Heat of Reaction Calculator Formula

Understand the math behind the heat of reaction calculator. Each variable explained with a worked example.

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Formulas Used

Heat of Reaction per Mole

heat_per_mole = heat_kj / moles

Variables

VariableDescriptionDefault
heat_kjTotal Heat Measured (q)(kJ)-50
molesMoles of Limiting Reagent(mol)0.1

How It Works

Heat of Reaction per Mole

Formula

ΔH_rxn = q / n

Where:

  • ΔH_rxn = heat of reaction per mole (kJ/mol)
  • q = total heat measured (kJ), negative for exothermic
  • n = moles of limiting reagent

    • This converts calorimetry data into a molar enthalpy value for the reaction.

    Worked Example

    A reaction releases 50 kJ of heat using 0.1 mol of limiting reagent.

    heat_kj = -50moles = 0.1
    1. 01ΔH_rxn = q / n
    2. 02ΔH_rxn = -50 / 0.1
    3. 03ΔH_rxn = -500 kJ/mol

    Frequently Asked Questions

    What is heat of reaction?

    Heat of reaction is the enthalpy change per mole of reaction as written. It depends on the balanced equation and the stoichiometric coefficients used.

    How do I determine the sign of q?

    If the temperature of the surroundings increases, the reaction is exothermic and q is negative. If the temperature decreases, q is positive (endothermic).

    Is heat of reaction the same as enthalpy of reaction?

    At constant pressure, yes. The heat measured at constant pressure equals ΔH. At constant volume (bomb calorimeter), q equals ΔU, not ΔH.

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