What does a positive enthalpy change mean?

The system takes in heat overall — the process is endothermic at constant pressure. A negative ΔH means heat is released, an exothermic process.

How do I calculate enthalpy change from internal energy?

ΔH = ΔU + PΔV. Add the pressure-volume work to the internal energy change. With P in kPa and ΔV in litres, PΔV comes out in joules, so multiply by 0.001 to get kJ.

Why is the pressure-volume term included?

A system that expands pushes against its surroundings and spends energy doing it; one that contracts gains energy. The PΔV term folds that work into the energy accounting, which is what makes enthalpy the natural quantity at constant pressure.

Is enthalpy change the same as heat?

At constant pressure, yes — the heat exchanged equals ΔH. That's why reaction enthalpies measured in an open calorimeter are useful. Enthalpy is still a state function, so it can also be found from other data.

What is molar enthalpy change?

The enthalpy change divided by the moles of substance involved, in kJ/mol. It normalizes the result so reactions run at different scales can be compared, and it's how thermochemical data is tabulated.

Can I enter pressure in pascals or atmospheres?

The calculator expects kilopascals. Convert first: 1 atm ≈ 101.325 kPa, and 1 kPa = 1,000 Pa. Using the wrong pressure unit scales the PΔV term incorrectly.

Enthalpy Calculator

Enthalpy is the bookkeeping quantity chemists use for energy at constant pressure — the condition almost every open-flask reaction and everyday heating process actually runs at. It bundles together two things: the change in a system’s internal energy, and the work the system does (or has done on it) as it expands or contracts against the surrounding pressure. This calculator combines those into a total enthalpy change and, dividing by the amount of substance, a molar enthalpy change.

The enthalpy change formula

is the enthalpy change, is the internal energy change, is the pressure, and is the volume change. The term is the pressure-volume work. With in kilopascals and in litres, the product is in joules, so a factor of 0.001 converts it to kilojoules. The molar enthalpy is:

Worked example

The calculator’s defaults are kJ, kPa, L, and mol.

Pressure-volume work:

Total enthalpy change:

Molar enthalpy change:

The positive sign means the process is endothermic — it absorbs energy overall. The expansion () adds a small positive contribution on top of the internal energy change.

Reading the signs

Quantity	Positive means	Negative means
ΔH	endothermic (heat absorbed)	exothermic (heat released)
ΔU	internal energy rose	internal energy fell
ΔV	system expanded	system compressed
PΔV	work done by the system	work done on the system

Where enthalpy calculations are used

Reaction thermochemistry — combining ΔU from a bomb calorimeter with a PΔV term to report ΔH.
Comparing reactions on a per-mole basis against tabulated standard enthalpies of formation and combustion.
Phase changes — enthalpies of fusion and vaporization are ΔH values at constant pressure.
Process and reactor design, where the heat load of a step is its enthalpy change times the throughput.

Assumptions and unit care

The formula assumes constant pressure and the specific units the calculator uses: ΔU in kJ, P in kPa, ΔV in L, n in mol. Enter pressure in atmospheres or pascals without converting and the PΔV term is off by a large factor (1 atm ≈ 101.325 kPa). For gas-phase problems where P, V, and T move together, cross-check with the ideal gas law calculator; to convert between a mass and the moles this calculator needs, use the molar mass calculator.

Converting units before you enter them

The calculator expects kilopascals and litres. Convert first:

Atmospheres to kPa: multiply by 101.325. At 2 atm and L, kJ.
Millilitres to litres: divide by 1,000. A 250 mL expansion is 0.250 L.
Pascals to kPa: divide by 1,000.

A compression example

When the system shrinks, is negative and the work term lowers the enthalpy change. For kJ, kPa and L:

The process is exothermic, and the compression makes it very slightly more so.

When gases are made or used up

For reactions involving gases at constant temperature, the work term can be written as . If a reaction produces one extra mole of gas at 298 K:

For reactions involving only liquids and solids, the volume change is tiny and is almost exactly .

Common mistakes

Getting the sign of wrong. Expansion is positive; compression is negative.
Forgetting the 0.001. kPa times L gives joules, not kilojoules.
Dividing by the wrong amount. Molar enthalpy is per mole of the substance or reaction you name, so state which.
Mixing up ΔU and ΔH. A bomb calorimeter measures ΔU at constant volume; a coffee-cup calorimeter measures ΔH at constant pressure.

About the formula: The enthalpy change is calculated with ΔH = ΔU + PΔV. When pressure is in kilopascals and volume is in liters, PΔV is converted from joules to kilojoules by multiplying by 0.001.