Advanced Placement (AP) Biology Practice Exam

What does free energy measure in a system?

1. Energy that cannot perform work

2. Energy available to do work under uniform conditions

3. Total energy content of the system

4. Energy required for reaction completion

The correct answer is: Energy available to do work under uniform conditions

Free energy, often represented as Gibbs free energy in the context of biological systems, quantifies the energy within a system that is available to perform useful work at constant temperature and pressure. This concept is pivotal in the study of thermodynamics as it relates to chemical reactions, metabolism, and cellular processes. When considering the conditions of a system, free energy reflects the amount of energy that can be extracted from a system and harnessed for processes such as synthesizing molecules, driving biochemical reactions, or performing mechanical work. It helps predict whether a process will occur spontaneously; negative changes in free energy indicate that a reaction can proceed without an external input of energy, while a positive change indicates that external energy is required. The other options are distinct concepts that do not pertain directly to the definition of free energy. For instance, energy that cannot perform work describes internal energy forms that may not be utilized in biological functions. Total energy content includes all forms of energy in a system, not just the usable portion designated by free energy. Lastly, the energy required for reaction completion pertains to the activation energy and the energy barrier of a reaction, rather than the available energy after the system reaches equilibrium. Thus, the understanding of free energy focuses specifically on its role in predicting and driving