What is Hess’s Law?
Hess’s Law states that the total enthalpy change of a reaction is the sum of the enthalpy changes of individual steps, regardless of the reaction path. This calculator determines the enthalpy change (\(\Delta H_{\text{reaction}}\)) using either:
- Summation of Steps Method
- Enthalpy of Formation Method
How the Calculation is Done
1. Summation of Steps Method
When a chemical reaction occurs in multiple steps, the overall enthalpy change is the sum of the enthalpy changes of each step:
\[ \Delta H_{\text{reaction}} = \sum \Delta H_{\text{steps}} \]
This calculator allows input for up to six steps, where each step’s enthalpy change (\(\Delta H\)) is entered in kJ/mol. The formula used:
\[ \Delta H_{\text{reaction}} = \Delta H_{\text{step1}} + \Delta H_{\text{step2}} + \Delta H_{\text{step3}} + \Delta H_{\text{step4}} + \Delta H_{\text{step5}} + \Delta H_{\text{step6}} \]
Only non-empty fields are included in the calculation.
2. Enthalpy of Formation Method
This method calculates the reaction enthalpy using the standard enthalpies of formation (\(\Delta H_f\)) of reactants and products:
\[ \Delta H_{\text{reaction}} = \sum \Delta H_f (\text{products}) – \sum \Delta H_f (\text{reactants}) \]
- \(\sum \Delta H_f (\text{products})\) = sum of the enthalpy of formation values of all products.
- \(\sum \Delta H_f (\text{reactants})\) = sum of the enthalpy of formation values of all reactants.
- Standard unit: kJ/mol.
Units Used
All enthalpy values in this calculator are in kJ/mol, which is the standard unit in thermodynamics for enthalpy change per mole of substance.
Frequently Asked Questions (FAQs)
1. What is Hess’s Law used for?
Hess’s Law is used to calculate the enthalpy change of a reaction when direct measurement is difficult. It helps determine reaction enthalpies from known stepwise reactions or standard enthalpy of formation values.
2. Why are only non-empty values included in the calculation?
If a step or formation enthalpy is unknown or irrelevant, leaving the field empty ensures that it is not mistakenly included in the summation.
3. What happens if I enter a negative enthalpy value?
Negative values indicate exothermic reactions (heat released). The calculator correctly processes both positive and negative values.
4. What is the difference between the two calculation methods?
- Summation of Steps Method is used when reaction enthalpies of intermediate steps are known.
- Enthalpy of Formation Method is used when standard enthalpy of formation values are available for reactants and products.
5. Can I enter more than six steps in the Summation of Steps Method?
Currently, this calculator supports up to six steps. If more steps are needed, sum the additional steps manually and enter the total in one of the available fields.
