Why does hydrochloric acid rust nails?

A. Aya,

If you go back to the home page, find the on line library and go to the only article on corrosion. Rust is a specific corrosion-IE on iron. If you read this very carefully, you will find the answer. It will also give you a wonderful reference for a multitude of other things that cause corrosion. If you only look at a reaction of HCl with pure iron, you will get either ferrous chloride or ferric chloride as a product. If you keep the iron in the acid a relatively short time, it will activate the iron and it will rust very rapidly, oxidizing the iron to ferric oxide. You can get ferrous oxide in some cases. Note the word oxide-it takes oxygen to form it. There is enough oxygen in tap water to actually have iron rust when under water. Now take and boil the water and your rust formation will be far slower while under water because you forced out most of the oxygen. A wonderful cleaner for young experimenters is a saturated solution of bicarbonate of soda-the soda power in your kitchen cabinet. To a quart of this, add 5 drops of cheap liquid soap. The pH of this solution is about 9-10 and is far safer than using lye / sodium hydroxide ⇦liquid caustic soda in bulk on Amazon [affil link] .

Mechanically clean your parts first then alkaline clean, rinse about 1 min in a 25% solution of pool acid a very quick rinse and then submerge in your tap water or boiled water. If you use the schools 37% HCl, make it up a 20 % solution as it is stronger than pool acid. Does any of this help?

A. Hydrochloric acid attacks metal and will dissolve it . . . but it dissolves rust even faster than it dissolves metal. So it is used in industry to de-rust metal before painting or plating. But after acid dipping the metal is very prone to flash rusting, and most metals will rust terribly if subjected to hydrochloric acid fumes.

I don't think that equation is the usual way for the reaction to proceed, Ho. I understand that it's probably more like:

6HCl + Fe₂O₃ + Fe --> 3FeCl₂ + 3H₂O

This produces ferrous chloride rather than ferric chloride (although dissolved oxygen may eventually oxidize the ferrous to ferric), but it doesn't explain the hydrogen release.

I believe that the hydrogen comes from additional iron reacting with the acid per:

2HCl + Fe --> FeCl₂ + H₂

Regards,

Hi, Holly.

When you can't seem to get a straight answer, sometimes the problem is a poorly phrased or poorly understood question like "what is the right size square peg to put in this round hole?" :-)

HCl will dissolve any rust that is on the steel item and, more slowly, dissolve the steel item.

If you have a situation where the acid cannot keep the dissolved metal in solution, that dissolved metal will become rust. Imagine that you immerse an iron nail in HCl or vinegar ⇦in bulk on eBay or Amazon [affil links] , and the watery solution dries up, the dissolved iron can't stay dissolved, and now turns to rust. Some acids like HCl and vinegar have a sharp smell and that's because they are actually gasses dissolved in water; the actual acid fumes can leave the liquid just as the carbonation leaves Coke and it goes flat. As the acid fumes drift away, there may be insufficient acid left in the liquid to support the dissolved metal, and rust develops.

Regards,

Hi, Rachael.

I think your confusion may possibly stem from not clearly understanding the terms you are using. Corrosion and rust are not always the same thing. Corrosion is any reaction between the liquid and the iron that causes some of the iron to be lost, consumed, or dissolved. That reaction will have reaction products, but those reaction products may or may not be rust (Fe2O3).

Hydrochloric acid attacks iron very aggressively and corrodes it, but the corrosion products (per the formulas higher up on the page) tend to be things like FeCl2 rather than Fe2O3. And the acid attacks rust even more aggressively, so it removes rust. There are numerous liquids you can test: bleach (careful, don't mix anything else with it), vinegar, rubbing alcohol, nail polish remover (careful, flammable), motor oil, various fruit juices, coffee, tea, etc. Good luck.

Regards,

No, Claire. Rust is not a base, and acid does not "neutralize" it. It's explained in my response to Ho Jing that acids react with rust to form soluble ferric salts.

Regards,

Hi, Amber.

As you say, you're expected to measure which one loses the most weight. Asking people the answer before you do the experiment will corrupt it. You may redo it if you got it "wrong", or accept the results if you got it "right" even if only due to some random fluke. You may miss things that actually happened, and "see" things that didn't. This is called "junk science" and you must avoid it.

