enthalpy/entropy

dependent variables

• [\(R](gas\ constant) = N_A k = 8.314462618 J/(mol·K)\)
• [\(v](speed)m/s\)
• [\(a](acceleration)= v / T = m/s^2\)
• [\(F](force) = ma = Ma = kg\cdot m / s^2 \equiv N\)
• [\(W] = Fd = N \cdot m = kg\cdot m^2/ s^2 \equiv J\)

Physical

\[W=∫_{Path}​δW \]

\[∫_Γ​δW=∫_Γ​P(V,T)dV \]

​\(δ/‌đ‌\) means path-dependent, which is an "Inexact differential".

Definitions

• H = U + pV
  \(kg\cdot m^2 / s^2 + \frac{kg\cdot m / s^2}{m^2} \cdot m^3 = kg\cdot m^2 / s^2 ≡ J\)
  We care J/mol.

\(ΔH=ΔU+pΔV\)
\(ΔU=Q_P-p\Delta V\)
\(ΔH=Q_P=∑(生成物键能)−∑(反应物键能)\)
\(Q_P\)​‌：恒压热交换（Heat Transfer at Constant Pressure）
\(Exothermic: ΔH<0\)
\(Endothermic: ΔH>0\)

• \(C_p = (\frac{∂H}{∂T})_p = T(\frac{∂S}{∂T})_p\)
  \((\frac{∂S}{∂T})_p = \frac{C_p}{T}\)
  \(dS = (\frac{∂S}{∂T})_pdT+(\frac{∂S}{∂p})_Tdp\)

if we use the dH, it should be a total differentiation but it's a multiple variables function.
\(_p\) means in the same pressure. so \(dp = 0\)

\(dS = (\frac{∂S}{∂T})_pdT = \frac{C_p}{T}dT\)

• \(dS = δQ_{rev} / T\)
  the fundamental definition of entropy change and only holds exactly for reversible paths. Physics seem like to use formula to define the definition.

$dS_{sys}= \frac{δQ_{sys}}{T} = \frac{−dH_{env}}{T} $

This is a statement about reversible processes(because entropy is defined) at constant pressure, where the system and its surroundings are at the same temperature, T.

\(S = ∮\frac{​δQ_{rev}}{T} = 0\)

• \(S = k_B ln \Omega\)
  \(dS_{isolated}​≥0\)
  the Boltzmann constant k is 1.380 649 x 10–23 J/K, Ω = microstates

Microstate count (Ω): The total number of all possible distinct configurations of a system at the microscopic level.
Entropy (S) is essentially a logarithmic measure of the "disorder" or "likelihood" of a system.
Therefore, all spontaneous criteria are unified in the direction in which the system always evolves towards the increase of Ω_total/Ω_isolated (the number of microstates of the system + environment).

so \(dΩ_{isolated} \geq 0, dS_{\text{isolated}} \geq 0\) in the spontaneous process.

• ΔS_isolated = ΔS_total = ΔS_sys + ΔS_env ≥ 0
  \(ΔS_{env} = -ΔH_{sys} / T\) constant pressure
  ΔS_total = ΔS_sys - ΔH_sys / T ≥ 0
  ΔS_sys - ΔH_sys / T ≥ 0
  \(ΔH_{sys} / T - ΔS_{sys} \leq 0\)
  \(Gibbs\ energy = ΔH - TΔS \leq 0\) in the non-isolated system.

Is it thermodynamics actually simple in math?

1. The State Space: A Differentiable Manifold

The foundation is the concept of a state space.

• Mathematical Definition: The set of all possible equilibrium states of a thermodynamic system forms a differentiable manifold, ( \mathcal{M} ).
• Interpretation: A manifold is a space that locally looks like ( \mathbb{R}^n ). Each point ( p \in \mathcal{M} ) represents a unique equilibrium state of the system (e.g., specified by coordinates like pressure ( P ), volume ( V ), and temperature ( T ), which are related by an equation of state).
• Coordinates: We can describe points on this manifold using coordinate functions. For a simple gas, a common coordinate chart is ( (P, V) ), or ( (T, V) ), etc. The number of coordinates needed is the number of degrees of freedom.

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2. State Functions: Differential 0-Forms

• Mathematical Definition: A state function is a smooth map (a scalar field) from the manifold to the real numbers:
  [
  F: \mathcal{M} \to \mathbb{R}
  ]
• Interpretation: It assigns a number (e.g., internal energy ( U ), entropy ( S )) to every state ( p \in \mathcal{M} ), regardless of how the system arrived at that state.
• Differential: The differential of a state function is its exterior derivative, denoted ( dF ). This is a differential 1-form.
  • In coordinates ( (x^1, x^2) ) (e.g., ( (T, V) )), it is expressed as:
    [
    dF = \frac{\partial F}{\partial x^1}dx1 + \frac{\partial F}{\partial x^2}dx2
    ]
  • Key Property (Exactness): The 1-form ( dF ) is called an exact differential. By definition, the exterior derivative of a 0-form is always exact. A fundamental property is that the second exterior derivative vanishes:
    [
    d(dF) = 0
    ]
    This is equivalent to the Clairaut-Schwarz theorem (symmetry of second partial derivatives).

