Acetylene in nature is formed from the thermal decomposition of long-chain hydrocarbons at high temperatures (1700 K). Acetylene is a hydrocarbon composed of two carbon and two hydrogen atoms. The decomposition reaction for acetylene is:
C₂H₂(g) → 2 C(s) + H₂(g)
Use bond energies to explain why this reaction is improbable. The bond energies are:
H-H: 432 kJ/mol
C≡C: 839 kJ/mol
C-H: 413 kJ/mol

• A. ΔH_rxn = 12.5 kJ/mol; This reaction is improbable because it is endothermic.
• B. ΔH_rxn = 1233 kJ/mol; This reaction is improbable because it requires a lot of energy.
• C. ΔH_rxn = 0 kJ/mol; The enthalpy of reaction is 0 kJ/mol so the reaction is at equilibrium.
• D. ΔH_rxn = −245 kJ/mol; This reaction can easily take place because it is exothermic.

Answer: (B)