chemistry

Polysilicon hydride

Polysilicon hydride is a chemistry topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Polysilicon hydride rather than just read about it. In short: Polysilicon hydrides are polymers containing only silicon and hydrogen. They have the formula ( SiH n ) x {\textstyle {\ce {(SiH_{n})_{x}}}} where 0.2 ≤ n ≤ 2.5 and x is the number of monomer units.

Key takeaways

Polysilicon hydride belongs to chemistry; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Polysilicon hydride to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Polysilicon hydride from memory before moving on to harder problems.

Reference excerpt

Polysilicon hydrides are polymers containing only silicon and hydrogen. They have the formula ( SiH n ) x {\textstyle {\ce {(SiH_{n})_{x}}}} where 0.2 ≤ n ≤ 2.5 and x is the number of monomer units. The polysilicon hydrides are generally colorless or pale-yellow/ocher powders that are easily hydrolyzed and ignite readily in air. The surfaces of silicon prepared by MOCVD using silane (SiH4) consist of a polysilicon hydride.

Synthesis Polysilicon hydrides are much less thermally stable than the corresponding alkanes (CnH2n+2). They are kinetically labile, with their decomposition reaction rate increasing with increases in the number of silicon atoms in the molecule. Consequently, the preparation and isolation of polysilicon hydrides is difficult for species containing more than a few silicon atoms. Greater catenation of the Si atoms can be obtained with the halides (SinX2n+2 with n = 14) for the fluorides. Thus the polymeric silicon hydrides are formed along with smaller silicon hydride oligomers and hydrogen gas from the spontaneous but slow decomposition, as well as from the accelerated thermolysis, of acyclic and cyclic liquid silanes that are higher in molecular weight than monosilane (SiH4) and disilane (Si2H6). Polysilicon hydrides are intermediates in the high-temperature conversion of mono- and disilane to silicon and hydrogen gas. In the following idealized sequence cyclopentasilane is the polysilicon hydride intermediate:

5 SiH4 → Si5H10 + 5 H2 Si5H10 → 5 Si + 5H2 Polymeric silicon hydrides may be prepared by hydrolysis of certain silicides. Acid hydrolysis of calcium monosilicide (CaSi) produces (SiH2)x. CaSi consists of zigzag silicon chain with the formula (Si2−)n. This chain is preserved in the hydrolysis. This reaction was reported in 1921 by the German chemists Lothar Woehler (1870–1952) and Friedrich Mueller. In 1923, German chemists Alfred Stock (1876–1946) and Friedrich Zeidler (1855–1931) found the (SiH)x polymer is formed along with silane gas by the action of sodium amalgam on dichlorosilane (SiH2Cl2). The reaction is proposed to first produce the disodium dihydrogen silanide (Na2SiH2) via a Wurtz-like reaction. The Na2SiH2 dissolves in the mercury to generate the diradical SiH2, which then forms the final products. Polysilicon hydrides may also be produced by the dehalogenation of polysilicon halides. An illustrative reaction is the debromination of HSiBr3 with Mg in ether:

HSiBr3 + 3/2 Mg → 1/x (SiH)x + 3/2 MgBr2

Macromolecular structure When n = 2 in (SiHn)x, the polymer is termed polysilene, which has a quasi-one-dimensional (zigzag) chain structure in which each silicon atom is bonded to two other silicon skeletal atoms and two hydrogen atoms. Thus the Si atom is tetravalent (has four bonds). If n = 1, quasi-two-dimensional (corrugated sheets) or random three-dimensional silicon networks termed polysilynes are obtained in which each silicon atom is bonded to three other silicon skeletal atoms and one hydrogen atom giving, again, a tetravalent Si atom. In the polysilenes and the polysilynes, the backbone is made exclusively of silicon atoms; the pendent- or side-groups, are not shared between skeletal atoms of the backbone chain.

Compression of silane itself under extremely high pressures (> 90 GPa) for long periods of time (~8 months) produces a material that differs from those shown above in that the silicon atoms are not bonded to one another but, rather, are connected via bridging hydrogen atoms. The silicon atoms are 8-fold coordinated by hydrogen (i.e. each silicon atom is bonded to 8 hydrogen atoms) forming a tetragonal structure.

Inorganic derivatives – siloxene In addition to the polysilicon halides, another related compound, in which the hydrogen is partially replaced by an inorganic group, is siloxene. Siloxene was first observed before the polysilicon hydrides, but it was initially thought to be a polymeric silicon hydride itself. Siloxene is structurally derived from layered polysilyne by replacing one-half of the hydrogen atoms with OH groups. It has the chemical formula Si2H2O (or [Si6H3(OH)3]x) and it is prepared by reacting calcium disilicide (CaSi2), which has a puckered layer of silicon atoms, with dilute hydrochloric acid. This yellow solid was prepared from that route as early as 1900 by Charles Schenck Bradley (1853–1929), a one-time associate of Thomas Edison. However, its structure, the intra-polymer Si-Si bond order, and true chemical formula were not determined for some time afterwards Bradley had erroneously concluded it was the silicon analog, (Si2H2)x, of the unsaturated (double-bond containing) polymer Polyacetylene and referred to as silico-acetylene.

Organic derivatives – polysilanes and polysilynes

When hydrogen atoms in linear polysilene are replaced with organic substituents, particularly alkyl and aryl groups, polysilanes are obtained. Although organosilicon polymers can be considered structural derivatives of polysilicon hydrides, they are not synthesized directly from them. Traditionally, polysilanes are prepared by the Wurtz-like sodium or potassium metal-mediated reductive coupling (dehalogenation) of dihaloorganosilanes (R2SiX2) in an inert solvent (e.g. toluene), very much like Burkhard's original method. Polysilynes have been similarly prepared from trihaloorganosilanes (RSiX3).

Worked examples

Example 1 — a first encounter with Polysilicon hydride

Start with the simplest possible case. Write down what Polysilicon hydride claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In chemistry, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Polysilicon hydride before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Polysilicon hydride ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Polysilicon hydride

In research: Polysilicon hydride appears in chemistry research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Polysilicon hydride in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Polysilicon hydride is common in secondary-school and first-year university syllabi. It links to neighbouring topics Inorganic polymers, Inorganic silicon compounds, Silicon hydrides, so understanding it makes those chapters shorter.
In everyday life: Look for Polysilicon hydride outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Polysilicon hydride in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Polysilicon hydride means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Polysilicon hydride out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Polysilicon hydride in simple terms?

Why does Polysilicon hydride matter?

Because it connects several chemistry ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Polysilicon hydride?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Polysilicon hydride.