Lecture 3 Evaluation of Entropy

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Lecture 3 Evaluation of Entropy

If we wish to designate S by a proper name we can say of it that it is the transformation content of the body, in the same way that we say of the quantity U that it is the heat and work content of the body. However, since I think it is better to take the names of such quantities as these, which are important for science, from the ancient languages, so that they can be introduced without change into all the modern languages, I proposed to name the magnitude S the entropy of the body, from the Greek word η τροπη, a transformation. I have intentionally formed the word entropy so as to be as similar as possible to the word energy, since both these quantities, which are to be known by these names, are so nearly related to each other in their physical significance that a certain similarity in their names seemed to me advantageous. Rudolf Clausius 1865, Annalen der Physik und Chemie, 125, 353- (1865). Earlier paper: Annalen der Physik und Chemie, 79, 368-97, 500-24 (1850)

What controls the direction of spontaneous change? According to first law, energy is conserved in the process. Therefore, energy of an isolated system is not reduced (or changed) during a process. It can be considered that energy of the system under consideration alone is undergoing change. However, there are also processes such as the expansion of ideal gas into vacuum, where energy is constant. While the energy is constant, the process may redistribute it in several packets. It can be seen that in all processes energy is partitioned and distributed into several smaller quantities. In second law, we introduce a new state function called Entropy which characterises spontaneous changes. The redistribution may be understood in terms of entropy.

In the case of surroundings, the temperature does not change during heat transfer. The internal energy changes during the energy transfer, ds sur = dq sur,rev /T sur = dq sur /T sur In adiabatic change, dq sur = 0 and S sur = 0

This breaking of processes into several sub processes means that we can write, S = q rev /T The summation extends over all the sub-processes. Thus, it is possible for entropy to reduce in some processes and increase in some others so as to increase the overall entropy. Thus, partitioning of energy into various units while keeping the total energy constant is the key to spontaneity.

Evaluation of entropy du = dq dw = dq rev dw rev dw rev = PdV TdS = dq rev du = TdS PdV For an ideal gas, du = nc v dt P = nrt/v nc v dt = TdS (nrt/v) dv ds = nc v dt/t + nr dv/v

If Cv is constant for the temperature range, 2 1 ds = n C v 2 1 dt/t + nr 2 1 dv/v S = n C v ln T 2 /T 1 + nr ln V 2 /V 1 In terms of P and T, V 2 = RT 2 /P 2 ; V 1 = RT 1 /P 1 V 2 /V 1 = RT 2 /P 2 P 1 /RT 1 = T 2 P 1 /T 1 P 2 S = n C v ln T 2 /T 1 + nr ln T 2 /T 1 nrlnp 2 /P 1 Since V 2 /V 1 = T 2 P 1 /P 2 T 1 = n (C v +R) ln T 2 /T 1 nrlnp 2 /P 1 = n C p ln T 2 /T 1 nrlnp 2 /P 1 Enthalpy has units cal/deg which is referred as the enthalpy unit.

For an isothermal process, S T = nr ln V 2 /V 1 = -nr ln P 2 /P 1 For isochoric process, S V = nc v ln T 2 /T 1 For isobaric process S p = nc p ln T 2 /T 1

A relation can be obtained in terms of P and V also. S = nc v ln T 2 /T 1 + nr ln V 2 /V 1 T 2 /T 1 = P 2 V 2 /P 1 V 1 = nc v ln P 2 /P 1 + nc v ln V 2 /V 1 + nr ln V 2 /V 1 = nc v ln P 2 /P 1 + nc p ln V 2 /V 1

Entropy change for a phase change TdS = du + PdV A constant pressure, TdS = d (U + PV) = dh ds = dh/t S = H/T Depending on the process, H = L, L, etc. v f

Variation of enthalpy with temperature ds = dq rev /T = dh/t = C p dt/t constant pressure ds = dq rev /T = du/t = C v dt/t constant volume These equations can be integrated to get variation in entropy. If heat capacities are expressed as a function of temperature, S can be evaluated accurately.

Variation with temperature Example Let us say a solid is heated to melt and the liquid is made to boil. S(T) = S(0) + T 0 (C p (s)dt/t) + ( fus H/T f ) + Tb Tf (C p (l)dt/t) + ( eva H/T b ) + T Tb C p (g)dt/t

C p /T or C v /T S = 2 1C/T dt T

Entropy changes with specific processes Heat change associated with a process such as phase transition is q = H, Therefore S = H/T. When H is positive such as melting, S is positive. Standard molar entropy of vaporization of liquids is a constant of value, 85 J/K/mol. This is called the Trouton s rule. With this rule, enthalpy of vaporization of liquids at their normal boiling points can be calculated. When liquids are associated, the entropies are generally larger.

Entropies of phase transitions (Table) Entropies of phase transitions at 1 atm ( S m /JK -1 mol -1 ) System Fusion Evaporation He 6.3 19.7 Ar 14.2 71.7 H 2 8.4 44.4 N 2 11.42 73.16 H 2 O 22.0 109.0 NH 3 28.9 97.21 CH 3 OH 18.1 104 CH 3 COOH 40.4 61.9

Entropy temperature diagram q 2 T 2 A B T dq = 0 dq = 0 T 1 D q 1 C S 1 S 2

Heat absorbed q 2 = T 2 (S 2 S 1 ) Heat evolved during isothermal compression q 1 = T 1 (S 1 S 2 ) = T 1 (S 2 S 1 ) This value of energy is the energy unavailable for work. Heat absorbed = work done = q 2 q 1 = (T 2 T 1 ) (S 2 S 1 ) This product is the area ABCD

The heat energy transferred cannot be completely used for work. Energy unavailable for work is proportional to S. Maximum work W max = Efficiency x heat absorbed = ((T 2 T 1 )/ T 2 ) x q 2 = q 2 (1 T 1 /T 2 ) = q 2 T 1 q 2 /T 2 =q 2 T 1 S

Entropy change in irreversible processes q irr T 2 T 1 A B Isothermal expansion Adiabatic Expansion Isothermal Compression, T 1 T 2 T 1

Three reversible steps, which are shown in the picture. 1. Isothermal expansion at T 2 till heat q is absorbed (at A). 2. Additional expansion till temperature reaches T 1 3. Isothermal compression till q is transferred to B. At the reservoirs, At A, heat lost = -q at T 2 At B, heat gained = q at T 1

Total entropy change S net = q/t 1 q/t 2 Since T 2 >T 1, S net > 0 Entropy change in a reversible process is zero. S system = Q rev /T S surrounding = -Q rev /T S total = Q uni = Q rev /T Q rev /T = 0 nr ln V 2 /V 1 nrlnv 2 /V 1 = 0 Entropy of the final state is higher for irreversible processes. Irreversible processes are spontaneous. For spontaneous process S > 0

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