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Why does thermodynamics only apply to macroscopic systems at equilibrium?
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Kurt Franzen
Why does thermodynamics only apply to macroscopic systems at equilibrium?
The mathematics of equilibrium systems are far simpler than others
It isn't strictly true that thermodynamics doesn't apply to systems that are not at equilibrium only that the simple theorems and formulae of equlibrium thermodynamics only apply to systems at equilibrium.
The reason why most discussion and most teaching is about those equilibrium systems is because the mathematical results are far simpler and far easier to teach. They are also widely useful even with systems that are not exactly at equilibrium. Chemists can understand the key features of many reactions, for example, by understanding the equilibrium thermodynamics of the reaction (like the heat of formation of the products). Exceptions can be explained by understanding kinetic barriers in the reaction pathway.
But when details of non-equilibrium behaviours are required, the behaviour of many systems is so complicated there are no easy rules to teach or to talk about.
Non-equilibrium thermodynamics is actually a very large field and key results have won Nobel prizes (Prigogine won the 1977 Nobel in Chemistry for work on key aspects of it). Arguably a great deal of the complexity of the universe and life itself is the result of the complexity and structures that can arise when systems are not at equilibrium. So you could even argue that our ability to understand any thermodynamics is a result of the complexity the can arise from non-equilibrium systems. If the subject is that complex, there are not going to be many simple formulae or generalisations to teach or talk about.
And that is probably the real reason why some people think thermodynamics only applies to systems at equilibrium: anything else is too complicated to talk about so educators and textbook writers tend to avoid the subject.
The mathematics of equilibrium systems are far simpler than others
It isn't strictly true that thermodynamics doesn't apply to systems that are not at equilibrium only that the simple theorems and formulae of equlibrium thermodynamics only apply to systems at equilibrium.
The reason why most discussion and most teaching is about those equilibrium systems is because the mathematical results are far simpler and far easier to teach. They are also widely useful even with systems that are not exactly at equilibrium. Chemists can understand the key features of many reactions, for example, by understanding the equilibrium thermodynamics of the reaction (like the heat of formation of the products). Exceptions can be explained by understanding kinetic barriers in the reaction pathway.
But when details of non-equilibrium behaviours are required, the behaviour of many systems is so complicated there are no easy rules to teach or to talk about.
Non-equilibrium thermodynamics is actually a very large field and key results have won Nobel prizes (Prigogine won the 1977 Nobel in Chemistry for work on key aspects of it). Arguably a great deal of the complexity of the universe and life itself is the result of the complexity and structures that can arise when systems are not at equilibrium. So you could even argue that our ability to understand any thermodynamics is a result of the complexity the can arise from non-equilibrium systems. If the subject is that complex, there are not going to be many simple formulae or generalisations to teach or talk about.
And that is probably the real reason why some people think thermodynamics only applies to systems at equilibrium: anything else is too complicated to talk about so educators and textbook writers tend to avoid the subject.
I'd agree with Li Zhi that this is a very broad question. Let me make a few points and see if they clear up the confusion.
Macroscopic systems - The gist here is that thermodynamics is based on statistics so you need a large enough system (i.e. enough molecules or atoms) for statistics to apply. Thus you can't get the Maxwell-Boltzmann distribution from 3 molecules of gas sealed in some container. You need millions of molecules - a macroscopic system.
initial and final states of system - The foundations of thermodynamics works with the energy difference between some starting point and some end point.
Think of it sort of like rolling a rock downhill. At the top of the hill the rock has some potential energy. Part way down the hill the rock stops and has a lower potential energy. So the difference in potential energy can be calculated by the height difference.
Naively using the rock and hill analogy, thermodynamics says that rocks roll downhill not uphill. Also in order to move the rock uphill you have to input energy into the rock-hill system.
Dynamics - Since thermodynamics is about the energy difference between the initial state and the final state, (basic) thermodynamics doesn't deal with chemical kinetics. But (basic) thermodynamics and chemical kinetics can be combined to become a specialized field of study such as non-equilibrium thermodynamics.
I'd agree with Li Zhi that this is a very broad question. Let me make a few points and see if they clear up the confusion.
Macroscopic systems - The gist here is that thermodynamics is based on statistics so you need a large enough system (i.e. enough molecules or atoms) for statistics to apply. Thus you can't get the Maxwell-Boltzmann distribution from 3 molecules of gas sealed in some container. You need millions of molecules - a macroscopic system.
initial and final states of system - The foundations of thermodynamics works with the energy difference between some starting point and some end point.
Think of it sort of like rolling a rock downhill. At the top of the hill the rock has some potential energy. Part way down the hill the rock stops and has a lower potential energy. So the difference in potential energy can be calculated by the height difference.
Naively using the rock and hill analogy, thermodynamics says that rocks roll downhill not uphill. Also in order to move the rock uphill you have to input energy into the rock-hill system.
Dynamics - Since thermodynamics is about the energy difference between the initial state and the final state, (basic) thermodynamics doesn't deal with chemical kinetics. But (basic) thermodynamics and chemical kinetics can be combined to become a specialized field of study such as non-equilibrium thermodynamics.
