Acid & Basic Anhydrides

[SaintJude]

---

Members do not see this ad.

Kaplan MCAT Qbank Discrete

Which of the following electron configuration represents the element that would form the most acidic anhydride?

Answer Choices:
Shows electron configuration of
A. Na
B. Al
C. Si
D. P

Answer is....


D. Which leads me to my question: Why are metals usually basic anhydrides and non-metals lead to acidic anhydrides?
[Btw, I had to look it up: An anhydride is any chemical compound obtained by the removal of water from another compound]

 

[MedPR]

Kaplan MCAT Qbank Discrete

Which of the following electron configuration represents the element that would form the most acidic anhydride?

Answer Choices:
Shows electron configuration of
A. Na
B. Al
C. Si
D. P

Answer is....


D. Which leads me to my question: Why are metals usually basic anhydrides and non-metals lead to acidic anhydrides?
[Btw, I had to look it up: An anhydride is any chemical compound obtained by the removal of water from another compound]

I would've guessed Phosphorous only because it's the most electronegative in that list. As far as I know, a Phosphoric anhydride (or any anhydride I guess) is an oxyacid. The rule with oxyacids is more oxygens = more acidic, and in the case of equal oxygens (which I assume is the case here) more electronegative central atom = more acidic.

If your question is based on fact, I would assume the metal anhydrides are basic because they are easily protonated and the lone pairs on the oxygen(s) are more likely to be donated, forming single bonds between the Oxygen and the central metal atom.

I believe anhydrides always include double bonded oxygens, and I've never seen Na, Al, or Si double bonded to anything.

 

[dranet]

If you look at where the metals are on the periodic table you'll see they areon the left of the periodic table "Alkaline" part and are electropositive (low electronegativity). The non metals appear on the on the right side of the periodic table in the "acidic" part and tend to be electronegative (higher electronegativity). Easy way to remember.
The metals are electron donors (base) and form oxides with oxygen. If you add water they form hydroxides The non metals form oxides also but you add water they become acids.
Now your question. Using POE you can throw out A and B they made basic anhydrides. Next compare C and D they are more to the right of the periodic table and possibilities.You need to compare their electronegativities because acid anhydrides have the non metal with similar electronegativity to oxygen.Si hase electron of 1.9, P 2.19 and O is 3.44. The difference in electronegativity of Si and O is largest so its the situation akin to the basic anhydrides ( Na at 0.93). The P has the least difference so its the candidate forming the most acid anhydride...so D is your answer.

 

[Pisiform]

As MedPR said, the greater the electronegativity, the greater would be the inductive effect and more acidic would be the compound.And metals lose electron, they are lewis base.