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!Urgent ! I need expert chemist advice, please, not much time left!

I'm an eleventh grader doing a science project, removing rust from pennies using salt and white vinegar ⇦in bulk on eBay or Amazon [affil links] . I know that the chemical formula for rust is Fe₂O₃-iron oxide, in other words. but if pennies are made of copper, how can that be? shouldn't it be CuO (copper oxide) instead? Also, I'd like to know how the vinegar(made from grain, diluted with water with 5 percent acidity) would help remove the rust, since both acid and water are the things that cause rust in the first place. Please help me understand, I don't have much time left. Thanks.

Mehdi K
student - Bronx, New York, United States
April 2, 2008

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Hi, Mehdi. See our FAQ on Cleaning Pennies which will, I believe, answer all. You're right, it's not rust, it's oxides of copper. Acids do dissolve the corrosion products of metals, like rust and copper oxides, faster than they attack the base metal.

Regards,

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
April 4, 2008

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May 17, 2008

ACIDS HAVE LOTS OF PROTONS (H+ ions). Acids have a low pH!

What is pH? "pH is the negative log of the hydrogen ion concentration". Get that ?!? Every time the pH drops by one unit, it means that there are 10X more hydrogen ions ("proton") is the better (more formal) way to refer to the hydrogen ion, (H⁺). Likewise, when the pH goes up one unit, there are 10x fewer protons in the water, so the deficit of proton yield hydroyl groups (OH^-), and the solution is referred to as a base, or, more formally, as "alkaline".

So if you put Fe oxide in acid, all those protons drive the equilibrium toward the association of protons with the OH groups to form water. In other words, the Fe-oxide is reduced from Fe(III) to Fe(II) (the (III) or (II) refers to the oxidate stare of the ion, and is the correct formal way to say that the (Fe⁺³) is reduced to (FE⁺²). I'll talk more about the terms oxidation and reduction later).

By having all those positively-charged proton, the acid "reduces" the iron oxide..i.e., it steals an electron from the Fe-oxide ...that electron came from the extra electron in oxygen (oxygen (O^-2) has a negative charge because it has an extra electron)

(Fe⁺³)OOH + H⁺ => Fe(⁺²) + 2 H₂O

To help remember, the trick to figure out if something is oxidized or reduced is to remember

LOGR

If something loses (L) an electron it is oxized (O)
Conversely if something gains (G) an electron, it is reduced (R).

The acid reduces Fe⁺³-oxide to Fe⁺² (i,e, the H+ ...the + sign indicates it's deficient an electron and is hungry for an electron from the oxygen in the Fe-oxide.

By the way, What happens to the Fe⁺² that produced when you reduce the Fe⁺³?
Well, Fe⁺² is very soluble in water, so it just disappears as just another atom in the water.

Fe⁺³, on the other hand is insoluble in water and exists as small Fe-oxide particles in water...those particles form a red suspension in the water...did you ever see reddish water come out of the faucet...the red color is from the Fe-oxide particles (maybe washed off rusty pipes).

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The same sort of reaction happens with copper oxide

(Cu⁺²O) is reduced by the acid. Think of it as

copper oxide(Cu⁺⁺ with O^-) + acid (H⁺) => the spare electron in the oxygen is stolen by the electron-hungry H⁺ to form water H₂O (remember, acids have lots of protons).

The copper (Cu⁺⁺) is soluble in water and floats away.


Sorry if you didn't like my folksy story-telling explanation....there are much more formal ways to describe the reactions...read your text book, or even Google it.

I'm a senior professor, but have always translated complicated processes and reactions in a cartoon. I don't do this intentionally, it's just become a habit that started when I was in high school...I owe my whole career to my knack for simplifying things to a cartoon...and my students seem to like it too (in between the smirks and laughter).

Good luck! Learn your chemistry and use it to help solve our global problems with climate and pollutants!

Prof. John

JOHN MCCARTHY
- LOUDON, Tennessee, USA