Enthalpy Formula
Use standard heats of formation to find reaction enthalpy
Reaction Enthalpy
DeltaH_rxn = sum(n * DeltaH_f, products) - sum(n * DeltaH_f, reactants)
DeltaH_f values are standard molar enthalpies of formation (kJ/mol). Multiply by stoichiometric coefficients n.
Steps
- Balance the chemical equation.
- Look up DeltaH_f for each compound (kJ/mol).
- Multiply DeltaH_f by stoichiometric coefficients for products and reactants.
- Subtract reactant sum from product sum to get DeltaH_rxn.
Example: Combustion of CH4
Reaction: CH4 + 2 O2 → CO2 + 2 H2O (l)
DeltaH_f (kJ/mol): CH4 = -74.8, O2 = 0, CO2 = -393.5, H2O(l) = -285.8
Products sum = 1(-393.5) + 2(-285.8) = -965.1
Reactants sum = 1(-74.8) + 2(0) = -74.8
DeltaH_rxn = -965.1 - (-74.8) = -890.3 kJ
Exothermic (negative sign).
Common Mistakes
Unbalanced equation
DeltaH calculations are invalid if coefficients are wrong.
Sign errors
Remember: products minus reactants, not the reverse.
Wrong phase data
Use DeltaH_f values for the correct phase (g, l, s, aq).
Mixing units
Keep everything in kJ/mol; convert if needed.
FAQ
What is standard state?
Most tables use 1 bar and 25 C; check the reference conditions.
Can I use bond energies instead?
Yes for estimates; heats of formation are more accurate for specific reactions.
Why is O2 set to zero?
Elements in their standard state have DeltaH_f = 0 by definition.