Free energy change, symbolized as Delta G, is a powerful concept used across chemistry and biology to predict whether a process will happen on its own. It represents the portion of energy within a system that is available to perform useful work at a constant temperature and pressure. Understanding this value allows scientists to determine the feasibility of chemical reactions or physical changes without the need for continuous external energy input. This calculation is foundational for studying everything from metabolic pathways in the human body to industrial chemical synthesis.
Defining Free Energy Change and Spontaneity
The physical significance of free energy change lies in its ability to measure the capacity of a system to drive a process forward. A process is considered “spontaneous” if it proceeds naturally toward its conclusion once started, similar to a ball rolling down a hill. Spontaneity does not mean the reaction is fast; it only means it does not require a constant supply of outside energy to keep going.
When a reaction releases free energy, that energy becomes available to power other necessary functions, such as muscle contraction or active transport across a cell membrane. Conversely, processes that consume free energy require an input from another source to proceed. The sign and magnitude of the calculated Delta G are the ultimate predictors of a reaction’s inherent driving force.
The Fundamental Calculation: Enthalpy, Entropy, and Temperature
The primary method for calculating the standard free energy change (Delta G-naught) involves combining three fundamental thermodynamic factors into the Gibbs equation. This equation mathematically balances the energy released or absorbed with the degree of disorder created during a reaction. The formula is expressed as Delta G = Delta H – T Delta S, where each component contributes to the final outcome.
The first factor, Delta H, represents the change in enthalpy, which is essentially the heat absorbed or released during the reaction at constant pressure. Reactions that release heat have a negative Delta H and tend to favor spontaneity, while those that absorb heat (positive Delta H) are less favored. Enthalpy is often the dominant factor in determining the energy exchange of a process.
The second factor, Delta S, is the change in entropy, which measures the change in randomness or disorder of the system. Processes that increase disorder, such as a solid turning into a gas, have a positive Delta S and contribute favorably to a reaction’s spontaneity. A higher degree of molecular freedom after the reaction suggests a more favorable outcome.
The final component is the absolute temperature, \(T\), which must always be expressed in Kelvin for this calculation. Temperature serves as a weighting factor for the entropy term, determining how significantly the change in disorder impacts the overall free energy. At higher temperatures, the T Delta S term becomes larger, making the entropy change a much stronger driver of spontaneity.
The standard free energy change, Delta G-naught, is specifically calculated under standard conditions. These conditions are defined as 1 atmosphere pressure, 1 molar concentration for all solutions, and usually a specified temperature, often 298 Kelvin (25 degrees Celsius).
Calculating Change Under Non-Standard Conditions
While the standard free energy change (Delta G-naught) provides a useful baseline, most chemical and biological reactions occur under non-standard conditions. Concentrations of reactants and products in a living cell, for example, rarely match the 1 molar standard used in the initial calculation. To determine the actual free energy change (Delta G) under these real-world conditions, an adjustment must be made based on the current composition of the system.
This adjustment uses the relationship Delta G = Delta G-naught + RT ln Q, which connects the standard value to the current state of the reaction. Here, \(R\) is the ideal gas constant, and \(T\) is the absolute temperature in Kelvin. The new variable, \(Q\), is the reaction quotient, which reflects the ratio of product concentrations to reactant concentrations at any given moment.
If the system has a high concentration of reactants relative to products, \(Q\) will be small, and the Delta G will be driven toward a more negative, spontaneous value. Conversely, a high concentration of products makes \(Q\) large, pushing the reaction toward a less spontaneous state.
The standard free energy change, Delta G-naught, is also directly related to the equilibrium constant, \(K\). At equilibrium, the reaction is perfectly balanced, meaning the actual free energy change (Delta G) is zero, and the reaction quotient (\(Q\)) equals the equilibrium constant (\(K\)). In this special case, the equation simplifies to Delta G-naught = -RT ln K, establishing a direct link between the standard energy potential and the ultimate ratio of products to reactants the system will achieve.
Interpreting the Final Value
Once the calculation for the free energy change is complete, the sign of the final value provides a clear prediction about the reaction’s behavior. A negative Delta G value indicates that the reaction is spontaneous, or exergonic, meaning it will release free energy and proceed naturally without continuous external intervention. This energy release can then be utilized to power other processes within the system.
A positive Delta G value signals a non-spontaneous, or endergonic, reaction that requires a continuous input of energy to proceed. In biological systems, these energy-requiring reactions are often made possible through a process called reaction coupling. A highly exergonic reaction (negative Delta G) is linked to an endergonic reaction (positive Delta G) so the net change of the combined process is still negative.
Finally, a Delta G equal to zero signifies that the system is at equilibrium. At this point, the forward and reverse reaction rates are balanced, and there is no net change in the concentrations of reactants or products over time. The magnitude of the Delta G value indicates how far the reaction is from this equilibrium state.