A) Écrire l'équation stæchiométrique de la réaction de...

a) Écrire l'équation stæchiométrique de la réaction de formation du chlorure d'hydrogène $\mathrm{HCl}$. Quels nombres stœchiométriques faut-il choisir pour que l'enthalpie molaire de réaction de référence corresponde à la formation d'une mole de $\mathrm{HCl}$ $\left(\Delta_{\mathrm{f}} H_{\mathrm{m}}^{\mathrm{o}}(\mathrm{HCl})=-92,5 \mathrm{~kJ} \cdot \mathrm{mol}^{-1}\right)$ ? b) Que représente la réaction dont l'équation stœchiométrique s'écrit : $$ \mathrm{H}+\mathrm{Cl}=\mathrm{HCl} $$ et pour laquelle $\Delta_{\mathrm{r}} H_{\mathrm{m}}^{\mathrm{o}}(\mathrm{HCl})=-432 \mathrm{~kJ} \cdot \mathrm{mol}^{-1}$ ? c) Retrouver la valeur de l'enthalpie molaire de réaction de référence pour la formation du chlorure d'hydrogène à partir de chlore et de dihydrogène, connaissant leurs énergies de liaison. $\mathrm{D}(\mathrm{Cl}-\mathrm{Cl})=243 \mathrm{~kJ} \cdot \mathrm{mol}^{-1}$ $\mathrm{D}(\mathrm{H}-\mathrm{H})=436 \mathrm{~kJ} \cdot \mathrm{mol}^{-1}$ $\mathrm{D}(\mathrm{H}-\mathrm{Cl})=432 \mathrm{~kJ} \cdot \mathrm{mol}^{-1}$

a) Écrire l'équation stæchiométrique de la réaction de formation du chlorure d'hydrogène $\mathrm{HCl}$.
Quels nombres stœchiométriques faut-il choisir pour que l'enthalpie molaire de réaction de référence corresponde à la formation d'une mole de $\mathrm{HCl}$ $\left(\Delta_{\mathrm{f}} H_{\mathrm{m}}^{\mathrm{o}}(\mathrm{HCl})=-92,5 \mathrm{~kJ} \cdot \mathrm{mol}^{-1}\right)$ ?
b) Que représente la réaction dont l'équation stœchiométrique s'écrit :
$$
\mathrm{H}+\mathrm{Cl}=\mathrm{HCl}
$$
et pour laquelle $\Delta_{\mathrm{r}} H_{\mathrm{m}}^{\mathrm{o}}(\mathrm{HCl})=-432 \mathrm{~kJ} \cdot \mathrm{mol}^{-1}$ ?
c) Retrouver la valeur de l'enthalpie molaire de réaction de référence pour la formation du chlorure d'hydrogène à partir de chlore et de dihydrogène, connaissant leurs énergies de liaison.
$\mathrm{D}(\mathrm{Cl}-\mathrm{Cl})=243 \mathrm{~kJ} \cdot \mathrm{mol}^{-1}$
$\mathrm{D}(\mathrm{H}-\mathrm{H})=436 \mathrm{~kJ} \cdot \mathrm{mol}^{-1}$
$\mathrm{D}(\mathrm{H}-\mathrm{Cl})=432 \mathrm{~kJ} \cdot \mathrm{mol}^{-1}$

Key Concepts

Stoichiometric Equation

A stoichiometric equation represents a chemical reaction with the correct proportions of reactants and products, ensuring that mass and atoms are conserved. It provides the quantitative basis for calculating the amounts of substances that react or are produced, and it is essential for defining the reaction in a manner that aligns with thermodynamic quantities, such as reaction enthalpy.

Enthalpy of Formation

The enthalpy of formation is the standard enthalpy change when one mole of a compound is formed from its constituent elements in their standard states. This concept is key in thermochemistry because it provides a reference value that can be used to calculate the enthalpy changes of other reactions through Hess's law, ensuring consistency and comparability of thermodynamic data.

Reaction Enthalpy

Reaction enthalpy is the overall energy change associated with a chemical reaction. It can be calculated using the enthalpies of formation of the products and reactants, or alternatively through bond energies. Understanding reaction enthalpy is fundamental in designing reactions and predicting whether a reaction is exothermic or endothermic.

Bond Dissociation Energy

Bond dissociation energy is the amount of energy required to break a chemical bond in a molecule, resulting in isolated atoms or fragments. This concept is crucial for estimating reaction enthalpies based on the energy required to break bonds in the reactants versus the energy released when new bonds are formed in the products.

Hess's Law

Hess’s law states that the total enthalpy change for a chemical reaction is the same, regardless of the pathway taken, because enthalpy is a state function. This principle allows one to calculate the enthalpy change of a reaction indirectly using known enthalpy changes of intermediate steps, such as bond breaking and formation, or using formation enthalpies.

Reference Reaction and Standard States

In thermochemistry, a reference reaction is chosen to represent a process in a standard and reproducible way, often used to define standard enthalpy changes like the enthalpy of formation. Standard states are the physical forms of substances under specified standard conditions (usually 1 bar pressure and a specified temperature), which must be used to ensure consistency in thermodynamic calculations.

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