What this calculator does
The enthalpy formula for a chemical reaction is ΔH = ΣH(products) − ΣH(reactants): the total enthalpy of everything that comes out, minus the total enthalpy of everything that goes in. A negative result means the reaction releases heat (exothermic); a positive result means it absorbs heat from its surroundings (endothermic). This calculator takes those two totals directly and does the subtraction, along with converting to a per-mole figure.
In practice the products and reactants totals are usually built from standard enthalpies of formation (ΔHf°) for each substance, multiplied by how many moles of it appear in the balanced equation, then summed. Getting those individual ΔHf° values right is a chemistry lookup task in itself; this calculator picks up from there, once the two totals are known.
The formula
Sum the enthalpy of formation times moles for every product to get the products total, do the same for every reactant, then subtract: ΔH = ΣH(products) − ΣH(reactants). Dividing by the moles of reaction as written in the balanced equation gives the enthalpy change per mole.
| Term | Meaning |
|---|---|
| ΔH | Enthalpy change of the reaction, in kilojoules. |
| ΣH(products) | The sum of each product's enthalpy of formation, multiplied by its stoichiometric coefficient. |
| ΣH(reactants) | The same sum for the reactants side of the balanced equation. |
| Exothermic / endothermic | A negative ΔH releases heat; a positive ΔH absorbs it from the surroundings. |
The inputs explained
| Field | What to enter |
|---|---|
| Total enthalpy of products (sum of ΔHf × moles for each product) (kJ) | The combined enthalpy of formation of all the products, each multiplied by its number of moles in the balanced equation, summed together. |
| Total enthalpy of reactants (sum of ΔHf × moles for each reactant) (kJ) | The same combined figure for the reactants side. Elements in their standard state have a formation enthalpy of zero. |
| Moles of reaction (as written) | The number of moles of reaction as the balanced equation is written, used only to work out the per-mole figure. |
When to use it
Working out heat released by a combustion reaction
Combustion reactions are strongly exothermic, and knowing the enthalpy released per mole of fuel is the starting point for sizing a burner, boiler or engine around that heat output.
Checking a reaction is thermodynamically favourable to run
A strongly endothermic reaction needs continuous heat input to proceed and will not run on its own, which matters when planning a process that assumed it would.
Comparing enthalpy change across scaled-up batches
Multiplying the products and reactants totals by the batch size before entering them here scales the whole calculation, and the per-mole result stays a fixed property of the reaction regardless of batch size.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
Enthalpy change for some common formation reactions
Standard formation enthalpies for a handful of common compounds, entered as the products total against zero for the elements they form from.
| Reaction | Enthalpy change (ΔH) | Reaction is |
|---|---|---|
| CO2(g) formation | -393.50 kJ | Exothermic (releases heat, ΔH negative) |
| H2O(l) formation | -285.80 kJ | Exothermic (releases heat, ΔH negative) |
| HCl(g) formation | -92.40 kJ | Exothermic (releases heat, ΔH negative) |
| H2O(g) formation | -241.80 kJ | Exothermic (releases heat, ΔH negative) |
| CH4(g) formation | -74.80 kJ | Exothermic (releases heat, ΔH negative) |
How the per-mole figure changes with the scale of reaction
The same total enthalpy change, expressed as written for different multiples of the balanced equation.
| Moles of reaction | Enthalpy change (ΔH) | Enthalpy change per mole of reaction |
|---|---|---|
| 0.5 | -393.50 kJ | -787.000 kJ/mol |
| 1 | -393.50 kJ | -393.500 kJ/mol |
| 2 | -393.50 kJ | -196.750 kJ/mol |
| 3 | -393.50 kJ | -131.167 kJ/mol |
| 5 | -393.50 kJ | -78.700 kJ/mol |
Questions
What is the enthalpy formula?
ΔH = ΣH(products) − ΣH(reactants): the total enthalpy of the products minus the total enthalpy of the reactants, using each substance's standard enthalpy of formation multiplied by its moles in the balanced equation.
How do I calculate enthalpy for a reaction I don't have ΔHf values for?
You need to look up or measure the standard enthalpy of formation for each substance involved, from a chemistry data table or reference source. This calculator does the subtraction and per-mole conversion once those totals are known; it does not supply the underlying ΔHf figures.
What is the difference between this and specific heat?
Specific heat, Q = mcΔT, is about warming or cooling a fixed mass of one substance by a temperature change, with no chemical reaction involved. Enthalpy of reaction is about the heat released or absorbed as reactants convert into different products, which is a distinct chemical property of the reaction itself, not a temperature change.
Why is enthalpy of formation zero for elements?
Standard enthalpy of formation is measured relative to elements in their most stable form at standard conditions, so by definition those reference elements are assigned zero. Everything else is measured relative to that baseline.
For whether a reaction with this ΔH will actually proceed on its own, see the Gibbs free energy calculator, which also needs the entropy change. For heating or cooling a substance without a reaction, see heat energy and specific heat.