April 19, 2015

Homework Help: chemistry

Posted by Christine on Sunday, April 29, 2007 at 5:53pm.

Write a net ionic equation for the reaction of the aqueous species ammonia and hydrofluoric acid.

I don't think mine is coming out right- I don't know how NH3 reacts with other species, because I end up getting hydrogen as both an anion and a cation in the same species.

NH3(aq) is NH3 + HOH ==> NH4^+ + OH^-
HF ==> H^+ + F^-
NH4F is soluble in water.

So if NH4F is soluble in water, would the net ionic equation be
H+ + OH- --> H2O?

Or would the net ionic equation deal with the NH4F?

I didn't indicate above that HF is a weak acid but it is.
I have never seen an equation like this before. Check my thinking.
NH3(aq) is NH3 + HOH ==> NH4^+ + OH^-
and if we were trying to be exact we would write NH4^+(aq) + OH^-(aq).
NH3 + HOH + HF ==> NH4^+ + OH^- + H^+ + F^-
NH3 + HOH + HF ==> NH4^+ + HOH + F^-
but HOH cancels on each side. Since the problem specifically states aqueous ammonis, then I think the following is reasonable.
NH3(aq) + HF ==> NH4^+(aq) + F^-(aq)
The problem is that I don't know how your prof counts an aqueous solution of NH3. It is ammonia gas dissolved in water and it is a weak base which dissociates slightly into NH4^+ and OH^- but there is a good deal of NH3 remaining undissociated. In addition, do we show the HF dissolving in the solution, too, and writing it as HF(aq). I don't know. But I think what I have written above is reasonable. The prof may simply be looking for
NH3(aq) + HF ==> NH4^+(aq) + F^-(aq) with the possible addition of HF(aq) replacing HF.

I am a little uncomfortable with the answer I wrote above BECAUSE, most of the NH3 stays in the form of NH3 and most of the HF stays in the form of HF; therefore, the molecular species far out number the ionic ones. In such cases we usually defer to the molecular form. However, writing "no reaction" just doesn't seem appropriate. I hope I haven't completely confused you.

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