A potential-energy profile for the gas-phase reaction
A₂(g) + B₂(g) → 2 AB(g)
is described below.
• The vertical axis is labeled “Potential Energy, kJ mol⁻¹,” and the horizontal axis is labeled “Reaction Coordinate.”
• The reactant line at the far left is at 0 kJ mol⁻¹ (chosen as reference).
• Two distinct uphill segments are shown before the transition state peak:
– Breaking one A–A bond corresponds to a vertical rise of 420 kJ mol⁻¹.
– Breaking one B–B bond corresponds to an additional vertical rise of 265 kJ mol⁻¹.
• After the peak, the curve drops in two stages toward the products:
– Forming the first A–B bond lowers the energy by 370 kJ mol⁻¹.
– Forming the second A–B bond lowers the energy by another 370 kJ mol⁻¹, placing the product line at –55 kJ mol⁻¹ relative to reactants.
Use the profile to analyze the reaction energetics.
(a) Identify which specific chemical bonds are broken and which are formed as the reaction proceeds from reactants to products.
(b) Using the numerical changes indicated on the profile, explain whether the reaction is overall endothermic or exothermic. In your explanation, compare the total energy required to break bonds with the total energy released when new bonds form.
(c) Suppose a catalyst were added that provides an alternate pathway with a lower maximum on the potential-energy curve while leaving the energies of reactants and products unchanged. Describe how the profile would differ and explain whether the catalyst would alter the enthalpy change you discussed in part (b).