Standard Enthalpy of Formation Calculator

Calculate the total heat of reaction (ΔH°rxn) using Hess's Law by summing the enthalpies of formation of products and reactants.

Sum of Products ΔH°f

Sum of Reactants ΔH°f

Standard Enthalpy of Reaction (ΔH°rxn)

-318.70 kJ

Reaction TypeExothermic (Releases Heat)

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Calculating the Heat of Reaction

How do we know how much heat a chemical reaction will release before we even mix the chemicals together?

We use Hess's Law and the Standard Enthalpy of Formation ( $\Delta H^\circ_f$ ). The Enthalpy of Formation is the precise amount of heat absorbed or released when exactly 1 mole of a compound is forged directly from its pure elemental building blocks.

The Summation Rule

Enthalpy is a "State Function." This means the total heat of a reaction only depends on where you start (Reactants) and where you finish (Products). It doesn't matter how many bizarre intermediate steps occur in the middle.

Because of this, we can calculate the total heat of any chemical reaction simply by mathematically subtracting the baseline energy of the Reactants from the final energy of the Products.

The Equation

\begin{aligned} \Delta H^\circ_{rxn} = \Sigma(n \cdot \Delta H^\circ_{f, \text{products}}) - \Sigma(m \cdot \Delta H^\circ_{f, \text{reactants}}) \end{aligned}

Where:

\Delta H^\circ_{rxn}

Total Enthalpy of Reaction

\Delta H^\circ_{f}

Standard Enthalpy of Formation

\Sigma

Summation

n, m=

Stoichiometric Coefficients

Interpreting the Result

Negative Result (Exothermic): The products contain less stored energy than the reactants. The "missing" energy was blasted outward into the surrounding environment as heat, causing the beaker to feel hot.
Positive Result (Endothermic): The products contain more stored energy than the reactants. The reaction had to steal heat from the surrounding environment to fuel itself, causing the beaker to feel freezing cold.

Frequently Asked Questions

Exactly zero. Elements in their most stable natural state (like $O_2$ gas or solid Carbon graphite) are the baseline definition of 'zero' on the thermodynamic scale. They do not have to be 'formed' because they already exist natively in the universe.

Because in a chemical reaction, the Reactants are being destroyed (which requires us to reverse their formation energy, making it negative), while the Products are being created (keeping their formation energy positive).

Yes. If the balanced equation produces 3 moles of water, you must multiply the textbook Enthalpy of Formation for water by 3 before adding it to your total summation.

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