Group 6 Acid Strength: H2O vs. H2S vs. H2Se vs. H2Te

[AdkinsJr]

Homework Statement

What is the strongest acid among the following: H2O, H2S, H2Se, H2Te

2. The attempt at a solution
I noticed they are in the same group so I think the idea is to pick the compound with the group 6 element that is largest in atomic size...I think it's H2Te because atomic size increases as you go down a group in the periodic table, and the heavier the atom the greater tenancy it will have to dissociate in a given group. Is this correct?

 

[Borek]

Yes, H₂Te is the most acidic. Easy to check - google for first dissociation constants of all four compounds.

If nothing else works, stick to facts ;)

 

[AdkinsJr]

Thanks for the tip, I thought it'd be easy to google and tried but wasn't sure what to search... dissociation constants is a good idea.

 

[epenguin]

You can google the substances themselves. Then there are lots of tables. Here is one specifically referring to your question. http://www.chemrat.com/ChemHog2/Organic Chem_files/pKtrends.doc. You have to know what pK means, but it you haven't done it, it suffices that it is the pH at which the acid is half-dissociated. But then you have to know what pH means.

So we'd class H₂Se as a weakish acid a bit more acidic than acetic. H₂Te is getting to be strongish. This may be the first and last time you meet it. And even H₂Se not often. The real thing you're likely to met later is that H₂S is a weak acid as are very many common RSH ((sulphydryl aka thiols aka mercaptans) which include important biological molecules like the amino acid cysteine. Their dissociation occurs over the neutral or physiological range, pK's round 6.75 - 8. Weak acids but much stronger than the corresponding alcohols.

If the rationalisations you are expected to learn about factors of acid strength make sense, OK, whenever they don't, don't worry, because the ones that have got fossilised in textbooks are all inadequate less-than-half truths that leave out the fundamental role of the solvent.

 

[DeeNos]

Your reasoning is partially correct. The strength of an acid is determined by its ability to donate a proton (H+ ion). In general, the stronger the bond between the hydrogen and the other atom in the molecule, the weaker the acid. This is because a strong bond is less likely to break and release a proton.

In the case of H2O, the bond between hydrogen and oxygen is very strong, making it a weak acid. H2S, H2Se, and H2Te all have weaker bonds between hydrogen and sulfur, selenium, and tellurium, respectively, making them stronger acids. And as you mentioned, the atomic size also plays a role, with H2Te having the weakest bond and therefore being the strongest acid among the group 6 elements.

However, it is important to note that there are other factors that can also affect acid strength, such as the polarity of the molecule and the stability of the resulting ions. So while your reasoning is generally correct, it is not the only factor that determines acid strength.