Name: calculating reaction heat with hess's law
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Enthalpies of Reaction

Para una reacción que ocurre en condiciones estándar, se utiliza una ecuación general para calcular el cambio de entalpía estándar de la reacción. Esta ecuación se resuelve encontrando la diferencia entre la suma de las entalpías estándar de formación de los productos y la suma de las entalpías estándar de formación de los reactivos, cada una multiplicada por su coeficiente estequiométrico. Considere la combustión de 2 moles de gas acetileno con 5 moles de gas oxígeno para formar 4 moles de gas dióxido de carbono y 2 moles de vapor de agua en condiciones estándar.La entalpía de la reacción es igual a la suma de cuatro veces la entalpía de formación de gas dióxido de carbono y dos veces la entalpía de formación de vapor de agua, menos la suma de dos veces la entalpía de formación de gas acetileno y cinco veces la entalpía de formación de gas oxígeno. La ecuación de entalpía se deriva de combinar dos conceptos:la entalpía estándar de formación y la ley de Hess. El primer término representa las entalpías estándar de formación de los productos;la formación de dióxido de carbono del carbono y el oxígeno ecuación 1, y la formación de agua a partir de hidrógeno y oxígeno ecuación 2.Las entalpías estándar conocidas de formación del dióxido de carbono y el agua son 393, 5 kilojulios negativos y 241, 8 kilojulios negativos, respectivamente. Dado que la combustión produce 4 moles de dióxido de carbono, el delta H1 sería la entalpía estándar de formación de dióxido de carbono multiplicado por 4, que sería 1574 kilojulios negativos. La combustión también produce 2 moles de agua, entonces el cambio de entalpía el delta H2 sería la entalpía estándar de formación de agua multiplicada por 2, que sería 483, 6 kilojulios negativos.Esto produce 2058 kilojulios negativos como la entalpía estándar neta de formación de los productos. El segundo término representa la descomposición del acetileno en carbono e hidrógeno ecuación 3. Esta es la reacción inversa para la entalpía de formación estándar del reactivo, y por lo tanto el valor de su entalpía, 227, 4 kilojulios, está precedido por un signo negativo, que también se puede ver en la ecuación de la entalpía.Debido a que la reacción consume 2 moles de acetileno, el delta H3 sería el negativo de la entalpía de formación estándar del acetileno multiplicado por 2, lo que equivale a 453, 4 kilojulios negativos. La entalpía de formación estándar de oxígeno es cero. Por lo tanto, la entalpía de formación estándar neta de los reactivos es 453, 4 kilojulios negativos.Recuerde de la ley de Hess, que si se lleva a cabo una reacción de un paso en varios pasos, luego se suman las entalpías de cada paso, esto es igual al cambio de entalpía neta. Sustituir los valores por las entalpías de formación en la ecuación da la entalpía de la reacción como 2511 kilojulios negativos.

<ol>
	<li>Davis, Thomas W. "Un malentendido común de la ley de Hess". Revista de Educación Química 28, no. 11 (1951): 584.</li>
	<li>Lee, A. L., H. L. Feldkirchner, F. C. Schora y J. J. Henry. "Calor de reacción del hidrógeno y el carbón". Diseño y Desarrollo de Procesos Químicos de Ingeniería Industrial 7, no. 2 (1968): 244-249.</li>
</ol>

entalpías de reacción

La ley de Hess se puede utilizar para determinar el cambio de entalpía de cualquier reacción siempre que estén disponibles las correspondientes entalpías de formación de los reactivos y productos. La reacción principal puede dividirse en reacciones escalonadas: (i) descomposición de los reactantes en sus elementos constituyentes, para los cuales los cambios de entalpía son proporcionales al negativo de las entalpías de formación de los reactantes, −ΔHf°(reactantes), seguido de (ii) recombinaciones de los elementos (obtenidos en el paso 1) para formar los productos, con los cambios de entalpía proporcionales a las entalpías de formación de los productos, ΔHf° (productos). El cambio de entalpía estándar de la reacción general es, por lo tanto, igual a: (ii) la suma de las entalpías estándar de formación de todos los productos más (i) la suma de los negativos de las entalpías estándar de formación de los reactantes, como se indica en la siguiente ecuación, donde ∑ representa “la suma de” y n representa los coeficientes estequiométricos.
 
