Entropy generation in chemical reactions

In summary, the conversation discusses the first and second laws of thermodynamics and their relation to internal energy, heat, and work in a closed system. The first law states that the change in internal energy is equal to the heat added to the system minus the work done by the system. The second law introduces the concept of entropy and states that the change in entropy is equal to the heat added to the system divided by the temperature plus the entropy generated within the system. By combining these laws and assuming constant entropy and volume, it can be shown that a decrease in internal energy (dU<0) is possible with a positive entropy generation (Sgen>0). This can be seen in the example of mixing salt and ice, where the ice
  • #1
Urmi Roy
753
1
So from the first law for a closed system,

dU=dQ-dW=dQ-PdV

From the second law,

dS=dQ/T + Sgenerated (i.e. the entropy generated)

Putting expression of dQ from second law into first law,
dU=T*dS-T*Sgen-PdV

If s and v are constant,
dU= -T*Sgen>0

Hence dU<0
This is a derivation that was given in class

My questions are as follows:
1. When we say s is constant, does it mean there is not heat flow into/out of the system?

2.If dU<0, does the temperature decrease? I find it hard to understand how there can be a positive Sgen and decrease in temperature/ internal energy simultaneously!
 
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  • #2
Urmi Roy said:
My questions are as follows:
1. When we say s is constant, does it mean there is not heat flow into/out of the system?

2.If dU<0, does the temperature decrease? I find it hard to understand how there can be a positive Sgen and decrease in temperature/ internal energy simultaneously!

1. No, you can imagine that dQ=Sgen/T
2. What happens if you mix salt and ice in a thermo jar?
 
  • #3
About your Q2...not too sure, but I know that having a salt in water lowers the freezing point of water...So the ice melts back?
 
  • #4
Yes, and gets cooler!
 
  • #5


1. When we say s is constant, it means that the entropy of the system remains constant. This does not necessarily mean that there is no heat flow into or out of the system. It could mean that the heat flow into the system is equal to the heat flow out, resulting in no change in entropy.

2. No, a decrease in internal energy does not necessarily mean a decrease in temperature. The change in internal energy, dU, is a function of both heat flow (dQ) and work done (dW). In the case of a chemical reaction, the decrease in internal energy may be due to the release of energy in the form of heat, but this does not necessarily mean a decrease in temperature. Temperature is a measure of the average kinetic energy of the particles in a system, and it can change due to a variety of factors, not just changes in internal energy.
 

1. What is entropy generation in chemical reactions?

Entropy generation is the measure of the disorder or randomness in a system. In chemical reactions, it refers to the increase in disorder or randomness that occurs during the reaction.

2. How is entropy generation related to the second law of thermodynamics?

The second law of thermodynamics states that the total entropy of a closed system will always increase over time. In chemical reactions, the increase in entropy generation represents the increase in disorder and chaos, which aligns with the second law of thermodynamics.

3. What factors affect the amount of entropy generation in a chemical reaction?

The amount of entropy generation in a chemical reaction is affected by factors such as temperature, pressure, and the number and complexity of molecules involved. Generally, higher temperatures and pressures lead to higher entropy generation, and more complex reactions tend to have higher entropy generation as well.

4. Can entropy generation be reversed or reduced in a chemical reaction?

Entropy generation in a chemical reaction cannot be reversed, as it is a natural result of the reaction. It can, however, be reduced by optimizing reaction conditions, such as temperature and pressure, to minimize the disorder and randomness that occurs.

5. How is entropy generation calculated in chemical reactions?

Entropy generation in chemical reactions can be calculated using the change in entropy of the system before and after the reaction, as well as the temperature at which the reaction occurs. This can be expressed mathematically as ΔS = ΔH/T, where ΔS is the change in entropy, ΔH is the change in enthalpy, and T is the temperature in Kelvin.

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