Quick answer
The enthalpy of a reaction is ΔH° = Σ n·ΔHf°(products) − Σ n·ΔHf°(reactants). Products summing to −593.66 kJ and reactants to −791.44 kJ give ΔH° = +197.78 kJ, an endothermic reaction.
Enter coefficients and formation enthalpies for reactants and products. The calculator returns the reaction enthalpy and whether it is endothermic or exothermic.
Related tools: Specific Heat Calculator: Specific heat capacity or heat required from Q = m c ΔT. Electron Configuration Calculator: Ground-state electron configuration of an element or monatomic ion, with noble-gas shorthand and unpaired electrons. Pressure Calculator: Pressure from force and area, with unit conversions.
How to use the Enthalpy Calculator
- Choose what to calculate: Reaction enthalpy from enthalpies of formation and ΔH = ΔU + p·ΔV at constant pressure.
- Optionally, enter reactant 1 coefficient. It starts at 0.
- Optionally, enter reactant 1 ΔHf° (kJ/mol). It starts at 0.
- Optionally, enter reactant 2 coefficient. It starts at 0.
- Optionally, enter reactant 2 ΔHf° (kJ/mol). It starts at 0.
- Optionally, enter reactant 3 coefficient. It starts at 0.
- Optionally, enter reactant 3 ΔHf° (kJ/mol). It starts at 0.
- Optionally, enter product 1 coefficient. It starts at 0.
- Optionally, enter product 1 ΔHf° (kJ/mol). It starts at 0.
- Optionally, enter product 2 coefficient. It starts at 0.
- Optionally, enter product 2 ΔHf° (kJ/mol). It starts at 0.
- Optionally, enter product 3 coefficient. It starts at 0.
- Optionally, enter product 3 ΔHf° (kJ/mol). It starts at 0.
- Optionally, enter change in internal energy ΔU. It starts at 0.
- Choose δU unit: Joules and Kilojoules.
- Optionally, enter pressure p. It starts at 1.
- Choose pressure unit: atm, Pa, kPa, bar and psi.
- Optionally, enter change in volume ΔV. It starts at 0.
- Choose volume unit: Litres, Cubic metres and Millilitres.
- Press Calculate to see the result, and use Reset to start again.
| Input | What to enter | Allowed values |
|---|---|---|
| What to calculate | Choose one: Reaction enthalpy from enthalpies of formation; ΔH = ΔU + p·ΔV at constant pressure | Starts at Reaction enthalpy from enthalpies of formation |
| Reactant 1 coefficient (optional) | A number | starts at 0 |
| Reactant 1 ΔHf° (kJ/mol) (optional) | A number | starts at 0 |
| Reactant 2 coefficient (optional) | A number | starts at 0 |
| Reactant 2 ΔHf° (kJ/mol) (optional) | A number | starts at 0 |
| Reactant 3 coefficient (optional) | A number | starts at 0 |
| Reactant 3 ΔHf° (kJ/mol) (optional) | A number | starts at 0 |
| Product 1 coefficient (optional) | A number | starts at 0 |
| Product 1 ΔHf° (kJ/mol) (optional) | A number | starts at 0 |
| Product 2 coefficient (optional) | A number | starts at 0 |
| Product 2 ΔHf° (kJ/mol) (optional) | A number | starts at 0 |
| Product 3 coefficient (optional) | A number | starts at 0 |
| Product 3 ΔHf° (kJ/mol) (optional) | A number | starts at 0 |
| Change in internal energy ΔU (optional) | A number | starts at 0 |
| ΔU unit | Choose one: Joules; Kilojoules | Starts at Joules |
| Pressure p (optional) | A number | starts at 1 |
| Pressure unit | Choose one: atm; Pa; kPa; bar; psi | Starts at atm |
| Change in volume ΔV (optional) | A number | starts at 0 |
| Volume unit | Choose one: Litres; Cubic metres; Millilitres | Starts at Litres |
Formula and method
Formula
ΔH°(reaction) = Σ n·ΔHf°(products) − Σ n·ΔHf°(reactants); ΔH = ΔU + p·ΔV
IEEE-754 double precision; shown to at most 6 decimal places.
Hess's law: the enthalpy change of a reaction is the coefficient-weighted sum of the standard enthalpies of formation of the products minus those of the reactants. Enter each coefficient from your balanced equation and look up ΔHf° (kJ/mol, at 298.15 K) in a reference such as the NIST Chemistry WebBook; elements in their standard state are 0. The calculator does not supply the ΔHf° values itself, so the result is only as good as the values you enter. The second mode is the definition H = U + pV at constant pressure, ΔH = ΔU + pΔV.
