2.10
Funções de Estado e Integrais de Linha
Um processo termodinâmico é um caminho através de uma sequência de estados que leva um sistema de um estado inicial a um estado final. Em um processo…
Um sistema termodinâmico sofre uma mudança de estado quando uma ou mais propriedades que definem seu estado mudam.
Processos que vão do mesmo estado inicial para o mesmo estado final por caminhos diferentes são considerados processos diferentes.
Em um processo cíclico, o sistema retorna ao seu estado inicial, então funções de estado como temperatura, pressão e volume retornam aos valores originais. No entanto, o calor e o trabalho trocados durante o ciclo podem ser diferentes de zero.
Em um processo reversível, o sistema está constantemente próximo ao equilíbrio, e qualquer mudança infinitesimal pode restabelecer tanto o sistema quanto o ambiente em seus estados originais. O trabalho nesse processo é dado por dwrev = -p dV, e o calor dq, quando dividido pela temperatura, corresponde à variação da entropia.
Um processo isotérmico mantém a temperatura constante durante todo o processo, enquanto um processo adiabático não envolve troca de calor, então q e dq são iguais a zero.
Um processo isocórico mantém o volume constante, enquanto um processo isobárico mantém a pressão estável durante todo o processo. Para um processo de pressão constante, verifica-se que ΔH = qp.
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Q1: What happens to state properties during a cyclic thermodynamic process?
In a cyclic process, the system returns to its initial state, so state properties like temperature, pressure, and volume return to their original values. However, heat and work exchanged during the cycle can be nonzero. The net heat and net work over the complete cycle need not be zero, even though all state function changes equal zero.
Q2: How does a reversible process differ from an irreversible process?
A reversible process keeps the system infinitesimally close to equilibrium, allowing any infinitesimal change to restore both system and surroundings to original states. Work in reversible processes follows dwrev = -pdV. Irreversible processes proceed with finite changes and cannot restore the system and surroundings without external work, occurring when friction or finite compositional, pressure, or temperature changes are present.
Q3: What distinguishes an isothermal process from an adiabatic process?
An isothermal process maintains constant temperature by placing the system in a thermal bath at fixed temperature, while other properties like volume may change. An adiabatic process involves no heat exchange between system and surroundings, achieved using adiabatic walls, so q and dq equal zero. Both processes can change internal energy and work, but through different mechanisms.
Q4: Why is work zero in a constant-volume process?
In a constant-volume process, system volume remains fixed using rigid walls, so no pV work occurs. Since work is given by dwrev = -pdV, when dV equals zero, the work term becomes zero regardless of pressure changes. This contrasts with constant-pressure processes, where volume can change and work is performed on or by the system.
Q5: How is enthalpy related to heat in a constant-pressure process?
In a constant-pressure process, the heat absorbed or released equals the change in enthalpy: ΔH = qp. This relationship makes enthalpy particularly useful for analyzing chemical reactions and physical changes occurring at constant pressure, such as processes in open containers at atmospheric pressure.
Q6: What defines different thermodynamic process types?
Common process types are defined by what remains constant: isothermal processes hold temperature T constant, adiabatic processes maintain zero heat transfer (q = 0), isochoric processes keep volume V constant with zero work, and isobaric processes maintain constant pressure p. Each constraint determines how the system's energy and work are distributed during state changes.
Q7: How do different paths affect the relationship between initial and final states?
Processes connecting the same initial and final states via different paths are considered different processes. Although state functions like temperature, pressure, and volume depend only on initial and final states, the heat and work exchanged differ for each path. This distinction is central to understanding state functions and exact differentials in thermodynamics.