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Relações Termodinâmicas de Maxwell
As relações termodinâmicas de Maxwell são muito úteis na resolução de problemas em termodinâmica. Cada uma das relações de Maxwell relaciona uma deriv…
Considere a forma diferencial dos potenciais termodinâmicos; As equações de Maxwell podem ser derivadas de potenciais termodinâmicos.
A energia interna é uma função da entropia e do volume. Por ser um diferencial exato, sua derivada de segunda ordem em relação ao volume e à entropia não depende da ordem de diferenciação.
Substituindo as expressões de temperatura e pressão obtém-se a primeira relação de Maxwell.
A entalpia depende da entropia e da pressão. Tomando a derivada de segunda ordem e substituindo as expressões para temperatura e volume, obtém-se a segunda relação de Maxwell.
A energia livre de Helmholtz depende do volume e da temperatura. Expressando-o como uma derivada de segunda ordem e substituindo as expressões de pressão e entropia, obtém-se a relação de Maxwell da terceira.
A energia livre de Gibbs é uma função da pressão e da temperatura. Diferenciando-o duas vezes e substituindo as relações de volume e entropia, obtém-se a quarta relação de Maxwell.
As equações de Maxwell são usadas para resolver problemas termodinâmicos complexos, pois relacionam um diferencial parcial entre quantidades que são difíceis de medir experimentalmente com um diferencial parcial entre quantidades facilmente mensuráveis.
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Q1: How are Maxwell's relations derived from thermodynamic potentials?
Maxwell's relations are derived from the symmetry of second-order derivatives of thermodynamic potentials. Since all thermodynamic potentials are exact differentials, their second derivatives do not depend on the order of differentiation. By expressing each potential in terms of its natural variables and taking partial derivatives, then substituting the definitions of thermodynamic quantities like temperature and pressure, the four Maxwell relations emerge mathematically.
Q2: What are the four Maxwell relations and their natural variables?
The four Maxwell relations correspond to different thermodynamic potentials: internal energy depends on entropy and volume; enthalpy depends on entropy and pressure; Helmholtz free energy depends on volume and temperature; and Gibbs free energy depends on pressure and temperature. Each relation connects partial derivatives of these variables, enabling conversion between difficult and easy-to-measure quantities in thermodynamic calculations.
Q3: Why do Maxwell's relations convert hard-to-measure quantities to easily measurable ones?
Maxwell's relations relate partial derivatives of quantities difficult to measure experimentally, such as entropy changes, to partial derivatives of easily measurable quantities like temperature, volume, and pressure. For example, entropy change with volume at constant temperature is complex to measure directly, but Maxwell's relations connect it to pressure change with temperature at constant volume, which is straightforward to measure in a laboratory.
Q4: What role does the exactness of differentials play in Maxwell's relations?
All thermodynamic potentials are exact differentials, meaning their second-order derivatives are path-independent and do not depend on the order of differentiation. This mathematical property is fundamental to deriving Maxwell's relations. When you differentiate a thermodynamic potential twice with respect to different variables, the mixed partial derivatives are equal, yielding the symmetry relationships that define Maxwell's equations.
Q5: How do you apply Maxwell's relations to solve complex thermodynamic problems?
Maxwell's relations simplify complex thermodynamic problems by substituting difficult-to-measure partial derivatives with easily measurable ones. When solving problems, identify which Maxwell relation connects your unknown quantity to measurable variables. Then use the relation to rewrite the problem in terms of temperature, pressure, or volume changes that can be determined experimentally or calculated directly.
Q6: Which thermodynamic potential corresponds to each Maxwell relation?
Internal energy, the first potential, yields the first Maxwell relation. Enthalpy, the second potential, produces the second relation. Helmholtz free energy, the third potential, generates the third relation. Gibbs free energy, the fourth potential, produces the fourth relation. Each potential's natural variables determine which thermodynamic quantities appear in its corresponding Maxwell relation.
Q7: What is the significance of partial derivatives in Maxwell's thermodynamic relations?
Partial derivatives in Maxwell's relations represent how one thermodynamic variable changes with respect to another while holding a third variable constant. These derivatives are central because they describe measurable physical properties. Maxwell's relations establish mathematical equivalences between different partial derivatives, allowing experimentalists to determine entropy changes indirectly by measuring more accessible quantities like pressure and temperature variations.