Why is hydrofluoric acid so dangerous if it is a weak acid?

Why is hydrofluoric acid so dangerous if it is a weak acid?

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I've read that hydrofluoric acid (HF) is extremely dangerous to touch, but what exactly makes it so toxic? It's weak acid ($K_a = 7.2 imes 10^{-4}$) and dissociates approximately 1/1000 as much as Hydrochloric acid does in water, so why is it so much more dangerous? Wouldn't a stronger acid cause more damage? What exactly happens when human skin comes into contact with Hydrofluoric acid that causes it to be so lethal?

The confusion arises from the term weak, which has only to be interpreted in chemical terms.

Weak acid, as you say, just means that the acid does not readily dissociate, not that its effects are weak! Just to say one, HF corrodes glass, something that not even smoking HCl does.

Wikipedia has a nice summary of HF toxicity (see also the references in the article itself):

Hydrofluoric acid is a highly corrosive liquid and is a contact poison. It should be handled with extreme care, beyond that accorded to other mineral acids. Owing to its low dissociation constant, HF as a neutral lipid-soluble molecule penetrates tissue more rapidly than typical mineral acids. Because of the ability of hydrofluoric acid to penetrate tissue, poisoning can occur readily through exposure of skin or eyes, or when inhaled or swallowed. Symptoms of exposure to hydrofluoric acid may not be immediately evident. HF interferes with nerve function, meaning that burns may not initially be painful. Accidental exposures can go unnoticed, delaying treatment and increasing the extent and seriousness of the injury.
Once absorbed into blood through the skin, it reacts with blood calcium and may cause cardiac arrest. Burns with areas larger than 25 square inches (160 cm2) have the potential to cause serious systemic toxicity from interference with blood and tissue calcium levels. In the body, hydrofluoric acid reacts with the ubiquitous biologically important ions Ca2+ and Mg2+. Formation of insoluble calcium fluoride is proposed as the etiology for both precipitous fall in serum calcium and the severe pain associated with tissue toxicity.

This question appears to have a revived interest? I would like to clarify this Q&A you don't mind.

  1. HF(aq), like other hydrogen halides, does completely give up its proton to $ce{H2O}$ but since F is the most eletronegative element on earth, it will pair up with hydronium ion in the aqueous environment, making the proton "less available", at least to a pH electrode. This is one of strongest hydrogen bond known.

$ce{HF(aq) -> F^-… H3O+}$

Thus HF(aq) has a depressed pKa in water whilst its "real acidity" can be much higher.

  1. Homo-association of HF only occurs in very high concentration (close to unity). You can estimate the extent of homo-association of anhydrous HF by measuring its conductivity. Anhydrous HF has lower conductivity (<1.6 $mu$S/cm) than deionized water (~5 $mu$S/cm) [1] so it's probably not terribly important in driving disassociation of HF, esp in water.

  2. HF causes deep burn, often with a delayed onset because while the skin prevents charged raw hydronium ion from penetrating too deeply, HF or the hydronium-flouride ion pair can be absorbed and releases the hydronium ion (by exchanging with water abundant in tissue) at deeper level. (Thus it's often reported as more painful than other acid burn)

  3. Although fluoride in ppm level is anti-microbial and prevents cavity (by reacting with calcium mineral in tooth), high concentration of fluoride is toxic because, as cited in previous answer, fluoride interferes with calcium metabolism in forming insoluble calcium fluoride. It causes skeletal fluorosis and suppresses some essential metabolic enzymes.

  4. HF is corrosive because of its proton; it's corrosive to bones because of its fluoride. HF corrodes glass and HCl doesn't because silicon tetrafluoride is a more stable complex than the highly reactive silicon tetrachloride. It has little to do with whether the acid is weak or strong.

I am not a chemist and unfortunately I will have to refer to wikipedia, but the ionization constant for HF (10−3.15, according to wikipedia) does not take homo-association into account.

Homo-association is an association between a base and its conjugate acid through a hydrogen bond. This process stabilizes the conjugate base, thereby increasing ionization. Often this behavior increases acidity, especially at high concentrations. In case of HF (double arrows indicate equilibria):

2HF <- -> H2F+ + F (autoionization of HF)
HF + F <- -> HF2 (homoassociation)

The formation of the bifluoride anion (HF2) encourages the ionization of HF by stabilizing and removing F from solution hence pulling the dissociation equilibrium of HF (HF + H2O <- -> H3O+ + F-) to the right. Thus, the usual ionization constant for hydrofluoric acid (10−3.15) understates the acidity of concentrated solutions of HF.

