Hess (2 etapas)
- Created by
- Renato Passos, Eng. de Software
- Reviewed by
- Renato Passos, Eng. de Software
Last updated: Apr 18, 2026
About this calculator
The Hess's Law (2-step) calculator determines the total enthalpy change (ΔH) of a chemical reaction split into two intermediate steps. It uses the formula ΔH = ΔH1 + ΔH2, summing the enthalpy changes of partial reactions. It's ideal for problems where a complex reaction is broken into two simpler steps with known ΔH1 and ΔH2 values.
It algebraically combines thermochemical equations of the steps, respecting reaction direction and stoichiometric coefficients. For instance, if the first step releases heat (negative ΔH1) and the second absorbs it (positive ΔH2), the calculation accounts for these signs. Inverting equations is necessary when aligning reactants and products correctly.
Use this tool when the overall reaction can't be measured directly but has enthalpy data for intermediate steps. Common applications include combustion calculations, compound formation, or industrial processes. Be cautious with sign errors and stoichiometric balancing, as they affect the final accuracy.
Frequently asked questions
How does the calculator combine the two steps?
It sums the enthalpy changes (ΔH1 and ΔH2) of the intermediate reactions, respecting their signs and balanced coefficients. If a step is reversed, the corresponding ΔH sign also changes.
Do I need to balance chemical equations before using the calculator?
Yes, balanced equations are required for accurate results. Incorrect stoichiometric coefficients will affect the ΔH calculation.
What if there are more than two steps?
This calculator is designed for two steps. For multiple steps, use thermodynamic calculation tools that handle multi-step reactions.
Why does the ΔH sign change when I reverse a reaction?
The ΔH sign flips because reversing the reaction direction alters whether heat is absorbed or released. For example, if ΔH1 is -100 kJ for the forward reaction, it becomes +100 kJ in reverse.