How is ammonia more acidic than methane? My teacher told me it is so as I am confused as ammonia is basic in nature and methane is neutral so how can ammonia be more acidic than methane?
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1$\begingroup$ Does this answer your question? Strength of weak acids and bases: Is methane better base than ammonia? $\endgroup$– MithoronCommented Apr 25, 2020 at 14:24
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2 Answers
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For ammonia, there are 2 different acido-basic reactions:
$$\ce{NH3 + H+ <=> NH4+}$$
$$\ce{NH3 <=> NH2- + H+}$$
In the former, ammonia is relatively strong base with weak conjugated acid $\ce{NH4+}$.
In the latter, ammonia is very weak acid with very strong conjugated base $\ce{NH2-}$.
It is related to the liquid ammonia autodissociation:
$$\ce{2 NH3(l) <=> NH4+ + NH2-}$$
The base $\ce{NH2-}$ cannot exist in water, as it reacts with it as with an acid:
$\ce{NH2- + H2O -> NH3 + OH-}$
Methane is an extremely weak acid, weaker than ammonia,
$$\ce{CH4 <=> CH3- + H+}$$
making $\ce{CH3-}$ much stronger base than $\ce{NH2-}$.
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The question is a very good one. I upvoted it because it shows how a student can see quite clearly what the teacher is either too unfamiliar with or too familiar with.
The basicity of ammonia and the neutrality of methane is the situation in the world of the student. These characteristics are readily observable. But the teacher is describing a quite different world.
Chemists are never really satisfied with the status quo, so they do reactions where extra energy is added. While methane is neutral with respect to water, because it is unreactive toward water, I could rip a proton off if I tried hard enough, and if I did that, I would get CH3-, which is extraordinarily basic and would itself rip a proton off almost everything else to go back to CH4. Compounds like methyl sodium or sodium methide (NaCH3) are so air-sensitive that they are handled in special equipment. Some alkali salts of hydrocarbons can be dissolved in solvents, like butyl lithium in hexane, but they are quite air-sensitive too.
It's easier to rip a proton off NH3 to form NH2-. Amides are easier to handle: NaNH2 is a solid that melts at 200C, and can be handled (if quick) in dry air, but it will react with water vapor, pulling off a proton to give NH3 and NaOH. If you thought NaOH was a strong base, think how much stronger a base NaNH2 is! But NaNH2 won't pull a proton off of CH4, or hydrocarbons in general, so NH2- is not a strong enough base, or CH4 can be said to be a weaker acid.
answered Apr 25, 2020 at 14:05
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$\begingroup$ Just want to ask one more thing is there any other nature of a molecule except acidic,basic and neutral as if it is not so how can the total nature of a molecule can exceed 100%? $\endgroup$ Commented Apr 25, 2020 at 14:37
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$\begingroup$ Not sure what you are asking. We don't say that a molecular nature has 100% of anything; we describe certain properties - they can go from zero to some large number (like polarity, or dipole moment), or from diamagnetic to paramagnetic (repelled by or attracted to a magnetic field), or colored (photon absorption energy). Perhaps buoyancy could go from - to + (something sinks or floats), but that would be characteristic of density of a quantity greater than a single molecule. Do you want to clarify? $\endgroup$ Commented Apr 26, 2020 at 19:07