 <img alt="Eq1" src="https://www.jovecore.com/CDNSource/lessons/802/802_Equations-01.png" style="height: 40px;" />
 
 La ecuación usualmente es ligeramente reorganizada para ser escrita de la siguiente manera: 
 
 <img alt="Eq2" src="https://www.jovecore.com/CDNSource/lessons/802/802_Equations-02.png" style="height: 40px;"" />
 
 El siguiente ejemplo muestra en detalle por qué esta ecuación es válida y cómo utilizarla para calcular el cambio de entalpía estándar para una reacción:
 
 <img alt="Eq3" src="https://www.jovecore.com/CDNSource/lessons/802/802_Equations-03.png" style="height: 40px;" />
 
 Aquí se utiliza la forma especial de la ley de Hess y los valores de calor de formación para los reactivos y productos: ΔHf° (HNO3) = −206,64 kJ/mol; ΔHf° (NO) = +90.2 kJ/mol; ΔHf° (NO2) = +33 kJ/mol; ΔHf° (H2O) = −285,8 kJ/mol. 
 
 <img alt="Eq4" src="https://www.jovecore.com/CDNSource/lessons/802/802_Equations-04.png" style="height: 40px;" />
 
 <img alt="Eq5" src="https://www.jovecore.com/CDNSource/lessons/802/802_Equations-05.png" style="height: 40px;" />
 
 <img alt="Eq6" src="https://www.jovecore.com/CDNSource/lessons/802/802_Equations-06.png" style="height: 40px;" />
 
 <img alt="Eq7" src="https://www.jovecore.com/CDNSource/lessons/802/802_Equations-07.png" style="height: 40px;" />
 
 
 Este texto ha sido adaptado de <a href="https://openstax.org/books/chemistry-2e/pages/5-3-enthalpy">Openstax, Química 2e, Sección 5.3: Entalpía.</a>

Entalpías de Reacción

La ley de Hess se puede utilizar para determinar el cambio de entalpía de cualquier reacción si las entalpías correspondientes de formación de los reactivos y productos ...

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calculating reaction heat with hess's law

Calculating reaction heat with Hess's law uses standard enthalpies of formation to find the enthalpy change of a reaction. When the formation values for the reactants and products are known, the overall reaction can be broken into simpler stepwise reactions. This approach helps show how the heat change for the full reaction is built from smaller parts.

First, the reactants are thought of as being decomposed into their component elements. The enthalpy change for this step is proportional to the negative of the standard enthalpies of formation of the reactants, written as -ΔHf°(reactants). Next, those elements are recombined to form the products, and this step has an enthalpy change proportional to ΔHf°(products).

The standard enthalpy change of the overall reaction is found by adding the sum of the standard enthalpies of formation of all the products and the sum of the negatives of the standard enthalpies of formation of the reactants. In the equation, Σ means &quot;the sum of&quot; and n is the stoichiometric coefficient. The formula is usually rearranged into a simpler form for calculation.

A worked example shows why the equation works and how to use it to calculate the standard enthalpy change of a reaction. In that example, the heat of formation values used are ΔHf°(HNO3) = -206.64 kJ/mol, ΔHf°(NO) = +90.2 kJ/mol, ΔHf°(NO2) = +33 kJ/mol, and ΔHf°(H2O) = -285.8 kJ/mol. This method follows the special form of Hess's law described in OpenStax Chemistry 2e, Section 5.3: Enthalpy.

Calculating Reaction Heat with Hess's Law

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Calculating reaction heat with Hess's law lets chemists find enthalpy changes without measuring every reaction directly. Some reactions are hard or impossible to test in the lab. Even when a reaction can be measured, it is often easier to calculate the heat change from other known enthalpy values. Hess's law says that if a process can be written as the sum of several stepwise processes, the enthalpy change for the total process equals the sum of the enthalpy changes for the steps. This works...

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enthalpies of reaction

Calculating reaction heat with Hess's law uses standard enthalpies of formation to find the enthalpy change of a reaction. When the formation values for the reactants and products are known, the overall reaction can be broken into simpler stepwise reactions. This approach helps show how the heat change for the full reaction is built from smaller parts. First, the reactants are thought of as being decomposed into their component elements. The enthalpy change for this step is proportional to the...

key terms and concepts

Use Hess's law and standard enthalpies of formation to calculate reaction heat. Learn the stepwise method and follow a worked chemistry example.

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Calculating Reaction Heat with Hess's Law

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