Enter the known reactions and target reactions into the calculator to determine the required change in enthalpy and multipliers.

Hess’s Law Combine Reactions
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Hess's Law Formula

The following formula is used to calculate the reaction enthalpy change using Hess's Law (using enthalpies of formation): 

\Delta H_{\mathrm{rxn}} = \sum \nu \Delta H_f(\text{products}) - \sum \nu \Delta H_f(\text{reactants})

Variables:

  • ΔHrxn is the reaction enthalpy change (kJ per mole of reaction, as written)
  • ν is the stoichiometric coefficient for each species in the balanced chemical equation
  • ΔHf is the (molar) enthalpy of formation for each species (typically tabulated in kJ/mol for the species)

To calculate the reaction enthalpy change, multiply each species’ enthalpy of formation by its stoichiometric coefficient (ν), sum these values for the products, and subtract the corresponding stoichiometric sum for the reactants. The result is the reaction enthalpy change for the reaction as written, regardless of the path taken.

What is Hess's Law?

Hess's Law, named after Russian chemist Germain Hess, states that the total enthalpy change during the complete course of a chemical reaction is the same whether the reaction is made in one step or in several steps. This is because enthalpy is a state function, meaning its value depends only on the current state of the system and not on the path taken to reach that state. Therefore, the total enthalpy change in a chemical reaction is independent of the route by which the chemical reaction takes place.

How to Calculate Hess's Law?

The following steps outline how to calculate the total enthalpy change (ΔH) using Hess's Law:

  1. Determine the enthalpy of formation (ΔHf) for each product involved in the reaction.
  2. Determine the enthalpy of formation (ΔHf) for each reactant involved in the reaction.
  3. Multiply each ΔHf by the stoichiometric coefficient (ν) for that species and sum to get ΣνΔHf(products) and ΣνΔHf(reactants).
  4. Subtract the stoichiometric sum of the reactants from the stoichiometric sum of the products: ΔHrxn = ΣνΔHf(products) − ΣνΔHf(reactants).
  5. After inserting the values and calculating the result, check your answer with the calculator above.

Example Problem:

Use the following variables as an example problem to test your knowledge:

ΣνΔHf (products) = -120 kJ/mol (of reaction)

ΣνΔHf (reactants) = -80 kJ/mol (of reaction)

Calculate the reaction enthalpy change (ΔHrxn) using the formula: ΔHrxn = ΣνΔHf (products) - ΣνΔHf (reactants).