Hydrogen compounds Carbon

C—H bond energy makes up 416 kJ/mol and yields only to the bond C–F. These compounds are named hydrocarbons. Their quantity is approximately 5 million and they make the article of study of organic chemistry. We we will consider CH₄ only.

Methane. CH₄ - at ordinary conditions colourless gas, without a smell and taste. B.p. = -161C, M.p. = -184 C. Molecule it is comparative small and unpolar. For this reason it is heavily liquified and badly dissolved in water.

Production. There is no necessity in synthesis of CH₄, although it can be synthesized. Reaction C with H₂ though exothermic, but has a reversible character and at ordinary conditions does not occure:

С + Н₂  СН₄ Н^о = -75,3 kJ/mol

Toward forming the CH₄ reaction proceeds at heating of reagents in presence the crushed nickel catalyst at high pressures.

In laboratory. 1. heating of mixture CH₃COOH with a natron lime:

CH₃COOH + NaOH = Na₂CO₃ + CH₄

2. hydrolysis of aluminium carbide:

Al₄C₃ + 12H₂O = 4Al(OH)₃ + 3CH₄

Properties. CH₄ easily gets ignited and burns with large emission of heat:

СН₄ + 2О₂ = СО₂ + 2Н₂О Н = -800 kJ^.mol^-1

Its mixture with air is explosive. It is visible from equation, that a mixture in which one volume of CH₄ is mixed with two volumes of O₂ is most explosive. Such correlation is easily achieved and from there follows a necessity to be extremely careful at use of natural gas. The mixture of methane with air at ordinary pressure ignites at  700C. Methane is toxic gas. For its exposure a little stinking mercaptane is added to natural gas.

CH₄, as well as the other saturated hydrocarbons, at ordinary conditions is very inert. It does is not oxidized by such strong oxidants, as acidified solutions of KMnO₄ K₂Cr₂O₇ are. For inorganic chemistry are important the reactions of its partial oxidization:

CH₄ + 1/2 О₂ = СО + 2Н₂ Н = -27.2 kJ/mol

and conversion:

СН₄ + Н₂О = СО + 3Н₂ Н = 204.6 kJ/mol

In industry these reactions are used for preparation of hydrogen.

Compounds Silicon

Silicon compounds

Si has oxidation state (+4) in compounds with non-metals; its oxidation state is (- 4) with the most active metals. Pure ionic compounds of this element are absent because a hypothetical Si ⁺⁴ ion has enormous charge and small size that cannot be stabilized by any chemical environment. Therefore, it is very unstable.

Silicides of metals. Si can form binary compounds with metals. In the molten state, it interacts with Mg, Ca, Fe, Pt, Bi and etc. with the formation of silicides: Li₃Si, CaSi, CaSi₂ and etc.

Silicides of active metals are ionic-covalent compounds that decompose by water or acids:

4CaSi + 9H₂O = CaSiO₃ + 3SiH₄ + 3Ca(OH)₂

Silicon with d-metals forms metal-like, hard, and resistant to chemical attack substances of complex structure: Mn₅Si₃, Cr₃Si, CrSi₂ and etc.

Si has no interaction with Zn, Al, Sn, Pb, Ag, Au and so it can be recrystallised from their melts. Hydrogen compounds Silicon

Hydrogen does not react directly with silicon (G₂₉₈ (SiH₄) = +57,2 kJ/mol; G₂₉₈ (Si₂H₆) = +126,1 kJ/mol) but hydrogen compounds of silicon are well known that form a homologous series of compounds (silanes, Si_nH_2n+2) analogous to the alkanes. They can be obtained indirectly.

Preparation: interaction of silicides of metals with diluted acid:

Mg₂Si + 4HCl = 2MgCl₂ + SiH₄

At these conditions, the product is a mixture of silanes: SiH₄ to silane Si₈H₁₈, but their yield does not exceed 25% because silane is easily destroyed by water.

P

roperties. Silane is a volatile toxic substance. Bonds Si–Si and Si—H are much weaker than in hydrocarbons, so the reducing properties of silanes are expressed much stronger. They selfignite with great heat on air:

SiH₄^-1 + 2O₂⁰ = SiO₂ + 2H₂O  H  ₂₉₈ =-1244 kJ / mol

hydrolyze with water rapidly:

SiH₄^-1 + 4H⁺¹OH = H₄SiO₄ + 4H₂⁰

Since EN (Si) is less than EN (H), the oxidation state of hydrogen in silanes is (-1) and therefore interaction with H₂O is a redox reaction of disproportionation where reducing agent is a hydrogen of silane and oxidant is a hydrogen of water.

At the presence of alkali, the latter reaction proceeds faster:

SiH₄ + 2NaOH + H₂O = Na₂SiO₃ + 4H₂

Silanes interact with halogens explosively:

SiH₄ + 4X₂ = SiX₄ + 4HX

When heated they decompose easily into elemental substances. This property is used for pure Si extraction.