Consider the two hypothetical elementary reactions below
CHEMISTRY · Single correct · A Level · Chemistry · Energetics · Energetics
Problem
Consider the two hypothetical elementary reactions below.
• Reaction X: breaks two C–H bonds (bond energy ≈ 410 kJ mol⁻¹ each) and forms one C=C bond (bond energy ≈ 610 kJ mol⁻¹).
• Reaction Y: breaks one C–C bond (bond energy ≈ 350 kJ mol⁻¹) and forms two C–H bonds (bond energy ≈ 410 kJ mol⁻¹ each).
Which statement is most consistent with these bond-energy data?
(a) Reaction Y is more likely to be endothermic because breaking a C–C bond requires more energy than is released when forming C–H bonds
(b) Reaction X is more likely to be exothermic because forming a C=C bond releases more energy than breaking two C–H bonds requires
(c) Reaction Y is neither endothermic nor exothermic because the energies for breaking and forming bonds are approximately equal
(d) Reaction X is more likely to be endothermic because the energy required to break bonds exceeds the energy released when new bonds form