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How is Gibbs free energy defined? At its most basic, Gibbs free energy (G) measures the useful work that can be extracted from chemical reactions that occur in an environment at constant temperature and pressure. All physical processes have an associated change in Gibbs free energy, which determines whether or not the process will occur spontaneously.
For this article, we’ll define Gibbs free energy as the total energy available in a chemical reaction, which we can usually measure in units of kilojoules per mole (kJ/mol). Though it might seem confusing at first, you’ll understand how to use and understand Gibbs free energy in no time. First, let’s start with an explanation of what enthalpy and Gibbs free energy are and how they relate to each other.
Gibbs Free Energy Defined
First, let’s go over a brief refresher on enthalpy and enthalpy definition (aka, heat content). Then we can move on to how Gibbs’s free energy relates. In chemistry, enthalpy refers to how much energy must be expended—or put in—to break down a substance into its parts.
Essentially, you can think of enthalpy as stored energy. The equation below shows what’s needed for an exothermic reaction: When reactants are fully broken down into products, their total H changes from H=O+H-OH+H-O-H; a negative sign means that there are fewer molecules than there were before.
Applications of Gibbs Free Energy
Gibbs free energy also predicts whether a chemical reaction will occur spontaneously as opposed to needing an outside source of energy, like light, heat, or electricity. Many important industrial processes rely on Gibbs’s free energy measurements for process control; for example, control of peroxide decomposition on a bleaching line for producing pulp paper in a pulp mill.
How does Gibbs’s free energy work?
The Gibbs free energy, named after Josiah Willard Gibbs, provides a general framework for determining whether chemical reactions will occur or not at constant temperature and pressure.
Gibbs free energy has a general form of: G = H – TS where H is enthalpy, T is temperature and S is entropy.
How does Gibbs’s free energy work?
Gibbs free energy, or Gibbs energy of formation, is simply a measure of how difficult a chemical reaction will be. Free energy can be defined in terms of Gibbs free energy, which can be defined as Gibbs free energy = ΔH – TΔS where ΔH = enthalpy change and TΔS = absolute temperature times entropy change. T
herefore, Gibbs’s free energy gives you an idea about how much work could be done by a system (not if it will happen). If there is negative Gibbs free energy, then spontaneous change would occur. For example, water spontaneously changes into ice at 0 °C without any input of heat because its enthalpies are lower at that temperature than at higher temperatures.
Why is Gibbs’s free energy negative?
For chemists, Gibbs free energy has one major advantage over enthalpy: It is usually expressed as a negative value. The convention in chemistry is that negative values of Gibbs free energy represent spontaneous processes. That is, when an endothermic reaction occurs in a closed system, it absorbs heat from its surroundings.
Likewise, an exothermic reaction releases heat into its surroundings. If you plot an endothermic process on a graph against temperature (the third dimension) versus entropy (the second dimension), you get something called a TDS diagram. A plot of an exothermic process looks very similar but with reversed signs: lower temperatures and higher entropies are associated with spontaneous reactions.
Conclusion
Gibbs free energy has been used in many engineering applications that require reactions to take place at certain temperatures or pressures. It can be used in thermodynamic calculations to help predict whether a chemical reaction will produce any work. The Enthalpy – Gibbs Free Energy relationship gives us information about the spontaneity of reactions, which is crucial in determining whether or not a reaction will occur.
