Si on chauffe le carbure de calcium CaC2 en présence de diazote, on observe l'équilibre : CaC2(

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Si on chauffe le carbure de calcium CaC2 en présence de...

Si on chauffe le carbure de calcium $\mathrm{CaC}_2$ en présence de diazote, on observe l'équilibre : $$ \mathrm{CaC}_2(\mathrm{~s})+\mathrm{N}_2(\mathrm{~g}) \rightleftharpoons \mathrm{CaCN}_2(\mathrm{~s})+\mathrm{C}(\mathrm{s}) \quad\left(\Delta_{\mathrm{r}} H_{\mathrm{m}}^{\mathrm{o}}=-291 \mathrm{~kJ} \cdot \mathrm{mol}^{-1}\right) $$ À l'air libre, sous la pression de $1 \mathrm{bar}$, cette réaction donne lieu à un équilibre à $1400^{\circ} \mathrm{C}$. a) Quelle est la valeur de sa constante d'équilibre ? b) Quelle évolution observerait-on en abaissant la température à $1300^{\circ} \mathrm{C}$ ?

Si on chauffe le carbure de calcium $\mathrm{CaC}_2$ en présence de diazote, on observe l'équilibre :
$$
\mathrm{CaC}_2(\mathrm{~s})+\mathrm{N}_2(\mathrm{~g}) \rightleftharpoons \mathrm{CaCN}_2(\mathrm{~s})+\mathrm{C}(\mathrm{s}) \quad\left(\Delta_{\mathrm{r}} H_{\mathrm{m}}^{\mathrm{o}}=-291 \mathrm{~kJ} \cdot \mathrm{mol}^{-1}\right)
$$

À l'air libre, sous la pression de $1 \mathrm{bar}$, cette réaction donne lieu à un équilibre à $1400^{\circ} \mathrm{C}$.
a) Quelle est la valeur de sa constante d'équilibre ?
b) Quelle évolution observerait-on en abaissant la température à $1300^{\circ} \mathrm{C}$ ?

Key Concepts

Exothermic Reaction

An exothermic reaction is one that releases heat into its surroundings (indicated by a negative enthalpy change, ?rH°). In the context of equilibrium, this means that heat can be considered as a product. As a result, reducing the temperature will generally shift the equilibrium toward the product side according to Le Chatelier’s principle, thereby increasing the equilibrium constant.

Temperature Dependence of Equilibrium (van 't Hoff Equation)

The equilibrium constant of a reaction changes with temperature according to the van 't Hoff equation, which relates the temperature dependence of K to the standard reaction enthalpy (?rH°). For exothermic reactions (where ?rH° is negative), an increase in temperature decreases the value of K, whereas a decrease in temperature increases K, reflecting the shift of equilibrium in response to temperature changes.

Le Chatelier’s Principle

Le Chatelier’s Principle states that if a dynamic equilibrium is disturbed by changing the conditions, the system will adjust itself to partially counteract the effect of the disturbance. For exothermic reactions, lowering the temperature removes heat from the system, which the equilibrium counteracts by shifting in the direction that produces heat—toward the products—increasing the yield of products and the equilibrium constant.

Chemical Equilibrium

Chemical equilibrium is the state in which the concentrations of the reactants and products remain constant over time because the rates of the forward and reverse reactions are equal. In heterogeneous systems, even if one or more reactants or products are in a different phase (such as solids in this case), equilibrium principles still apply based on the components that change concentration, typically the gases or solutes.

Equilibrium Constant (K)

The equilibrium constant is a numerical value that expresses the ratio of the concentrations (or activities) of products to reactants at equilibrium, each raised to the power of their stoichiometric coefficients. In reactions involving pure solids, the activities of those solids are taken as constant (typically equal to 1), and only the concentrations of gases (or solutions) affect the equilibrium constant value. This constant depends on temperature, so its value will vary if the temperature is changed.

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