Question:
Which of the following is a unit of enthalpy?
Which of the following is a unit of enthalpy?
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To read thermodynamic tables (such as steam tables or refrigerant charts):
- Specific enthalpy (\(h\)) is always expressed in energy per unit mass: \(\text{kJ/kg}\).
- Pressure (\(P\)) is expressed in \(\text{kPa}\) or \(\text{bar}\).
- Specific volume (\(v\)) is expressed in \(\text{m}^3\text{/kg}\).
- Specific enthalpy (\(h\)) is always expressed in energy per unit mass: \(\text{kJ/kg}\).
- Pressure (\(P\)) is expressed in \(\text{kPa}\) or \(\text{bar}\).
- Specific volume (\(v\)) is expressed in \(\text{m}^3\text{/kg}\).
Updated On: Jul 23, 2026
- kJ/kg
- kcal
- kWkgf
- kPa
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The Correct Option is A
Solution and Explanation
Step 1: Understanding the Concept:
Enthalpy (\(H\)) is a thermodynamic property defined as the sum of the internal energy (\(U\)) of a system and the product of its pressure (\(P\)) and volume (\(V\)):
\[ H = U + PV \]
Specific enthalpy (\(h\)) is the enthalpy per unit mass of a substance:
\[ h = \frac{H}{m} \]
Step 2: Detailed Explanation:
Let us analyze the units of each option to identify the correct unit for enthalpy:
- (A) kJ/kg (Kilojoules per kilogram):
Kilojoule (\(\text{kJ}\)) is a metric unit of energy, and kilogram (\(\text{kg}\)) is the unit of mass.
Therefore, \(\text{kJ/kg}\) is the standard SI unit for specific enthalpy, representing the energy content per unit mass of a working fluid (such as steam or refrigerant).
- (B) kcal (Kilocalorie):
Kcal is a unit of total heat energy, but it lacks the mass dimension, so it is not used as a unit for specific enthalpy.
- (C) kWkgf (Kilowatt-kilogram-force):
This is an irregular, non-standard unit combination that does not represent enthalpy.
- (D) kPa (Kilopascal):
Kilopascal is a unit of pressure (\(\text{kN/m}^2\)), not energy.
Therefore, \(\text{kJ/kg}\) is the correct unit for specific enthalpy.
Step 3: Final Answer:
The standard unit for specific enthalpy is kJ/kg.
Enthalpy (\(H\)) is a thermodynamic property defined as the sum of the internal energy (\(U\)) of a system and the product of its pressure (\(P\)) and volume (\(V\)):
\[ H = U + PV \]
Specific enthalpy (\(h\)) is the enthalpy per unit mass of a substance:
\[ h = \frac{H}{m} \]
Step 2: Detailed Explanation:
Let us analyze the units of each option to identify the correct unit for enthalpy:
- (A) kJ/kg (Kilojoules per kilogram):
Kilojoule (\(\text{kJ}\)) is a metric unit of energy, and kilogram (\(\text{kg}\)) is the unit of mass.
Therefore, \(\text{kJ/kg}\) is the standard SI unit for specific enthalpy, representing the energy content per unit mass of a working fluid (such as steam or refrigerant).
- (B) kcal (Kilocalorie):
Kcal is a unit of total heat energy, but it lacks the mass dimension, so it is not used as a unit for specific enthalpy.
- (C) kWkgf (Kilowatt-kilogram-force):
This is an irregular, non-standard unit combination that does not represent enthalpy.
- (D) kPa (Kilopascal):
Kilopascal is a unit of pressure (\(\text{kN/m}^2\)), not energy.
Therefore, \(\text{kJ/kg}\) is the correct unit for specific enthalpy.
Step 3: Final Answer:
The standard unit for specific enthalpy is kJ/kg.
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