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A more general rule is any polymerization reaction. Since you chain molecules together you reduce the number of freely moving particles. Thus such reaction always leads to a decrease of entropy. This also means that any polymerization reaction needs to exothermic or it would not work. In some special cases like polymethylmethacrylate (PMMA, also known as ...


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There exist much simpler examples, like the synthesis of water according to $$\ce{2 H2(g) + O2(g) -> 2 H2O(l)}$$ This reaction is exothermic. As the entropies of $\ce{H2},$ $\ce{O2}$ and $\ce{H2O(l)}$ (in $\pu{J/mol})$ are respectively $131,$ $205$ and $70$, the reaction has a negative change of entropy.


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A very simple reaction which is readily accessible for study is the dimerisation of nitrogen dioxide: $$\ce{2 NO2(g) -> N2O4 (g)}$$ This process is has a standard reaction enthalpy change of $\rm{\Delta_r H^o=-57.2\ kJ\ mol^{-1}}$ and a standard reaction entropy change of $\rm{\Delta_r S^o=-175.83\ J\ mol^{-1}\ K^{-1}}$. With these values, the standard ...


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The nitridation/oxidation of transition elements is exothermic and decreases the entropy, since that one of the reactants ($\ce{O2}$, $\ce{N2}$) is gaseous, but at the end is dissolved in the crystal structure of the resulting product. For example the formation of titanium nitride: $$\ce{Ti(s) + 1/2 N2(g) -> TiN(s)}$$ It is higly exothermic and it ...


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It is not exactly a reaction, but consider crystallization of $\ce{KClO3}$ solution. If you put together solid $\ce{KClO3}$ with its solution of particular concentration, the crystalization becomes spontaneous below particular temperature, where solution is the saturated solution for given temperature. It is exothermic ($\Delta H \lt 0$), as the dissolution ...


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This has nothing to do with any process or process path. Enthalpy is a function of state (a property of the material comprising the system), and depends only on the internal energy, pressure, and volume in the initial and final states. For certain kinds of processes(such as constant pressure), the change in enthalpy between the initial and final states ...


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