State 2 T 2, V 1 isothermal W 3 = RT 1 ln (V 4/V 3) -q 1 = w 3 q=0, w 2= - CV T State 3 T 1
The net work done by the system (w)=(-w 1)+(-w 2)+w 3+w 4 w=RT 2 ln. V 2/V 1 - CV(T 2 -T 1) + RT 1 ln. V 4/V 3 + CV(T 2 -T 1) w=RT 2 ln. V 2/V 1+RT 1 ln. V 4/V 3 1 2 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 3 (V 4/V 1) =T 2/T 1 4 Comparing both the equations, we get:
V 4/V 3 = V 1/V 2 5 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 6 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 2 lln. V 2/V 1 7
CARNOT CYCLE State 1 Thot q=0, w=CV T w= -n. RThotln(V 2/V 1) q=-w isothermal adiabatic State 4 Tcold State 2 Thot isothermal w= -n. RTcoldln(V 4/V 3) q=-w q=0, w=-CV T State 3 Tcold
It will arouse changes while the heat transfers from low temp. substance to high temp. one. 114
It will arouse changes while the heat transfers from low temp. substance to high temp. one. 121
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