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3. Path-Dependent Quantities: Differential 1-Forms

• Mathematical Definition: Path-dependent quantities like infinitesimal work (( \delta W )) and heat (( \delta Q )) are mathematically represented as differential 1-forms on ( \mathcal{M} ), which are not exact.
• Notation: We use ( \delta ) or ( d ) with a slash (( \not d )) to emphasize that they are not the differential of any state function. Let's denote such a general 1-form as ( \omega ).
  [
  \omega = M_i dx^i
  ]
  (Using Einstein summation notation).
• Interpretation: A 1-form is an object that can be integrated along a path (curve). The result of the integral ( \int_\gamma \omega ) depends on the path ( \gamma ), not just its endpoints. This captures the essence of path-dependence.

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4. The Fundamental Dichotomy: Exact vs. Closed Forms

This is the core of the mathematical rigor.

• Exact Form: A 1-form ( \omega ) is exact if there exists a 0-form (state function) ( F ) such that:
  [
  \omega = dF
  ]

• Closed Form: A 1-form ( \omega ) is closed if its exterior derivative is zero:
  [
  d\omega = 0
  ]
  In coordinates, for ( \omega = M dx + N dy ), this means:
  [
  d\omega = \left( \frac{\partial N}{\partial x} - \frac{\partial M}{\partial y} \right) dx \wedge dy = 0 \quad \Rightarrow \quad \frac{\partial M}{\partial y} = \frac{\partial N}{\partial x}
  ]
  This is the integrability condition.

• The Deep Mathematical Result (Poincaré Lemma):

  • Every exact form is closed. (If ( \omega = dF ), then ( d\omega = d(dF) = 0 )).
  • On a simply connected manifold, every closed form is exact.
  • Conclusion: On the state space of thermodynamics (which is typically contractible and hence simply connected), the concepts of closed and exact are equivalent.

This gives us a powerful test:

A 1-form ( \omega ) is a state function differential (i.e., ( \omega = dF )) if and only if it is closed (( d\omega = 0 )).

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5. Application to Thermodynamics

Let's apply this formalism to the first law.

• The First Law as a Geometry Statement:
  [
  dU = \delta Q - \delta W
  ]
  Here, ( dU ) is an exact 1-form (it's the differential of the state function ( U )). The right-hand side is a sum of two 1-forms that are individually not exact.

• Work 1-Form (( \delta W )):
  For a simple hydrostatic system, ( \delta W = P dV ).
  Let's test if it's closed:
  [
  \omega_W = P dV \quad \text{(Consider coordinates } (T, V) \text{)}
  ]
  [
  d\omega_W = dP \wedge dV = \left( \frac{\partial P}{\partial T} dT + \frac{\partial P}{\partial V} dV \right) \wedge dV = \frac{\partial P}{\partial T} dT \wedge dV
  ]
  Since ( \frac{\partial P}{\partial T} ) is generally not zero (e.g., in an ideal gas, ( P = nRT/V ), so ( \partial P/\partial T = nR/V \neq 0 )), we have ( d\omega_W \neq 0 ). Therefore, ( \omega_W ) is not closed, and hence not exact. Its integral ( W = \int \delta W ) is path-dependent.

• Heat 1-Form (( \delta Q )):
  From the first law, ( \delta Q = dU + \delta W = dU + P dV ).
  Let's test if it's closed:
  [
  d(\delta Q) = d(dU + P dV) = d(dU) + d(P dV) = 0 + d\omega_W \neq 0
  ]
  So ( \delta Q ) is also not closed and not exact. Its integral ( Q = \int \delta Q ) is path-dependent.

• Entropy and the Second Law:
  The second law postulates the existence of an integrating factor ( \frac{1}{T} ) for the heat 1-form ( \delta Q ). Mathematically, this means:
  [
  dS = \frac{1}{T} \delta Q
  ]
  Here, ( \frac{1}{T} ) is the integrating factor that converts the inexact 1-form ( \delta Q ) into an exact 1-form ( dS ), where ( S ) is the state function entropy. This is a profound geometric result: it says that while "heat" is not a property of a state, "heat divided by temperature" is.

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Summary in Mathematical Language

Concept	Thermodynamics	Differential Geometry
System State	Equilibrium State	Point ( p ) on a manifold ( \mathcal{M} )
State Function	( F ) (e.g., ( U, S ))	Differential 0-form ( F: \mathcal{M} \to \mathbb{R} )
Infinitesimal Change	( dF ) (exact differential)	Exact 1-form ( \omega = dF )
Path-Dependent Qty	( \delta W, \delta Q )	Inexact 1-form ( \omega ), where ( d\omega \neq 0 )
Test for State Function	Path integral ( \oint dF = 0 )	Closedness: ( d\omega = 0 ) ( \iff ) ( \omega ) is exact (on simply connected ( \mathcal{M} ))
First Law	( dU = \delta Q - \delta W )	An exact 1-form equals a sum of inexact 1-forms.
Second Law	( dS = \frac{\delta Q_{rev}}{T} )	The 1-form ( \delta Q ) admits an integrating factor ( 1/T ), making it exact.

This geometric perspective reveals thermodynamics as the physics of differential forms on the state space manifold, where the laws of thermodynamics are constraints on which forms are exact and which are not.