Which term describes the heat absorbed or released during a chemical reaction?

• A. Heat of formation
• B. Hess's Law
• C. Enthalpy change
• D. Calorimeter

Answer: (C)

Heat absorbed or released during a chemical reaction is described by the enthalpy change, ΔH. Enthalpy is the heat content of a system at constant pressure, so the heat exchanged with the surroundings as the reaction occurs equals this change in enthalpy. If heat flows into the system, ΔH is positive (endothermic); if heat flows out, ΔH is negative (exothermic). Because enthalpy is a state function, its value depends only on the starting and ending states, not on the path taken to get there. This makes ΔH a useful way to characterize the heat aspect of a reaction under typical laboratory conditions. Related ideas include Hess's Law, which states that the total enthalpy change for a reaction is the same regardless of the route, and the heat of formation, which is a specific ΔH for forming a compound from its elements. A calorimeter, by contrast, is simply the instrument used to measure heat flow.