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Chemistry LibreTexts

Energetics

  • Page ID
    38976
  • Chemical Energetics: A Study Guide

    Discussion Questions

    • What is the driving force for chemical changes?
    • What is Chemical Energetics? What are the two factors governing chemical reactions?
    • Give some examples to illustrate how thermodynamic and kinetics affect chemical changes.

    Chemical Energetics

    In Inorganic Chemistry you have learned that the material world is made up of some 90 natural occuring chemical elements, some more abundant than others. They combine with each other to make up compounds, mixtures, solutions, coloids, aggregates, and composite materials. The atoms are made up of tiny nuclei and surround by electrons.

    The material world is constantly changing. The changes give rise to all sorts of phenomena, including light, thundder, flood, storms, and heat observed in the weather. Did you ever ask yourself what make the material change? Well, former scientists have analyzed the question, and they have provided a simple answer to this. Energy causes all the changes. However, the concept of energy and how it behalve are very difficult to master. On the other hand, once you have mastered it, you will appreciate the simplicity and realize the beauty of this concept.

    What is Chemical Energetics?

    Chemical energetics is the study of chemical changes caused by energy. Chemical energetics discusses both thermaldynamics and kinetics or reactions.

    Thermodynamics discusses changes based on amounts of energy. Since energy is conserved, energy transferred into a system is called internal energy. The amount of energy in a reaction remains the same regardless whether the reaction (or change) takes place in one or several steps. This principle is illustrated by the Hess's law, the application of which gives the estimate of energy in a process. Measurements of energy is called calorimetry, and they can be measured under constant volume or constant pressure. A system tends to minimize its Gibb's free energy, G, and such a tendency leads to the concept of chemical equilibria. These topics require lengthy discussion, and each is a sub-unit of study.

    Chemical Kinetics which deals with the rate of reactions. A good example to illustrate the two factors is the existence of diamond and graphite. At room temperature and pressure, thermaldynamics indicates that the stable form of carbon is graphite. From a thermaldynamic point of view, diamond should convert to graphite. But the reaction rate (kinetics) is so slow that there is no detectable change.

    The temperature dependence associated with chemical kinetics is discussed in terms of activation energy, which is often perceived as the energy barrier that has to be overcome in order for a reaction to proceed.

    However, kinetics of chemical reaction also deals with rate laws, elementary steps, and mechanicsms of reactions.

    The links given above have been prepared for other courses, and they have a slightly different style.

    Example 1

    Give another example in which a system is not stable from thermaldynamic point of view, but the rate of reaction is slow.

    SOLUTION
    Glass is a meta stable state compared to crystalline state. Arrangements of molecules or groups of atoms in glass does not have a long range order, and the state is often referred to as frozen solid. Polymers are in glassy states when cooled, because the long molecules do not have time to properly align themselves into crystalline states. Over a long period of time, the polymers will become more crystalline. As a result, they become brittle. For example, a pair of old glass frame belonging to your grandfather are very brittle.

    DISCUSSION
    Realizing that there two factors governing the changes in material let you understand some engineering problems.

    Questions

    1. Why does honey become opaque after a jar of honey is left in a cupboard or fridge for a long time?

    2. Ordinary tin is a silvery-white metal, is malleable, somewhat ductile, and has a highly crystalline structure. Due to the breaking of these crystals, a "tin cry" is heard when a bar is bent. The element has two allotropic forms. On warming, grey, or alpha-tin, with a cubic structure, changes at 13.2øC into white, or beta-tin, the ordinary form of the metal. White tin has a tetragonal structure. When tin is cooled below 13.2øC, it changes slowly from white to grey. This change is affected by impurities such as aluminium and zinc, and can be prevented by small additions of antimony or bismuth. The conversion was first noted as growths on organ pipes in European cathedrals, where it was thought to be the devils work. This conversion was also speculated to be caused microorganisms and was called "tin plague" or "tin disease".

      From the above discussion, what is the stable phase of tin at room temperature, 293 K?

    3. What is the crystal structure of the cubic or alpha tin?

    4. Why Napoleon's attempt to conquor Russia failed?

    Solutions

    1. crystallization takes place.

      Skill -
      Analyze a problem from the fundamental point of view.

    2. white (tetragonal) tin

      Skill -
      White tin change to grey tin even below freezing point of water at such a slow rate that you will not notice. However, rate is rapid once the phase transition is initiated. Thus, prolonged exposure to cold temperature causes the tin pest.

    3. diamond2.gifIt's called diamond tin.

      The diagram
      The diagram here show the structure of the cubic tin, which is the same as that of the diamond.

    4. material failure (see explanation)

      Just for fun
      The uniforms of his soldier have pewter buttons. The button crumbled and left his men exposed! There may be other reasons, but this one is interesting from engineering point of view.

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