Enthalpy (H) is a thermodynamic property of a system equal to the sum of its internal energy and the product of its pressure and volume: H = U + PV. In simple terms, enthalpy represents the total heat content of a system at constant pressure, making it the natural measure of heat exchanged in most real-world processes such as combustion, phase changes, and chemical reactions carried out in open vessels.
What is enthalpy in thermodynamics?
Enthalpy is a state function, meaning its value depends only on the current state of the system (pressure, temperature, composition) and not on the path taken to reach that state. Because most laboratory and industrial processes occur at constant atmospheric pressure rather than constant volume, enthalpy is more convenient than internal energy for tracking heat flow.
At constant pressure, the change in enthalpy of a system equals the heat added to or removed from it:
- ΔH = qp (heat exchanged at constant pressure)
- ΔH > 0 → endothermic process (system absorbs heat)
- ΔH < 0 → exothermic process (system releases heat)
What is the enthalpy formula and what do the terms mean?
The defining equation is:
H = U + PV
For a change between two states at constant pressure:
ΔH = ΔU + PΔV
Where each term carries a specific physical meaning:
- H — enthalpy, the total heat content of the system.
- U — internal energy, the sum of kinetic and potential energies of the molecules.
- P — absolute pressure of the system.
- V — volume occupied by the system.
- PΔV — the boundary (expansion) work done by or on the system.
This relationship follows directly from the first law of thermodynamics, ΔU = q − W, where W is the work done by the system. At constant pressure, W = PΔV, so qp = ΔU + PΔV = ΔH.
What are the units of enthalpy?
The table below summarises the common forms of enthalpy and their SI units used in engineering thermodynamics.
| Quantity | Symbol | SI Unit | Notes |
|---|---|---|---|
| Enthalpy (total) | H | joule (J) | Extensive property; scales with mass |
| Specific enthalpy | h | J/kg (kJ/kg) | Enthalpy per unit mass; used in steam tables |
| Molar enthalpy | Hm | J/mol (kJ/mol) | Enthalpy per mole; used in reaction chemistry |
| Enthalpy change | ΔH | J or kJ | Heat exchanged at constant pressure |
| Standard enthalpy of reaction | ΔH° | kJ/mol | Measured at 298.15 K and 1 bar |
What are the main types of enthalpy change?
Engineering and chemistry courses commonly distinguish several specific enthalpy changes, each defined for a particular process:
- Enthalpy of formation (ΔHf) — heat change when one mole of a compound forms from its elements in their standard states.
- Enthalpy of combustion (ΔHc) — heat released when one mole of a substance burns completely in oxygen.
- Enthalpy of fusion (ΔHfus) — heat absorbed to melt one mole of a solid at its melting point.
- Enthalpy of vaporisation (ΔHvap) — heat absorbed to convert one mole of liquid to vapour at its boiling point.
- Enthalpy of neutralisation — heat released when an acid and base react to form one mole of water.
Why is enthalpy significant in thermodynamics?
Enthalpy is central to thermodynamic analysis for several practical reasons:
- Constant-pressure processes: Most heating, cooling, and reaction processes happen in open systems at atmospheric pressure, where ΔH directly gives the heat transferred.
- Hess’s Law: Because enthalpy is a state function, the total enthalpy change of a reaction is independent of the route, allowing unknown reaction enthalpies to be calculated from known ones.
- Energy balances: Enthalpy is the working variable in the steady-flow energy equation for turbines, compressors, boilers, condensers, and heat exchangers.
- Phase and steam analysis: Specific enthalpy values from steam tables and Mollier (h–s) charts drive power-plant and refrigeration cycle calculations.
- Heat capacity link: At constant pressure, Cp = (∂H/∂T)P, connecting enthalpy directly to measurable temperature change.
How is enthalpy measured and demonstrated in a teaching lab?
