Concept 06 / 09The p-Block Elements
Group 15: Nitrogen, Ammonia, Nitric Acid and Oxides of Nitrogen
General trends of group 15 elements, anomalous nitrogen, trends in hydrides, preparation and properties of dinitrogen, ammonia by the Haber process, nitric acid by the Ostwald process and its oxidising reactions with metals, the brown ring test, and the structures and properties of oxides of nitrogen.
Group 15 trends. Nitrogen, phosphorus, arsenic, antimony, bismuth; configuration (half-filled, extra stable, giving high ionisation enthalpies). Oxidation states range from to ; the stability of decreases and of increases down the group (inert pair effect), so compounds are strong oxidants () and Bi is mainly . Metallic character increases down the group.
Hydrides : basic strength ; thermal stability decreases and reducing character increases in the same direction; bond angles fall from () to about . Boiling points: ( is raised by hydrogen bonding).
Anomalous nitrogen. Small size, high electronegativity and no d orbitals. It forms - multiple bonds (, bond enthalpy ), which makes very inert; its maximum covalency is ; the N-N single bond is weaker than P-P (lone pair repulsion). Phosphorus and the rest form single-bonded tetrahedral units and can expand their octets ().
Dinitrogen. Laboratory: ; also (on heating). Very pure from . Industrially by fractional distillation of liquid air. Inert at room temperature; combines with Li, Mg (), with in lightning (NO), and with in the Haber process.
Ammonia. Laboratory: . Haber process: , ; about , , iron oxide catalyst with and . Trigonal pyramidal (), hydrogen-bonded, very soluble, weakly basic. As a Lewis base it forms complexes: (deep blue); (dissolves).
Nitric acid. Ostwald process (see figure): catalytic oxidation of ammonia to NO, oxidation to , absorption in water. A strong acid and powerful oxidising agent; the products depend on concentration and on the metal:
Non-metals are oxidised to their highest oxoacids (, , , ). Al, Cr and Fe are rendered passive by concentrated (oxide film).
Brown ring test for nitrate: add solution, then concentrated down the side: , then (brown ring at the interface).
Oxides of nitrogen. (, colourless, neutral, laughing gas); NO (, colourless, neutral, paramagnetic); (, blue solid, acidic); (, brown, acidic, paramagnetic, dimerises to colourless ); (, colourless solid, acidic, anhydride of ).
Key Takeaways:
- Basicity and stability of hydrides decrease from to ; reducing power increases.
- () makes inert.
- Ostwald: over Pt/Rh.
- Dilute with Cu gives NO; concentrated gives .
- NO and are odd-electron, paramagnetic molecules.
- Pitfall: is not the highest-boiling group 15 hydride; and boil higher.
Brain-Teaser 1: How many moles of NO are produced when of copper reacts with excess dilute ?
Solution: give , so Cu gives NO. (Electron check: electrons; each N goes , taking electrons, so NO.)
Brain-Teaser 2: Why does dimerise but NO does not readily do so?
Solution: has its unpaired electron largely on nitrogen, and pairing it forms a stable N-N bond in (colourless, diamagnetic). In NO, the unpaired electron is in a delocalised orbital over both atoms, so dimerisation gains little; NO dimerises only in the liquid and solid states.
Brain-Teaser 3: What is the oxidation state of iron and the charge on NO in the brown ring complex ?
Solution: Conventionally the ligand is taken as (nitrosonium), after an electron transfer from , so iron is : gives .
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