Why is hydrofluoric acid so dangerous if it is a weak acid? - Biology


This page looks at the acidity of the hydrogen halides - hydrogen fluoride, hydrogen chloride, hydrogen bromide and hydrogen iodide. It starts by describing their physical properties and how they might be made and then explains what happens when they react with water to make acids like hydrofluoric acid and hydrochloric acid.

The hydrogen halides - background information

The hydrogen halides are colourless gases at room temperature, producing steamy fumes in moist air. Hydrogen fluoride has an abnormally high boiling point for the size of the molecule (293 K or 20°C), and could condense to a liquid on a cool day.

Hydrogen fluoride's boiling point is higher than you might expect because it forms hydrogen bonds.

Note: If you aren't happy about hydrogen bonding, you ought to follow this link before you go on.

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Fluorine is the most electronegative of all the elements and the bond between it and hydrogen is very polar. The hydrogen atom carries quite a lot of positive charge (+) the fluorine is fairly negatively charged (-).

In addition, each fluorine atom has 3 very active lone pairs of electrons. Fluorine's outer electrons are at the 2-level, and the lone pairs represent small highly charged regions of space. Hydrogen bonds form between the + hydrogen on one HF molecule and a lone pair on the fluorine of another one.

The other hydrogen halides don't form hydrogen bonds. The other halogens aren't as electronegative as fluorine, and so the bonds in HX are less polar. As well as that, their lone pairs are at higher energy levels. That makes the lone pairs bigger, and so they don't carry such an intensely concentrated negative charge for the hydrogens to be attracted to.

Note: If you aren't sure about why the electronegativity of the halogens changes as you go down the Group, you could follow this link.

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Making the hydrogen halides

There are several ways of making hydrogen halides, but the only one of interest at A' level is the reaction between an ionic halide (like sodium chloride) and an acid like concentrated phosphoric(V) acid, H3PO4, or concentrated sulphuric acid.

Making hydrogen chloride

You can add concentrated sulphuric acid to a solid chloride like sodium chloride in the cold. The concentrated sulphuric acid donates a hydrogen ion to a chloride ion to make hydrogen chloride. Because this is a gas, it immediately escapes from the system.

The full equation for the reaction is:

Sodium hydrogensulphate is also formed.

Concentrated phosphoric(V) acid behaves similarly. You would again add it to solid sodium choride. Once again, as soon as any hydrogen chloride is formed, it escapes as a gas. The ionic equation is:

. . . and the full one showing the formation of the salt, sodium dihydrogenphosphate(V) is:

Making the other hydrogen halides

All of the hydrogen halides can be made in an exactly similar way using concentrated phosphoric(V) acid. All you would need to do is swap the symbol Cl in the two equations for whichever other halogen you were interested in.

The situation is more complicated with concentrated sulphuric acid.

Hydrogen fluoride can be made in exactly the same way as hydrogen chloride using concentrated sulphuric acid, but hydrogen bromide and hydrogen iodide can't.

The problem is that concentrated sulphuric acid is a reasonably strong oxidising agent, and as well as producing hydrogen bromide or hydrogen iodide, some of the halide ions are oxidised to bromine or iodine. This problem doesn't happen with phosphoric(V) acid because it isn't an oxidising agent.

The acidity of the hydrogen halides

Hydrogen chloride as an acid

We are going to use the Bronsted-Lowry definition of an acid as a proton donor. Hydrogen chloride is an acid because it gives protons (hydrogen ions) to other things. We are going to concentrate on its reaction with water.

Hydrogen chloride gas is very soluble in water, reacting with it to produce hydrochloric acid. The familiar steamy fumes of hydrogen chloride in moist air are caused by the hydrogen chloride reacting with water vapour in the air to produce a fog of concentrated hydrochloric acid.

A proton is donated from the hydrogen chloride to one of the lone pairs on a water molecule.

A co-ordinate (dative covalent) bond is formed between the oxygen and the transferred hydrogen ion.