After you have done the project, please get back to us if you have any questions. Make sure you get supervision from the teacher because hydrochloric acid is dangerous. Good luck.

Regards,

A. Hi Rob. Please remember that every explanation of how & why in chemistry is a simplification, so my explanation is a simplification too. We learn very simplistic explanations in elementary school, slightly deeper ones in high school, deeper still as an undergrad; and if you go on for a doctorate you learn a lot about it that undergrads aren't exposed to. But the PhD graduates realize that even they are not studying at the quantum level. The late great Richard Feynman told us to ask how but never why. We need to know enough about something to make accurate predictions of the things that we want to predict, but we'll never see the ultimate reality :-)

With that disclaimer, younger students might see the equation as:
2HCl + Fe => H₂ + FeCl₂

At your level & mine, I think we see this equation:
2H⁺Cl^- + Fe⁰ <=> H⁰₂^ + Fe⁺²Cl^-₂

... and we understand that the iron wants more urgently to combine with the chlorine than the hydrogen does. i.e., that the lowest energy state will be when the balance shifts to the right. So the reaction will move that way and heat will be released as the reaction proceeds to the right, and heat would have to be added for the reaction to move back towards the left. Furthermore, as the reaction proceeds, the hydrogen will form hydrogen gas, which will leave the reaction milieu, preventing the reaction from moving back to the left anyway.

Exactly why we achieve a lower energy state with the products on the right side as compared to the left has to do with the energy states of the electrons shells of each atom (which is a little beyond my level); but a simplification that I am content with is that the electrons are shared between the hydrogen and chlorine on the left side of the equation, and they are shared between the iron and the chlorine on the right hand side of the equation, and the shared electrons "fit" better, at a more relaxed, lower energy state, when shared between iron and chlorine than when shared between hydrogen and chlorine.

Regards,

Rust

A. Hi Claire. According to Wikipedia, rust can be one of, or a mix of, Fe2O3.nH2O and iron(III) oxide-hydroxide (FeO(OH), Fe(OH)3). To get rust, one thing we have to do is raise iron's oxidation state from 0 to Plus 3.

Although when we look at the formula Fe(OH)3 it "looks" like iron will combine with 3 units of hydroxide, it probably doesn't happen much faster than geological time, because the formula is probably actually better written FeO(OH).H2O

Another way to look at the reason why it doesn't happen might be to look at the charges when we write:
Fe⁰ + Na⁺ + OH^- => ? There is nothing there to oxidize the iron from its Fe⁰ metallic state to the Fe⁺⁺⁺ state.

Regards,

A. Hi Claire,

In acid, you are dissolving iron from the metal (oxidation state 0) to your solution (oxidation state 2), and oxygen in the air oxidizes again your iron to Fe3+.

Rust is, as Ted mentions, a mixed not-so-crystalline solid made of oxides and hydroxides (and everything in between) of Fe3+ (Iron in its oxidation state 3). For this to happen, you must have Iron, oxygen and something to help the solid state reaction to happen, as water. In pure water you have H+ and OH- in approx 10^-7 M concentration.

When you have acid, you have H+ in much more concentration and they reduce to H2 oxidizing your metal. So, the oxidation reaction happens, and fast.

When you have alkali, you have much more OH-, but they can't reduce to H2 as OH- can't dissolve iron. There are some metals that can dissolve in alkalis (as Aluminium, Zinc, Copper, etc.) but not iron. So, iron is not oxidized by alkalis and the oxidation reaction is almost as slow as it wasn't covered in alkali.

I hope this cleared some doubts about this reactions. If I can be in any help, I'm just right here!

Best of lucks!

A. Hi Samantha. Sounds like a good project. What procedures and materials, if any, have been suggested? Please copy to here what you have been given as the starting point for the assignment.

Do you have a hypothesis? Do you know how you will measure 'rust removal'? I.e., are you expected to go by eye, or measure the mass change, or analyze the solution for iron after the immersion? A tricky issue is that HCl doesn't just dissolve rust, it also dissolves unrusted steel. Depending on what grade you're in, maybe look up and study Kleingarn to really impress the teacher.

Regards,