IMPORTANT FACTS ABOUT ENTROPY CLASS NOTES
IMPORTANT FACTS ABOUT
ENTROPY CLASS NOTES
Entropy is energy flow from order to disorder.
However, entropy is quantitative measure of
disorder or randomness or chaos in a system.
Thus, a highly ordered system has low entropy.
1.1 Entropy and Disorder
Entropy and disorder is related to equilibrium. Perfect internal disorder is equilibrium and random disorder is non equilibrium. In a closed system, the entropy of the system either remains constant or increases. For an irreversible process, the combined entropy of the system and its environment always increases. In order to decrease entropy, energy must be transferred from somewhere outside the system into the system. The SI units of entropy are J/K (joules/ Kelvin)
1.2 Decrease in Entropy
The entropy decreases with decrease of temperature and also by decrease in volume. Such decrease takes place in a refrigerator where cooling is taking place. However, this decrease in entropy possible at the expense of entropy increase of the surroundings due to the heat addition.
1.3 Increase of Entropy
Increase of entropy is decrease of availability. In other words, increase of entropy is decrease of available useful work.
1.4 Change of Entropy
Rather change in entropy is of importance in any process. In turn, it is expressed as
dS = dQ/T
But in a cyclic process, it is presented by an integral from the starting stage to the final stage.
Where dQ = heat energy transferred reversibly to the system from the surroundings
T = the absolute temperature at which the transfer occurs
Equations for change in entropy
(a) Change in entropy during phase change ds = dq/T
(b) Change in entropy for a unit mass
ds =s_{2}–s_{1} = R ln (v_{2}/v_{1}) + C_{v}ln (T_{2}/T_{1})
ds= C_{p} ln (v_{2}/v_{1}) + C_{v }ln (p_{2}/p_{1})
ds= C_{p} ln (T_{2}/T_{1}) — R ln (p_{2}/p_{1})
(c) Change in entropy for mass m

When a gas is heated under constant volume condition dS = S_{2}–S_{1} = m C_{v}ln (T_{2}/T_{1})

If a gas is heated under constant pressure condition dS = S_{2}–S_{1} =m C_{p} ln (T_{2}/T_{1})

Change of entropy during a reversible adiabatic process will be zero.

Entropy increases with the addition of heat

Entropy decreases with the extraction of heat from a system.
1.5 Change in entropy of a system and its surroundings from the heat of reaction
Reaction at constant pressure and temperature is expressed by the formula. ΔS = ΔH/T
Where ΔS is the change in entropy of the surroundings
ΔH is heat of reaction
T = Absolute Temperature (Kelvin)
During an exothermic reaction, the entropy of the surroundings increases.
If the reaction is endothermic, the entropy of the surroundings decreases.
Entropy features
(a) Difference of entropy is path independent.
(b) Exothermic reactions increase entropy. Exothermic reactions are spontaneous which result in more disorder and hence increase in entropy.
(c) Complexity of molecules show increase of entropy
(s (propane) > s (ethane) > s(methane)), s is entropy
(d) Phase change from solid to liquid and liquid to gas shows increase in entropy
Entropy (steam) > Entropy (water) > Entropy (ice)
(e)A macrostate is more complex than its own microstate. Hence, macro has more entropy than that of its microstate.
(f) In all natural occurring processes, there is increase of entropy since all processes in nature are irreversible.
(g) An equilibrium state has the maximum disorder and hence entropy in equilibrium state is highest.
(h) Entropy increases because of increase in temperature.
(i) Increase in entropy is quite less during increase of temperature but there is abnormal increase in entropy during phase change.
Firstly entropy (ice at 0^{0}C) > entropy (ice at –20^{0}C)
Secondly entropy of water (0^{o}C) >> entropy of ice (0^{0}C)
Thirdly entropy of water (100^{o}C) > entropy of water (0^{0}C)
Fourthly entropy of steam (100^{o}C) >> entropy of water (100^{0}C)
Molar entropy of steam is 189 J/K. mol and that of water is 70 J/K. mol.
(j) Gases combined to form liquid decrease entropy
For example
2H_{2} + O_{2} = 2H_{2}O
3 molecules of gases give two molecules of liquid, therefore entropy will DECREASE.
(k) Entropy CAN BE CREATED but CANNOT be destroyed. Therefore, entropy of a (system+ surroundings) cannot be reduced.
(l) Conversion of mechanical energy to heat energy is increase in disorder (increase of entropy) and vice versa.
(m) It is said that increase of entropy is decrease of useful energy or availability. Because, in every natural process, there exists an inherent tendency of dissipation of useful energy. Thus, increase of entropy is more randomness. In general, increase of entropy is non desirable.
(n) Heat transfer from higher to lower temperature is a natural process and is a irreversible process and there will be increase of entropy in the process.
(o) Entropy is related to microconstituents of matter and hence is complex and causes difficulty in understanding.
(p) Entropy is related to ABSOLUTE TEMPERATURE only.
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