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Wikipedia has the following reaction:

$\ce{Mg2Si + 4 HCl → 2 MgCl2 + SiH4}$

This seems to be a double displacement (metathesis) reaction, where $\ce{Si}$ has oxidation state $-4$. However, I thought that silane had silicon in the $+4$ oxidation state? For example, in this reaction: $\ce{3 SiO2 + 6 H2 + 4 Al → 3 SiH4 + 2 Al2O3}$, silicon remains in the $+4$ oxidation state.

Why is there the discrepancy between the oxidation state of silicon in silane?

Cyclopropane
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    If one reacted SiO2 with two reducing agents, why you'd expected it to not get reduced? – Mithoron Dec 28 '19 at 23:31
  • The answer seems to be in a couple of paragraphs further away. Also, have a look at Negative oxidation states of Si. – andselisk Dec 28 '19 at 23:37
  • Electronegativities: Mg < Al < Si < H < O. I would identify both reactions as redox, In the first, silicon and hydrogen change oxidation states. In the second, aluminum and hydrogen change oxidation states while silicon and oxygen don't. – Karsten Dec 28 '19 at 23:41
  • @KarstenTheis No, in the second reaction, silicon, aluminium and hydrogen all experience a change in oxidation state. Only oxygen does not experience a change in oxidation state. – Tan Yong Boon Dec 29 '19 at 00:28
  • @Mithoron are you saying that Si in SiH4 goes from +4 to -4? – Cyclopropane Dec 29 '19 at 00:38
  • @TanYongBoon See https://chemistry.stackexchange.com/questions/37209/negative-oxidation-states-of-si/37214 – Karsten Dec 29 '19 at 01:06
  • The reaction given by DrPepper implies 13 substances to work simultaneously : one gas and two solids. It is highly improbable. This reaction must be the sum of two or more reactions, like $$\ce{6 H2O + 4 Al->2 Al2O3 + 6 H2}$$ $$\ce{SiO2 + 4 H2-> SiH4 + 2H2O}$$ And the H2O com ing from the last equation starts a new reaction according to the first equation – Maurice Dec 29 '19 at 13:44

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Magnesium silicide is a somewhat strange beast, whose electronic structure has features that cannot be accounted for by just bonding between magnesium and silicon.

Basically, magnesium silicide has some properties of a salt and some properties of an intermetallic compound. As an intermetallic compound we can expect it to reduce hydrogen, just as an elemental metal would if it were as electropositive as magnesium or even silicon (which displaces hydrogen from steam when hot).

So hydrogen could reduced by magnesium silicide, and may thereby reach the -1 oxidation state (or zero, as magnesium silicide can also form elemental hydrogen from acid or water). With hydrogen thus reduced to the -1 state in silane, silicon goes from -4 to +4 which represents the oxidation of the silicide. The magnesium, already +2 in the silicide, remains there.

Oscar Lanzi
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