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Sodium nitrate (and other Group 1 nitrates, except lithium) upon strong heating, produces sodium nitrite and gaseous oxygen. Copper nitrate, on the other hand produces copper (II) oxide and nitrogen dioxide and oxygen upon heating. The difference in reactivity of these two salts can be explained on the basis of reactivity and charge density of the corresponding cations. Cu˄2+ is a small positive ion and has a lot of charge packed into a small volume of space. It has a high charge density and will have a marked distorting effect on the counter ion (NO3˄–, in this case). This distorts the anionic frame effectively and brings one of the oxygen atoms closer toward it. In addition to that, CuO possesses a high value of lattice energy. When heated, the rest of the anion (NO2) breaks free to produce metal oxide and nitrogen dioxide. In this way, a big gain in energy terms is accomplished. On the other hand, Na+ is a bigger cation has lesser charge spread over a larger volume of space. Its charge density will be lower, and it will cause less distortion to nearby NO3˄– ions. Na2O too has not so high value of lattice energy. So, energy factor is also not in favor of formation of Na2O. As a result, Na+ would not be able to snatch oxygen from NO3˄– upon heating. If still heated very strongly, it breaks down into smaller nitrite ion, with nitrogen still attached to the cation, and liberating oxygen in the process.
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