|Lecture Notes::Lec 21_29 June
© R. Paselk 2006
The Representative Elements, cont.
Group V consists of Nitrogen, Phosphorus, Arsenic, Antimony and Bismuth. Again we see a transition from non-metals (N & P) through semimetals (As & Sb) to a metal (Bi).
- Group V shows a good illustration of the trend from acidic to basic oxides going down the group:
- Nitrogen, phosphorus and arsenic form acidic oxides as expected for non-metals: e.g. HNO3, H3PO4, H3AsO4.
- Arsenic(III) oxide gives arsenite (from arsenous acid): As4O6(s) + 12OH- 4AsO33- + 6H2O
- Arsenic(V) oxide gives arsenate (from arsenic acid): As4O10(s) + 12OH- 4AsO43- + 6H2O
- Antimony forms amphoteric oxides (though a semimetal, its more metallic than arsenic).
- Basic: Sb2O3(s) + 6H+ + 6Cl- 2SbCl3(s) + 3H2O
- Acidic: Sb2O3(s) + 6OH- 2SbO33- + 3H2O
- Bismuth forms a basic oxide, Bi(III) oxide, Bi2O3, as expected for a metal.
- Bi2O3(s) + 6H+ + 6Cl- 2BiCl3(s) + 3H2O
- Group V elements commonly form three different molecular structures:
||Hybridization of M
|MX3 (e.g. PCl3)
|MX5 (e.g. PCl5)
|MX6 (e.g. PCl6-)
Properties of Group V
|Outer electron configuration
|Melting point (°C)
||-210 (bp = -196)
- Major constituent of the atmosphere (78% by volume)
- nitrogen is the second ranked industrial chemical in the US
- produced by distillation from liquid air (b.p. .
- Nitrogen can have a wide variety of oxidation states from -3 to +5::
||Weak base, see text
||Very reactive, very exothermic reaction with oxygen etc.
||Nitrous oxide or "laughing gas" Used for anesthesia, aerosol cans.
||Nitric oxide. Reacts readily with oxygen to give NO2.
||Dark blue solid.
||Brown, reactive gas. Responsible for brown color of L.A. smog. Reacts with oxygen to give NO plus atomic O, which reacts with oxygen to give ozone.
||Strong, oxidizing acid.
- Most inorganic nitrogen compounds are soluble, so not commonly found in mineral deposits (exception is in very dry climate locales - current or fossil)
- One of the four major elements of life on Earth (C, H, O, N) [Nitrogen cycle overhead]
- Nitrogen triple bond is very stable thermodynamically and kinetically.
- Converting nitrogen to usable forms ("fixing"), such as ammonia or nitrate, is thus a chemical activity of immense importance.
- A huge amount of nitrogen is used for the production of ammonia, which in turn is used mostly for fertilizer. [Ammonia synthesis overhead]
- Ammonia is extremely soluble in water, giving "ammonium hydroxide," with part of the ammonia reacting with water to give ammonium and hydroxide ions: NH3 + H2O 2NH4+ + OH-. Note that there is no ammonium hydroxide as a species!
- Ammonia can be converted to ammonium sulfate, the most widely used solid fertilizer, by aqueous reaction with sulfuric acid: 2NH3 + H+ + HSO4- 2NH4+ + SO4-.
- Ammonia can also be converted to nitric acid [overhead.
- Nitrogen has two important oxyacids:
- Nitrous acid, HNO2, is a weak acid, which reacts to give nitrite salts. Nitrites can serve as either oxidizing or reducing agents under different conditions. Nitrite is also a good complexing agent.
- Nitric acid HNO3, is a strong acid, and when concentrated is a powerful oxidizing agent. Most metals are dissolved by nitric acid because of its oxidizing properties, e.g. Cu(s) + 2NO3- + 4H+ Cu2+ + 2NO2(g)+ 2H2O.
- Some metals, such as iron, are "passivated" by the formation of a thin oxide coating by the acid and so are protected from both it and other acids.
- Occurs as three common allotropes (at least three others also exist):
- White phosphorus, P4, a very reactive, poisonous substance. It is a translucent white solid with P4 molecules in a cubic structure. It is stored under water to prevent its spontaneous combustion in air with the release of large amounts of heat (deltaH° = -2940 kJ/mol).
- Red phosphorus has "tubes" of phosphorus in a layered arrangement. It is much less reactive then white phosphorus, is less poisonous, and can be stored in air.
- Black phosphorus has a sheet structure and is the most stable form at room temperature.
- Phosphorus in the phosphate structure (PO43-) form is very important in biological systems, particularly in the form of di- or tri- polyphosphate forms. These acid anhydride bonds are thermodynamically unstable, but kinetically very stable unless a catalyst is present. Hydrolysis of these polyphosphate anhydride bonds releases a great deal of energy (about 30 kJ/mol) and serves as the major immediate form of chemical energy in organisms, driving muscle contraction, ion pumping etc.
- Occurs as three allotropes
- Gray: metallic appearance
- Yellow: non-metallic, consists of As4 molecules like white phosphorus molecules
- Occurs as the yellow pigment orpiment (As4S6)
- Very toxic, but essential as a trace element for blood formation. In the form of arsenate (AsO43-), it can substitute for the very similar phosphate in some enzyme sites, and as a result be incorporated instead of phosphate. However, unlike phosphate the bonds with arsenate are readily hydrolyzed, so the energy captured is immediately lost to the organism - not good!
- In ancient times arsenic was often used in bronze alloys. It took centuries to realize that working with these alloys was very hard on bronze smiths!
- Occurs as four allotropes
- Gray: metallic, but poor conductor.
- Yellow: non-metallic, consists of Sb4 molecules like white phosphorus molecules
- "explosive antimony" violently converts to gray antimony when scratched.
- Dense metal with dull metallic luster.
- Bismuth is (like water) unusual in expanding on freezing. Some of its alloys share this property and are used where very tight fits are needed (such as in making type from type metal.)
Group VI consists of Oxygen, Sulfur, Selenium, Tellurium, and Polonium. Again we see a trend towards increased metallicity as we go down the group. Oxygen and sulfur are case-study non-metals with no metallic properties of note. Selenium and Tellurium are also non-metallic, behaving much like sulfur (below), however, tellurium has some metallic tendencies. Finally, Polonium is somewhat metallic, considered a metal by some, and forms a basic oxide, but its chemistry is not well known since it has no stable isotopes.
Most commonly the Group VI elements pick up two electrons to give the inert gas valence shell configuration. They tend to form 2- ionic compounds with metals, as we've seen in lab for the sulfides.
Oxides and sulfides are the most common minerals formed by most metals (e.g. galena, PbS; pyrite, FeS; iron oxide, FeO+Fe2O3).
They form covalent compounds with non-metals (e.g. H2O, H2S, SO2, SF4, etc.).
- Note that all but oxygen form compounds with expanded valence shells, such as SF6.
- Tellurium and Polonium can also form +4 cations, showing the increasing metallicity of the group.
- Oxides of all but polonium and oxygen are acidic.
© R A Paselk
Last modified 29 June 2006