Enthalpy Calculator

Enthalpy is the cornerstone of thermochemistry: it quantifies the heat exchanged during a reaction at constant pressure, drives Gibbs free-energy calculations, and underpins every bond-energy estimate in physical chemistry. This tool brings together six interconnected calculations in a single page — no switching between tabs on separate websites, no unit-conversion errors, and no data sent to a server.

What each tab calculates

Hess’s Law implements the IUPAC-standard formation-enthalpy approach. Add as many product and reactant rows as your equation requires, enter each species’ standard enthalpy of formation (ΔHf° in kJ/mol), and the calculator applies ΔH°rxn = Σ n·ΔHf°(products) − Σ m·ΔHf°(reactants) in real time. The full working line shows both sums so you can verify each term.

Gibbs Free Energy (ΔG = ΔH − TΔS) lets you solve for any one of the four variables. Switch the “Solve for” drop-down to ΔG, ΔH, ΔS, or T and the remaining three become the inputs. The result panel flags whether the reaction is spontaneous, non-spontaneous, or at equilibrium at the chosen temperature.

Bond Enthalpy uses a table of 33 mean bond enthalpies (H−H through I−I, all standard gas-phase values). Build a list of bonds broken in reactants and bonds formed in products; the estimate ΔH_rxn ≈ Σ E(broken) − Σ E(formed) updates live. A caveat note reminds you this is an approximation — good for trend analysis, not precise thermodynamic cycles.

Kirchhoff’s Law corrects a tabulated ΔH to a different temperature: ΔH(T₂) = ΔH(T₁) + ΔC_p·(T₂ − T₁). Useful when literature data are at 298 K but your reaction runs at 500 K or higher.

Calorimetry converts a temperature-rise experiment directly into ΔH: q = m·c_p·ΔT gives the heat absorbed by the solution; dividing by moles and flipping sign gives ΔH_rxn per mole of limiting reagent.

q = nC_pΔT calculates the heat transferred when a substance is heated or cooled — available in both molar (n, C_p) and specific-heat (m, c) forms.

Worked example — combustion of hydrogen

The reaction H₂(g) + ½ O₂(g) → H₂O(l) at 298 K and 1 bar.

Using the Hess’s Law tab:

ΔH°rxn = (−285.83) − (0 + 0) = −285.83 kJ/mol

This is one of the most precisely measured reaction enthalpies in thermochemistry. The negative sign confirms the reaction is exothermic — consistent with hydrogen fuel cells releasing energy.

Now apply Gibbs: if ΔS° = −163.4 J/(mol·K) at 298 K, the Gibbs tab gives ΔG = −285.83 − 298 × (−163.4/1000) = −237.1 kJ/mol — spontaneous, as expected.

Formula reference

All calculations are 100% client-side. No values are uploaded, stored, or shared with any server.