Every general model is applicable to some range of physical phenomena. The Energ

Question

Every general model is applicable to some range of physical phenomena. The Energy-Interaction Model can be used to analyze virtually any process, but up to this point we have been using it to make sense of the phenomena of temperature changes and phase changes. The former are modeled as changes in thermal energy systems, with the indicator being temperature, and the latter are modeled as changes in bond energy systems, with the indicator being the mass of substance that is changing phase. But bond energy changes in chemical reactions just as it does in phase changes, so we use exactly the same approach when modeling chemical reactions. Consider the following chemical reaction, the hydration of calcium sulfate (plaster of Paris). Recall that when you mix water with the white power, the paste not only gets hard, but it also gets hot: (Ca2SO4)2 H2O + 3H2O Right arrow Ca2S04 2H2O + Heat to env. Has the bond energy of this total system increased, decreased, or stayed the same during this process (mixing water with the powder)? Use the energy interaction model to explain. The simplest way to answer this question is to model all of these different chemicals as being a single physical system with one thermal-energy system and one bond-energy system. (Note that the question asked about the total bond energy, not bond energies of the separate molecular species. If the question had asked about bond energy changes of particular molecular species, we would have to include separate bond energies in our model.)

Explanation / Answer

The total bond energy is decreased, since heat is going out. You can prove this if you do an enthalpy analysis of each bond. At the end, you will see that there is extra energy that won't fit in the system, therefore will go out as heat (exothermic) reaction

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