Key Takeaways
- Enthalpy change equals products minus reactants formation enthalpies
- Negative values indicate exothermic reactions (heat released)
- Positive values indicate endothermic reactions (heat absorbed)
- Standard enthalpy of formation for elements in their standard state is zero
- Hess's Law allows calculation of enthalpies for reactions that cannot be measured directly
What Is Enthalpy?
Enthalpy (H) is a thermodynamic property representing the total heat content of a system at constant pressure. The enthalpy change of a reaction represents the heat absorbed or released when reactants are converted to products under constant pressure conditions.
Understanding enthalpy is crucial for predicting whether reactions will release heat (useful for energy production) or require heat input (important for industrial processes). This calculator uses Hess's Law to determine the enthalpy change from standard formation enthalpies.
Hess's Law Explained
Delta H reaction = Sum Delta Hf (products) - Sum Delta Hf (reactants)
Hess's Law states that the total enthalpy change for a reaction is independent of the pathway taken. This means we can calculate the enthalpy change of any reaction by using the standard enthalpies of formation of products and reactants, even if we cannot measure the reaction directly.
How to Use This Calculator
- Look up formation enthalpies for all products and reactants from a thermodynamic data table
- Multiply each value by its stoichiometric coefficient in the balanced equation
- Sum the products and enter in the first field
- Sum the reactants and enter in the second field
- Click Calculate to find the enthalpy change
Example: Combustion of Methane
Reaction: CH4(g) + 2O2(g) -> CO2(g) + 2H2O(l)
Products:
- CO2(g): -393.5 kJ/mol x 1 = -393.5 kJ/mol
- H2O(l): -285.8 kJ/mol x 2 = -571.6 kJ/mol
- Sum: -965.1 kJ/mol
Reactants:
- CH4(g): -74.8 kJ/mol x 1 = -74.8 kJ/mol
- O2(g): 0 kJ/mol x 2 = 0 kJ/mol (element in standard state)
- Sum: -74.8 kJ/mol
Delta H = -965.1 - (-74.8) = -890.3 kJ/mol (Exothermic)
Common Standard Enthalpies of Formation
| Substance | Formula | Delta Hf (kJ/mol) |
|---|---|---|
| Water (liquid) | H2O(l) | -285.8 |
| Water (gas) | H2O(g) | -241.8 |
| Carbon dioxide | CO2(g) | -393.5 |
| Carbon monoxide | CO(g) | -110.5 |
| Methane | CH4(g) | -74.8 |
| Ethanol | C2H5OH(l) | -277.7 |
| Ammonia | NH3(g) | -45.9 |
| Sodium chloride | NaCl(s) | -411.2 |
Pro Tip: Elements in Standard State
The standard enthalpy of formation for any element in its most stable form at 25C and 1 atm is zero by definition. This includes O2(g), N2(g), H2(g), C(graphite), and metals in their standard states. This simplifies many calculations significantly.
Exothermic vs. Endothermic Reactions
The sign of the enthalpy change tells you about the energy flow in a reaction:
Exothermic Reactions (Negative Delta H)
- Heat is released to the surroundings
- Products have lower energy than reactants
- Surroundings get warmer
- Examples: Combustion, neutralization, respiration
Endothermic Reactions (Positive Delta H)
- Heat is absorbed from the surroundings
- Products have higher energy than reactants
- Surroundings get cooler
- Examples: Photosynthesis, dissolving ammonium nitrate, thermal decomposition
Frequently Asked Questions
Enthalpy is typically measured in kilojoules per mole (kJ/mol) or joules per mole (J/mol). Standard enthalpies of formation are always given per mole of substance formed. When calculating reaction enthalpies, always multiply by the stoichiometric coefficients from the balanced equation.
By definition, elements in their most stable form at standard conditions (25C, 1 atm) are assigned an enthalpy of formation of zero. This provides a reference point for measuring all other enthalpies. Since forming an element from itself requires no energy change, this convention makes thermodynamic calculations consistent and straightforward.
Not entirely. While exothermic reactions (negative enthalpy) are often spontaneous, Gibbs free energy is the true predictor of spontaneity. Gibbs free energy considers both enthalpy and entropy: Delta G = Delta H - T*Delta S. A reaction is spontaneous when Delta G is negative, which can occur even for endothermic reactions if entropy increases sufficiently.
Internal energy (U) is the total energy within a system. Enthalpy (H) equals internal energy plus pressure-volume work: H = U + PV. At constant pressure, enthalpy change equals the heat transferred, making it more practical for laboratory and industrial processes where pressure is typically constant.
Multiply each standard enthalpy of formation by its coefficient in the balanced equation before summing. For example, if 2H2O appears in products, use 2 x (-285.8) = -571.6 kJ. Remember: coefficients represent moles, so this accounts for the total enthalpy of all moles involved.