[thagza]

Hey all,
   I am trying to teach myself organic chemistry this summer and have run into my first problem. My book tells me that the structure of a molecule can affect its acidity. In this topic it says that a more electronegative element bears a negative charge more easily, giving a more stable conjugate base. Thus:

HNH₂ is less acid than H₂O which is less acidic than HF

This makes sense

I was doing alright following this rule until I hit this problem:

Write equations for the following acid base reactions and determine whether the equilibrium favors the products or the reactants.

I wrote the equation as:

H₂S + OH-  HS^- + H₂O

Along the line of reasoning of the book, the oxygen atom is more electronegative, and should thus be the more acid of the two because its conjugate base, the hydroxide ion, would be more stable. Thus, the reaction should favor the weaker acid, and so the reaction would favor the reactants. However, I know that H₂S is a stronger acid, and so the products should be favored here.

I figure that this is because the oxygen atom, being more electronegative than the S atom, means that the O-H bond is harder to break and so the other acid would be more acidic. However, this is not the conclusion I would have come to following my book.

Can somebody tell me if my reasoning is incorrect, and also can you all give me tips and what to look for in determining relative acid strength?

Thanks!

[ramboacid]

There are a lot of reasons for determining acidity, but in your case as you are comparing OH^- and HS^- would be the difference in the ionic radius of sulfur and oxygen ions. As sulfur ions are larger than oxygen ions, the bond radius is larger in HS^- than in OH^- and thus the bond is weaker, allowing the H⁺ to ionize more easily in HS^- than in OH^-. The greater ease of dissociation makes HS^- more acidic.

My book has these quick guidelines for acid strength:

• If anions are in the same period, then farther right means more acidic
• If anions are in the same group, then farther down means more acidic
• If working with oxoacids (acids in which O are covalently bound to p-block element i.e. HClO, H₃PO₄, etc.), the more oxidized the central atom is the more acidic it is. If there is the same number of O bonded, then the more electronegative the central atom is the more acidic the oxoacid will be.
  • HClO > HBrO > HIO
  • HClO₄ > HClO₃ > HClO₂ > HClO