According to Kirchhoff’s equation in thermochemistry, the heat of reaction depends primarily on which variable (through heat capacities)?

Introduction / Context:
Standard heats of reaction are tabulated at reference temperatures (often 298 K), but process conditions may differ. Kirchhoff’s equation provides a way to adjust the heat of reaction from one temperature to another using heat capacities, enabling accurate energy balances at operating conditions.

Given Data / Assumptions:

• Heats of formation and reaction are state functions.
• Heat capacities Cp(T) of reactants and products are known or approximated.
• Pressure effects on heats are usually negligible for condensed phases and small for gases over moderate ranges.

Concept / Approach:
Kirchhoff’s equation states that d(ΔH_rxn)/dT = ΔCp, where ΔCp is the sum of heat capacities of products minus reactants. Integrating between T1 and T2 gives ΔH_rxn(T2) = ΔH_rxn(T1) + ∫_(T1→T2) ΔCp dT. Thus, temperature is the controlling variable for adjusting heats of reaction via heat capacities. Neither reaction order nor molecularity appears in the thermodynamic relation, and pressure effects are secondary unless very high pressures are involved for gases.

Step-by-Step Solution:

Verification / Alternative check:
Using tabulated Cp polynomials, engineers routinely correct ΔH_rxn from 298 K to reactor temperature; these calculations match calorimetric data, validating the temperature dependence predicted by Kirchhoff’s equation.

Why Other Options Are Wrong:

• Pressure/Volume: may affect reaction enthalpy slightly for gases but are not the basis of Kirchhoff’s relation.
• Molecularity/Reaction order: kinetic concepts; do not enter thermodynamic state property adjustments.

Common Pitfalls:
Confusing temperature correction (thermodynamics) with rate changes (kinetics); neglecting ΔCp variation with temperature when large ranges are involved.

Final Answer:
temperature