Enthalpy of Reaction (Hess's Law) Calculator
Calculate the overall enthalpy change of a reaction by summing the enthalpies of formation of products and reactants.
Calculation Parameters
Specify your stoichiometry inputs.
Calculated Result
Enthalpy of Reaction (ΔH)
Expert Tip
Remember that standard enthalpies of formation for pure elements in their reference state (e.g., O₂ gas, solid C) are always 0 kJ/mol.
How this Calculator Works
This calculator computes the net enthalpy change of a chemical reaction using standard enthalpies of formation. Adjust the parameters using the sliders below:
- Enthalpy of Products (Σ ΔHf Products): Sum of enthalpies of formation for all products (Default: -394 kJ/mol)
- Enthalpy of Reactants (Σ ΔHf Reactants): Sum of enthalpies of formation for all reactants (Default: -111 kJ/mol)
The results are computed instantly and updated in the results panel on the right.
Formula & Methodology
Where:
- ΔH_reaction: The standard enthalpy change of the reaction (kJ/mol).
- Σ ΔHf (Products): Sum of standard enthalpies of formation of products.
- Σ ΔHf (Reactants): Sum of standard enthalpies of formation of reactants.
Step-by-Step Calculation Example
To calculate the enthalpy of reaction manually using the default inputs, follow these steps:
- Identify the input parameters. For example:
- Σ ΔHf (Products) = -394 kJ/mol
- Σ ΔHf (Reactants) = -111 kJ/mol
- Apply the formula:
ΔH_reaction = -394 - (-111) = -283 kJ/mol (Since ΔH < 0, the reaction is exothermic.)
- Verify the calculated value which is updated instantly in the results panel on the right.
Frequently Asked Questions
What is Hess's Law? ▼
Hess's Law states that total enthalpy change for a chemical reaction is independent of the pathway between initial reactants and final products.
How do you calculate ΔH°rxn from standard formation enthalpies? ▼
ΔH°rxn = Σ (n × ΔH°f(products)) - Σ (m × ΔH°f(reactants)).
What is the standard enthalpy of formation for pure elements? ▼
The standard enthalpy of formation (ΔH°f) for any element in its most stable standard state (at 25°C and 1 atm) is defined as exactly 0 kJ/mol.