Experiment 9. Instability of thiosulfuric acid

Dissolve some grains of Potassium Thiosulfate (Na₂S₂O₃) in 15-20 drops of distilled water and add 8-10 drops of Sulfuric acid. Observe turbidity of solution in 5-10 min. What is the reason? Write the reaction.

CHAPTER # 13. NITROGEN, PHOSPHORUS

1. Introduction in General, Organic and Biochemistry, 7^th Edition, by Morris Hein, Leo R. Best, Scott Pattison and Susan Arena, Brooks/Cole Publishing Co., 2001. (Chapter 10.3, pp. 299-302);

2. http://www.chemsoc.org/viselements/pages/Visual Elements Group 15.html

3. http://www.hrw.com/science/mc/index/htm

1. General characteristics

The elements of Group 5A (or the main sub-group of V group) are:

The most important members of this sub-group are Nitrogen and Phosphorus. The other elements will mostly not be considered here.

Appearance

The appearance of the Group 5A elements varies widely. Nitrogen is a colorless, odorless gas; Phosphorus exists in white, red and black solid forms (allotropic modifications); Arsenic is found in yellow and gray solid forms; Antimony is found in a metallic or amorphous gray form; and finally Bismuth is a white, crystalline, brittle metal. These appearances reflect the changing nature of the elements as the sub-Group is descended, from non-metal to metal.

General Reactivity

The elements of Group 5A show a marked trend towards metallic character on descending the Group. This trend is reflected both in their structures and in their chemical properties, as for example in the oxides, which become increasingly basic.

Occurrence

Nitrogen is found in the atmosphere, and makes up 78% of the air by volume. Phosphorus is not found free in nature, but occurs in several minerals and ores such as Phosphate rock (see Appendix 12). The other elements are all found in the elemental form in the Earth crust, but more frequently as minerals.

Physical Properties

The physical properties of this Group vary widely as Nitrogen is a gas, and the other elements are solids of increasingly metallic character.

Nitrogen exists as the diatomic molecule N₂. It is a colorless, odorless gas, which condenses to a colorless liquid at -196⁰C. The strength of the bond and the short bond length provide evidence for the bond between the N atoms being a triple bond: N N.

Phosphorus has at least two allotropes, red and white one. White phosphorus is a solid composed of covalent tetrahedral P₄ molecules, and red phosphorus is an amorphous solid, which has an extended covalent structure.

The covalent radii of the atoms increase on descending the sub-Group. However, the N atom is anomalously small and so it can multiple-bond to other N, C and O atoms.

Chemical Properties

Both Nitrogen and phosphorus exist in oxidation states +3 and +5 in their compounds. Nitrogen is very unreactive. The only element to react with Nitrogen at room temperature is Lithium, to form the nitride Li₃N. Magnesium also reacts directly, but only when ignited. Some microorganisms, however, have developed a mechanism for reacting directly with Nitrogen gas and building it into protein - this is called Nitrogen fixation, and is an important early step in the food chain.

Phosphorus is more reactive than Nitrogen. It reacts with metals to form phosphides, with Sulfur to form Sulfides, with halogens to form halides, and ignites in air to form oxides. It also reacts with both alkalis and concentrated nitric acid.

Industrial Information

For industrial use Nitrogen is obtained by fractional distillation of the air. It is used for the manufacture of ammonia by the Haber-Bosch Process:

N₂(g) + 3H₂(g) ↔ 2NH₃(g)

A catalyst of finely-divided Iron is required for this equilibrium reaction. The mixture is heated to 450°C at 250 atm of pressure. Working at high pressure increases the yield of ammonia, but this adds to the cost of the plant, and a compromise between cost and yield is needed. The reaction is exothermic so low temperatures increase the yield of ammonia, but this slows up the reaction so again a compromise is needed. At the temperatures and pressures used in practice about 15% conversion is attained. The ammonia is condensed and removed from the plant and the unreacted gases are recycled.

Ammonia has numerous uses - approximately 100 megatonnes are produced worldwide each year. It is used as a fertilizer both directly and after conversion to other fertilizers such as Ammonium Nitrate. It is also a raw material for nitric acid manufacture and for the production of nylon. Gaseous Nitrogen is used to provide an inert atmosphere for reactions, which cannot be carried out in Oxygen. It is also used as a carrier gas in liquid-gas chromatography. Phosphorus is used in match-heads and on safety.

Date: 2015-01-12; view: 757