When chemists or biologists use the term “thermodynamically favorable,” they are describing a process that has an intrinsic tendency to occur without needing a continuous external energy source. This concept is fundamental to understanding all chemical reactions, physical changes, and the complex machinery of life. A favorable reaction moves from a higher energy state to a lower energy state, releasing energy into its surroundings as it proceeds. This tendency toward a lower energy state drives systems toward equilibrium.
Defining Spontaneity
A thermodynamically favorable process is also referred to as a “spontaneous” process, although this term can be misleading. Spontaneity in this context does not mean the reaction happens suddenly or immediately, but rather that it is capable of proceeding on its own once initiated. Reactions that release usable energy as they proceed are often called exergonic reactions.
A common misconception is confusing thermodynamic favorability with the speed of a reaction, which is a separate field of study called kinetics. Thermodynamics predicts if a reaction is possible, whereas kinetics determines how fast that reaction will proceed. A reaction can be highly favorable but still occur at an imperceptible rate due to a large energy barrier. The classic example illustrating this difference is the conversion of diamond into graphite.
The transformation of diamond, a metastable form of carbon, into the more stable graphite is a thermodynamically favorable process at normal room temperature and pressure. However, this reaction requires an extremely high amount of energy, known as the activation energy, to break the strong carbon-carbon bonds in the diamond lattice. Consequently, diamonds do not noticeably turn into graphite even over millions of years, demonstrating that a favorable reaction can be incredibly slow, or kinetically unfavored.
Quantifying Favorability with Free Energy
The most accurate way to quantify thermodynamic favorability is by calculating the change in Gibbs Free Energy, symbolized as \(\Delta G\). This value represents the amount of energy within a system that is available to do useful work at a constant temperature and pressure. A reaction is designated as thermodynamically favorable, or spontaneous, only if the calculated value for \(\Delta G\) is negative.
Reactions with a negative \(\Delta G\) are called exergonic, indicating they release free energy to the surroundings. Conversely, if the calculation yields a positive \(\Delta G\), the reaction is termed endergonic, meaning it requires a continuous input of energy to proceed. When \(\Delta G\) is zero, the system is at chemical equilibrium, and there is no net change in the concentrations of reactants and products.
The magnitude of the negative \(\Delta G\) value indicates how far the reaction is from equilibrium and the maximum amount of work the reaction can theoretically perform. For instance, a reaction with a \(\Delta G\) of \(-50 \text{ kJ/mol}\) is more strongly favored than one with a \(\Delta G\) of \(-5 \text{ kJ/mol}\). This value only concerns the initial and final states of the reactants and products, providing no information about the energy barrier that must be overcome to start the reaction.
The Driving Forces of Chemical Change
The determination of Gibbs Free Energy depends on a combination of two distinct thermodynamic forces: the change in enthalpy (\(\Delta H\)) and the change in entropy (\(\Delta S\)). These two factors compete or cooperate to ultimately determine the sign of \(\Delta G\). Enthalpy is a measure of the heat content or the change in bond energy during a reaction. Reactions that release heat into the surroundings are called exothermic and have a negative \(\Delta H\).
Exothermic reactions, which involve forming stronger bonds in the products, generally contribute to favorability. For example, a combustion reaction releases a large amount of heat, which strongly drives the process toward being favorable. However, a reaction is not required to be exothermic to be favorable, as the second factor, entropy, plays a significant role.
Entropy (\(\Delta S\)) is a measure of the disorder or randomness within a system. Reactions that result in an increase in disorder, such as a solid turning into a gas, have a positive \(\Delta S\). An increase in entropy is a major driving force for favorability because systems naturally tend toward maximum disorder. The interplay between enthalpy and entropy is crucial, as a reaction that absorbs heat (endothermic, positive \(\Delta H\)) can still be favorable if it creates enough disorder.
The influence of entropy is modulated by temperature, which links enthalpy and entropy in the full free energy calculation. At high temperatures, the entropy factor becomes more influential, meaning that a highly disordered state can outweigh an unfavorable heat absorption. Conversely, at low temperatures, the enthalpy term dominates, and the reaction’s tendency to release heat becomes the primary driver of favorability.
Biological Relevance and Real-World Examples
The concept of thermodynamic favorability allows living organisms to function and maintain their highly ordered internal environments. Cells rely on a continuous supply of favorable, exergonic reactions to power the processes necessary for life. The most common of these energy-releasing reactions is the hydrolysis of adenosine triphosphate (ATP), the cell’s main energy currency.
The breakdown of ATP to adenosine diphosphate (ADP) and inorganic phosphate is a highly exergonic process. Under typical cellular conditions, the change in free energy for this reaction is approximately \(-50 \text{ kJ/mol}\) to \(-57 \text{ kJ/mol}\), making it strongly favorable. This significant energy release is then used to drive reactions that are otherwise unfavorable, such as muscle contraction, active transport across membranes, and the synthesis of complex molecules.
Unfavorable, or endergonic, biological processes, such as the building of proteins from amino acids, are made possible through a mechanism called reaction coupling. The energy released from the favorable ATP hydrolysis reaction is directly used to power the unfavorable protein synthesis reaction. Since Gibbs Free Energy is additive, the overall coupled reaction becomes thermodynamically favorable, ensuring that the necessary building blocks of life can be assembled spontaneously.