Hess’s Law Enthalpy Calculator
Enter total number of reactions in your pathway
Results:
Total Enthalpy Change: 0 kJ/mol
Step Breakdown:
| Step | ΔH Input | Multiplier | Reversed | Contribution |
|---|
What is Hess’s Law?
Hess’s Law states that the total enthalpy change during a chemical reaction is independent of the pathway between initial and final states. This fundamental principle of thermochemistry allows calculation of overall ΔH by summing enthalpy changes of individual reaction steps.
Hess’s Law Formula
The general formula for calculating enthalpy change using Hess’s Law:
ΔHtotal = Σ (coefficient × ΔHstep × reversal factor)
| Variable | Meaning | Unit | Range |
|---|---|---|---|
| ΔHstep | Enthalpy change of individual reaction | kJ/mol or J/mol | -500 to 500 kJ |
| Coefficient | Stoichiometric multiplier | Unitless | 0.1-10 |
Practical Examples
Example 1: Combustion of carbon to CO2 through intermediate CO formation
- Step 1: C + O2 → CO2 (ΔH = -393.5 kJ)
- Step 2: CO → C + ½O2 (Reversed, ΔH = +110.5 kJ)
- Total ΔH = -393.5 + 110.5 = -283 kJ
Using the Calculator
- Enter number of reaction steps
- Input ΔH values with proper signs
- Check reversal for inverted reactions
- Apply stoichiometric coefficients
- Select desired output units
Factors Affecting Enthalpy Calculations
- Reaction directionality
- Stoichiometric coefficients
- Phase changes
- Temperature dependence
- Measurement accuracy
- Side reactions