Pour déterminer indirectement les enthalpies de réaction, on...

Pour déterminer indirectement les enthalpies de réaction, on utilise souvent comme données des enthalpies de combustion facilement calculables à partir de données expérimentales. Déterminez l'enthalpie de la réaction de synthèse du méthanol, $$ \mathrm{CO}(\mathrm{g})+2 \mathrm{H}_2(\mathrm{~g})=\mathrm{CH}_3 \mathrm{OH} \text { (1) } $$ en utilisant les données suivantes : $$ \begin{array}{ll} 2 \mathrm{CO}+\mathrm{O}_2=2 \mathrm{CO}_2 & \Delta_{\mathrm{r}} H^{\circ}=-566 \mathrm{~kJ} \cdot \mathrm{mol}^{-1} \\ 2 \mathrm{H}_2+\mathrm{O}_2=2 \mathrm{H}_2 \mathrm{O}(\mathrm{g}) & \Delta_{\mathrm{r}} H^{\circ}=-484 \mathrm{~kJ} \cdot \mathrm{mol}^{-1} \\ \mathrm{CH}_3 \mathrm{OH}(\mathrm{l})+3 / 2 \mathrm{O}_2=\mathrm{CO}_2+2 \mathrm{H}_2 \mathrm{O}(\mathrm{g}) & \Delta_{\mathrm{r}} H^{\circ}=-638 \mathrm{~kJ} \cdot \mathrm{mol}^{-1} \end{array} $$

Pour déterminer indirectement les enthalpies de réaction, on utilise souvent comme données des enthalpies de combustion facilement calculables à partir de données expérimentales.
Déterminez l'enthalpie de la réaction de synthèse du méthanol,
$$
\mathrm{CO}(\mathrm{g})+2 \mathrm{H}_2(\mathrm{~g})=\mathrm{CH}_3 \mathrm{OH} \text { (1) }
$$
en utilisant les données suivantes :
$$
\begin{array}{ll}
2 \mathrm{CO}+\mathrm{O}_2=2 \mathrm{CO}_2 & \Delta_{\mathrm{r}} H^{\circ}=-566 \mathrm{~kJ} \cdot \mathrm{mol}^{-1} \\
2 \mathrm{H}_2+\mathrm{O}_2=2 \mathrm{H}_2 \mathrm{O}(\mathrm{g}) & \Delta_{\mathrm{r}} H^{\circ}=-484 \mathrm{~kJ} \cdot \mathrm{mol}^{-1} \\
\mathrm{CH}_3 \mathrm{OH}(\mathrm{l})+3 / 2 \mathrm{O}_2=\mathrm{CO}_2+2 \mathrm{H}_2 \mathrm{O}(\mathrm{g}) & \Delta_{\mathrm{r}} H^{\circ}=-638 \mathrm{~kJ} \cdot \mathrm{mol}^{-1}
\end{array}
$$

Key Concepts

Standard Reaction Enthalpy

Standard reaction enthalpy refers to the heat change associated with a reaction carried out under standard conditions (typically 1 atm pressure and 298 K temperature) with substances in their standard states. It provides a common reference that makes comparing and combining different reaction data consistent and meaningful.

Algebraic Manipulation of Chemical Equations

In order to apply Hess's Law, reactions can be reversed or scaled by coefficients. Reversing a reaction changes the sign of its enthalpy change, while multiplying the reaction by a coefficient multiplies the enthalpy change by that same factor. This manipulation is essential to combine reactions appropriately to yield the desired overall reaction.

Enthalpy of Combustion

The enthalpy of combustion is the heat released when a substance completely burns in oxygen under standard conditions. It is often measured experimentally and used as a reliable data point for calculating other reaction enthalpies indirectly, especially when direct measurement is challenging.

Hess's Law

Hess's Law states that the total enthalpy change for a reaction is the same, regardless of the pathway taken, as long as the initial and final conditions are identical. This principle allows one to determine the enthalpy change of a target reaction by combining other reactions with known enthalpy changes.

Transcript

Please give Ace some feedback