Reaction spontaneity

Definition

Reaction spontaneity refers to the inherent tendency of a chemical reaction to proceed without the need for continuous external energy input. A reaction is considered spontaneous if it occurs naturally under specified conditions, typically characterized by a decrease in free energy, meaning the system can reach a more stable state. This concept is closely tied to thermodynamic potentials, which provide insights into the favorability and directionality of reactions based on changes in energy and entropy.

5 Must Know Facts For Your Next Test

1. A reaction is spontaneous if the change in Gibbs free energy ($$ ext{ΔG}$$) is negative, indicating that the products are lower in energy than the reactants.
2. Spontaneity does not imply a rapid reaction; some spontaneous reactions can occur very slowly over time.
3. Temperature can influence reaction spontaneity by affecting both enthalpy and entropy, potentially changing the sign of $$ ext{ΔG}$$.
4. Reactions can be spontaneous in one direction but not in reverse; this means that while products may be favored, the reverse process could require additional energy input.
5. The second law of thermodynamics states that the total entropy of an isolated system can never decrease over time, reinforcing the concept of spontaneity through entropy increase.

Review Questions

• How does Gibbs free energy relate to reaction spontaneity?
  • Gibbs free energy is crucial in determining whether a reaction is spontaneous. If the change in Gibbs free energy ($$ ext{ΔG}$$) is negative, it indicates that the reaction can proceed spontaneously without additional energy. The relationship $$ ext{ΔG} = ext{ΔH} - T ext{ΔS}$$ illustrates how changes in enthalpy ($$ ext{ΔH}$$) and entropy ($$ ext{ΔS}$$), along with temperature (T), collectively influence the spontaneity of reactions.
• Discuss how temperature affects the spontaneity of a reaction and provide an example.
  • Temperature plays a significant role in determining reaction spontaneity by affecting both enthalpy and entropy changes. For example, consider the dissolution of salt in water. At lower temperatures, the dissolution might not be spontaneous due to unfavorable enthalpy changes. However, as temperature increases, the greater entropy from increased disorder may lead to a negative Gibbs free energy change, making the dissolution spontaneous at higher temperatures.
• Evaluate how entropy and enthalpy contributions can change the spontaneity of a reaction under different conditions.
  • The spontaneity of a reaction is evaluated through the interplay between enthalpy and entropy contributions as expressed in Gibbs free energy. Under certain conditions, an exothermic reaction with low entropy change may still be non-spontaneous at low temperatures. Conversely, endothermic reactions that lead to a significant increase in disorder (entropy) may become spontaneous at high temperatures. This highlights that understanding both factors is essential for predicting reaction behavior across varying environments.

Related terms

Gibbs Free Energy:

A thermodynamic potential that measures the maximum reversible work obtainable from a system at constant temperature and pressure; a key factor in determining reaction spontaneity.

Enthalpy:

A thermodynamic quantity equivalent to the total heat content of a system, often used to analyze heat transfer during reactions.

Entropy:

A measure of the disorder or randomness in a system, which plays a crucial role in determining the spontaneity of reactions according to the second law of thermodynamics.