MadSci Network: Chemistry |
While I'm not aware of anything specifically called "Gibbs Free Energy Equation", I can try and explain what the Gibbs energy (sometimes called Gibbs Free Energy) is and how it relates to enthalpy and entropy. The Gibbs energy is defined as G = H - TS, where H is the enthalpy, T is the temperature, and S is the entropy. But what does it represent? Basically, it is a way to talk about the way energy and entropy work in competing directions in determining the equilibrium state of a system. This is sometimes called "energy-entropy compensation". All other things being equal, systems will tend toward the state of lowest energy (enthalpy is a measure of energy that includes the energy due to the system having a pressure, but you don't need to worry about that detail). However, systems at temperatures above absolute zero do not just sit in the lowest-energy state, because the thermal energy causes the molecules to fly around and get into other states. The entropy can be thought of as a measure of how many different states a system samples due to thermal energy. At very low temperatures, the system doesn't get to sample all these other states, and the equilibrium is determined by the low-energy state. At very high temperatures, the intermolecular energies are relatively unimportant compared to the thermal energy, and the system tends to go to a state of maximum entropy. The competition between these two is represented by the Gibbs energy. The condition for equilibrium at a fixed temperature and pressure is that the Gibbs energy is a minimum -- when T is small that means minimizing the enthalpy and when T is big it means maximizing the entropy. As an example, consider some argon. At very low temperatures, the intermolecular attractions cause the atoms to freeze into a regular crystal lattice, which is the lowest energy state. This state has a low entropy since the atoms are stuck in one spot and can't sample other configurations. At high temperatures, the entropy is more important and the argon atoms fly around in a gaseous state with higher entropy, even though it means they have more energy than if they were sitting in a nice regular lattice. With regard to the second part of your question, one can't just talk about "higher temperature"; one has to talk about the other constraints on the system such as whether you are increasing the temperature while holding volume constant, or holding pressure constant, or what. However, under most constraints, both the enthalpy and entropy will increase with increasing temperature. The enthalpy because it is primarily a measure of the energy in a system, and raising the temperature generally means putting in energy. The entropy because more thermal energy generally allows the system to sample a wider range of states.
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