According to Hess’s law, which factor does the standard heat of reaction (enthalpy change at a stated temperature) NOT depend upon?

Introduction / Context:
Thermochemistry distinguishes state functions from path functions. The heat of reaction reported as an enthalpy change at a specified temperature is a state function result, central to energy balances and reactor heat-duty estimates.

Given Data / Assumptions:

• Enthalpy is a state function; it depends on initial and final states.
• Standard heats are tabulated at specified temperature, pressure, and physical states.
• Constant-pressure heat equals enthalpy change; constant-volume heat equals internal energy change.

Concept / Approach:
By Hess’s law, the enthalpy change for a reaction is independent of the path or mechanism. However, it does depend on the temperature and on the physical states of the species because these define the thermodynamic states. Reporting at constant pressure versus constant volume distinguishes ΔH from ΔU; the chosen condition matters for the type of heat reported but not for path dependence.

Step-by-Step Solution:

Verification / Alternative check:
Construct the reaction enthalpy from standard enthalpies of formation; the balanced sum is path-independent by definition.

Why Other Options Are Wrong:

• Physical state: affects enthalpy via latent/sensible contributions.
• Constant pressure vs constant volume: distinguishes ΔH and ΔU reporting.
• Reaction temperature: ΔH varies with T through heat capacities.

Common Pitfalls:
Assuming ΔH is numerically identical at any temperature or phase combination; always specify T and phase.

Final Answer:
the route or method by which the final products are obtained