Free energy change

Definition

Free energy change refers to the difference in free energy between the products and reactants of a chemical reaction, indicating the spontaneity and thermodynamic favorability of that reaction. It is a crucial concept in understanding chemical processes, as it helps predict whether a reaction will occur under specific conditions. The two primary forms of free energy, Gibbs free energy and Helmholtz free energy, are used in different contexts depending on the variables involved, such as temperature and pressure.

5 Must Know Facts For Your Next Test

1. A negative free energy change indicates that a reaction is spontaneous under given conditions, while a positive value suggests it is non-spontaneous.
2. Free energy change can be calculated using the equation $$ riangle G = riangle H - T riangle S$$, where $$ riangle H$$ is the change in enthalpy, $$T$$ is the absolute temperature, and $$ riangle S$$ is the change in entropy.
3. At equilibrium, the free energy change is zero, which means there is no net change in the concentrations of reactants and products over time.
4. The Gibbs free energy is commonly used for reactions occurring at constant temperature and pressure, while Helmholtz free energy is more applicable to reactions at constant temperature and volume.
5. Understanding free energy changes helps in predicting reaction pathways, understanding equilibrium positions, and assessing the feasibility of chemical processes.

Review Questions

• How do Gibbs and Helmholtz free energies differ in their applications to chemical reactions?
  • Gibbs and Helmholtz free energies serve different purposes based on the conditions of the reaction. Gibbs free energy is primarily used for reactions occurring at constant temperature and pressure, which are common in biological systems and everyday chemical reactions. In contrast, Helmholtz free energy applies to systems at constant temperature and volume, making it more suitable for certain physical processes where volume does not change. Both forms provide insight into the spontaneity and favorability of reactions but are used in contexts that reflect their respective conditions.
• What role does the sign of free energy change play in determining the spontaneity of a chemical reaction?
  • The sign of the free energy change indicates whether a reaction is spontaneous or non-spontaneous under specific conditions. A negative value for free energy change means that the reaction can proceed without needing additional external energy input, indicating that it is thermodynamically favorable. Conversely, a positive value suggests that the reaction requires input from an outside source to occur. Therefore, analyzing free energy change helps predict not just if a reaction will happen but also guides chemists in controlling conditions to favor desired outcomes.
• Evaluate how changes in temperature affect Gibbs free energy and the spontaneity of reactions.
  • Changes in temperature can significantly impact Gibbs free energy and thus influence the spontaneity of reactions. According to the equation $$ riangle G = riangle H - T riangle S$$, an increase in temperature can affect both terms depending on whether enthalpy ($$ riangle H$$) or entropy ($$ riangle S$$) changes are more dominant. For instance, if a reaction has a positive entropy change ($$ riangle S > 0$$) but also has an endothermic nature ($$ riangle H > 0$$), increasing temperature may eventually lead to a negative Gibbs free energy value, making the reaction spontaneous. This interplay highlights how critical temperature is in determining whether certain reactions will proceed under given conditions.

Related terms

Gibbs Free Energy: Gibbs free energy is a thermodynamic potential that measures the maximum reversible work obtainable from a system at constant temperature and pressure.

Helmholtz Free Energy: Helmholtz free energy is a measure of the useful work obtainable from a closed thermodynamic system at constant temperature and volume.

Spontaneity: Spontaneity describes whether a reaction occurs naturally without the input of external energy, which can be predicted by the sign of free energy change.