Il existe un procédé de synthèse des hydrocarbures à partir...

Il existe un procédé de synthèse des hydrocarbures à partir des mélanges $\mathrm{CO} /$ $\mathrm{H}_2$, par lequel on peut par exemple obtenir l'octane $\mathrm{C}_8 \mathrm{H}_{18}$ : $$ 8 \mathrm{CO}+17 \mathrm{H}_2=\mathrm{C}_8 \mathrm{H}_{18}+8 \mathrm{H}_2 \mathrm{O} \text { (l) } $$ Comment pourrait-on déterminer l'enthalpie de cette réaction, connaissant l'enthalpie de combustion de l'octane, $$ \mathrm{C}_8 \mathrm{H}_{18}+25 / 2 \mathrm{O}_2=8 \mathrm{CO}_2+9 \mathrm{H}_2 \mathrm{O} \text { (l) } \quad \Delta_{\mathrm{r}} H^{\circ}=-5480 \mathrm{~kJ} \cdot \mathrm{mol}^{-1} $$ (Les autres données éventuellement nécessaires se trouvent dans l'Annexe I).

Il existe un procédé de synthèse des hydrocarbures à partir des mélanges $\mathrm{CO} /$ $\mathrm{H}_2$, par lequel on peut par exemple obtenir l'octane $\mathrm{C}_8 \mathrm{H}_{18}$ :
$$
8 \mathrm{CO}+17 \mathrm{H}_2=\mathrm{C}_8 \mathrm{H}_{18}+8 \mathrm{H}_2 \mathrm{O} \text { (l) }
$$

Comment pourrait-on déterminer l'enthalpie de cette réaction, connaissant l'enthalpie de combustion de l'octane,
$$
\mathrm{C}_8 \mathrm{H}_{18}+25 / 2 \mathrm{O}_2=8 \mathrm{CO}_2+9 \mathrm{H}_2 \mathrm{O} \text { (l) } \quad \Delta_{\mathrm{r}} H^{\circ}=-5480 \mathrm{~kJ} \cdot \mathrm{mol}^{-1}
$$
(Les autres données éventuellement nécessaires se trouvent dans l'Annexe I).

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Key Concepts

Hess's Law

Hess's Law states that the overall enthalpy change for a reaction is the same, regardless of the number of intermediate steps or the pathway taken. This fundamental principle in thermochemistry allows one to determine the enthalpy of a reaction by combining known reactions, as long as their sum yields the overall reaction of interest.

Reaction Enthalpy

Reaction enthalpy is the heat change associated with a chemical reaction occurring at constant pressure. It provides valuable information on the energy absorbed or released during a reaction, and it can be calculated using standard enthalpies of formation or by combining known reaction enthalpies.

Enthalpy of Combustion

The enthalpy of combustion is the heat released when a substance undergoes complete oxidation in the presence of oxygen under standard conditions. It is an important energy parameter, especially in the context of fuels, and serves as a reference point for calculating other reaction enthalpies using Hess's Law.

Standard Enthalpy of Formation

The standard enthalpy of formation is the change in enthalpy when one mole of a compound is formed from its elements in their standard states. This concept is fundamental for constructing energy cycles and calculating reaction enthalpies, as it provides a baseline for comparing the energy content of different compounds.

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