General reactivity
Silicon is characterized by a pronounced dependence of its chemical activity on ambient temperature conditions. At normal room temperature, this substance is remarkably inert. Under such conditions, silicon practically does not react with the vast majority of reagents, keeping its structure unchanged.
The situation changes drastically upon significant heating. When the temperature reaches the melting point of 1414 °C, the crystal lattice breaks down, and molten silicon acquires the ability to actively interact with a wide range of chemical elements. In this aggregate state, it readily undergoes chemical transformations with both typical metals and non-metals. It is worth emphasizing that among all possible reaction products, oxygen compounds of silicon are distinguished by exceptional stability.
Interaction with simple substances and oxides
Reactions with simple substances and metal oxides reveal broad capabilities of this element. Interactions proceed in the following directions:
- With carbon (C): Upon contact with carbon at high temperatures, the synthesis of silicon carbide, also known by the trivial name carborundum, occurs. The chemical formula of this compound is SiC. A distinguishing feature of carborundum is its colossal hardness, due to the fact that the spatial structure of this substance is extremely close to the structure of diamond.
- With metal oxides: In reactions with metal oxides occurring under significant heating, silicon consistently acts as a reducing agent.
- With metals: Here, two fundamentally different options are possible. With some metallic elements, silicon forms true chemical compounds — binary phases called silicides (a characteristic example is magnesium silicide, $Mg_2Si$). With other metal representatives, such as aluminum (Al) or silver (Ag), chemical interaction with the formation of new substances does not occur — in this case, only alloys are formed.
Behavior toward acids and alkalis
The interaction of silicon with solutions of acids and bases has a number of unique features that require close attention.
Reactions with acids: No single concentrated acid on its own is capable of reacting with silicon. There is a single important exception: dissolution occurs exclusively in a binary mixture of concentrated nitric ($HNO_3$) and hydrofluoric ($HF$) acids. During this process, hexafluorosilicic acid is formed, along with the release of nitrogen(II) oxide and water. The reaction scheme is as follows: $$Si + HNO_3 + HF \rightarrow H_2[SiF_6] + NO + H_2O$$
Reactions with alkali solutions: A completely different picture is observed upon contact with aqueous alkali solutions. Under these conditions, silicon easily enters into a reaction accompanied by the intensive release of hydrogen. Such chemical behavior demonstrates a striking similarity between silicon and classical amphoteric metals such as zinc (Zn) or aluminum (Al). Reaction equation: $$Si + 2NaOH + H_2O = Na_2SiO_3 + 2H_2\uparrow$$
Briefly: answers to topical questions
With which metals does silicon not form silicides, but instead yields alloys?
Silicon does not form silicides with aluminum (Al) and silver (Ag), but forms alloys with them.
To memorize its behavior toward acids, use the association: "Silicon fears only a double blow." Single acids are not a threat to it; only a "cocktail" of hydrofluoric and nitric acids can dissolve it.
Class and exam questions
Does silicon react with individual concentrated acids?
No, this element does not react with individual acids alone. The only exception is the reaction with a mixture of hydrofluoric and nitric acids.
What is formed during the reaction of silicon with carbon?
Silicon carbide ($SiC$), or carborundum, is formed — a substance with a diamond-like structure and exceptionally high hardness.
With which elements does silicon behave like amphoteric metals?
The similarity with amphoteric metals (such as Al, Zn) is manifested in the fact that silicon is capable of reacting with aqueous alkali solutions with the evolution of hydrogen.
What else to study on this topic
- The mechanism of the redox reaction with a mixture of nitric and hydrofluoric acids.
- Comparison of the spatial structure of silicon carbide (carborundum) with the structure of diamond.
- Features of the formation and classification of binary compounds — silicides.
- Reasons for the exceptional stability of oxygen compounds of silicon.
- Comparison of the chemical behavior of silicon and typical amphoteric metals in an alkaline environment.
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