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Definition

Enthalpy refers to the thermodynamic potential of a system that combines its internal energy with the product of pressure and volume (H = U + PV), representing the total heat content available for transfer during constant-pressure processes.

Key characteristics

• State function: Depends only on current system state, not the path taken to reach it.
• Extensive property: Scales with system size (e.g., 2 kg water has twice the enthalpy of 1 kg under identical conditions).
• Pressure-dependent: Unlike thermal content, explicitly accounts for pressure-volume work.

Quantification

For simple systems, at constant pressure, enthalpy change (ΔH) with the temperature (T) can be calculated as:

where Cp is heat capacity at constant pressure.

For complex systems involving phase changes:

where:

• m is the mass of the substance
• C_p,1 is the heat capacity before phase change
• ΔH_phase is the enthalpy change during phase transformation (e.g., heat of vaporization)
• C_p,2 is the heat capacity after phase change

Example: For 100g of water heated from 25°C to 125°C at constant pressure:

• Sensible heat (25°C to 100°C): 100g × 4.18 J/g°C × 75°C = 31,350J
• Latent heat (vaporization at 100°C): 100g × 2256 J/g = 225,600J
• Sensible heat (steam 100°C to 125°C): 100g × 2.09 J/g°C × 25°C = 5,225J
• Total enthalpy change = 262,175J or 262.2kJ

Enthalpy is particularly useful for analyzing flow processes, chemical reactions, and heat transfer operations in engineering systems.

References

1. Perplexity A.I. Deepsearch assisted description, 7^th Apr 2025. 

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