Note: If you need to revise co-ordinate (dative covalent) bonding, you could follow this link. That page also describes the reaction between hydrogen chloride and ammonia - another reaction of hydrogen chloride as an acid.

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The equation for the reaction is:

The H3O + ion is the hydroxonium ion (also known as the hydronium ion or the oxonium ion). This is the ion that we are actually talking about when we write H + (aq).

When hydrogen chloride dissolves in water (to produce hydrochloric acid), almost 100% of the hydrogen chloride molecules react in this way. Hydrochloric acid is therefore a strong acid. A strong acid is one which is considered as fully ionised in solution.

Hydrobromic acid and hydriodic acid as strong acids

Hydrogen bromide and hydrogen iodide dissolve in (and react with) water in exactly the same way as hydrogen chloride does. Hydrogen bromide reacts to give hydrobromic acid hydrogen iodide gives hydriodic acid. Both of these are also strong acids.

Hydrofluoric acid as an exception

By contrast, although hydrogen fluoride dissolves freely in water, hydrofluoric acid is only a weak acid - similar in strength to organic acids like methanoic acid.

In the past, the explanation for this was often given as the very high strength of the H-F bond, which has to be broken when hydrogen fluoride forms ions. But this explanation falls to pieces if you look at the energetics of the reaction in detail.

It is always dangerous to look at the energetics of just one step in the whole series of energy changes which happen during a reaction.

In this case, you do indeed have to put a lot of energy in to break the hydrogen-fluorine bond, but you also get a very large amount of energy released when the fluoride ion becomes surrounded by water molecules in solution. The high hydration enthalpy of the fluoride ion more or less compensates for the high H-F bond strength.

Up to September 2014, this Chemguide page looked at the energetics of this in scary detail, but by accident I then came across a much better explanation of why hydrofluoric acid is weak, which is quite easy to understand:

Why hydrofluoric acid is weak

There is good spectroscopic evidence that hydrogen fluoride ionises fairly completely in solution in water, but instead of producing free hydroxonium ions, H3O + , and fluoride ions, there is such strong attraction between these that they form a strongly bound ion pair, H3O + .F -

The position of this equilibrium lies well to the right.

But to function as an acid, the hydroxonium ion needs to be free - not attached firmly to a fluoride ion,

and this equilibrium lies much further to the left.

So hydrofluoric acid is weak, not because ionisation is weak, but because the ions formed bind themselves together too strongly.

Note: If you want to follow this up, you will find a research article on this page from the Journal of the American Chemical Society. This will give you the first page of the article, which contains enough information for this level. If you have the right access, you can, of course, read the whole thing.

Questions to test your understanding

If this is the first set of questions you have done, please read the introductory page before you start. You will need to use the BACK BUTTON on your browser to come back here afterwards.

Why is #HF# a weak acid and #HCl# a strong acid?

The effects of both entropy and enthalpy reduce the acidity of #HF# compared to #HCl# and #HBr# .


#H-X + H_2O rightleftharpoons H_3O^+ + X^-#

Acid strength, i.e. the degree that the forward reaction goes to completion, depends on (i) the strength of the #HX# bond, and (ii) the degree that the halide is solvated.

For #HF# , there is better overlap in #H-X# because the fluoride ligand is smaller than the lower halides this is an enthalpy effect. Moreover, the fluoride ligand, #F^-# , is smaller and more polarizing, and more effectively solvated by solvent molecules. Since fluoride is smaller and more polarizing, it causes more solvent order and the forward reaction is disfavoured.

Since both entropy and enthalpy is disfavoured with respect to fluoride, the result is that #HF# is a poor acid compared to #HCl# , #HBr# , etc. The entropy effect is probably most significant.

Is Hydrofluoric Acid a Strong or Weak Acid?

Hydrofluoric acid (HF) is a weak acid mainly because it forms stable species after it dissociates.

Hydrofluoric acid or HF is an extremely powerful, corrosive acid. However, it’s classified as a weak acid rather than a strong acid. This makes HF the only hydrohalic acid that isn’t classified as a strong acid (e.g., HCl, HBr, HI).

