Acid base reasoning

[deleted647690]

Part of a question I did involved comparing the acidity of NH4+ vs. HF. I reasoned that:

NH4+ -----> yields a conjugate base: NH3

HF-------> yields a conjugate base: F-

Based on the fact that F- has a charge, whereas NH3 does not, I thought that F- would be considered a stronger, more reactive base. The negative charge would destabilize the base, making it more reactive. Thus, HF would be a weaker acid than NH4+. However, this was incorrect. What is wrong with this reasoning?

The correct reasoning would have been to look at the electronegativity difference between N and F. Since Fluorine is more electronegative, the bond between H and F is more polar, and thus more likely to break, making HF more acidic.

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[DrPatrickStar]

The way I think about it, HF is a stronger acid than NH4 due to electronegativity. The conjugate base of something strong will be weak, and vice versa. This is why NH3 would be right. Your reasoning is not correct because F- is not very reactive since it is at its octet. I wouldn’t dwell on it too much but the concept here is knowing the conjugate of something strong is weak and vice versa.

 

[deleted936470]

Part of a question I did involved comparing the acidity of NH4+ vs. HF. I reasoned that:

NH4+ -----> yields a conjugate base: NH3

HF-------> yields a conjugate base: F-

Based on the fact that F- has a charge, whereas NH3 does not, I thought that F- would be considered a stronger, more reactive base. The negative charge would destabilize the base, making it more reactive. Thus, HF would be a weaker acid than NH4+. However, this was incorrect. What is wrong with this reasoning?

The correct reasoning would have been to look at the electronegativity difference between N and F. Since Fluorine is more electronegative, the bond between H and F is more polar, and thus more likely to break, making HF more acidic.

The strongest acid among a pair will be the compound with the weakest conjugate base. HF/F- wins but this one is tricky because fluorine is an exception to some trends; do note that unlike other hydrogen halide acids, fluoride ion is VERY reactive and can forms fairly insoluble salts with alkaline earth metals, making it extremely dangerous if exposed to bone. While F fully follows the octet rule, it is not exactly an appropriate explanation for its basicity; its uniqueness here really has more to do with its extremely small atomic radii and thermochemistry contributing to its very poor nucelophilicity *unless* involved in salt formation. I wouldn't go beyond this. Just remember fluorine is a weird one & one of my favorites.

 

[deleted647690]

The strongest acid among a pair will be the compound with the weakest conjugate base. HF/F- wins but this one is tricky because fluorine is an exception to some trends; do note that unlike other hydrogen halide acids, fluoride ion is VERY reactive and can forms fairly insoluble salts with alkaline earth metals, making it extremely dangerous if exposed to bone. While F fully follows the octet rule, it is not exactly an appropriate explanation for its basicity; its uniqueness here really has more to do with its extremely small atomic radii and thermochemistry contributing to its very poor nucelophilicity *unless* involved in salt formation. I wouldn't go beyond this. Just remember fluorine is a weird one & one of my favorites.

Why is it your favorite?

 

[deleted936470]

Why is it your favorite?

will form compounds with noble gases xenon, krypton, and radon, considered weak acid at low concentrations, but practically and speaking from experience, it acts as a strong acid at high concentrations, partly due to aforementioned things in my last response. We've also found use for it in so many things... fluoride in toothpaste, chemotherapy, teflon, used for MRI, and much more etc. idk just geeks me out.