Delta H equals the sum of product formation enthalpies minus the sum for reactants.
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The enthalpy change of a reaction can be calculated from standard enthalpies of formation using delta H rxn equals the sum of n times delta Hf for products minus the sum of m times delta Hf for reactants, where n and m are the stoichiometric coefficients. The method works because the reaction can be imagined as first breaking all reactants into their elements and then forming the products from those elements, so the elemental steps cancel. Formation values for elements in their standard states are zero and need not be included. Coefficients must be applied carefully, because each formation value refers to one mole of compound. The result is the enthalpy at standard conditions, and it is reliable when accurate tables are available.
Key ideas: Use delta H rxn equals sum of n delta Hf of products minus the sum of m delta Hf of reactants. Write the balanced equation, list each species with its coefficient, multiply, and subtract. Include states, because gaseous and liquid water have different values.
Common mistakes and connections: Typical errors are reversing the subtraction, forgetting coefficients and omitting an element with a nonzero value in a nonstandard form. Remember that elements in standard state contribute zero. The method is a direct application of Hess's law and is preferred over bond enthalpies when accurate data exist.
Reactants and products both form from elements: delta H rxn = sum Hf(products) - sum Hf(reactants) = -965.1 - (-74.8) = -890.3 kJ for burning methane. The interactive lesson shows this as a live animation or chart.
For the combustion of methane, CH4 + 2O2 -> CO2 + 2H2O, the enthalpy change is -393.5 + 2(-285.8) - (-74.8) = -890.3 kJ.
Delta H equals the sum of product formation enthalpies minus the sum for reactants.
Reaction Enthalpy from Formation Data is a intermediate topic in Thermodynamics & Energetics. It builds on the vocabulary of beginner topics in this area.
Study these earlier topics in Thermodynamics & Energetics first: Enthalpy of Neutralization, Enthalpy of Solution, Hess's Law.
The next topics in Thermodynamics & Energetics are Bond Enthalpy Calculations and Enthalpy of Atomization.
Yes. The interactive lesson includes a self-check question on Reaction Enthalpy from Formation Data, and the Thermodynamics & Energetics quiz draws on every topic in this area.
Reaction Enthalpy from Formation Data is topic 25 of 91 in Thermodynamics & Energetics.