Why Hydrofluoric Acid Is a Weak Acid

The simple reason hydrofluoric acid is a weak acid is because it does not completely dissociate into its ions in water (the definition of a strong acid). Initially, HF actually does dissociate almost completely:

However, after the acid dissociates, it participates in other reactions with itself and with water. The behavior of hydrofluoric acid greatly depends on its concentration in water. The hydronium cation and fluoride anion are strongly attracted to one another and form H3O + · F – . The chemical bond in this species is strong enough to limit acidity, so hydrofluoric acid is a very weak acid in dilute solution.

Concentrated HF solutions behave much like a strong acid. This is due to homoassociation, in which a chemical bond forms between a base and conjugate acid:

Hydrogen bonding between hydrogen and fluorine stabilizes the FHF – bifluoride ion. Hydrogen bonding also causes HF to have a higher boiling point than the other hydrogen halides.

Is HF Polar?

Two factors play into acid strength: atom size and the polarity of the H-A bond (where A is the acid). Fluorine is highly electronegative, so the bond in HF is a polar covalent bond. The more polar a bond is, the easier it is to remove the proton or hydrogen from the acid. So, hydrochloric acid (HCl) is a stronger acid than hydrobromic acid (HBr). Based only on its polarity, one might expect HF to be stronger than HCl. But, HF participates in reactions after dissociation that make it a weaker acid!

Weak but Dangerous

Hydrofluoric acid causes a chemical burn and then poisons deeper tissues. ( Dr. Watchorn )

Although HF is a weak acid, it is highly corrosive. It attacks glass, even in dilute solution, so it must be stored in plastic containers. It’s hazardous to work with for a few reasons. First, HF interferes with nerve function, so exposure isn’t immediately painful like it would be with other acids. Second, it’s absorbed through tissue, so it does more than produce a chemical burn on skin or mucous membranes. Ultimately, the acid works its way deeper into tissues, causing bone damage and fluid buildup in the lungs. A hydrofluoric acid burn might not seem serious, but it requires immediate attention.

Health Hazard Data: Dermal Exposure

Dermal Exposure – HF is an inorganic acid that is highly corrosive and readily penetrates the skin, causing deep tissue layer destruction. Severity and rapidity of onset of signs and symptoms depends on the concentration, duration of exposure, and penetrability of the exposed tissue. Pain may be delayed.

  1. CONCENTRATIONS LESS THAN 20% – Erythema and pain may be delayed up to 24 hours, often not reported until tissue damage is extreme. In one study, 7% HF produced symptoms in 1 to several hours, 12% HF in less than one hour, and 14.5% HF immediately.
  2. CONCENTRATIONS 20 TO 50% – Erythema and pain may be delayed from 1 to 8 hours, and is often not reported until tissue damage is extreme.
  3. CONCENTRATIONS GREATER THAN 50% – Produces immediate burning, erythema, and tissue damage.

Decontamination – Remove all exposed clothing taking necessary precautions to prevent self- exposure. Immediately wash all exposed areas with copious amounts of water.

Calcium Gluconate or Carbonate Gel – Application of 2.5 to 33% calcium gluconate or carbonate gel or slurry, either placed into a surgical glove into which the affected extremity is then placed, or rubbed into the burn, is recommended. This therapy is more easily administered and less painful than infiltration. Use calcium gluconate for dermal treatment only.

Calcium Gluconate Infiltration – (Medical Assistance) – Continued tissue destruction and pain may be minimized by subcutaneous administration of calcium gluconate. Consider infiltration with CALCIUM GLUCONATE if (1) exposure results in immediate tissue damage or (2) erythema and pain persist following adequate irrigation. Infiltrate each square centimeter of affected dermis and subcutaneous tissue with about 0.5 mL of 10% CALCIUM GLUCONATE using a 30 gauge needle. Repeat as needed to control pain. Split or remove nails to treat nail bed burns. The earlier this is administered, the more rapidly symptoms resolve. CAUTION: Avoid administering large volumes of subcutaneous calcium gluconate as this will result in decreased tissue perfusion and potential necrosis.

DO NOT USE CALCIUM CHLORIDE – Calcium chloride is irritating to the tissues and may cause injury.

