On dit de l'acétylène C2 H2 (ou H-C ≡C-H ) que c'est un composé de formation endothermique, ce q

step 1 We can explain by the bonds break at high temperatures by using the delta G equation. Delta G is

On dit de l'acétylène C2 H2 (ou H-C ≡C-H ) que c'est un...

On dit de l'acétylène $\mathrm{C}_2 \mathrm{H}_2$ (ou $\mathrm{H}-\mathrm{C} \equiv \mathrm{C}-\mathrm{H}$ ) que c'est un composé de formation endothermique, ce qui explique sa forte tendance à se décomposer de façon exothermique violente (explosive). Selon les tables thermodynamiques, son enthalpie de formation est en effet $+227 \mathrm{~kJ}^{-1} \mathrm{~mol}^{-1}$. Mais comment un composé peut-il se former de façon endothermique, alors que toutes les enthalpies de liaison sont négatives?

On dit de l'acétylène $\mathrm{C}_2 \mathrm{H}_2$ (ou $\mathrm{H}-\mathrm{C} \equiv \mathrm{C}-\mathrm{H}$ ) que c'est un composé de formation endothermique, ce qui explique sa forte tendance à se décomposer de façon exothermique violente (explosive). Selon les tables thermodynamiques, son enthalpie de formation est en effet $+227 \mathrm{~kJ}^{-1} \mathrm{~mol}^{-1}$.
Mais comment un composé peut-il se former de façon endothermique, alors que toutes les enthalpies de liaison sont négatives?

Key Concepts

Reaction Energetics

Reaction energetics involves the study of energy changes during chemical reactions. It examines how the energy costs of breaking bonds compare to the energy released upon forming new bonds. A compound can have an endothermic formation if the energy required to break bonds and overcome activation barriers exceeds the energy released by new bond formation, making the formation process energetically uphill and potentially leading to instability.

Thermodynamic Pathways and Hess's Law

The observed enthalpy change for forming a compound is determined by the entire pathway of reactions from the elements to the compound. Hess's Law states that the total enthalpy change for a reaction is the same regardless of the path taken. In cases where intermediate steps are energetically unfavorable, such as the high energy required for initial bond breakage or rearrangement, the overall formation may be endothermic, even if individual bond formations release energy.

Bond Energy

Bond energy, or bond enthalpy, is the energy required to break a chemical bond in a molecule in the gas phase. Although individual bonds release energy upon formation (indicated by negative bond enthalpy values), the overall formation of a compound also involves various bond-breaking steps and reorganization of atoms, which can lead to a net endothermic process.

Enthalpy of Formation

The enthalpy of formation is the change in enthalpy when one mole of a compound is formed from its elements in their reference states. It reflects the overall energy change during the formation process, which can be positive (endothermic) or negative (exothermic) depending on the energy required or released, regardless of individual bond enthalpies being negative.

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