Thermochemistry concept check:\nHess’s law of constant heat summation deals with what fundamental quantity or relationship?

Introduction / Context:
Hess’s law is a cornerstone of thermochemistry. It allows chemists and engineers to compute heats of reaction that may be difficult to measure directly by algebraically combining reactions whose enthalpies are known, because total enthalpy change depends only on the initial and final states—not on the path taken.

Given Data / Assumptions:

• Chemical reactions are considered at a common reference state.
• Enthalpy is a state function; path independence applies.

Concept / Approach:
Hess’s law states that the total heat of reaction for a process is the sum of the heats of reaction for any set of steps into which the overall reaction can be divided. This law directly concerns changes in heat of reaction (enthalpy change, ΔH), not kinetics or equilibrium constants.

Step-by-Step Solution:

Verification / Alternative check:
Example: If A → B has ΔH1 and B → C has ΔH2, then A → C has ΔH = ΔH1 + ΔH2, independent of the path. This illustrates Hess’s law in action.

Why Other Options Are Wrong:

• Equilibrium constant: related to ΔG° and temperature via van ’t Hoff; not Hess’s law.
• Reaction rate: kinetics are unrelated to enthalpy summation.
• Entropy change of formation: not the central subject of Hess’s law.
• None of these: incorrect because the law clearly handles heats of reaction.

Common Pitfalls:
Confusing path independence (thermodynamics) with rate dependence (kinetics), or mixing up ΔH with ΔG and K. Always distinguish thermodynamic state functions from kinetic behavior.

Final Answer:
Changes in the heat of reaction