Advanced Placement (AP) Chemistry Practice Exam

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What does ΔH signify in thermodynamics?

Temperature change
Heat of a system at constant pressure
Randomness of the system
Gibbs free energy change

The correct answer is: Heat of a system at constant pressure

The symbol ΔH in thermodynamics represents the change in enthalpy of a system at constant pressure. Enthalpy is a thermodynamic quantity that reflects the total heat content of a system, which includes both the internal energy and the energy associated with the pressure and volume of the system. When a reaction occurs at constant pressure, ΔH indicates the amount of heat absorbed or released during the reaction. For example, if a chemical reaction releases heat to the surroundings, the ΔH will be negative, indicating an exothermic process. Conversely, if heat is absorbed, ΔH will be positive, indicating an endothermic process. Understanding ΔH is crucial in predicting how a chemical reaction will behave and how much energy will be involved. Other choices describe different thermodynamic concepts. Temperature change pertains to the variation in kinetic energy of particles, which does not directly equate to enthalpy change. Randomness of the system relates to entropy, not enthalpy, which measures the degree of disorder. Gibbs free energy change, represented by ΔG, indicates the spontaneity of a process and incorporates both enthalpy and entropy, but it is not the same as ΔH.