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Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: 1204oscar on March 13, 2020, 11:05:17 PM

Title: Net Ionic Equations for Acid-Base Reactions
Post by: 1204oscar on March 13, 2020, 11:05:17 PM
Hi all,
I am studying for my Chemistry final right now, and I am puzzled by this particular question:

"When hydrocyanic acid (HCN) is added to sodium hydroxide (NaOH), what is the net ionic equation for the reaction that occurs?"

I thought that the answer would simply be
H+ + OH-  ::equil:: H2O
However, the correct answer is
HCN + OH-  ::equil:: CN- + H2O.

Earlier in the course, we were taught that the net ionic equation for any reaction considers aqueous compounds as anions and cations, and therefore if a part of the aqueous compound doesn't react that part stays out of the net ionic equation. Wouldn't that be the case for HCN, since the CN- part of HCN doesn't change? The only reason I could see why that is is that HCN is such a weak acid with a small pKa value that it barely dissociates. If that is the reason why I got this problem wrong, then my question is at what specific pKa value do we no longer consider the dissociation of acids?
Title: Re: Net Ionic Equations for Acid-Base Reactions
Post by: MNIO on March 14, 2020, 12:17:26 AM
The issue is HCN is a weak acid and doesn't completely dissociate.

We do NOT write these equations if a species does not completely dissociate.
   molecular:      1 HCN(aq) + 1 NaOH(aq) ----> 1 H2O(l) + 1 NaCN(aq)
   total ionic:      1 H+(aq) + 1 CN-(aq) + 1 Na+(aq) + 1 OH-(aq) ---> 1 H2O(l) + 1 Na+ + 1 OH-
     net ionic:      1 H(+) + 1 OH(-) ----> 1 H2O(l)

We write this instead...
   molecular:      1 HCN(aq) + 1 NaOH(aq) ----> 1 H2O(l) + 1 NaCN(aq)
   total ionic:      1 HCN(aq) + 1 Na+(aq) + 1 OH-(aq) ----> 1 H2O(l) + 1 Na+(aq) + 1 OH-(aq)
     net ionic:      1 HCN(aq) + 1 OH(-)(aq) ----> 1 H2O(l) + 1 CN-(aq)

see the difference?  we have to leave HCN as a molecule.

see this link
https://chem.libretexts.org/Bookshelves/Introductory_Chemistry/Book%3A_Introductory_Chemistry_(CK-12)/21%3A_Acids_and_Bases/21.16%3A_Neutralization_Reaction_and_Net_Ionic_Equations_for_Neutralization_Reactions
Title: Re: Net Ionic Equations for Acid-Base Reactions
Post by: Borek on March 14, 2020, 05:30:01 AM
Quote from: 1204oscar on March 13, 2020, 11:05:17 PM
at what specific pKa value do we no longer consider the dissociation of acids?

It is not like we can draw an exact line, as degree of dissociation depends not only on pKa, but also on concentration. Acid with pKa=2 is dissociated 10% in 1 M solution, but 90% in 10-3 M solution (more or less, these are not exact numbers).

HCN is very weak, with pKa around 9, so even at very low concentrations its dissociation is negligible.