April 07, 2012

Thermodynamics - II Five Marks

1.  Describe / Explain / Mention / Write the characteristics of free energy G.
1. G is defined as (H - TS) where H and S are the enthalpy and entropy of the system respectively. T = temperature. Since H and S are state functions, G is a state function.
2. G is an extensive property while ΔG = (G2-G1) which is the free energy change between the initial (1) and final (2) states of the system becomes the intensive property when mass remains constant between initial and final states (or) when the system is a closed system.
3. G has a single value for the thermodynamic state of the system.
4. G and ΔG values correspond to the system only.
There are three cases of ΔG in predicting the nature of the process.
i) When, ΔG < 0 (negative), the process is spontaneous and feasible;
ii) ΔG = 0, the process is in equilibrium and
iii) ΔG > 0 (positive), the process is nonspontaneous and not feasible.
5. ΔG = ΔH – TΔS.
But according to I law of thermodynamics,
ΔH = ΔE + PΔV and ΔE = q – w.
ΔG = q – w + PΔV – TΔS
But ΔS = q / T and TΔS = q = heat involved in the process.
ΔG = q – w + PΔV – q = –w + PΔV
–ΔG = w – PΔV = network.
The decrease in free energy –ΔG, accompanying a process taking place at constant temperature and pressure is equal to the maximum obtainable work from the system other than work of expansion.
This quantity is called as the “net work” of the system and it is equal to (w – PΔV).
Net work = –ΔG = w – PΔV.
–ΔG represents all others forms of work obtainable from the system such as electrical, chemical or surface work etc other than P-V work.
2. Give / State / Write the various statements of second law of thermodynamics.
1. Kelvin – Planck statement of II 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.
2. Clausius statement of II law of thermodynamics
It is impossible to transfer heat from a cold body to a hot body by a machine without doing some work.
3. Entropy statement of II law of thermodynamics
A process accompanied by increase in entropy tends to be spontaneous.
4. Efficiency of a machine can never be cent percent.
5.  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
3. What are the characteristics of entropy?
1. The term ‘S’ entropy is evolved from the formulation of II law of thermodynamics as a thermodynamic state function.
2. Entropy change ‘ΔS’ of a system under a process is defined as the constant equal to the ratio of the heat change accompanying a process at constant temperature to the temperature of the system under process. The process should be reversible at that temperature.
ΔSrev = Δqrev / T(K)
Heat, q is not a state function , But for a reversible process Δq = (q2-q1) divided by temperature (T) of the process is a state function.
3. A spontaneous process is accompanied by increase in the ‘disorder’ (or) ‘randomness’ of the molecules constituting the system. Entropy increases in all spontaneous processes. Hence entropy may be regarded as a measure of disorder (or) randomness of the molecules of the system.
4. When a system undergoes a physical (or) a chemical process, there occurs a change in the entropy of the system and also in its surroundings. This total change in the entropy of the system and its surroundings is termed as the entropy change of the universe brought about by the process.
     For an isothermal process (T = constant), the entropy change of the universe during a reversible process is Zero.
     The entropy of the universe increases in an irreversible process.
5. The energy of the universe remains constant although the entropy of the universe tends to a maximum.
6. For a spontaneous process, at constant T, ΔS is positive (ΔS > 0).
    For an equilibrium process, ΔS is zero.
    For a non spontaneous process, ΔS is negative (ΔS < 0).
7. Units of entropy:
     The dimension of entropy are energy in terms of heat x temperature-1.
     The entropy is expressed as calories per degree which is referred to as the entropy units (eu).
     Since entropy also depends on the quantity of the substance, unit of entropy is calories per degree per mole (or) eu. per mole.
      cgs units of entropy is cal.K-1 denoted as eu.
      The SI unit is JK-1 and denoted EU. 1 eu = 4.184 EU.
8. Entropy change is related to enthalpy change as follows:
For a reversible and isothermal process, ΔS = Δqrev / T. Since ΔH is the heat absorbed (or) evolved in the process at constant T and pressure P. ΔS is also calculated from ΔH as ΔS = ΔH / T where T is the temperature of the process involving ΔH, amount of enthalpy change, at constant pressure.

4. Define Trouton’s rule. What are the substances that deviate from this rule?
The heat of vaporisation (ΔHvap) in calories per mole divided by the boiling point of the liquid in Kelvin is a constant equal to 21 cal deg-1 mole-1 and is known as the entropy of vapourisaiton.
ΔSvap = ΔHvap / Tb = 21 cal deg-1 mole-1
Substances that deviate from this rule are :
1. Low boiling liquids such as Hydrogen and Helium which boil only a little above 0 K.
2. Polar substances like Water, Alcohol which form Hydrogen bonded liquids and exhibit very high boiling points as well as high ΔHvap.
3. Liquids such as Acetic acid whose molecules are partially associated in the vapor phase and possess very low entropy vaporization which is very much less than 21 cals / mol / deg.


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