Thermochemistry of ammonia synthesis: For the reaction N2(g) + 3 H2(g) → 2 NH3(g), the overall process under Haber–Bosch conditions is best described as

Introduction / Context:
Knowing whether a reaction releases or absorbs heat guides temperature selection, heat integration, and equilibrium management in reactors. Ammonia synthesis is a flagship industrial reaction with strong temperature and pressure sensitivities dictated by both thermodynamics and kinetics.

Given Data / Assumptions:

• Reaction: N2 + 3 H2 → 2 NH3.
• Typical industrial conditions: elevated pressure, iron- or ruthenium-based catalyst.
• Standard thermochemical data indicate heat release.

Concept / Approach:
Ammonia formation reduces the number of gas moles (4 → 2) and forms strong N–H bonds, releasing heat. The standard enthalpy change at 25°C is negative (exothermic). Consequently, lower temperatures shift equilibrium toward NH3, while higher temperatures accelerate kinetics; industrial operation balances these via moderate temperatures and high pressures with interstage cooling and recycle.

Step-by-Step Solution:

Verification / Alternative check:
Handbooks report ΔH° ≈ −92 kJ per 2 mol NH3 (about −46 kJ per mol NH3), confirming exothermicity.

Why Other Options Are Wrong:

• Endothermic: incorrect—would require heat input to proceed.
• Autocatalytic: catalysts are external (Fe, Ru), not a product-driven catalysis.
• None/Athermal: contradicted by measured heat of reaction.

Common Pitfalls:
Confusing kinetic requirements (needing temperature) with thermodynamic heat effect; exothermic does not mean it proceeds rapidly without a catalyst.

Final Answer:
Exothermic.