Immediate signs and symptoms of exposure to hydrogen fluoride

  • Swallowing only a small amount of highly concentrated hydrogen fluoride will affect major internal organs and may be fatal.
  • Hydrogen fluoride gas, even at low levels, can irritate the eyes, nose, and respiratory tract. Breathing in hydrogen fluoride at high levels or in combination with skin contact can cause death from an irregular heartbeat or from fluid buildup in the lungs.
  • Even small splashes of high-concentration hydrogen fluoride products on the skin can be fatal. Skin contact with hydrogen fluoride may not cause immediate pain or visible skin damage(signs of exposure).
  • Often, patients exposed to low concentrations of hydrogen fluoride on the skin do not show effects or experience pain immediately. And, severe pain at the exposure site may be the only symptom for several hours. Visible damage may not appear until 12 to 24 hours after the exposure.
  • Depending on the concentration of the chemical and the length of time of exposure, skin contact with hydrogen fluoride may cause severe pain at the point of contact a rash and deep, slow-healing burns. Severe pain can occur even if no burns can be seen.
  • Showing these signs and symptoms does not necessarily mean that a person has been exposed to hydrogen fluoride. Other chemicals also can cause these effects.
  • Exposure to hydrogen fluoride can result in severe electrolyte problems.

ELI5: Why is HF so dangerous when it's a fairly weak acid?

From my chemistry knowledge HF is weaker than HCl yet it is common to find bench grade HCl, why isn't it the same with HF?

"Strong acid" and "weak acid" are technical terms that denote aspects of how the acids dissolve in water. Weak acids can be dangerous at high concentrations.

Just look in your kitchen. Household vinegar is about a 5% concentration of acetic acid, lemon juice is about 5% citric acid. These are perfectly safe concentrations - you can even eat them. Kick those concentrations up to 90% and you need fume hoods and rubber gloves to safely work with them.

Household vinegar is about a 5% concentration of acetic acid

Right, acetic acid is considered a weak acid, but glacial acetic acid (17M) will fuck your shit up.

100% citric acid isn't too dangerous. (White crystalline powder) you can buy it as a food additive.

I think that's partially because most of the cells in your body use it as fuel, so the body can detoxify it pretty quickly by oxidizing it.

It's sour tasting as fuck though.

Very high concentrations of HF can also have quite a respectable acidity though, not in its own right, but due to the way it undergoes autoionisation.

It's not unlike how even pure water is a little bit conductive, because a small amount autoionises into OH - and H +. In the case of HF the specific ion produced reacts with free non-ionised HF, and this acts to kind of solvate further fluoride ions, such that its acidity doesn't necessarily scale linearly with its concentration.

Essentially the more concentrated the HF the more its conjugate base F - can be encouraged to exist, making it more readily release its proton.

It reacts with damned near everything and seems to especially like breaking down organic compounds. It reacts with metals creating all sorts of halite salts which are also toxic.

That too, but that's pretty much par for the course when dealing with more exotic acids than hcl or sulfuric.

In terms of physical chemistry, hydrofluoric acid is a pretty weak acid.

This is because, unlike hydrochloric acid, most of the HF does not dissociate into a fluoride ion and a hydronium ion, the latter of which is responsible for the reactivity of most acids.

Hydrogen Fluoride whether in gas or in a solution, however, is extremely poisonous.

This is for two reasons. The first is that Hydrogen Fluoride is soluble in oils as well as water, and being a small molecule it can easily pass through the skin into the blood stream, and into tissues.

More importantly, it easily complexes calcium ions, rendering them unusable by the body.

Calcium is used by body systems in a huge number of ways especially as a signalling ion. Your body depends on calcium levels being very tightly controlled.

Exposure to HF causes tissue necrosis, hypotension, breathing difficulty, paralysis, and cardiac arrest.

Worse, one of the first symptoms can be numbness in the effected area so your may not notice that your skin is peeling off.

The reactivity of hydrofluoric acid is not due to it's acidity, but rather, due to it having a higher electron affinity than oxygen. So it can steal electrons from oxygen, thereby replacing the oxygen in normally highly stable/inert substances like silicon dioxide, and rendering them much more soluble.

Why is hydrofluoric acid so dangerous if it is a weak acid? - Biology

This page explains the terms strong and weak as applied to acids. As a part of this it defines and explains what is meant by pH, Ka and pKa.

It is important that you don't confuse the words strong and weak with the terms concentrated and dilute.

As you will see below, the strength of an acid is related to the proportion of it which has reacted with water to produce ions. The concentration tells you about how much of the original acid is dissolved in the solution.

It is perfectly possible to have a concentrated solution of a weak acid, or a dilute solution of a strong acid. Read on . . .

