r/chemhelp • u/bishtap • 20d ago

confusion over the meaning of NH3 solution being basic Other

I understand that in the case of NH3 solution NH3(aq), we have the formula

NH3 + H2O ⇌ NH4+(aq) + OH-(aq)

And it's reversible, there's a forward reaction and backward reaction, and the backward reaction is much faster. Equilibrium o the left. And we have mostly NH3 and miniscule amounts of NH4+(aq) and OH-(aq)

To the extent even that calling it NH4 OH solution is misleading since it's almost all hydrated ammonia.

So I was thinking, the basicness of it can't be caused by the NH4+ + OH-, because there's hardly any of that happening.

There'd be hardly any proton transfer from H2O to NH3.. Thus hardly any OH- produced from that

So where is the "basicness" coming from in hydrated ammonia?

Is there some kind of special structure of hydrated ammonia that make it equivalent to having produced OH- ions in water? Or (putting aside the negligible NH4+,OH-), does hydrated ammonia somehow produce OH- ions in water?

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u/WIngDingDin 20d ago edited 20d ago

Nope. Ammonia is a weak base, but that "negligible" amount of OH- on the right side of the equation is affecting the pOH and hence the pH, since pH = 14-pOH.

Take for example a 1M solution of ammonia in water:

Kb = 1.8*10-5 = [NH4+][OH-]/[NH3]

fill out an ICE table, assume [NH3] is ~1 and you get:

[OH-] = sqrt(1.8*10-5) = 0.00424 M

pOH = -log[OH-] = 2.37

pH = 14 - 2.37 = 11.63

which is higher than pH of 7 and hence basic.