Difficulty: Easy
Correct Answer: Number of intermediate chemical reactions involved
Explanation:
Introduction / Context:
Hess's law states that the enthalpy change of a process depends only on the initial and final states of the system, not on the pathway taken. This powerful idea allows engineers and chemists to compute heats of reaction by combining tabulated steps, even if the overall reaction cannot be measured directly. Identifying what enthalpy change is independent of helps avoid conceptual errors in energy balances.
Given Data / Assumptions:
Concept / Approach:
Because enthalpy is a state function, ΔH depends only on initial and final states. Therefore, ΔH is independent of the number or nature of intermediate steps. However, ΔH does depend on conditions such as temperature, pressure, and phases of reactants and products, because those define the initial and final states. Any option suggesting independence from these state-defining parameters would be incorrect.
Step-by-Step Solution:
Identify the property: enthalpy is a state function.State what follows: ΔH is path independent.Find the option representing the path details: number of intermediate reactions.Select that option as the independent factor.
Verification / Alternative check:
Construct hypothetical cycles that begin and end at the same state; the sum of ΔH around a closed loop is zero, confirming path independence and the validity of Hess's law.
Why Other Options Are Wrong:
Temperature, pressure, and physical states define the state of the system, so ΔH changes when they change.
Common Pitfalls:
Confusing path independence of ΔH with path independence of heat q in general processes; only under constant pressure with negligible kinetic and potential energy changes does heat equal ΔH.
Final Answer:
Number of intermediate chemical reactions involved
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