Worked examples
These examples are verified: each one is an automated test case that the live calculator must reproduce exactly.
Example 1
Inputs: What to calculate = Reaction enthalpy from enthalpies of formation; Reactant 1 coefficient = 2; Reactant 1 ΔHf° (kJ/mol) = -395.72; Reactant 2 coefficient = 0; Reactant 2 ΔHf° (kJ/mol) = 0; Reactant 3 coefficient = 0; Reactant 3 ΔHf° (kJ/mol) = 0; Product 1 coefficient = 2; Product 1 ΔHf° (kJ/mol) = -296.83; Product 2 coefficient = 1; Product 2 ΔHf° (kJ/mol) = 0; Product 3 coefficient = 0; Product 3 ΔHf° (kJ/mol) = 0; Change in internal energy ΔU = 0; ΔU unit = Joules; Pressure p = 1; Pressure unit = atm; Change in volume ΔV = 0; Volume unit = Litres.
| Result | Value |
|---|---|
| Reaction enthalpy ΔH° | 197.78 kJ |
| Σ products | -593.66 kJ |
| Σ reactants | -791.44 kJ |
| Type | Endothermic (absorbs heat) |
Example 2
Inputs: What to calculate = Reaction enthalpy from enthalpies of formation; Reactant 1 coefficient = 1; Reactant 1 ΔHf° (kJ/mol) = -74.87; Reactant 2 coefficient = 2; Reactant 2 ΔHf° (kJ/mol) = 0; Reactant 3 coefficient = 0; Reactant 3 ΔHf° (kJ/mol) = 0; Product 1 coefficient = 1; Product 1 ΔHf° (kJ/mol) = -393.51; Product 2 coefficient = 2; Product 2 ΔHf° (kJ/mol) = -285.83; Product 3 coefficient = 0; Product 3 ΔHf° (kJ/mol) = 0; Change in internal energy ΔU = 0; ΔU unit = Joules; Pressure p = 1; Pressure unit = atm; Change in volume ΔV = 0; Volume unit = Litres.
| Result | Value |
|---|---|
| Reaction enthalpy ΔH° | -890.3 kJ |
| Σ products | -965.17 kJ |
| Σ reactants | -74.87 kJ |
| Type | Exothermic (releases heat) |
Example 3
Inputs: What to calculate = Reaction enthalpy from enthalpies of formation; Reactant 1 coefficient = 1; Reactant 1 ΔHf° (kJ/mol) = -100; Reactant 2 coefficient = 0; Reactant 2 ΔHf° (kJ/mol) = 0; Reactant 3 coefficient = 0; Reactant 3 ΔHf° (kJ/mol) = 0; Product 1 coefficient = 1; Product 1 ΔHf° (kJ/mol) = -100; Product 2 coefficient = 0; Product 2 ΔHf° (kJ/mol) = 0; Product 3 coefficient = 0; Product 3 ΔHf° (kJ/mol) = 0; Change in internal energy ΔU = 0; ΔU unit = Joules; Pressure p = 1; Pressure unit = atm; Change in volume ΔV = 0; Volume unit = Litres.
| Result | Value |
|---|---|
| Reaction enthalpy ΔH° | 0 kJ |
| Σ products | -100 kJ |
| Σ reactants | -100 kJ |
| Type | Thermoneutral |
The first line is the headline result (reaction enthalpy ΔH°). Below it the enthalpy Calculator lists σ products, σ reactants and type, which describe the same case from other angles. IEEE-754 double precision; shown to at most 6 decimal places.
Sign convention
A negative ΔH releases heat to the surroundings (exothermic), a positive ΔH absorbs it (endothermic). Elements in their standard states have ΔHf° = 0.
From ΔU to ΔH
The page also supports ΔH = ΔU + pΔV for constant-pressure processes. Use standard values from a data table for formation enthalpies.
How the result changes with reactant 1 coefficient
Holding every other input at the first worked example, this table changes only reactant 1 coefficient. It shows how sensitive the answer is to that one value; each row was produced by running this calculator.
| Reactant 1 coefficient | Reaction enthalpy ΔH° | Σ products | Σ reactants |
|---|---|---|---|
| 1 | -197.94 kJ | -593.66 kJ | -395.72 kJ |
| 2 | 197.78 kJ | -593.66 kJ | -791.44 kJ |
| 4 | 989.22 kJ | -593.66 kJ | -1,582.88 kJ |
| 8 | 2,572.1 kJ | -593.66 kJ | -3,165.76 kJ |
What happens with invalid input
These messages are produced by the calculator itself, so you know what to fix.
- Reactant 1 coefficient: entering "0" shows "Enter at least one coefficient above zero." beside that field and no result.
