What does Hess's law state about the enthalpy change of a reaction?

• A. Enthalpy depends on mechanism and steps
• B. The total enthalpy change is the sum of the enthalpy changes of the steps, independent of the path.
• C. Enthalpy only depends on temperature
• D. Enthalpy change is always zero

Answer: (B)

Enthalpy change for a reaction is a state function, so it depends only on the starting and ending states, not on the path taken. Because of this, Hess's law says the total enthalpy change of a reaction equals the sum of the enthalpy changes of the individual steps used to reach the products, regardless of how those steps are arranged. In practice, you can add known reactions to build the overall reaction and add their ΔH values, or use standard enthalpies of formation to find the overall ΔH. This means the heat absorbed or released is the same whether the reaction occurs in one step or through multiple steps. The other statements don’t fit: enthalpy does not depend on the mechanism or steps, it depends on the state; it isn’t determined solely by temperature; and enthalpy change is not always zero.