2.5
Entalpia e capacità termica
L'entalpia (H) è una grandezza termodinamica che combina l'energia interna di un sistema con il prodotto della sua pressione e volume. Può essere rapp…
L'entalpia, H, è una funzione di stato che combina l'energia interna di un sistema con pressione e volume.
La variazione dell'entalpia è uguale al calore assorbito o rilasciato dal sistema a pressione costante.
Consideriamo due cilindri, A e B, contenenti un gas ideale ma in condizioni diverse.
Il cilindro A è sigillato, quindi quando viene riscaldato, tutto il calore applicato aumenta l'energia interna del sistema. Il cilindro B, invece, permette l'espansione al riscaldamento e utilizza il calore sia per il lavoro che per aumentare l'energia interna del sistema.
Per il cilindro A che opera a volume costante, l'energia interna aumenta con la temperatura. Per il cilindro B, mantenuto a pressione costante, l'energia fornita è uguale alla variazione dell'entalpia, che aumenta anch'essa con la temperatura.
Un grafico dell'entalpia rispetto alla temperatura mostra che l'entalpia di un sistema aumenta man mano che la temperatura aumenta a pressione costante. La pendenza della tangente a questa curva dà la capacità termica a pressione costante, Cp.
Allo stesso modo, un grafico dell'energia interna rispetto alla temperatura mostra che l'energia interna aumenta a volume costante, e la pendenza della sua tangente definisce la capacità termica a volume costante, Cv.
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Q1: What is enthalpy and how does it relate to internal energy?
Enthalpy (H) is a state function combining a system's internal energy with the product of pressure and volume, expressed as H = U + pV. Since internal energy, pressure, and volume are state functions, enthalpy is also a state function. However, only changes in enthalpy (ΔH) can be measured, not absolute values for specific substances.
Q2: Why does enthalpy change equal heat at constant pressure?
At constant pressure, the change in enthalpy (ΔH) equals the heat (q) absorbed or released by the system. When a system loses heat to surroundings, like burning wood, ΔH is negative and the process is exothermic. When a system gains heat, like a cold pack, ΔH is positive and the process is endothermic.
Q3: How do constant volume and constant pressure conditions affect heat distribution differently?
In a sealed cylinder at constant volume, all applied heat increases internal energy. In an open cylinder at constant pressure, heat is distributed between work done during expansion and increasing internal energy. This difference explains why heat capacity varies between constant volume (Cv) and constant pressure (Cp) conditions.
Q4: What does the slope of an enthalpy-temperature plot represent?
The slope of the tangent to an enthalpy versus temperature plot at constant pressure gives the heat capacity at constant pressure (Cp). Similarly, the slope of an internal energy versus temperature plot at constant volume defines the heat capacity at constant volume (Cv). These slopes quantify how much heat is needed to raise temperature.
Q5: What is the difference between heat capacity and molar heat capacity?
Heat capacity (C) is the amount of heat required to raise a substance's temperature by one degree Celsius and depends on the amount of substance, making it an extensive property. Molar heat capacity is heat capacity divided by moles, making it an intensive property independent of substance amount.
Q6: How does temperature affect a system's enthalpy at constant pressure?
A system's enthalpy increases as its temperature rises at constant pressure. This relationship is linear and can be visualized on an enthalpy-temperature plot, where the positive slope indicates that higher temperatures correspond to higher enthalpy values. The rate of this increase is quantified by heat capacity at constant pressure.
Q7: Why can't we measure absolute enthalpy values for substances?
Absolute enthalpy values cannot be measured because enthalpy depends on internal energy, pressure, and volume—all of which have arbitrary reference points. Only changes in enthalpy (ΔH) between two states can be determined experimentally, making it a practical measure for thermodynamic calculations and applications.