- Pressure p: entering "-1" shows "Pressure cannot be negative." beside that field and no result.
Choosing between the Enthalpy Calculator and similar calculators
Each row shows what a related calculator on this site asks for and what it returns, so you can pick the one that matches your question.
| Calculator | You enter | It returns |
|---|---|---|
| [Specific Heat Calculator](/science/specific-heat-calculator/) | solve for, mass and temperature change | Specific heat |
| [Electron Configuration Calculator](/science/electron-configuration/) | element and charge | Electron configuration of H |
| [Pressure Calculator](/science/pressure-calculator/) | force and area | Pressure |
| [EER Calculator](/science/energy-efficiency-ratio-calculator/) | cooling capacity and power input | EER |
| [Rainwater Harvesting Calculator](/science/rainwater-harvesting-calculator/) | roof catchment area, rainfall and collection efficiency | Water collected |
Assumptions and limitations
- Values of ΔHf° are user-supplied
- Up to three reactants and three products
- Leave unused coefficients at 0
If a required value is empty, not a number, or outside its allowed range, the form marks that field with a message and shows no result instead of a misleading number. The automated checks deliberately try invalid entries for reactant 1 coefficient and pressure p to confirm this.
Frequently asked questions
Where do I find ΔHf° values?
In a thermodynamic data table.
Is ΔH the same as heat?
At constant pressure, yes.
What does standard state mean?
1 bar and a specified temperature, usually 25 °C.
What does the Enthalpy Calculator calculate?
Enthalpy change of a reaction from enthalpies of formation (Hess's law), or ΔH = ΔU + pΔV at constant pressure. It returns Reaction enthalpy ΔH°, Σ products, Σ reactants and Type.
What formula does the Enthalpy Calculator use?
ΔH°(reaction) = Σ n·ΔHf°(products) − Σ n·ΔHf°(reactants); ΔH = ΔU + p·ΔV. Hess's law: the enthalpy change of a reaction is the coefficient-weighted sum of the standard enthalpies of formation of the products minus those of the reactants.
What do I need to enter in the enthalpy Calculator?
You enter What to calculate, ΔU unit, Pressure unit and Volume unit. Optional: Reactant 1 coefficient, Reactant 1 ΔHf° (kJ/mol), Reactant 2 coefficient, Reactant 2 ΔHf° (kJ/mol), Reactant 3 coefficient, Reactant 3 ΔHf° (kJ/mol), Product 1 coefficient, Product 1 ΔHf° (kJ/mol), Product 2 coefficient, Product 2 ΔHf° (kJ/mol), Product 3 coefficient, Product 3 ΔHf° (kJ/mol), Change in internal energy ΔU, Pressure p and Change in volume ΔV.
How is the result rounded or limited?
IEEE-754 double precision; shown to at most 6 decimal places. Values of ΔHf° are user-supplied.
What are the limits of this calculator?
Values of ΔHf° are user-supplied. Up to three reactants and three products. Leave unused coefficients at 0.
What does the “What to calculate” option change?
It sets how the enthalpy is worked out. The choices are Reaction enthalpy from enthalpies of formation and ΔH = ΔU + p·ΔV at constant pressure.
What is an example of a enthalpy result?
With what to calculate Reaction enthalpy from enthalpies of formation, reactant 1 coefficient 2, reactant 1 ΔHf° (kJ/mol) -395.72, reactant 2 coefficient 0 and reactant 2 ΔHf° (kJ/mol) 0, the reaction enthalpy ΔH° is 197.78 kJ.
Where does the method come from?
The method follows: Hess's law of constant heat summation (any general-chemistry text); Enthalpies of formation: look up in the NIST Chemistry WebBook (https://webbook.nist.gov/chemistry/); Definition of enthalpy, H = U + pV; 1 atm = 101,325 Pa exactly (NIST SP 811).
Related calculators
- Specific Heat Calculator — Specific heat capacity or heat required from Q = m c ΔT.
- Electron Configuration Calculator — Ground-state electron configuration of an element or monatomic ion, with noble-gas shorthand and unpaired electrons.
- Pressure Calculator — Pressure from force and area, with unit conversions.
- EER Calculator — Energy efficiency ratio of an air conditioner and its COP equivalent.
- Viscosity Calculator — Convert between dynamic viscosity (cP) and kinematic viscosity (cSt).
Sources
- Hess's law of constant heat summation (any general-chemistry text)
- Enthalpies of formation: look up in the NIST Chemistry WebBook (https://webbook.nist.gov/chemistry/)
- Definition of enthalpy, H = U + pV; 1 atm = 101,325 Pa exactly (NIST SP 811)
Content last reviewed 2026-11-04. Formulas and worked examples are checked by automated tests against independently computed values.