ΔG = ΔH - TΔS

Definition

ΔG, or the change in Gibbs free energy, is a fundamental concept in thermodynamics that describes the spontaneity and feasibility of a chemical process. It is defined by the equation ΔG = ΔH - TΔS, where ΔH is the change in enthalpy, T is the absolute temperature, and ΔS is the change in entropy. This equation provides a comprehensive understanding of the driving forces behind chemical reactions and physical transformations.

5 Must Know Facts For Your Next Test

1. The sign of ΔG determines the spontaneity of a process: a negative ΔG indicates a spontaneous process, a positive ΔG indicates a non-spontaneous process, and a ΔG of zero indicates a process at equilibrium.
2. The magnitude of ΔG indicates the driving force of the process, with a larger negative value of ΔG corresponding to a greater driving force for the process to occur.
3. Processes with a positive ΔH and a positive ΔS, or a negative ΔH and a negative ΔS, will have a negative ΔG and be spontaneous.
4. Processes with a positive ΔH and a negative ΔS, or a negative ΔH and a positive ΔS, may have a positive ΔG and be non-spontaneous.
5. The temperature of the system can significantly affect the value of ΔG, as it is directly proportional to the term TΔS.

Review Questions

• Explain the relationship between ΔG, ΔH, and ΔS, and how it determines the spontaneity of a process.
  • The relationship between ΔG, ΔH, and ΔS is described by the equation ΔG = ΔH - TΔS. This equation shows that the spontaneity of a process is determined by the balance between the change in enthalpy (ΔH) and the change in entropy (ΔS), modified by the absolute temperature (T). If ΔG is negative, the process is spontaneous, meaning it will occur naturally. If ΔG is positive, the process is non-spontaneous and will not occur naturally without the input of external energy. The magnitude of ΔG indicates the driving force of the process, with a larger negative value corresponding to a greater driving force.
• Describe how the temperature of the system can affect the value of ΔG and the spontaneity of a process.
  • The temperature of the system is a crucial factor in determining the value of ΔG, as it is directly proportional to the term TΔS in the equation ΔG = ΔH - TΔS. As the temperature increases, the value of TΔS also increases, which can change the overall sign and magnitude of ΔG. For processes with a positive ΔH and a positive ΔS, or a negative ΔH and a negative ΔS, an increase in temperature will make ΔG more negative, increasing the spontaneity of the process. Conversely, for processes with a positive ΔH and a negative ΔS, or a negative ΔH and a positive ΔS, an increase in temperature can make ΔG more positive, decreasing the spontaneity of the process.
• Analyze the role of enthalpy and entropy in determining the feasibility of a chemical process, and provide examples to illustrate their influence on ΔG.
  • Enthalpy (ΔH) and entropy (ΔS) are the two key factors that determine the feasibility of a chemical process, as reflected in the equation ΔG = ΔH - TΔS. Processes with a negative ΔH (exothermic) and a positive ΔS (increase in disorder) will have a negative ΔG and be spontaneous, such as the combustion of fuels. Processes with a positive ΔH (endothermic) and a negative ΔS (decrease in disorder) will have a positive ΔG and be non-spontaneous, such as the formation of complex organic molecules from simpler precursors. However, if the positive ΔH is offset by a large positive ΔS, the process may still be spontaneous, as seen in the dissolution of many salts in water. Understanding the interplay between enthalpy and entropy is crucial for predicting the feasibility and spontaneity of chemical processes.

Related terms

Enthalpy (ΔH): Enthalpy is the sum of a system's internal energy and the work done by or on the system due to changes in pressure and volume. It represents the total energy released or absorbed during a chemical reaction or physical process.

Entropy (ΔS): Entropy is a measure of the disorder or randomness of a system. It quantifies the amount of energy that is unavailable for useful work during a process, and it increases as a system becomes more disordered.

Spontaneity: Spontaneity refers to the natural tendency of a process to occur without the input of external energy. A spontaneous process is one that occurs naturally and has a negative value of ΔG.