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Enthalpy Change Calculator

Work out enthalpy change instantly with clear inputs, formula shown and shareable results.

About Enthalpy Change Calculator

Enthalpy is defined as H = U + PV, so at constant pressure dH = dU + P.dV. Adding 5 kJ of internal energy while expanding 0.02 m3 against atmospheric pressure costs 2027 J of expansion work, making the enthalpy change 7027 J. This is why constant-pressure heat capacities exceed constant-volume ones.

Formulas

Enthalpy change

dH = dU + P dV at constant pressure

Definition

H = U + P V

Worked example

With the default values below, Enthalpy Change Calculator returns Enthalpy Change of 7,026.5. Change any input above and every figure here is recalculated in your browser.

Inputs used

Change in internal energy
5,000 J
Constant pressureminimum 0 kPa
101.325 kPa
Change in volume
0.02 m3

Results

Enthalpy Change
7,026.5 J
Pressure-Volume Work
2,026.5 J
Share Of Enthalpy That Is Expansion Work
28.84%
Same Change In Kilojoules
7.0265

Step by step

  1. 1Values used
  2. 2Enthalpy change
  3. 3Definition
  4. 4Enthalpy Change= 7,026.5 J
  5. 5Pressure-Volume Work= 2,026.5 J
  6. 6Share Of Enthalpy That Is Expansion Work= 28.84
  7. 7Same Change In Kilojoules= 7.0265

Assumptions and limits

  • The result is derived only from the 3 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
  • Figures are rounded for display to 4 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

Why bother with enthalpy at all?

Because most chemistry happens in open vessels at constant pressure, where the heat exchanged equals dH directly. It saves accounting for expansion work every time.

Is dH always bigger than dU?

Only when the system expands. A reaction that contracts, such as gas-consuming combustion, has dH smaller than dU.

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