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Name: Michelle
Status: educator
Grade: 9-12
Location: IN
Country: USA
Date:  Summer 2011  

Question:
The teachers in my department had a disagreement on which was the "strongest" acid. Some thought hydrofluoric acid because it seemed like it would dissociate well due to the electronegativity difference between the H and the F. We found this argument was incorrect, because it does not dissociate well. Some thought it was hydrofluoric acid because they have heard stories about how nasty it is when you spill it on yourself. We found this was incorrect because "strong acid" does not equate to "most dangerous". After looking it up, we saw it was a weak acid, but very dangerous to work with. We understand the definition of "strong acid" vs "weak acid", but were wondering what causes an acid to be so corrosive, since it seems to not be related to dissociation of the H.

Replies:
Hi Michelle.

Many acids present dangers other than their mere acidity. F- is pretty damaging to bio-molecules, and it steals Calcium from bones pretty strongly in order to form highly stable and insoluble calcium fluoride. That is why HF is dangerous, quite out of proportion to the damage its weak acidity could do. Sulfuric acid is dangerous because it strongly steals water from tissue. Nitric acid can make pale brown stains on your skin, because the NO3- ion is an oxidzer considerably stronger than the H+ ion. (It does require and consume H+ in order to be that strong.)

I tend to think of HCl as the prototypical example of the most danger that can be created purely from strong acidity. It can do substantial damage, but it may not be instant and dramatic.

It is not a completely pure case. HCL is also dangerous because it's volatile, and breathing it carries it to the unprotected wet lining of your lungs.

So perhaps Perchloric acid is a nearly perfect example of something that acts only by acidity. It's about the strongest acid, it's not volatile and up to 50% in water it is not immediately toxic or oxidizing. But most of us never need it so we rarely encounter it. It is an extremely strong oxidizer in principle, but very slow-acting in water solutions, so I think it tends to cause eventual cancer rather than immediate chemical burns. Frankly, it is boring. I suppose it is good that it is so non-dramatic that there is little temptation to play with it.

HF is a weak acid because high electronegativity difference is not the only thing that matters. Size is the other thing. Strong dissociation in water is encouraged by a combination of high difference and large ionic radius. F- is very small, so the H+ can get very close, and that improves the energy standing of the combined HF species compared with the separated ions surrounded by water. Small size also makes electron-sharing (covalent bonds) more effective.

Jim Swenson


Hi Michelle,

The high reactivity, and danger, of aqueous HF does not come so much from the H(+), because, as you noted, HF is a weak acid (with a ka = 6.4 x 10^-4) so there is little of the H(+) present. The danger comes from two issues: (1) the counter-ion, F(-) is very reactive, and (2) the HF molecule can act as a polar covalent molecule and can diffuse through many substances, including skin.

For example, since the counter-ion of most strong acids are not as reactive as F(-), it is the H(+) that reacts with skin. Since the H(+) reacts with skin on contact, the top layer of skin is most affected, irritation or burns are immediately noticeable, and can be treated right away. HF on the other hand, having very little H(+) can lay on the skin without being noticed, the HF -as a molecule- can diffuse through the skin, and with some ionization, the H(+) may react throughout the depth of penetration. More importantly, the F(-) can reach blood vessels and bone where it can bind with other cations. On reaching the calcium of bone, it can form CaF2.

This also leads to issues of containment. Since HF can either diffuse as a molecule or as ions, and the F(-) will readily react with some of the atoms of the container, careful choice of container must be made to avoid multiple possibilities of containment failure.

Greg (Roberto Gregorius) Canisius College


Michelle,

An acid is strong when all of the hydrogen ions dissociate from all the formula units almost completely. In other words, the strength of an acid describes the behavior. Hydrofluoric acid does not dissociate completely so it is a weak acid. The corrosive effect of an acid is due--in large part--to the hydrogen ions in solution, which is due to a combination of the acid strength AND the concentration. The nastiness of hydrofluoric acid on organic tissues is very much due to the fluoride ion. So the corrosive effect of an acid--generally--is due to the concentration of the hydrogen ion, which is due--in turn--to BOTH the strength AND the concentration of the acid. That said, some corrosive effects, or poisonous effects, can be due to the acid anion--the part of the acid formula unit that remains after the hydrogen ion dissociates. This is the case with hydrofluoric acid. Another example of a dangerous anion in a weak acid is hydrocyanic acid, also called hydrogen cyanide or Prussic acid. Interestingly, it is the electronegativity in combination with the small fluoride ion size that makes it more difficult for the hydrogen to dissociate from the fluoride ion. The explanation is rather complex. If you really want to get into it, check into the following web page: http://www.chemguide.co.uk/inorganic/group7/acidityhx.html .

Ray Tedder


Hi Michelle.

Many acids present dangers other than their mere acidity. F- is pretty damaging to bio-molecules, and it steals Calcium from bones pretty strongly in order to form highly stable and insoluble calcium fluoride. That is why HF is dangerous, quite out of proportion to the damage its weak acidity could do. Sulfuric acid is dangerous because it strongly steals water from tissue. Nitric acid can make pale brown stains on your skin, because the NO3- ion is an oxidzer considerably stronger than the H+ ion. (It does require and consume H+ in order to be that strong.)

I tend to think of HCl as the prototypical example of the most danger that can be created purely from strong acidity. It can do substantial damage, but it may not be instant and dramatic.

It is not a completely pure case. HCL is also dangerous because it's volatile, and breathing it carries it to the unprotected wet lining of your lungs.

So perhaps Perchloric acid is a nearly perfect example of something that acts only by acidity. It's about the strongest acid, it's not volatile and up to 50% in water it is not immediately toxic or oxidizing. But most of us never need it so we rarely encounter it. It is an extremely strong oxidizer in principle, but very slow-acting in water solutions, so I think it tends to cause eventual cancer rather than immediate chemical burns. Frankly, it is boring. I suppose it is good that it is so non-dramatic that there is little temptation to play with it.

HF is a weak acid because high electronegativity difference is not the only thing that matters. Size is the other thing. Strong dissociation in water is encouraged by a combination of high difference and large ionic radius. F- is very small, so the H+ can get very close, and that improves the energy standing of the combined HF species compared with the separated ions surrounded by water. Small size also makes electron-sharing (covalent bonds) more effective.

Jim Swenson


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