Spontaneous process

Definition

A spontaneous process is a physical or chemical change that occurs without the need for external energy input, driven by the natural tendency of systems to increase their entropy. This concept is crucial in understanding how reactions proceed and the conditions under which they can occur, often linked to the changes in Gibbs free energy, which determines whether a reaction can happen spontaneously at constant temperature and pressure.

5 Must Know Facts For Your Next Test

1. For a process to be spontaneous, the change in Gibbs free energy ( extDelta G) must be negative, indicating that energy is released as the reaction proceeds.
2. Spontaneous processes often lead to an increase in entropy ( extDelta S), reflecting the natural tendency of systems to move towards greater disorder.
3. Not all spontaneous processes occur quickly; some may take a long time to happen due to kinetic barriers.
4. Exothermic reactions are often spontaneous, but endothermic reactions can also be spontaneous if they result in a larger increase in entropy.
5. The temperature can influence spontaneity; a reaction that is non-spontaneous at one temperature may become spontaneous at another due to changes in Gibbs free energy.

Review Questions

• How does the change in Gibbs free energy relate to the spontaneity of a process?
  • The change in Gibbs free energy ( extDelta G) is a key factor in determining whether a process is spontaneous. When extDelta G is negative, it indicates that the reaction can occur spontaneously, meaning that the system can move toward a lower energy state without additional energy input. Conversely, if extDelta G is positive, the process is non-spontaneous and requires energy to proceed.
• Discuss how entropy and enthalpy contribute to the understanding of spontaneous processes.
  • Entropy (S) reflects the degree of disorder in a system, while enthalpy (H) represents its total heat content. For a spontaneous process to occur, there must be a favorable balance between these two factors. The relationship between them is captured by Gibbs free energy ( extDelta G = extDelta H - T extDelta S). If an increase in entropy outweighs any endothermic enthalpy changes, it can result in a negative extDelta G, leading to spontaneity.
• Evaluate how temperature affects the spontaneity of endothermic reactions and provide an example.
  • Temperature plays a crucial role in determining whether an endothermic reaction can be spontaneous. For example, consider the dissolution of ammonium nitrate in water, which absorbs heat (endothermic). At lower temperatures, this process may not be spontaneous due to unfavorable enthalpy changes. However, as temperature increases, the positive contribution from entropy (increased disorder as solute particles disperse) may become sufficient to result in a negative extDelta G. Thus, at higher temperatures, this reaction becomes spontaneous.

Related terms

Entropy:

A measure of the disorder or randomness in a system, where higher entropy indicates a greater degree of disorder.

Gibbs Free Energy:

A thermodynamic potential that combines enthalpy and entropy to predict the direction of a chemical reaction, with negative values indicating a spontaneous process.

Equilibrium:

A state in which the forward and reverse reactions occur at equal rates, resulting in constant concentrations of reactants and products.