Strong acids

Explaining the term "strong acid"

Note: If you don't know what the Bronsted-Lowry theory of acids is, you should read about theories of acids and bases on another page in this section. You don't need to spend time reading about Lewis acids and bases for the purposes of this present page.

Use the BACK button on your browser when you are ready to return to this page.

When an acid dissolves in water, a proton (hydrogen ion) is transferred to a water molecule to produce a hydroxonium ion and a negative ion depending on what acid you are starting from.

These reactions are all reversible, but in some cases, the acid is so good at giving away hydrogen ions that we can think of the reaction as being one-way. The acid is virtually 100% ionised.

For example, when hydrogen chloride dissolves in water to make hydrochloric acid, so little of the reverse reaction happens that we can write:

At any one time, virtually 100% of the hydrogen chloride will have reacted to produce hydroxonium ions and chloride ions. Hydrogen chloride is described as a strong acid.

A strong acid is one which is virtually 100% ionised in solution.

Other common strong acids include sulphuric acid and nitric acid.

You may find the equation for the ionisation written in a simplified form:

This shows the hydrogen chloride dissolved in the water splitting to give hydrogen ions in solution and chloride ions in solution.

This version is often used in this work just to make things look easier. If you use it, remember that the water is actually involved, and that when you write H + (aq) what you really mean is a hydroxonium ion, H3O + .

Note: You should find out what your examiners prefer on this. You are unlikely to find this from your syllabus, but should look at recent exam papers and mark schemes. If you are doing a UK-based exam and haven't got copies of your syllabus and past papers, you should have! Follow this link to find out how to get hold of them.

pH is a measure of the concentration of hydrogen ions in a solution. Strong acids like hydrochloric acid at the sort of concentrations you normally use in the lab have a pH around 0 to 1. The lower the pH, the higher the concentration of hydrogen ions in the solution.

Note: If you are asked to define pH in an exam, simply write down the expression in black. Never try to define it in words - it is a waste of time, and you are too likely to miss something out (like mentioning that the concentration has to be in mol dm -3 ). In the expression, above, the square brackets imply that, so you don't need to mention it.

Working out the pH of a strong acid

Suppose you had to work out the pH of 0.1 mol dm -3 hydrochloric acid. All you have to do is work out the concentration of the hydrogen ions in the solution, and then use your calculator to convert it to a pH.

With strong acids this is easy.

Hydrochloric acid is a strong acid - virtually 100% ionised. Each mole of HCl reacts with the water to give 1 mole of hydrogen ions and 1 mole of chloride ions

That means that if the concentration of the acid is 0.1 mol dm -3 , then the concentration of hydrogen ions is also 0.1 mol dm -3 .

Use your calculator to convert this into pH. My calculator wants me to enter 0.1, and then press the "log" button. Yours might want you to do it in a different order. You need to find out!

Note: If you want more examples to look at and to try yourself (with fully worked solutions given), you may be interested in my chemistry calculations book. This also includes the slightly more confusing problem of converting pH back into hydrogen ion concentration.

Weak acids

Explaining the term "weak acid"

A weak acid is one which doesn't ionise fully when it is dissolved in water.

Ethanoic acid is a typical weak acid. It reacts with water to produce hydroxonium ions and ethanoate ions, but the back reaction is more successful than the forward one. The ions react very easily to reform the acid and the water.

At any one time, only about 1% of the ethanoic acid molecules have converted into ions. The rest remain as simple ethanoic acid molecules.

Most organic acids are weak. Hydrogen fluoride (dissolving in water to produce hydrofluoric acid) is a weak inorganic acid that you may come across elsewhere.

Note: If you are interested in exploring organic acids further, you will find them explained elsewhere on the site. It might be a good idea to read the rest of this page first, though.

If you want to know why hydrogen fluoride is a weak acid, you can find out by following this link. But beware! The explanation is very complicated and definitely not for the faint-hearted!

These pages are in completely different parts of this site. If you follow either link, use the BACK button to return to this current page.

Comparing the strengths of weak acids

The position of equilibrium of the reaction between the acid and water varies from one weak acid to another. The further to the left it lies, the weaker the acid is.

Note: If you don't understand about position of equilibrium follow this link before you go any further.

