Which statement about Hess's Law is true?

• A. Enthalpy depends on the path
• B. Enthalpy is not conserved
• C. Enthalpy is a state function
• D. Only reactions with gases follow Hess's Law

Answer: (C)

Hess's Law rests on the idea that enthalpy is a state function, meaning its value depends only on the current state of the system (temperature, pressure, composition) and not on how the system got there. Because enthalpy is a state function, the total enthalpy change for a reaction is the same whether the reaction occurs in one step or through multiple steps. This lets us add up the enthalpy changes of individual steps to find the overall enthalpy change, which is why Hess's Law is so useful for calculating unknown enthalpies from known ones, such as using standard enthalpies of formation or combustion. The other statements don’t fit: saying enthalpy depends on the path contradicts its state-function nature; saying enthalpy is not conserved is a mischaracterization since what matters here is how enthalpy changes between states, with energy conservation still underpinning everything; and Hess's Law applies to reactions in all phases, not just gases.