MCAT assessment: Strongest base?

[LuminousTruth]

From one of the Self assessment questions, it was asking which of the following ions in reaction 2 is the strongest base?

Reaction 2: Mg3N2 + H2O -> Mg(OH)2 + NH3.

A) N3-
B) OH-
C) Mg2+
D) H+

The correct answer was A.
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I narrowed it down to OH- and N3-.

I looked on the electronegativity table and oxygen was more electonegative than N so I thought OH was the answer but it turned out N3 was actually the stronger base.

Can someone explain why?

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[Ace-Co-A]

The equation shows that ammonia is formed as a result of placing Mg3N2 in water. So, the protons on water (H2O) were stripped to protonate N3-, forming ammonia, NH3. If the N3- was basic enough to strip protons from a very weak acid like water, then clearly it is a stronger base than OH-.

If what you suggest was correct, the equation they gave you would have made no sense, because the N3- was protonated but the OH- was not. (A stronger base is protonated more readily than is a weaker one.)

 

[sciencebooks]

I solved A as well but I'm also curious... Say they just say you N3- and OH- alone... Would we still say N3- was more basic because it's more negative charged/has more electrons to donate? Does the electronegativity pattern OP mentioned only come into play when all else is equal?

 

[Trayshawn]

Just look at A, its a VERY small compound with a VERY large charge. You rarely see nonmetals with that high a charge. Nitrogen is simply too small and this gives it an incredibly high charge density. It will do whatever it can to remove that charge. Even NH2- (just one negative charge) is a stronger base than OH-. Nitrogen is simply too small to handle its own negative charge.

Same analogy goes for oxygen. If you ever see an O^-2, expect to see some serious deprotonation going on whenever possible.

Electronegativity plays a role when charge and size are about equal. The compound that is more electronegative is more "okay" with handling a negative charge and has less of a desire to get rid of it.
This is why even NH2- is a stronger base than OH-, nitrogen is less electronegative than oxygen and is more eager to get rid of that negative charge since its so unstable.

 

[sciencebooks]

Just look at A, its a VERY small compound with a VERY large charge. You rarely see nonmetals with that high a charge. Nitrogen is simply too small and this gives it an incredibly high charge density. It will do whatever it can to remove that charge. Even NH2- (just one negative charge) is a stronger base than OH-. Nitrogen is simply too small to handle its own negative charge.

Same analogy goes for oxygen. If you ever see an O^-2, expect to see some serious deprotonation going on whenever possible.

Electronegativity plays a role when charge and size are about equal. The compound that is more electronegative is more "okay" with handling a negative charge and has less of a desire to get rid of it.
This is why even NH2- is a stronger base than OH-, nitrogen is less electronegative than oxygen and is more eager to get rid of that negative charge since its so unstable.

I know you were answering the OP but you inadvertently answered my side question above too so thanks! Happy I was thinking about it properly, 🙂.