You are also going to need to know about equilibrium constants, Kc for homogeneous equilibria. There is no point in reading any more of this page unless you do!

If you follow either link, use the BACK button to return to this current page.

The acid dissociation constant, Ka

You can get a measure of the position of an equilibrium by writing an equilibrium constant for the reaction. The lower the value for the constant, the more the equilibrium lies to the left.

The dissociation (ionisation) of an acid is an example of a homogeneous reaction. Everything is present in the same phase - in this case, in solution in water. You can therefore write a simple expression for the equilibrium constant, Kc.

Here is the equilibrium again:

You might expect the equilibrium constant to be written as:

However, if you think about this carefully, there is something odd about it.

At the bottom of the expression, you have a term for the concentration of the water in the solution. That's not a problem - except that the number is going to be very large compared with all the other numbers.

In 1 dm 3 of solution, there are going to be about 55 moles of water.

Note: 1 mole of water weighs 18 g. 1 dm 3 of solution contains approximately 1000 g of water. Divide 1000 by 18 to get approximately 55.

If you had a weak acid with a concentration of about 1 mol dm -3 , and only about 1% of it reacted with the water, the number of moles of water is only going to fall by about 0.01. In other words, if the acid is weak the concentration of the water is virtually constant.

In that case, there isn't a lot of point in including it in the expression as if it were a variable. Instead, a new equilibrium constant is defined which leaves it out. This new equilibrium constant is called Ka.

Note: The term for the concentration of water hasn't just been ignored. What has happened is that the first expression has been rearranged to give Kc (a constant) times the concentration of water (another constant) on the left-hand side. The product of those is then given the name Ka.

You don't need to worry about this unless you really insist! All you need to do is to learn the format of the expression for Ka.

You may find the Ka expression written differently if you work from the simplified version of the equilibrium reaction:

This may be written with or without state symbols.

It is actually exactly the same as the previous expression for Ka! Remember that although we often write H + for hydrogen ions in solution, what we are actually talking about are hydroxonium ions.

This second version of the Ka expression isn't as precise as the first one, but your examiners may well accept it. Find out!

To take a specific common example, the equilibrium for the dissociation of ethanoic acid is properly written as:

If you are using the simpler version of the equilibrium . . .

Note: Because you are likely to come across both of these versions depending on where you read about Ka, you would be wise to get used to using either. For exam purposes, though, use whichever your examiners seem to prefer.

The table shows some values of Ka for some simple acids:

acid Ka (mol dm -3 )
hydrofluoric acid 5.6 x 10 -4
methanoic acid 1.6 x 10 -4
ethanoic acid 1.7 x 10 -5
hydrogen sulphide 8.9 x 10 -8

These are all weak acids because the values for Ka are very small. They are listed in order of decreasing acid strength - the Ka values get smaller as you go down the table.

However, if you aren't very happy with numbers, that isn't immediately obvious. Because the numbers are in two parts, there is too much to think about quickly!

To avoid this, the numbers are often converted into a new, easier form, called pKa.

pKa bears exactly the same relationship to Ka as pH does to the hydrogen ion concentration:

If you use your calculator on all the Ka values in the table above and convert them into pKa values, you get:

Note: Notice that unlike Ka, pKa doesn't have any units.

Notice that the weaker the acid, the larger the value of pKa. It is now easy to see the trend towards weaker acids as you go down the table.

The lower the value for pKa, the stronger the acid.

The higher the value for pKa, the weaker the acid.

Note: If you need to know about Ka and pKa, you are quite likely to need to be able to do calculations with them. You will probably need to be able to calculate the pH of a weak acid from its concentration and Ka or pKa. You may need to reverse this and calculate a value for pKa from pH and concentration. I can't help you with these calculations on this site, but they are all covered in detail in my chemistry calculations book.


Season 1

Jesse disposing Emilio's corpse

Walter White and Jesse Pinkman first use hydrofluoric acid, stolen from the high school science lab, to dispose of the corpse of Emilio Koyama. However, Jesse disregards Walter's instructions about using a plastic container, instead choosing to pour the acid onto the body placed in a bathtub. This results in the acid dissolving through the bathtub and the floor, scattering Emilio's liquified remains into the hallway (" Cat's in the Bag. ") and forcing Walt and Jesse to clean up the bloody mess. (" . and the Bag's in the River ")

While it is not shown onscreen, it is assumed that the corpse of Emilio's cousin, Krazy-8, was disposed of in a similar fashion.