Going back to my rock-hill analogy, the difference in potential energy depends only only the height of the rock. The rock is at "equilibrium" only when it is still.More
So thermodynamics works by starting with some "standard state" and then calculating the energy differences from that standard state to some other state. Chemists could do that 100 years ago. It isnt practical to measure thermodynamic properties in an absolute sense since it isnt practical to start with all substances at absolute zero.More
Im oversimplifying this, but consider the rock. If I want to measure the "total energy" of the rock while moving I need to know how fast the rock is moving the slope of the hill, the direction of the rock, the friction between the rock and the hill and so on. So it is a much more complicated problem. But is easy to measure the difference in potential energy of the still rock at two different heights on the hill.More
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So without chemical kinetics just thermodynamics, is it always valid at equilibrium only ?More
The mathematics of equilibrium systems are far simpler than others
It isn't strictly true that thermodynamics doesn't apply to systems that are not at equilibrium only that the simple theorems and formulae of equlibrium thermodynamics only apply to systems at equilibrium.
The reason why most discussion and most teaching is about those equilibrium systems is because the mathematical results are far simpler and far easier to teach. They are also widely useful even with systems that are not exactly at equilibrium. Chemists can understand the key features of many reactions, for example, by understanding the equilibrium thermodynamics of the reaction (like the heat of formation of the products). Exceptions can be explained by understanding kinetic barriers in the reaction pathway.
But when details of non-equilibrium behaviours are required, the behaviour of many systems is so complicated there are no easy rules to teach or to talk about.
Non-equilibrium thermodynamics is actually a very large field and key results have won Nobel prizes (Prigogine won the 1977 Nobel in Chemistry for work on key aspects of it). Arguably a great deal of the complexity of the universe and life itself is the result of the complexity and structures that can arise when systems are not at equilibrium. So you could even argue that our ability to understand any thermodynamics is a result of the complexity the can arise from non-equilibrium systems. If the subject is that complex, there are not going to be many simple formulae or generalisations to teach or talk about.
And that is probably the real reason why some people think thermodynamics only applies to systems at equilibrium: anything else is too complicated to talk about so educators and textbook writers tend to avoid the subject.
The mathematics of equilibrium systems are far simpler than others
It isn't strictly true that thermodynamics doesn't apply to systems that are not at equilibrium only that the simple theorems and formulae of equlibrium thermodynamics only apply to systems at equilibrium.
The reason why most discussion and most teaching is about those equilibrium systems is because the mathematical results are far simpler and far easier to teach. They are also widely useful even with systems that are not exactly at equilibrium. Chemists can understand the key features of many reactions, for example, by understanding the equilibrium thermodynamics of the reaction (like the heat of formation of the products). Exceptions can be explained by understanding kinetic barriers in the reaction pathway.
But when details of non-equilibrium behaviours are required, the behaviour of many systems is so complicated there are no easy rules to teach or to talk about.
Non-equilibrium thermodynamics is actually a very large field and key results have won Nobel prizes (Prigogine won the 1977 Nobel in Chemistry for work on key aspects of it). Arguably a great deal of the complexity of the universe and life itself is the result of the complexity and structures that can arise when systems are not at equilibrium. So you could even argue that our ability to understand any thermodynamics is a result of the complexity the can arise from non-equilibrium systems. If the subject is that complex, there are not going to be many simple formulae or generalisations to teach or talk about.
And that is probably the real reason why some people think thermodynamics only applies to systems at equilibrium: anything else is too complicated to talk about so educators and textbook writers tend to avoid the subject.
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I'd agree with Li Zhi that this is a very broad question. Let me make a few points and see if they clear up the confusion.
Macroscopic systems - The gist here is that thermodynamics is based on statistics so you need a large enough system (i.e. enough molecules or atoms) for statistics to apply. Thus you can't get the Maxwell-Boltzmann distribution from 3 molecules of gas sealed in some container. You need millions of molecules - a macroscopic system.
initial and final states of system - The foundations of thermodynamics works with the energy difference between some starting point and some end point.
Think of it sort of like rolling a rock downhill. At the top of the hill the rock has some potential energy. Part way down the hill the rock stops and has a lower potential energy. So the difference in potential energy can be calculated by the height difference.
Naively using the rock and hill analogy, thermodynamics says that rocks roll downhill not uphill. Also in order to move the rock uphill you have to input energy into the rock-hill system.
Dynamics - Since thermodynamics is about the energy difference between the initial state and the final state, (basic) thermodynamics doesn't deal with chemical kinetics. But (basic) thermodynamics and chemical kinetics can be combined to become a specialized field of study such as non-equilibrium thermodynamics.
I'd agree with Li Zhi that this is a very broad question. Let me make a few points and see if they clear up the confusion.
Macroscopic systems - The gist here is that thermodynamics is based on statistics so you need a large enough system (i.e. enough molecules or atoms) for statistics to apply. Thus you can't get the Maxwell-Boltzmann distribution from 3 molecules of gas sealed in some container. You need millions of molecules - a macroscopic system.
initial and final states of system - The foundations of thermodynamics works with the energy difference between some starting point and some end point.
Think of it sort of like rolling a rock downhill. At the top of the hill the rock has some potential energy. Part way down the hill the rock stops and has a lower potential energy. So the difference in potential energy can be calculated by the height difference.
Naively using the rock and hill analogy, thermodynamics says that rocks roll downhill not uphill. Also in order to move the rock uphill you have to input energy into the rock-hill system.
Dynamics - Since thermodynamics is about the energy difference between the initial state and the final state, (basic) thermodynamics doesn't deal with chemical kinetics. But (basic) thermodynamics and chemical kinetics can be combined to become a specialized field of study such as non-equilibrium thermodynamics.
More
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