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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?

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    $\begingroup$ If one reacted SiO2 with two reducing agents, why you'd expected it to not get reduced? $\endgroup$ – Mithoron Dec 28 '19 at 23:31
  • $\begingroup$ The answer seems to be in a couple of paragraphs further away. Also, have a look at Negative oxidation states of Si. $\endgroup$ – andselisk Dec 28 '19 at 23:37
  • $\begingroup$ 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. $\endgroup$ – Karsten Theis Dec 28 '19 at 23:41
  • $\begingroup$ @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. $\endgroup$ – Tan Yong Boon Dec 29 '19 at 0:28
  • $\begingroup$ @Mithoron are you saying that Si in SiH4 goes from +4 to -4? $\endgroup$ – DrPepper Dec 29 '19 at 0:38

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