Why is stirring a hot solution a demonstration of the 2nd law of thermodynamics?

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In summary, my chemistry teacher recently made a statement that must be one of the most stupid statements ever made. She claims that this is a demonstration of the 2nd law of thermodynamics. Apparently, she doesn't understand entropy or thermodynamics at all and is using a false analogy to try and prove her point. She refuses to admit she was wrong, even after being proven wrong. Crystallization is an exothermic reaction and can't proceed if you add energy, which is why her example is completely false.
  • #1
Indian Fruitloop
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My chemistry teacher recently made something that must be one of the most stupid statements ever made:
She claimed that this was a demonstration of the 2nd law of thermodynamics
"We can usethe analogy of dissolving sugar in coffee as an example of increasing entropy, in this case - disorder. No matter how you stirred, the sugar would not separate from the coffee again because that would be bringing order out of chaos."

Isn't that ridiculous? Crystallisation is an exothermic reaction and could anyone be so dumb as to think that an exothermic reaction could proceed when you add mechanical energy to it.

It just shows hercomplete lack of understanding of thermodynamics in her insistence of using a false 'analogy' involving the spontaneity of an exothermic process depending on the addition of mechanical energy.

But she refuses to admit she was wrong!
 
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  • #2
Your teacher's statement is perfectly correct. Whether you stir or not,crystallization will not take place. That is assuming that the temperature does not change and there is no evaporation. It is a good example of increasing entropy.
 
  • #3
Isn't that ridiculous? Crystallisation is an exothermic reaction and could anyone be so dumb as to think that an exothermic reaction could proceed when you add mechanical energy to it.

It just shows hercomplete lack of understanding of thermodynamics in her insistence of using a false 'analogy' involving the spontaneity of an exothermic process depending on the addition of mechanical energy.

But she refuses to admit she was wrong!

spontaneity really has no direct relation to enthalpy, it's determined by the change in the total entropy of the system. dG=dH-TdS, pertains to a constant temperature, constant pressure process, all the variables refer to the system, but it's derived from when for a spontaneous process

dSsys>dSsurr, dSsys>-dq/T, for constnat pressure

dSsys>dH/T, TdSsys-dH>0, we assign the function G=H+TS

dG=dH+TdS, dG>0 for constant temperature, pressure process

)dStotal>0 for a spontaneous process, dSsys-dSsurr>0)
 
  • #4
WTF?!?

When she added stirring she invalidated the whole example. Because she is trying to make an EXOTHERMIC reaction dependent on adding mechanical energy.

Crystallization is exothermic. Stirring is adding energy. Exothermic reactions can't proceed if you add energy (unless you include activation energy which doesn't count).

She tries to pretend that stirring is adding disorder to the system, that if the two solutions were at the same temperature, cool enough for crystals to form, the stirred solution would not crystallise because it was having disorder added to it.

In her example where she said no matter "how much you stirred" the implication is that this exothermic process depends on the addition of mechanical energy.

Which is just totally ignorant.
 
  • #5
Crystallization is exothermic. Stirring is adding energy. Exothermic reactions can't proceed if you add energy (unless you include activation energy which doesn't count).

So why is it that when you stir a hot solution, the solution cools faster. You're increasing the collisional frequency between molecules, the energy is dissipated by stirring the solution.

I think you're making a lot of assumptions here. Just because a reaction is exothermic doesn't require that the process be spontaneous, in fact, it has no direct bearing on whether a reaction will proceed in many cases. Total change in entropy of the system and surroundings determines spontaneity, and when your teacher was referring to the situation of stirring, she probably was referring to increasing the entropy of the system and perhaps additional chances for crystallization (of course, the chances are incredibly small).
 

1. What is thermodynamics?

Thermodynamics is a branch of physics that studies the relationship between heat, temperature, energy, and work. It explains how these factors influence the behavior of systems, such as gases, liquids, and solids.

2. What are the laws of thermodynamics?

The laws of thermodynamics are fundamental principles that govern all physical processes. They include the zeroth law, which states that when two systems are in thermal equilibrium with a third system, they are also in thermal equilibrium with each other; the first law, which is the law of conservation of energy; the second law, which states that the total entropy of an isolated system always increases over time; and the third law, which states that the entropy of a perfect crystal at absolute zero temperature is zero.

3. How does thermodynamics apply to real-world situations?

Thermodynamics has countless real-world applications, such as in engines, refrigerators, power plants, and chemical reactions. It helps us understand and optimize energy usage and conversion, and it is crucial in fields like engineering, chemistry, and environmental science.

4. What is the difference between heat and temperature?

Heat and temperature are related but distinct concepts. Heat is a form of energy that is transferred from one system to another due to a temperature difference. Temperature, on the other hand, is a measure of the average kinetic energy of the particles in a system. In other words, heat is the transfer of energy, while temperature is a measure of the intensity of that energy.

5. Can thermodynamics be used to predict the future?

No, thermodynamics cannot be used to predict the future. The laws of thermodynamics only describe the behavior of a system in equilibrium or as it changes from one equilibrium state to another. They do not predict the specific outcomes of complex systems, which are influenced by many other factors and variables.

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