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So I was dealing with a reaction the other day, which is also briefly written in the title:

$$\ce{AlH3(s) + LiH(aq) -> Li+(aq) + AlH4-(aq)}$$

The strange thing is:

  • Arrhenius definition cannot explain the reaction, since there is no $\ce{OH-}$ in the reaction formula.

  • Also Brønsted–Lowry definition cannot explain the reaction, because the stuff which $\ce{LiH}$ gives to $\ce{AlH3}$ is not a proton (hydrogenium, $\ce{H+}$) but a protide (hydride, $\ce{H-}$).

  • And as far as I know, this is the exact definition of Lewis acid and base written in my chemistry textbook:

  • Lewis acid: A substance which can receive unshared electron pair
  • Lewis base: A substance which can give unshared electron pair

leading to a conclusion that the above reaction also cannot be explained through the Lewis definition. Below are my questions:

  • Is there anything wrong in my thought?
  • Is that really an acid-base reaction? If it is, what kind of acid-base reaction is it?
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  • $\begingroup$ "(aq)" really? $\endgroup$ – Mithoron Sep 28 '17 at 14:04
  • $\begingroup$ @Mithoron Yes, that is how it was written. $\endgroup$ – PenPoint Sep 29 '17 at 7:51
  • $\begingroup$ Then better get a new book, 'cause you'd sooner get a nuke in your basement then aqueous LiH. $\endgroup$ – Mithoron Sep 29 '17 at 14:06
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This is indeed a Lewis acid-base reaction.

The hydride ion has a lone pair which is, in general, not bound to anything. The aluminium atom has at least one unpopulated orbital (typically taken to be a $\mathrm p_z$ orbital in a planar $\ce{AlX3}$ environment). This makes hydride a Lewis base and aluminium a Lewis acid. The association of hydride to aluminium is thus a Lewis acid-base reaction.

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  • $\begingroup$ @PenPoint Hydride is the Lewis base. Lithium is technically also a Lewis acid, but it is not receiving here. $\endgroup$ – Jan Sep 28 '17 at 12:21

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