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\begin{align} \ce{4Fe(NO3)2 &-> 2Fe2O3 + 8NO2 + O2}\tag{1} \\ \ce{2NaNO3 &-> 2NaNO2 + O2}\tag{2} \end{align}

Why do nitrates of different metals decompose in different ways?

Why do we get a nitrite with sodium, but an oxide with iron(II)?

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    $\begingroup$ iron (II) -> iron (III); sodium oxides are not really very stable; not comparable reaction conditions. I am quite certain, that you are not looking for reaction mechanisms in any of your recent questions. A reaction mechanism is a detailed step-by-step description in terms of elementary reactions (also including transition states). $\endgroup$ – Martin - マーチン May 26 '16 at 8:22
  • $\begingroup$ @Martin-マーチン - then I'm not sure which tag to pick. I'm looking for something to help me memorize, something logically explaining the differences. I wonder why can't we have Fe(NO2)2, for instance. My latest questions are from USE Exam Sample Question No.37, considered one of the hardest in the exam. The question contains a textual description of a chain of reactions which you should write down, guessing the products. I've already memorized some interesting reactions, like SO2 + H2S (occurs in volcanoes) and am plowing further. $\endgroup$ – CowperKettle May 26 '16 at 8:49
  • $\begingroup$ Maybe good tags would be [inorganic-chemistry] [redox]. $\endgroup$ – Martin - マーチン May 26 '16 at 9:18
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    $\begingroup$ For the thermal decomposition of sodium nitrate see Eli S. Freeman, J. Phys. Chem. 1956, 60 (11), 1487–1493. $\endgroup$ – Martin - マーチン May 26 '16 at 11:13
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Iron(II) cation is moderately reducing- an iron(II) cation has a valence electron configuration of 3d5 4s1, and by giving up an electron in its 4s orbital the cation will transform into an iron(III) cation with 3d5 valence electron configuration, which, with every d orbital half filled, has a higher degree of symmetry over 3d5 4s1 electron configuration and hence is more stable (thermodynamically favored).

Sodium(I) cation, on the contrary, has a highly stable valence electron configuration of 2s2 2p6, with all its 2s and 2p orbitals filled, identical to the electron configuration of the chemically inert noble gas neon, and either removing or adding one electron to the sodium(I) ion will lead to an electron configuration much less stable and hence thermodynamically unfavorred.

You may notice that in the decomposition reaction, the iron(II) cation is oxidized into iron (III), which is favored, and the sodium cation remain intact. Put it in another way, we may imagine a first step thermal decomposition reaction of iron(II) nitrate resembling decomposition of sodium nitrate and yields iron(II) nitrite and oxygen gas as products. Then we easily notice that iron(II) nitrite is reducing, and oxygen gas is a strongly oxidant, hence the two products of the first step decomposition can further react with each other to produce iron(III) oxide and nitrogen dioxide.

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    $\begingroup$ Welcome to Chemistry.SE! Take the tour to get familiar with this site. Mathematical expressions and equations can be formatted using $\LaTeX$ syntax. I disagree that $\mathrm{[Ar]\,3d^5\,4s^1}$ has a lower symmetry than $\mathrm{[Ar]\,3d^5}$. $\endgroup$ – Martin - マーチン May 26 '16 at 11:11

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