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💏intro to chemistry review

Nonspontaneous Processes

Written by the Fiveable Content Team • Last updated September 2025
Written by the Fiveable Content Team • Last updated September 2025

Definition

Nonspontaneous processes are physical or chemical changes that do not occur naturally or on their own without the input of external energy. These processes require an energy source or external work to be performed in order to proceed, unlike spontaneous processes which occur naturally and without any energy input.

5 Must Know Facts For Your Next Test

  1. Nonspontaneous processes require an input of energy, such as heat or work, to proceed and overcome the unfavorable change in Gibbs free energy.
  2. The change in Gibbs free energy, $\Delta G$, is the key indicator of spontaneity, with $\Delta G > 0$ indicating a nonspontaneous process.
  3. Nonspontaneous processes tend to have a positive change in enthalpy, $\Delta H > 0$, meaning they are endothermic and absorb energy from the surroundings.
  4. Examples of nonspontaneous processes include the charging of a battery, the melting of ice, and the synthesis of complex organic molecules.
  5. Nonspontaneous processes can be driven to completion by coupling them with a spontaneous process that provides the necessary energy input, such as the use of a battery to power an electric motor.

Review Questions

  • Explain the relationship between Gibbs free energy and the spontaneity of a process.
    • The change in Gibbs free energy, $\Delta G$, is the key indicator of the spontaneity of a process. For a process to be spontaneous, the change in Gibbs free energy must be negative ($\Delta G < 0$), indicating that the process will occur naturally and release energy. Conversely, a positive change in Gibbs free energy ($\Delta G > 0$) indicates a nonspontaneous process that requires an input of energy to proceed. The magnitude of $\Delta G$ determines the extent of spontaneity or the driving force for a process to occur.
  • Describe the role of enthalpy in the spontaneity of a process and how it relates to nonspontaneous processes.
    • Enthalpy, $\Delta H$, is a measure of the total energy of a system, including the energy required to create the system and the work done by or on the system. Nonspontaneous processes typically have a positive change in enthalpy, $\Delta H > 0$, meaning they are endothermic and absorb energy from the surroundings. This positive change in enthalpy opposes the spontaneity of the process, as the system must overcome this energy barrier to proceed. The relationship between enthalpy and Gibbs free energy, $\Delta G = \Delta H - T\Delta S$, shows that a positive $\Delta H$ contributes to a positive $\Delta G$, indicating a nonspontaneous process that requires an external energy input to occur.
  • Analyze how nonspontaneous processes can be driven to completion by coupling them with a spontaneous process.
    • Nonspontaneous processes, which have a positive change in Gibbs free energy ($\Delta G > 0$), can be driven to completion by coupling them with a spontaneous process that provides the necessary energy input. This coupling allows the overall process to have a negative change in Gibbs free energy, making it spontaneous and favorable. For example, the nonspontaneous process of charging a battery can be driven by the spontaneous process of a chemical reaction within the battery that releases energy. Similarly, the synthesis of complex organic molecules, a nonspontaneous process, can be driven by coupling it with a spontaneous process, such as the hydrolysis of ATP. In this way, the energy released from the spontaneous process can be used to power the nonspontaneous process, enabling it to occur.