Spontaneity and Gibbs Free Energy - SS1 Chemistry Lesson Note
In thermodynamics, the concept of spontaneity refers to whether a process or a chemical reaction will occur naturally without any external intervention. A spontaneous process is one that occurs on its own accord, driven by the inherent properties of the system. On the other hand, a non-spontaneous process requires an input of energy or an external influence to occur.
The Gibbs Free Energy (G) is a thermodynamic function that is closely related to the spontaneity of a process. It is named after the American scientist Josiah Willard Gibbs. The Gibbs Free Energy incorporates both the enthalpy (H) and the entropy (S) of a system, providing a measure of the system's ability to do work.
The Gibbs Free Energy is given by the equation:
G = H - TS
Where:
- G represents the Gibbs Free Energy
- H represents the enthalpy of the system
- T represents the absolute temperature
- S represents the entropy of the system
The sign convention for the Gibbs Free Energy is as follows:
- If G is negative (G < 0), the process is spontaneous in the forward direction.
- If G is positive (G > 0), the process is nonspontaneous in the forward direction.
- If G is zero (G = 0), the system is at equilibrium.
The Gibbs Free Energy allows us to determine the spontaneity of a process based on the sign of ΔG, the change in Gibbs Free Energy. The equation for ΔG is:
ΔG = ΔH - TΔS
Where:
- ΔG represents the change in Gibbs Free Energy
- ΔH represents the change in enthalpy
- ΔS represents the change in entropy
- T represents the temperature
The relationship between ΔG and spontaneity is as follows:
- If ΔG is negative (ΔG < 0), the process is spontaneous in the forward direction.
- If ΔG is positive (ΔG > 0), the process is nonspontaneous in the forward direction.
- If ΔG is zero (ΔG = 0), the system is at equilibrium.
Based on this relationship, we can make the following conclusions:
- If ΔG is negative and TΔS is positive (ΔH is negative and ΔS is positive), the process is always spontaneous at all temperatures.
- If ΔG is positive and TΔS is negative (ΔH is positive and ΔS is negative), the process is always non-spontaneous at all temperatures.
- If ΔG is positive and TΔS is positive (ΔH is positive and ΔS is positive) or if ΔG is negative and TΔS is negative (ΔH is negative and ΔS is negative), the spontaneity depends on the temperature. At low temperatures, the process is nonspontaneous, while at high temperatures, the process becomes spontaneous.
It is important to note that the Gibbs Free Energy not only indicates the spontaneity of a process but also provides information about the maximum work that can be obtained from the system. When a process occurs spontaneously, the decrease in Gibbs Free Energy corresponds to the maximum work that can be extracted from the system. This relationship is described by the equation:
ΔG = -wmax
Where:
- wmax represents the maximum work that can be obtained from the system
In summary, the Gibbs Free Energy is a crucial thermodynamic parameter that determines the spontaneity of a process. By comparing the change in Gibbs Free Energy (ΔG) with the temperature (T), enthalpy (ΔH), and entropy (ΔS) changes, we can predict whether a process will occur spontaneously or not. Additionally, the Gibbs Free Energy provides insights into the maximum work that can be obtained from a system undergoing a spontaneous process.