Explain why HF is a weak acid, whereas HCl, HBr, and HI are all strong acids.

Name: Problem 72, Chapter 20: Chemistry
Uploaded: 2020-05-19T20:09:01-08:00
Duration: 6 min 47 s
Channel: Matthew Bittner
Description: Explain why HF is a weak acid, whereas HCl, HBr, and HI are all strong acids.

step 1 From 66 is an interesting question. It asks why hydrofluoric acid is a weak acid, while HCL, HBR

Explain why HF is a weak acid, whereas HCl, HBr, and HI are...

Key Concepts

Acid Strength

Acid strength refers to an acid's ability to donate its proton (H+) in solution, which is quantified by the acid dissociation constant (Ka). A strong acid fully ionizes in solution, releasing a high concentration of H+, whereas a weak acid only partially ionizes, resulting in fewer free protons.

Bond Strength

The strength of the bond between hydrogen and the atom it is attached to is a key determinant of acid behavior. A stronger H–X bond requires more energy to break, meaning the acid is less likely to donate its proton. In cases where the bond is very strong, even a highly electronegative atom may not facilitate complete dissociation.

Conjugate Base Stability

The stability of the conjugate base (the species remaining after the proton is lost) greatly influences acid strength. A more stable conjugate base can better accommodate the negative charge, driving the proton dissociation process. Factors such as atomic size and polarizability contribute to the stability, with larger atoms dispersing the charge more effectively.

Electronegativity and Atomic Size

Electronegativity and atomic size together impact both bond strength and conjugate base stability. While a highly electronegative element tends to pull electron density towards itself, resulting in a stronger bond with hydrogen, a larger atomic size allows for better dispersion of the negative charge once the proton is lost, enhancing the stability of the conjugate base. This interplay explains why acids with larger halogen atoms often dissociate more readily than those with smaller, more tightly bound atoms.

Transcript

00:01 From 66 is an interesting question.

00:03 It asks why hydrofluoric acid is a weak acid, while hcl, hbr, and hi are all strong acids.

00:14 This is a good question, and a lot of people might be wondering that, because if you look at the periodic table, f, clbr, and i are all in the same group.

00:27 They're all halogens.

00:28 So why is hf the only one? that's a weak acid.

00:33 Well, let's start by looking at what happens when these acids are put in water, right? so if we have aqueous hydrofluidazin, then we're going to be in a state of equilibrium with hydrogen ions and fluoride ions, right? and this is going to be the same for all of these things, right? so like h cl is going to be an equilibrium with h plus and c l minus and then hbrr and these are all aqueous by the way i'm just not going to write it out for anything but hbrr is an equilibrium with hydrogen ions and bromide ions and then h i is an equilibrium with hydrogen ions and iodide ions.

01:55 So these are all very similar when i'm equest in water.

01:59 This right here is a weak acid.

02:06 Well, these guys right here are all strong.

02:16 So why is this the case? well, what makes a strong acid? a strong acid is something that is willing to give up.

02:25 It's proton it's most likely to give it's proton so basically we're saying that these these guys right here are all more willing to give up that hydrogen ion give up that proton to a base than what would make a molecule more an acid more likely to this proton well the more stable its base the more likely it is to give this proton.

03:00 Because if the base, and these bases, are stable by themselves, then they'll be more likely to form, right? if you have a base that isn't so stable by itself, then it's not going to want to give up the proton because it's not going to want to have that base.

03:23 So knowing that now, we try to figure out which are the most stable bases.

03:30 Well, fluoride, the reason that, well, let's start here, let's talk about chloride bromide in the iodide...

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