Which principle states that the total energy in a closed system remains constant?

• A. First Law of Thermodynamics
• B. Second Law of Thermodynamics
• C. Law of Mass Action
• D. Law of Conservation of Mass

Answer: (A)

The principle that states that the total energy in a closed system remains constant is known as the First Law of Thermodynamics. This law is essentially a statement of the conservation of energy, which implies that energy can neither be created nor destroyed, but can only change forms. In practical terms, this means that the energy in a closed system is constant over time unless it is altered by external work or heat transfer.

For example, in a chemical reaction occurring within a closed container, the total energy before the reaction (in terms of the energies of reactants) will equal the total energy after the reaction (in terms of the energies of products plus any heat absorbed or released). This principle is foundational in understanding energy transfers and transformations in physical and chemical processes.

Other principles mentioned, such as the Second Law of Thermodynamics, specifically address the direction of energy transformations and the concept of entropy, indicating that energy spontaneously tends to disperse or spread out. The Law of Mass Action pertains to the rates of chemical reactions based on the concentrations of reactants and products, while the Law of Conservation of Mass states that mass is neither created nor destroyed in a chemical reaction, although it is a different concept than energy conservation.