Thermodynamic arrow of time. Equating Entropy and Disorder?

[swansont]

It's also questionable to claim that enthalpy is applicable here, and how much classical thermodynamics would tell you.

[imatfaal]

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Michel

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[studiot]

puppypower you may like to look at Mollier diagrams.

 

They plot the relationship between enthalpy and entropy.

 

https://www.google.co.uk/search?hl=en-GB&source=hp&biw=&bih=&q=mollier+diagram&gbv=2&oq=mollier&gs_l=heirloom-hp.1.0.0j0i10j0j0i10l3j0j0i10l2j0.1593.4156.0.6875.7.7.0.0.0.0.219.985.0j6j1.7.0....0...1ac.1.34.heirloom-hp..0.7.985.aV-4v27-bBE

 

 

 

Swansont

It's also questionable to claim that enthalpy is applicable here, and how much classical thermodynamics would tell you.

 

It is not often explained, but the relationship between entropy and the 'arrow of time' is classical and arises thus.

 

 

Most equations (note I said equations, there are a few laws but lots of equations) in thermodynamics do not contain time.

Time is not a (thermodynamic) state variable.

This is also true of the second law that leads to the arrow statement.

 

However thermodynamics is about (thermodynamic) processes and processes are loosely about changes of (thermodynamic) state.

 

Underlying the arrow of time is the assumption that a system cannot be in two (thermodynamic) states at once.

So a variable is required that can be used as the independent variable in charting progress of a process.

This variable is chosen to be time.

 

So in changing from one defined state to another the second law suggests that entropy is a state variable that can never decrease in value at the end of any such change of state.

 

A (thermodynamic) state is defined as complete set of values of all (thermodynamic) variables, so whatever happens the other variables must adjust to meet this requirement on entropy.

 

So the only permitted changes are those which produce a compatible set of values of state variables for the second state in the process.

 

This is colloquially known as the arrow of time.

 

When you break it down there are several indirect steps to lead to the result.

[swansont]

It is not often explained, but the relationship between entropy and the 'arrow of time' is classical and arises thus.

 

What's the enthalpy (or change in enthalpy) of the cup? That's what I was referring to.