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Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: confusedstud on October 26, 2018, 06:23:40 AM

Title: Why don't anions cause aqueous solutions to turn basic like how aqua ions do
Post by: confusedstud on October 26, 2018, 06:23:40 AM

When Na+ is in water it slightly acidifies it because the water molecules surrounding it in a shell would HABE their O-H bind polarised and as a result, the hydrogen atoms become more susceptible to removal by another water molecule to produce H3O+ ions.

However, why isn't the inverse said about anions. For example if Cl- is in water, wouldn't the hydrogens face the anion leaving the O pointed away. And now the partial negative charge on oxygen becomes greater allowing it to deprotonate the water molecules around it to produce OH- ions which makes the solution basic. Why don't we talk about this phenomenon?

When I google about anion acidity they point towards the anion acting as a base itself with an equilibrium reaction with water, but in the case of aqua acids that's not the case (we only talk about the aqua acids forming a dative bond to the metal ion centre, or in Na+'s case ion-dipole interactions) so shouldn't anions have the same but opposite effect?

Title: Re: Why don't anions cause aqueous solutions to turn basic like how aqua ions do
Post by: AWK on October 29, 2018, 05:49:48 AM

Compare Cl^- and CN^- in water. Why solution of  Na3PO4 is higly basic?

Title: Re: Why don't anions cause aqueous solutions to turn basic like how aqua ions do
Post by: confusedstud on November 03, 2018, 08:57:34 AM

Quote from: AWK on October 29, 2018, 05:49:48 AM

Compare Cl^- and CN^- in water. Why solution of  Na3PO4 is higly basic?

Prof, CN- in water would have an equilibrium with water to form OH- and HCN making the solution basic. But Cl- doesn't have that effect as it has a very high pKb (evident by HCl having a very small pKa). Likewise for the PO43- ion.

But in the mechanism I propose for increasing the basicity of the solution doesn't involve this mechanism. Instead its similar to that of the aqua acids: https://imgur.com/a/eItp2Fd

In the aqua acid's case, we would say that the hydrogen becomes more partially positive making it more prone to deprotonation to produce a hydroxonium which acidifies the solution. So I was thinking that the same situation should happen for the anion, but makes the solution more basic instead.

Title: Re: Why don't anions cause aqueous solutions to turn basic like how aqua ions do
Post by: Borek on November 03, 2018, 09:27:45 AM

Even if the effect exists (I don't see why it shouldn't), it is dwarfed by several other, much stronger effects. First, anions are typically much larger than cations, so they can't attract water molecules so strongly, second, large anions are typically products of weak acids dissociation, so their hydrolysis is already dominating the solution.

Title: Re: Why don't anions cause aqueous solutions to turn basic like how aqua ions do
Post by: confusedstud on November 04, 2018, 01:14:26 PM

Thanks Prof Borek, I can finally give this a rest.

Title: Re: Why don't anions cause aqueous solutions to turn basic like how aqua ions do
Post by: Borek on November 04, 2018, 01:18:18 PM

don't "prof" us :)