4
$\begingroup$

I am working on reaction predictions for the AP Chemistry and I need some help with this problem. On a practice quiz I tried to use the rule of "Metal + Water = Metal Oxide" so I answered with $\ce{2Li + O^{2-} -> Li2OH}$. I got zero points for this answer meaning both reactants and products were wrong. What is the correct answer and how would you get to the correct answer?

$\endgroup$
  • $\begingroup$ I think that's what you were trying to do with the "exponent" on the oxygen, if not feel free to fix it to reflect the problem. It would also be good if you explained your rationale for choosing that answer, so someone can help correct your process. Thanks! $\endgroup$ – jonsca Feb 19 '13 at 4:17
  • $\begingroup$ I wrote how I got the answer, thanks for the exponent formatting! $\endgroup$ – Krikor Ailanjian Feb 19 '13 at 4:25
  • 3
    $\begingroup$ Hi, this question will probably be closed on account of it being too localised and homeworky. A general pointer, however, is that your products and reactants need to be stoichiometrically equivalent ('balanced'). You have hydrogen in your products but not in your reactants. Your reactants do not actually include water. Moreover, the alkali metals are famous for producing a certain... explosive gas... when reacted with water. $\endgroup$ – Richard Terrett Feb 19 '13 at 4:26
  • $\begingroup$ @RichardTerrett: Nah, he has shown some work (he could show more though), and it's a simple conceptual error. It probably won't be closed :) $\endgroup$ – ManishEarth Feb 19 '13 at 5:19
  • $\begingroup$ @RichardTerrett Ordinarily, and in its original form, I would have agreed, but since this was a chance for someone knowledgeable to correct his rationale I think it could be useful in the future. $\endgroup$ – jonsca Feb 20 '13 at 3:21
5
$\begingroup$

I think the major error you made is applying what you call the "Metal + Water = Metal Oxide" rule. This may apply to some metals, but not all. Specifically, Lithium is a Group 1 metal, the so-called Alkali Metals. This means that Group 1 metals tend not to form oxides with water, but rather form hydroxides with water. That is, the metal ions bond with hydroxide ion, $\ce{OH-}$, which has a -1 charge. If you have ever seen this reaction (lots of YouTube videos if you have not done the reaction yourself), you will see that lithium in water produces lots of gas. And in some videos, you will see that this gas can be set on fire! Given the elements involved, that's a good indication that the gas is hydrogen, a very flammable gas.

So the most likely equation for lithium in water forms an hydroxide of lithium along with hydrogen gas. Is that enough information for you to propose a different equation than what you answered?

$\endgroup$
4
$\begingroup$

I think you here looking for this:

$$\ce{aM(s) + bH2O(l) -> M_a(OH)_b(s) + $\tfrac b2$\,H2(g)}$$

Then, if you remove the water from the medium you are left with the $\ce{LiOH}$. The thing is, the lithium breaks up the water and molecular hydrogen is released. Nevertheless, this ionic compound ($\ce{LiOH}$) is soluble in water, hence the aqueous forms.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.