The gas-phase conversion of to proceeds by the two-step mechanism shown

CHEMISTRY · Single correct · AP · Chemistry · Thermochemistry · Thermochemistry

Problem

The gas-phase conversion of AB2\text{AB}_{2} to A+B2\text{A}+\text{B}_{2} proceeds by the two-step mechanism shown. Step 1 AB2→AB+B\text{AB}_{2}\rightarrow \text{AB}+\text{B} Step 2 AB+B→A+B2\text{AB}+\text{B}\rightarrow \text{A}+\text{B}_{2} Average bond energies (kJ mol−1\text{kJ\,mol}^{-1}): A–B 250 B–B 180 Assume AB2\text{AB}_{2} contains two equivalent A–B bonds and AB\text{AB} contains one A–B bond. Which option correctly describes both the overall ΔH\Delta H and the relative position of the intermediate AB\text{AB} on a potential-energy diagram? (a) Overall ΔH≈+70 kJ\Delta H\approx +70\,\text{kJ}; the intermediate is higher than the reactants but lower than the products. (b) Overall ΔH≈+320 kJ\Delta H\approx +320\,\text{kJ}; the intermediate is higher in energy than the reactants but lower than the products. (c) Overall ΔH≈+320 kJ\Delta H\approx +320\,\text{kJ}; the intermediate is higher in energy than both reactants and products. (d) Overall ΔH≈−320 kJ\Delta H\approx -320\,\text{kJ}; the intermediate is lower in energy than both reactants and products.

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