ChemBench

Enthalpy Calculator

Heat content, accounting for expansion work.

ΔU internal energy (J)

Pressure (Pa)

ΔV volume change (m³)

Enthalpy change

41,013.25 J

What you entered

ΔU internal energy (J): 40000Pressure (Pa): 101325ΔV volume change (m³): 0.01

1
PΔV
101325 × 0.01= 1013.25 J
2
ΔH = ΔU + PΔV
40000 + 1013.25= 41013.25 J

Result

Enthalpy change: 41,013.25 J

ΔH for this process is 41,013.25 J.

Enthalpy from internal energy and pressure-volume work

H equals U plus PV. When a system expands against constant pressure it does work, and that work is added to the internal energy.

Internal energy U	Pressure	Volume change	PV work	Enthalpy H
1,000 J	101,325 Pa	0.001 m3	101.33 J	1101.33 J
5,000 J	101,325 Pa	0.01 m3	1,013.25 J	6013.25 J
-2,000 J	101,325 Pa	0.005 m3	506.63 J	-1493.38 J
10,000 J	202,650 Pa	0.002 m3	405.30 J	10405.30 J
0 J	101,325 Pa	0.0224 m3	2,269.68 J	2269.68 J

101,325 Pa is exactly one standard atmosphere. The last row is one mole of gas expanding into its full molar volume at STP, which costs about 2.27 kJ of work.

Why enthalpy isn't just internal energy

Enthalpy adds a correction term (PΔV) to internal energy to account for work done by or on a system as it expands or contracts — at constant pressure, ΔH directly equals the heat absorbed or released.

Why chemists prefer enthalpy over internal energy

Most chemistry happens in open containers at constant atmospheric pressure, not constant volume — enthalpy is the quantity that directly matches the heat you'd actually measure in that common setup.

Frequently asked questions

A reaction changes internal energy by -50 kJ and does 5 kJ of PV work on its surroundings — what is ΔH?

ΔH = ΔU + PΔV = -50 + 5 = -45 kJ. The enthalpy change is slightly less negative than ΔU because some energy went to expansion work.

How is enthalpy different from the Gibbs free energy calculator?

Enthalpy (ΔH) measures total heat exchange. Gibbs free energy (ΔG = ΔH - TΔS) also factors in entropy to determine spontaneity. A reaction can be exothermic (negative ΔH) yet non-spontaneous if entropy strongly decreases.

Is ΔH positive or negative for an exothermic reaction?

Negative. An exothermic reaction releases heat to the surroundings, so the system's enthalpy decreases. Endothermic reactions absorb heat and have positive ΔH.