American Chemical Society (ACS) Chemistry Test 2025 – 400 Free Practice Questions to Pass the Exam

Question: 1 / 400

What does Hess's Law state about enthalpy changes in a reaction carried out in a series of steps?

∆H is the average of all steps

∆H equals the sum of enthalpy changes for the individual steps

Hess's Law states that the total enthalpy change for a chemical reaction is equal to the sum of the enthalpy changes for each step in the process, regardless of the pathway taken from reactants to products. This principle is grounded in the first law of thermodynamics, which asserts that energy cannot be created or destroyed but can only change forms. In essence, it indicates that enthalpy is a state function, which means it depends only on the initial and final states of the system, not on the specific route taken to reach that state.

This law allows chemists to calculate the reaction enthalpy even if it is challenging to measure directly in a laboratory setting. By breaking down a reaction into multiple steps with known enthalpy changes, the overall change in enthalpy can be calculated simply by adding these values together. Thus, the correctness of the answer hinges on this fundamental understanding of enthalpy as it applies to reaction pathways.

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Each step has an equal value of ∆H

∆H is independent of the steps taken

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