Enthalpy
Enthalpy is a thermodynamic state function defined as the sum of the internal energy of a system and the product of its pressure and volume, representing the total heat content of a system at constant pressure. At constant pressure, the change in enthalpy equals the heat exchanged between the system and its surroundings, making it the central quantity in calorimetry, chemical reactions, and engineering heat-exchange calculations. Positive ΔH indicates an endothermic process (heat absorbed), while negative ΔH indicates an exothermic process (heat released).
Key Formula
H = U + pV
LaTeX: H = U + pV
| Symbol | Meaning | Unit |
|---|---|---|
| H | Enthalpy of the system | J |
| U | Internal energy of the system | J |
| p | Pressure of the system | Pa |
| V | Volume of the system | m³ |
Worked Example
Problem
The combustion of methane at constant pressure releases 890.3 kJ/mol. 2 moles of methane are burned. Calculate ΔH for the reaction.
Solution
Step 1: Standard enthalpy of combustion of CH₄: ΔH_c° = −890.3 kJ/mol (negative = exothermic). Step 2: For 2 moles: ΔH = 2 × (−890.3 kJ/mol) = −1780.6 kJ. Step 3: The system releases 1780.6 kJ to the surroundings.
Answer
ΔH = −1780.6 kJ (exothermic; heat is released to surroundings)
Standard Enthalpies of Combustion of Common Fuels
| Fuel | Formula | ΔH_c° (kJ/mol) | Phase | Application |
|---|---|---|---|---|
| Hydrogen | H₂ | −285.8 | Gas | Fuel cells, rockets |
| Methane | CH₄ | −890.3 | Gas | Natural gas, heating |
| Ethanol | C₂H₅OH | −1366.8 | Liquid | Biofuel, beverages |
| Glucose | C₆H₁₂O₆ | −2803 | Solid | Cellular respiration |
| Octane | C₈H₁₈ | −5471 | Liquid | Petrol engines |
| Carbon | C | −393.5 | Solid | Coal combustion |
Interactive Tools
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Related Terms
First Law of Thermodynamics
The First Law of Thermodynamics states that energy cannot be created or destroyed, only converted from one form to another, making it a statement of conservation of energy applied to thermodynamic systems. For any process, the change in internal energy of a system equals the heat added to the system minus the work done by the system on its surroundings. This principle underpins the analysis of engines, refrigerators, and all energy-conversion devices in engineering and science.
Entropy
Entropy is a thermodynamic state function that quantifies the degree of disorder, randomness, or the number of microstates available to a system at a given macrostate. Macroscopically, it is defined via the Clausius inequality as the ratio of reversible heat exchange to absolute temperature; microscopically, Boltzmann's formula connects it to the number of microscopic configurations. Entropy always increases in irreversible processes in isolated systems, driving systems toward equilibrium and explaining the thermodynamic arrow of time.
Heat Engine
A heat engine is a device that converts thermal energy into mechanical work by exploiting the temperature difference between a high-temperature heat source (hot reservoir) and a low-temperature heat sink (cold reservoir). The engine absorbs heat Q_H from the hot reservoir, converts part of it to useful work W, and rejects the remainder Q_C to the cold reservoir, operating in a cyclic process. The thermal efficiency of a heat engine is always less than 100% due to the Second Law of Thermodynamics, and the maximum theoretical efficiency is set by the Carnot efficiency.
From Greek "enthalpein" meaning "to heat within," from "en" (in) + "thalpein" (to heat). The term was introduced by Heike Kamerlingh Onnes around 1909, though the concept was developed earlier by J. Willard Gibbs. The symbol H honours Hess, though historically the exact origin is debated.