standard entropy

(noun)

Entropy of a substance at 1 atm pressure.

Related Terms

Examples of standard entropy in the following topics:

  • Standard Entropy

    • The standard entropy of a substance (its entropy at 1 atmospheric pressure) helps determine if a reaction will take place spontaneously.
    • The standard entropy of a substance is its entropy at 1 atm pressure.
    • Some typical standard entropy values for gaseous substances include:
    • Scientists conventionally set the energies of formation of elements in their standard states to zero.
    • The standard entropy of reaction helps determine whether the reaction will take place spontaneously.

  • Standard Free Energy Changes

    • The standard Gibbs Free Energy is calculated using the free energy of formation of each component of a reaction at standard pressure.
    • These same definitions apply to standard enthalpies and internal energies.
    • Don't confuse these thermodynamic standard states with the "standard temperature and pressure" (STP) widely employed in gas law calculations.
    • To accomplish this, combine the standard enthalpy and the standard entropy of a substance to get the standard free energy of a reaction:
    • The other factor to keep in mind is that enthalpy values are normally given in $\frac{kJ}{mole}$ while entropy values are given in $\frac{J}{K\times mole} $ .

  • Solvent Effects

    • From this description of ion formation in solution, it should be clear that both enthalpy and entropy factors will be important to the outcome of an ionization process.
    • Although these two inorganic salts have similar standard enthalpies of solution in water, their standard entropies are quite different.
    • One might expect this entropy change to be positive, since a single molecule in the solid state produces two or more ionic species, accompanied by an increase in system disorder.
    • The overall entropy change for solution of NaCl is positive, reflecting the increased disorder of ionization, but the entropy change for CaF2 solution is strongly negative thanks to the solvation shell structure required by the resulting ions.
    • These different entropy changes are incorporated in the free energy of solution, which is exergonic for NaCl, but endergonic for CaF2.

  • Struggle against entropy

  • Order to Disorder

    • Entropy is a measure of disorder, so increased entropy means more disorder in the system.
    • Entropy is a measure of disorder.
    • There is a large increase in entropy in the process.
    • The mixing decreases the entropy of the hot water but increases the entropy of the cold water by a greater amount, producing an overall increase in entropy.
    • Entropy is a measure of disorder.

  • The Third Law

    • Entropy is related to the number of possible microstates, and with only one microstate available at zero kelvin the entropy is exactly zero.
    • Nernst proposed that the entropy of a system at absolute zero would be a well-defined constant.
    • This law provides an absolute reference point for the determination of entropy. ( diagrams the temperature entropy of nitrogen. ) The entropy (S) determined relative to this point is the absolute entropy represented as follows:
    • Temperature–entropy diagram of nitrogen.
    • Absolute value of entropy can be determined shown here, thanks to the third law of thermodynamics.

  • Spontaneous and Nonspontaneous Processes

    • 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 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.
    • 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.
    • Since the overall ΔS = ΔSsurroundings + ΔSsystem, the overall change in entropy is still positive.
    • 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.

  • Changes in Energy

    • For isolated systems, entropy never decreases.
    • Increases in entropy correspond to irreversible changes in a system.
    • The entropy of a system is defined only if it is in thermodynamic equilibrium.
    • However, the entropy of the system of ice and water has increased more than the entropy of the surrounding room has decreased.
    • Ice melting in a warm room is a common example of increasing entropy.

  • Living Systems and Evolution

    • It is possible for the entropy of one part of the universe to decrease, provided the total change in entropy of the universe increases.
    • But it is always possible for the entropy of one part of the universe to decrease, provided the total change in entropy of the universe increases.
    • How is it possible for a system to decrease its entropy?
    • However, there is a large total increase in entropy resulting from this massive heat transfer.
    • Formulate conditions that allow decrease of the entropy in one part of the universe

  • Microstates and Entropy

    • With more available microstates, the entropy of a system increases.
    • As a result, entropy (denoted by S) is an expression of disorder or randomness.
    • With more available microstates, the entropy of a system increases.
    • This is the basis of an alternative (and more fundamental) definition of entropy:
    • Therefore, the entropy of a solid is less than the entropy of a liquid, which is much less than the entropy of a gas: