Spontaneous and Nonspontaneous Processes



 

Learning Objective

  • Describe the differences between spontaneous and nonspontaneous processes.

Key Points

    • A spontaneous process is capable of proceeding in a given direction without needing to be driven by an outside source of energy.
    • The laws of thermodynamics govern the direction of a spontaneous process, ensuring that if a sufficiently large number of individual interactions are involved, then the direction will always be in the direction of increased entropy.
    • An endergonic reaction (also called a nonspontaneous reaction) is a chemical reaction in which the standard change in free energy is positive and energy is absorbed.
    • Endergonic processes can be pushed or pulled by coupling them to highly exergonic reactions.

Terms

  • exergonicDescribing a reaction that releases energy to its surroundings.
  • endergonicDescribing a reaction that absorbs energy from its surroundings.
  • entropyA thermodynamic property that is the measure of a system’s thermal energy per unit temperature that is unavailable for doing useful work.

There are two types of processes (or reactions): spontaneous and non-spontaneous. Spontaneous changes, also called natural processes, proceed when left to themselves, and in the absence of any attempt to drive them in reverse. The sign convention of changes in free energy follows the general convention for thermodynamic measurements. This means a release of free energy from the system corresponds to a negative change in free energy, but to a positive change for the surroundings. Examples include:

  • a smell diffusing in a room
  • ice melting in lukewarm water
  • salt dissolving in water
  • iron rusting.

The laws of thermodynamics govern the direction of a spontaneous process, ensuring that if a sufficiently large number of individual interactions (like atoms colliding) are involved, then the direction will always be in the direction of increased entropy.

The Second Law of Thermodynamics

The second law of thermodynamics states that for any spontaneous process, the overall ΔS must be greater than or equal to zero; yet, spontaneous chemical reactions can result in a negative change in entropy. This does not contradict the second law, however, since such a reaction must have a sufficiently large negative change in enthalpy (heat energy). The increase in temperature of the reaction surroundings results in a sufficiently large increase in entropy, such that the overall change in entropy is positive. That is, the ΔS of the surroundings increases enough because of the exothermicity of the reaction so that it overcompensates for the negative ΔS of the system. Since the overall ΔS = ΔSsurroundings + ΔSsystem, the overall change in entropy is still positive.

Spontaneous Processes

Spontaneity does not imply that the reaction proceeds with great speed. For example, the decay of diamonds into graphite is a spontaneous process that occurs very slowly, taking millions of years. The rate of a reaction is independent of its spontaneity, and instead depends on the chemical kinetics of the reaction. Every reactant in a spontaneous process has a tendency to form the corresponding product. This tendency is related to stability.

Nonspontaneous Processes

An endergonic reaction (also called a nonspontaneous reaction or an unfavorable reaction) is a chemical reaction in which the standard change in free energy is positive, and energy is absorbed. The total amount of energy is a loss (it takes more energy to start the reaction than what is gotten out of it) so the total energy is a negative net result. Endergonic reactions can also be pushed by coupling them to another reaction, which is strongly exergonic, through a shared intermediate.Saul Steinberg from The New Yorker illustrates a nonspontaneous process here.