Silicon oxide (IV)

Chemical properties and specific reactions

Despite the fact that silicon dioxide (IV) exhibits acidic properties, its chemical inertness at room temperature is very high. It does not react with water at all and is also resistant to aqueous alkali solutions. The acidic character of this compound is fully revealed only in specific reactions and at high temperatures.

The main feature is the reaction with hydrogen fluoride (hydrofluoric acid). This is the only acid capable of dissolving this oxide. During the interaction, oxygen atoms are exchanged for fluorine atoms. Such a reaction is thermodynamically feasible because the resulting chemical bond between silicon and fluorine is comparable in its phenomenal strength to the bond between silicon and oxygen. The reaction proceeds with the formation of silicon tetrafluoride and water.

The second group of reactions, where silicon dioxide behaves as a typical acidic oxide, consists of processes occurring during fusion, i.e., under high-temperature conditions:

• When reacting with molten alkalis, salts of silicic acid (silicates) are formed, and water is released as steam.
• When fused with salts of volatile acids, such as alkali metal carbonates, silicon dioxide displaces carbon dioxide, also forming silicates.

Quartz and melting thermodynamics

In nature, silicon dioxide is most commonly found in the form of quartz, the most widespread crystalline modification. The forms in which quartz is found in the environment are striking in their diversity: from microscopic grains of sand to giant single crystals whose mass can reach one ton. Physically, this is a colorless, completely transparent, and extremely refractory substance.

The melting process of quartz has a complex, stage-by-stage character. Upon heating, the substance does not simply transition into a liquid state, but first transforms into another crystalline modification known as tridymite. It is precisely tridymite that subsequently melts upon reaching a temperature of 1710 °C.

The melt of silicon dioxide has a unique microstructure. Silicon atoms in it do not exist in isolation, but are firmly bound together by means of oxygen bridges. Such a network structure causes an extremely high viscosity of the molten mass, since the movement of atoms within the liquid occurs very slowly and requires significant energy expenditure.

Quartz glass: amorphous state

If the viscous melt of silicon dioxide is subjected to cooling, a special material is formed — quartz glass. Due to the high viscosity of the liquid and the slow movement of particles as the temperature decreases, atoms simply do not have time to rearrange and take strictly defined places in a regular crystal lattice.

As a result, a glass-like substance is formed, which from a physicochemical standpoint represents an amorphous state, or a "solid" liquid. The key and most valuable property of such material is its extremely low coefficient of thermal expansion.

In practice, this means phenomenal resistance of quartz glass products to so-called thermal shock. Laboratory crucibles, tubes, and flasks made of this material do not crack at all during sharp, abrupt temperature changes. For example, a red-hot object made of quartz glass can be safely immersed in ice water, and it will retain its integrity, which is completely impossible when using ordinary silicate glass.

Additional silicon compounds

In addition to oxides, silicon chemistry includes other important compounds that require attention when studying the topic. One such substance is silane, a binary compound of silicon and hydrogen. Unlike refractory dioxide, silane is a gaseous substance. Its most important and dangerous chemical property is its ability to spontaneously ignite upon direct contact with atmospheric oxygen.

Class and exam questions

What else to study on this topic

• Thermodynamics and mechanism of silicon fluoride formation during reaction with hydrofluoric acid
• Structure and properties of oxygen bridges in silicate melts
• Crystallographic differences between quartz and its high-temperature modification, tridymite
• Vitrification process: physics of transition from a viscous melt to an amorphous state
• Properties and reactions of silane in comparison with hydrocarbons (methane)

Other topics in "Chapter 12. Group IVA Elements"

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