ENTROPIA

(UFPI) A entropia é uma medida:

a) da velocidade de uma reação.

b) da não-utilização da energia total de um sistema na realização de trabalho útil.

c) da energia livre da reação.

d) do "conteúdo calorífico" do sistema.

e) do grau de desorganização de um sistema.

Evidentemente é a opção "e", porém uma das definições termodinâmicas não seria a opção "b" , seria? Agradeço uma justificativa.

Shakitay escreveu:(UFPI) A entropia é uma medida:

a) da velocidade de uma reação.

b) da não-utilização da energia total de um sistema na realização de trabalho útil.

c) da energia livre da reação.

d) do "conteúdo calorífico" do sistema.

e) do grau de desorganização de um sistema.

Evidentemente é a opção "e", porém uma das definições termodinâmicas não seria a opção "b" , seria? Agradeço uma justificativa.

Shakitay, pelo que entendi do TEXTO abaixo não seria "medida da não utilização da energia", mas sim "medida de quanta energia não está disponível" (grifo em VERMELHO):

Entropy and the Second Law of Thermodynamics: Disorder and the Unavailability of Energy

LEARNING OBJECTIVES

[size=13]By the end of this section, you will be able to:
[/size]

• Define entropy.
  
  
• Calculate the increase of entropy in a system with reversible and irreversible processes.
  
  
• Explain the expected fate of the universe in entropic terms.
  
  
• Calculate the increasing disorder of a system.
  
  

There is yet another way of expressing the second law of thermodynamics. This version relates to a concept called entropy. By examining it, we shall see that the directions associated with the second law—heat transfer from hot to cold, for example—are related to the tendency in nature for systems to become disordered and for less energy to be available for use as work. The entropy of a system can in fact be shown to be a measure of its disorder and of the unavailability of energy to do work.

MAKING CONNECTIONS: ENTROPY, ENERGY, AND WORK

Recall that the simple definition of energy is the ability to do work. Entropy is a measure of how much energy is not available to do work. Although all forms of energy are interconvertible, and all can be used to do work, it is not always possible, even in principle, to convert the entire available energy into work. That unavailable energy is of interest in thermodynamics, because the field of thermodynamics arose from efforts to convert heat to work.
We can see how entropy is defined by recalling our discussion of the Carnot engine. We noted that for a Carnot cycle, and hence for any reversible processes,


Rearranging terms yields


for any reversible process. Q_c and Q_h are absolute values of the heat transfer at temperatures T_c and T_h, respectively. This ratio of 

[ltr]��[/ltr]
 is defined to be the change in entropy ΔS for a reversible process, 
[ltr]ΔS=([/ltr]


[ltr])rev�=(��)rev[/ltr]
, where Q is the heat transfer, which is positive for heat transfer into and negative for heat transfer out of, and T is the absolute temperature at which the reversible process takes place. The SI unit for entropy is joules per kelvin (J/K). If temperature changes during the process, then it is usually a good approximation (for small changes in temperature) to take T to be the average temperature, avoiding the need to use integral calculus to find ΔS.
The definition of ΔS is strictly valid only for reversible processes, such as used in a Carnot engine. However, we can find ΔS precisely even for real, irreversible processes. The reason is that the entropy S of a system, like internal energy U, depends only on the state of the system and not how it reached that condition. Entropy is a property of state. Thus the change in entropy ΔS of a system between state 1 and state 2 is the same no matter how the change occurs. We just need to find or imagine a reversible process that takes us from state 1 to state 2 and calculate ΔS for that process. That will be the change in entropy for any process going from state 1 to state 2. (See Figure 2.)


Now let us take a look at the change in entropy of a Carnot engine and its heat reservoirs for one full cycle. The hot reservoir has a loss of entropy 
[ltr]ΔSh=[/ltr]


[ltr]Δ�h=−�h�h[/ltr]
, because heat transfer occurs out of it (remember that when heat transfers out, then Q has a negative sign). The cold reservoir has a gain of entropy
[ltr]ΔSc=[/ltr]


[ltr]Δ�c=�c�c[/ltr]
, because heat transfer occurs into it. (We assume the reservoirs are sufficiently large that their temperatures are constant.) So the total change in entropy is ΔS_tot = ΔS_h + ΔS_c .
Thus, since we know that 

[ltr]=[/ltr]


[ltr]�h�h=�c�c[/ltr]
 for a Carnot engine, 
[ltr]ΔStot=[/ltr]


[ltr]=[/ltr]


[ltr]=0Δ�tot=�h�h=�c�c=0[/ltr]
.
This result, which has general validity, means that the total change in entropy for a system in any reversible process is zero.
The entropy of various parts of the system may change, but the total change is zero. Furthermore, the system does not affect the entropy of its surroundings, since heat transfer between them does not occur. Thus the reversible process changes neither the total entropy of the system nor the entropy of its surroundings. Sometimes this is stated as follows: Reversible processes do not affect the total entropy of the universe. Real processes are not reversible, though, and they do change total entropy. We can, however, use hypothetical reversible processes to determine the value of entropy in real, irreversible processes. Example 1 illustrates this point.

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