Thermodynamically Favored

4 Key excerpts on "Thermodynamically Favored"

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  eBook - ePub

  Survival Guide to General Chemistry

  • Patrick E. McMahon, Rosemary McMahon, Bohdan Khomtchouk(Authors)
  • 2019(Publication Date)
  • CRC Press

    (Publisher)

  Reactants and products can exchange roles; solids, liquids and gases can be interconvertible. It is valuable to determine the expected direction of a reversible chemical process. The expected direction for a reversible chemical reaction refers to determining which molecules will be favored as being products and which will be favored as being reactants under a defined set of conditions. Chemical Spontaneity refers to the direction (forward or reverse) a chemical reaction proceeds as a function of certain criteria. A spontaneous chemical process is one which, once started, will continue on its own (that is, by itself) without any additional stimulus of any type, such as the addition of required energy; this is also termed product favored. A non-spontaneous chemical process is one which, even once started, will not continue its own; this process requires continuous input of required energy in order to proceed; this is also termed reactant favored. Spontaneity does not indicate if the reaction will occur or how fast a reaction will occur (the rate of reaction). It answers the question: “If a reaction starts, what is the probable result and what is the resultant energy change?” E NTHALPY AND C HEMICAL S PONTANEITY Potential energy change is one main criterion for determining the spontaneous direction of a chemical process. Potential energy change is most conveniently measured as the enthalpy change (Δ H) for a reaction. One general measure of enthalpy is the difference in bond strengths between products and reactants

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  eBook - ePub

  Computational, Education, and Materials Science Aspects

  • Ponnadurai Ramasami(Author)
  • 2022(Publication Date)
  • De Gruyter

    (Publisher)

  11 ]

  The entropy of an isolated system increases in the course of a spontaneous change.

  This requires the understanding of the nature of the entropy (S) state function and how to consider and calculate its changes.

  When the system is not isolated, one has to consider the total entropy change, i.e., the sum of the entropy changes in the system where the process occurs and in its surrounding. Therefore, the criterion for a process to be spontaneous is expressed as

  (11.1)

  Δ

  S system

  + Δ

  S surroundings

  >0

  When considering chemical reactions, in place of evaluating ΔS system and ΔS surroundings , it is more convenient to utilise the Gibbs free energy function G = H−TS. Then, the spontaneity criterion (Eq. (11.1) ) is written as

  (11.2)

  Δ G  < 0

  that is, a process is spontaneous if is accompanied by a free energy decrease.

  11.2  The context and the approaches

  The results and analyses presented in this work pertain to a systematic investigation of the difficulties encountered by underprivileged students in their approach to chemistry concepts and applications, carried out at the University of Venda (UNIVEN, South Africa) during the 1997–2018 years and considering the courses taught by the author (all the physical chemistry courses, first year general chemistry and third year process technology). UNIVEN is a Historically Black University (HBU), thus also qualified as “historically disadvantaged”. The meaning of “historically disadvantaged” has been explained in previous works (e.g., [12 ], [13 ], [14

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  eBook - ePub

  Materials Kinetics

  Transport and Rate Phenomena

  • John C. Mauro(Author)
  • 2020(Publication Date)
  • Elsevier

    (Publisher)

  For example, the lower-left quadrant of the figure shows the case of a reaction that is enthalpically favorable (Δ H < 0) but entropically unfavorable (Δ S < 0). Since Δ G = Δ H − T Δ S, the spontaneity of the reaction depends on the temperature of the system. At low temperatures, the entropic term (− T Δ S) contributes less because of the smaller value of T. Hence, at low temperatures the Gibbs free energy of the reaction is dominated by the enthalpy of the reaction, i.e., the heat of reaction. This means that Δ G < 0 at low temperatures, since the enthalpy is dominant and Δ H < 0. However, at high temperatures the entropic term becomes dominant, leading to Δ G > 0. Hence, such an enthalpy-driven reaction would be spontaneous at low temperatures but non-spontaneous at high temperatures. The opposite is true in the upper-right quadrant of Figure 1.5, which shows a reaction that is favored by entropy (Δ S > 0) but not by enthalpy (Δ H > 0). Again, the entropic term is dominant at high temperatures and the enthalpic term is dominant at low temperatures. Therefore, in this case of an entropy-driven reaction, the process is spontaneous at high temperatures but non-spontaneous at low temperatures. One of the most common themes in thermodynamics is the competition between enthalpy and entropy. As we shall see throughout this volume, the competition between enthalpic and entropic effects has a profound impact on all of materials physics, including the associated kinetic processes of materials. 1.4. Microscopic Basis of Entropy Every good story needs a hero, and the hero of our story is Ludwig Boltzmann, an Austrian physicist born in Vienna in 1844. While the concept of atoms, i.e., the indivisible fundamental units of matter, dates back to the ancient Greek philosophers, most notably Democritus (ca. 460–370 B.C.), Boltzmann was the first to develop the physics connecting atomic theory and thermodynamics

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  Chemistry

  With Inorganic Qualitative Analysis

  • Therald Moeller(Author)
  • 2012(Publication Date)
  • Academic Press

    (Publisher)

  G to equilibrium and electrochemistry is then discussed. Thermodynamic calculations of heats of reactions, equilibrium conditions, phase changes, and rates of reaction at temperatures other than 25°C are demonstrated. The last section applies thermodynamics to a biochemical reaction.

  t hermodynamics (from the Greek “therme” meaning heat or energy, and “dynamis” meaning power ) is the study of the transformation of energy from any one form to another. A complete study of thermodynamics involves such topics as heat, work, changes in state, and chemical energy .

  When applied to chemical systems, thermodynamics serves as a valuable tool in predicting such things as the spontaneity of a chemical reaction, the relationship between the amounts of products and reactants once equilibrium has been established, and the amount of energy absorbed or released during a reaction. However, thermodynamics cannot predict the reaction mechanism nor the reaction rate—these are in the realm of chemical kinetics, which has been explored in

  Chapter 15 .

  In Chapter 5

  we discussed thermochemistry, the branch of thermodynamics that deals with heat in chemical reactions and with ΔH, the change in enthalpy, or heat content, of substances. At this point you may be wondering what more there is to be learned about thermodynamics. From

  Chapter 5 you know that exothermic reactions can be spontaneous and endothermic reactions are not likely to be spontaneous. Isn’t this enough? As you can probably guess, the answer is no. If you go back to Section 5.3 ,

  you will see that there we emphasized the words “often” and “usually” in talking about reaction spontaneity. In this chapter we investigate all of the thermodynamic forces and explain why some changes with positive ΔH are spontaneous, while some changes with negative ΔH are not spontaneous

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