Enthalpy describes the total heat-related energy change of a system at constant pressure. It is useful for evaluating chemical reactions, phase changes, heating processes, and energy transfers. This calculator combines internal energy change with pressure-volume work and reports both total and molar enthalpy change.
How to Calculate Enthalpy Change (step by step)
Step One: Identify the internal energy change
Start with the system’s change in internal energy, ΔU. Use kilojoules for this calculator so the final enthalpy result is also reported in kilojoules.
Change in internal energy = 25 kJ
Step Two: Calculate pressure-volume work
Multiply pressure by the change in volume. Because kilopascals multiplied by liters produces joules, multiply by 0.001 to convert the result to kilojoules.
PΔV = 100 kPa × 5 L × 0.001 = 0.5 kJ
Step Three: Add the pressure-volume term
Add the pressure-volume contribution to the internal energy change. This gives the total enthalpy change.
ΔH = ΔU + PΔV
ΔH = 25 kJ + 0.5 kJ = 25.5 kJ
Step Four: Calculate molar enthalpy
To compare reactions involving different quantities of material, divide total enthalpy change by the amount of substance in moles.
Molar ΔH = 25.5 kJ ÷ 1.5 mol = 17 kJ/mol
What your results mean
The calculator reports an enthalpy change of 25.500 kJ for the default values. Because the result is positive, the system absorbs energy overall. The expansion also contributes positive pressure-volume work, increasing the enthalpy change above the internal energy change alone.
The molar result is 17.000 kJ/mol. This normalizes the energy change to one mole, which is useful when comparing reactions or published thermochemical data. For chemistry work involving specific substances, you may also need a molar mass calculator to convert between mass and amount of substance.
Sign conventions and assumptions
A positive change in volume represents expansion, while a negative change represents compression. The formula assumes pressure is expressed in kPa, volume in liters, internal energy in kJ, and amount in mol. For more advanced thermodynamic calculations, compare this result with the ideal gas law calculator, especially when pressure, temperature, and volume are related through gas behavior.
Frequently asked questions
Does a negative enthalpy change mean heat is released?
Yes. A negative ΔH generally indicates an exothermic process, where the system releases heat under constant-pressure conditions.
Can I enter pressure in pascals?
Not directly. Convert pressure to kilopascals first, or the PΔV term will be scaled incorrectly. One kilopascal equals 1,000 pascals.
Why does the molar result depend on the amount of substance?
Total enthalpy change increases with the amount of material involved. Dividing by moles expresses the result on a per-mole basis, making values easier to compare across samples.