What is the difference between ΔH and ΔU?

ΔU is the change in internal energy of the system. ΔH is the enthalpy change, which also accounts for work done by expanding or contracting gases at constant pressure. They are related by ΔH = ΔU + ΔnRT, where Δn is the change in moles of gas.

What is Δn in the formula?

Δn is the change in moles of gaseous species: moles of gaseous products minus moles of gaseous reactants. Liquids and solids are excluded because they contribute negligibly to PΔV work.

What value of R should I use?

This calculator uses R = 8.314 J/(mol·K). This is the universal gas constant in SI units. When ΔU is in joules and T is in kelvin, the result is in joules.

When are ΔH and ΔU essentially equal?

When Δn = 0 (equal moles of gas on both sides) or when the reaction involves only solids and liquids, the PΔV term is negligible and ΔH ≈ ΔU.

What does a negative ΔH mean?

A negative ΔH means the reaction is exothermic — it releases heat to the surroundings. A positive ΔH indicates an endothermic reaction that absorbs heat.

Enthalpy Change Calculator

Calculate enthalpy change (ΔH) from internal energy change and moles of gas, or directly from heat of reaction.

Inputs

Calculation mode

ΔH from ΔU (ideal gas)

ΔH = ΔU + ΔnRT

Heat at constant pressure

ΔH = q_p

Internal energy change (ΔU)

Change in gas moles (Δn)

Gas moles of products minus gas moles of reactants

Temperature (T)

Result unit

Unit conversions

Unit	ΔH

Enter values and click Calculate ΔH

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Summary

The Enthalpy Change Calculator computes ΔH (enthalpy change) for chemical reactions using two methods: the thermodynamic relation ΔH = ΔU + ΔnRT, or direct input of heat measured at constant pressure. It supports unit conversion between kJ/mol, kcal/mol, and eV, and shows a step-by-step derivation so you can follow the arithmetic.

How it works

Select the calculation mode: "ΔH from ΔU" or "Heat at Constant Pressure".
Enter the required values with their units.
For the ΔU mode, provide internal energy change, moles of gas produced minus consumed (Δn), and temperature.
Click Calculate to see ΔH with a step-by-step breakdown.
Use the unit selector to view the result in kJ/mol, kcal/mol, or eV.

Use cases

Determine enthalpy change for a chemical reaction from calorimetry data.
Convert between internal energy and enthalpy in thermochemistry problems.
Verify ΔH values for combustion, neutralization, or formation reactions.
Study thermodynamics for chemistry exams or coursework.
Calculate heat released or absorbed at constant pressure in lab settings.
Cross-check theoretical ΔH from ΔU using the ideal gas correction term.

Frequently Asked Questions

Last updated: 2026-06-11 · Reviewed by Nham Vu