Enthalpy of Reaction

In a given reaction, the energy produced or absorbed by the whole reaction is equal to the energy of the products minus the energy of the reactants.

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Enthalpy of Reaction

Enthalpies of Formation (∆H_f^o)

• Enthalpies of formation represent the enthalpy change while making a given molecule.
• These are known values you will be given (on a chart).

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• Equation:

∆H_rxn (Total) = ∆H_f^o (products) - ∆H_f^o (reactants)

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1. Start with a balanced reaction.
2. Use table values to put ∆H_f^o values under each molecule
3. Multiply each value by the number of moles present for each
4. Add the energy of all the reactants and the energy of all the products separately
5. Subtract (products) – (reactants).

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What is the ∆H_rxn for this reaction?

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4NH_{3 (g)} + 5O_{2 (g)} 🡪 4NO _(g) + 6H₂O _(g)

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2HNO_{3 (aq)} + Na₂CO_{3 (s)} 🡪2NaNO_{3 (aq)} + H₂O_(l) + CO_2(g)

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