Season 4

Walt, Mike, and Jesse disposing of Victor's corpse

Later, Walter, Jesse, and Mike Ehrmantraut use hydrofluoric acid to dissolve the body of Victor, who Gustavo Fring killed by slicing his throat with a box cutter. This time they are more successful, as they put Victor's body into a plastic barrel beforehand. Along with the body they dissolve the box cutter and the gun Jesse used to kill Gale Boetticher. (" Box Cutter ")

Later, at Gustavo Fring's chicken farm, a sniper (Gaff) shoots and kills one of Gus's henchmen. Mike and Jesse take the unnamed henchman down to the superlab in a laundry basket, where Walt is waiting. Walt is shocked, but is told by Mike to grab a spare barrel. The barrel with the dissolved body is then later seen loaded onto a truck of hazardous materials (" Bug ")

Season 5

Todd, Walt, and Mike disposing of the young boy

In Season 5, Walter, Mike, and Todd Alquist use hydrofluoric acid to dissolve the corpse of Drew Sharp, a young boy shot by Todd after witnessing their train heist (" Buyout "), to make sure he was never found by the authorities. The group also uses the acid to dissolve Drew's bike. Later in the season, Walt and Todd use hydrofluoric acid to dispose of Mike's body after Walt shot him in a fit of rage (" Gliding Over All ").

2 Answers 2

Hydrofluoric acid is toxic and corrosive, but actually isn't that strong of an acid compared to other hydrohalic acids the fluorine has a very good orbital overlap with hydrogen and is also not very polarizable, therefore it resists to readily donate its proton. Hydrochloric acid is much stronger, and as it has several uses from pH-balancing pool water to preparing concrete surfaces, it's available by the gallon from any hardware store. However, it isn't very good at dissolving bodies either while it will eventually work by breaking down the connective tissues, it will make a huge stink and take several days to dissolve certain types of tissues and bones.

The standard body-dissolving chemical is lye aka sodium hydroxide. The main source is drain clog remover, because most drain clogs are formed by hair and other bio-gunk that accumulates naturally when humans shower, exfoliate etc. It works, even though the body's overall chemistry is slightly to the basic side of neutral (about 7.35-7.4), because the hydroxide anion is a strong proton acceptor. That means that it strips hydrogen atoms off of organic molecules to form water (alkaline hydrolysis, aka saponification), and as a result, those organic molecules are turned into simpler molecules with lower melting points (triglycerides are turned into fatty acids, saturated fats are dehydrogenated to form unsaturated fats, alkanes become alcohols, etc). Sodium hydroxide is also a ready source of the sodium ion sodium salts are always water-soluble (at least I can't think of a single one that isn't). The resulting compounds are thus either liquids or water-soluble alcohols and salts, which flush down the drain. What's left is the brittle, insoluble calcium "shell" of the skeleton if hydrolyzed by sodium hydroxide, the resulting calcium hydroxide ("slaked lime") won't dissolve completely but is relatively easy to clean up.

It isn't the amount of HF that is notable it is whether HF will work at all in this context. HF sounds like a good way to dissolve bodies as it has a fearsome and dangerous reputation in chemistry. But that reputation is based on the fact that it is a dangerous biological poison not on its ability to "dissolve" flesh. It is, in fact, a weak acid and not that good as a way to turn flesh into a flushable mush.

From my answer on the site (explaining the script choice):

I think the use of Hydrofluoric Acid was script-driven rather than fact driven: it sounds scary rather than being a good choice. Also it allows for the possibility of the darkly comic bathtub scene where the acid dissolves a ceramic bath because Jessie ignores Walter's instructions (which establishes Walter's expertise and Jessie's lack of it).

Moreover, we don't have to rely on theory here, the experiment has been done. Periodic Videos tested the idea using raw chicken legs and several flesh-dissolving alternatives. This video shows that, while HF has some remarkable effects on flesh, dissolving it into a mush isn't something it does quickly.

Watch the video: Why is hydrofluoric acid, HF, a weak acid compared to other hydrogen halides? (July 2022).


  1. Saewald

    Well done, I liked it!

  2. Zulkishakar

    Please, tell more in detail..

  3. Seely

    Your question how to regard?

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