In an engineering or science teaching laboratory, enthalpy changes are most often determined by calorimetry. A calorimeter isolates the process so the heat released or absorbed can be quantified using:
q = m · c · ΔT
where m is mass, c is specific heat capacity, and ΔT is the measured temperature change. For a constant-pressure (open) calorimeter, this measured heat equals ΔH for the process.
Typical demonstrations include:
- Bomb calorimeter — measures enthalpy of combustion of solid and liquid fuels (after correcting from constant volume to constant pressure).
- Coffee-cup / simple calorimeter — measures enthalpy of neutralisation, dissolution, and reaction.
- Steam apparatus and Marcet boiler — relate saturation pressure and temperature to demonstrate enthalpy of vaporisation.
What to ask a supplier before buying calorimetry equipment
When sourcing thermodynamics teaching equipment for a college laboratory, use this checklist:
- Accuracy & range: What is the temperature resolution and measurement range of the instrument?
- Standards compliance: Is the equipment built to recognised quality standards (e.g. ISO 9001:2015, CE)?
- Capacity: Is it suited to batch sizes typical for undergraduate practicals?
- Safety: What pressure ratings, relief mechanisms, and insulation are provided?
- Manuals & support: Are experiment manuals, calibration data, and spares available?
- Export readiness: Can the supplier provide CIF quotations, export documentation, and after-sales support to your country?
Selection criteria for a thermodynamics lab
- Curriculum fit — equipment that maps to your syllabus experiments (first law, calorimetry, steam properties, heat transfer).
- Durability — corrosion-resistant materials and robust construction for repeated student use.
- Repeatability — stable, reproducible results across batches and operators.
- Serviceability — local or shippable spares and clear maintenance procedures.
Understanding enthalpy moves from theory to insight when students measure ΔH for themselves on reliable, well-calibrated apparatus. Scientico India is an ISO 9001:2015 and CE certified manufacturer and exporter of engineering and science lab equipment, supplying calorimeters and complete thermodynamics setups to engineering colleges and universities across 60+ countries. Explore our full range of Thermodynamics Lab Equipment to equip your laboratory.
Frequently Asked Questions
What is enthalpy in simple words?
Enthalpy is the total heat content of a system at constant pressure. It is defined as H = U + PV, combining the system’s internal energy (U) with the product of its pressure (P) and volume (V). Its change, ΔH, tells you how much heat a process absorbs or releases at constant pressure.
What is the formula for enthalpy?
The defining formula is H = U + PV, where H is enthalpy, U is internal energy, P is pressure and V is volume. For a change at constant pressure, ΔH = ΔU + PΔV, and this equals the heat exchanged, q_p.
What is the SI unit of enthalpy?
The SI unit of enthalpy is the joule (J). Specific enthalpy (per unit mass) is expressed in J/kg or kJ/kg, while molar enthalpy (per mole) is expressed in J/mol or kJ/mol.
What is the difference between enthalpy and internal energy?
Internal energy (U) is the total kinetic and potential energy of a system’s molecules. Enthalpy (H) adds the pressure-volume term PV, so H = U + PV. Internal energy change equals heat at constant volume, while enthalpy change equals heat at constant pressure.
How is enthalpy change measured in a lab?
Enthalpy change is measured by calorimetry using q = m·c·ΔT, where m is mass, c is specific heat capacity and ΔT is the temperature change. A constant-pressure calorimeter gives ΔH directly, while a bomb calorimeter measures combustion enthalpy after correcting from constant volume.
Lab Equipment Featured in This Guide
Manufactured in-house by Scientico India — ISO 9001:2015 & CE certified, exported to 60+ countries. Request a CIF quote within 24 hours.
Saturation Pressure ThermoFlux – 7069View details & get quote →
Thermal and Light Radiation | ThermoFlux – 7031View details & get quote →
Marcet Boiler | ThermoFlux – 7047View details & get quote →
Free and Forced Convection Heat Exchanger | ThermoFlux – 7027View details & get quote →
Heat Conduction and Convection ThermoFlux – 7106View details & get quote →
Extended Surface Heat Transfer Module | ThermoFlux – 7049/4View details & get quote →