Gibbs Free Energy (GFE) is a thermodynamic concept used to understand chemical and physical processes. It indicates whether a process, such as a chemical reaction, can occur without a continuous external energy input. Scientists use GFE to predict the direction of a process under conditions of constant temperature and pressure. The calculation of this energy combines the effects of heat exchange and the degree of disorder within a system. Evaluating the change in GFE determines the inherent driving force behind a transformation.
Defining Gibbs Free Energy
Gibbs Free Energy, represented by the symbol \(G\), is a measure of a system’s capacity to perform work. It accounts for the energy that is available, or “free,” to be converted into useful work, excluding work related to volume changes. \(G\) combines the system’s internal energy, pressure, volume, temperature, and entropy into one comprehensive value.
In practical chemistry, the focus is placed on the change in Gibbs Free Energy, symbolized as \(\Delta G\). This value is calculated when a process moves from its initial state to its final state. The \(\Delta G\) reflects the maximum amount of non-expansion work that can be extracted from a system and ultimately predicts the feasibility of a reaction.
The Link Between Gibbs Energy and Work
The fundamental nature of Gibbs Free Energy is rooted in its relationship to energy transfer and work. Since GFE represents the maximum amount of non-volume-change work a system can perform, its units must fundamentally be units of energy.
The capacity to do work and the measurement of energy share the same scale. For instance, GFE quantifies the portion of stored chemical potential energy in a battery that is available to power a circuit. Therefore, any unit used to measure energy transferred in a process, such as heat or work, is the appropriate unit for GFE.
Standard Units of Measurement
Since Gibbs Free Energy quantifies available energy, its standard International System of Units (SI) unit is the Joule (J). The Joule is the measure of work or energy, defined as one kilogram meter squared per second squared (\(\text{kg}\cdot\text{m}^2/\text{s}^2\)). Because energy changes in chemical reactions are often substantial, the Kilojoule (kJ), equal to 1,000 Joules, is the more common unit for reporting total \(\Delta G\).
For chemical reactions, the unit is typically expressed as Kilojoules per mole (\(\text{kJ}/\text{mol}\)) or Joules per mole (\(\text{J}/\text{mol}\)). The “per mole” component standardizes the measurement, representing the energy change for one mole of substance undergoing the reaction. This molar basis allows for direct comparison of energy changes between different reactions.
Interpreting Gibbs Energy Values
The sign of the calculated \(\Delta G\) value provides the most practical insight into the nature of a reaction.
Negative \(\Delta G\) (Spontaneous)
A negative \(\Delta G\) (\(\Delta G < 0[/latex]) indicates that a process is thermodynamically favored and will proceed spontaneously without needing a continuous external energy source. These are known as exergonic reactions, meaning the system releases available energy. A common example is the rusting of iron, which proceeds naturally once initiated.
Positive [latex]\Delta G\) (Non-Spontaneous)
A positive \(\Delta G\) (\(\Delta G > 0\)) signifies a non-spontaneous process, termed an endergonic reaction. This means it will not occur on its own and requires a continuous input of energy from the surroundings to drive it forward, such as charging a battery. The magnitude of the positive value indicates the minimum amount of energy that must be supplied to force the transformation to happen.
Zero \(\Delta G\) (Equilibrium)
When \(\Delta G\) is exactly zero (\(\Delta G = 0\)), the system is at equilibrium. At this point, the forward and reverse reaction rates are equal, and there is no net change in the concentrations of reactants and products.
Speed vs. Feasibility
While GFE determines if a reaction is possible, it offers no information about the speed at which the reaction will take place. For instance, the conversion of diamond to graphite has a negative \(\Delta G\), indicating it is spontaneous, but the process is extremely slow. Therefore, a negative \(\Delta G\) guarantees the transformation is energetically favorable, not that it will happen quickly.