# Oxidation number of Si in SiH4

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?

• 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 Theis 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 0:28
• @Mithoron are you saying that Si in SiH4 goes from +4 to -4? – Cyclopropane Dec 29 '19 at 0:38