Calculate standard reaction enthalpy from up to four reactants and four products using supplied formation enthalpies and coefficients from a balanced equation. The tool does not balance equations or supply preset species data.

Supply a balanced equation and matching species phases at one specified temperature. Unused row: leave both entries blank. This tool does not balance or validate chemical equations.

Rows 3โ€“4 and display precision

Standard Reaction Enthalpy (ฮ”Hยฐrxn) from ฮ”Hยฐf Formula

The following example outlines the steps and information needed to calculate the standard enthalpy change of reaction (ฮ”Hยฐrxn) using tabulated standard enthalpies of formation (ฮ”Hยฐf).

ฮ” H^ยฐแตฃโ‚“โ‚™ = ฮฃ nu ฮ” H^ยฐf(products) - ฮฃ nu ฮ” H^ยฐf(reactants)

Variables:

  • ฮ”Hยฐrxn is the standard enthalpy change of reaction (kJ/mol of reaction as written)
  • ฮฝ is the stoichiometric coefficient for each species (use positive values on each side)
  • ฮ”Hยฐf is the standard enthalpy of formation of each species (kJ/mol), defined for formation of 1 mol of the substance from its elements in their reference states at the specified temperature and standard pressure (1 bar). Tables often use 298.15 K; standard state alone does not fix temperature

To calculate ฮ”Hยฐrxn from ฮ”Hยฐf values, sum ฮฝฮ”Hยฐf for the products and subtract the sum ฮฝฮ”Hยฐf for the reactants (elements in their standard states have ฮ”Hยฐf = 0).

How to Calculate ฮ”Hยฐrxn from ฮ”Hยฐf?

The following steps outline how to calculate the standard enthalpy change of reaction.

  1. Write the balanced chemical equation and identify each species on the reactant and product sides.
  2. Find ฮ”Hยฐf for each species in the correct phase at a common specified temperature. Elements in their reference states have ฮ”Hยฐf = 0. Select the correct kJ/mol or thermochemical kcal/mol input unit for each row.
  3. Compute ฮฃ(ฮฝฮ”Hยฐf) for the products and ฮฃ(ฮฝฮ”Hยฐf) for the reactants.
  4. Subtract: ฮ”Hยฐrxn = ฮฃ(ฮฝฮ”Hยฐf)products โˆ’ ฮฃ(ฮฝฮ”Hยฐf)reactants.
  5. Check your answer with the calculator above.

Example Problem: 

Compute ฮ”Hยฐrxn for the formation of liquid water: 2 H2(g) + O2(g) โ†’ 2 H2O(l). Use ฮ”Hยฐf[H2O(l)] = โˆ’285.83 kJ/mol and ฮ”Hยฐf for H2(g) and O2(g) (standard states) = 0.

ฮฃ(ฮฝฮ”Hยฐf) products = 2 ร— (โˆ’285.83) = โˆ’571.66 kJ per mol of reaction as written

ฮฃ(ฮฝฮ”Hยฐf) reactants = 2 ร— 0 + 1 ร— 0 = 0 kJ per mol of reaction as written

ฮ”Hยฐrxn = โˆ’571.66 โˆ’ 0 = โˆ’571.66 kJ per mol of reaction as written (exothermic). This doubled water-formation equation produces 2 mol of water; the molar formation enthalpy of Hโ‚‚O(l) is โˆ’285.83 kJ/mol. Use the balanced equation basis consistently.