MadSci Network: Chemistry

Re: Why does a metal nail rust the most in vinegar as opposed to soda and water

Date: Thu Apr 13 15:22:06 2000
Posted By: David Reibstein, Staff, Princeton Materials Institute, Princeton University
Area of science: Chemistry
ID: 955295975.Ch

Some previous questions on this topic have been answered by other Mad Scientists, and you might want to look at them too, especially Why is Coca-Cola corrosive?.  However, I will try to provide a little more explanation of what's going on here chemically, as best I can.  I'll assume you have had at least a little chemistry in school.

First, I have to assume that the metal nail you used is made of iron.  As I will try to explain below, any acid will promote the formation of rust.  So that's why you found (or I assume you found) that the nail rusted the least in plain water.

Both vinegar and sodas are acidic.  The thing that makes a solution more acidic or less acidic is the concentration of hydrogen ions, represented by H+.  Soda is acidic because of the dissolved carbon dioxide (which is what makes it bubbly).  The carbon dioxide combines with water to make carbonic acid, H2CO3.  The chemical reaction is:

CO2   +  H2O -->  H2CO3
And vinegar is acidic because it contains acetic acid, which releases H+ into the solution.  Now, the amount of  H+ ions in a solution depends on two variables: (1) how much acid is in the solution and (2) how strong the acid is.  As you may know, we usually represent the acidity of a solution by the quantity pH, with lower pH meaning a more acidic solution.  As stated in one of the previous answers, sodas usually range in pH from 2.5 - 4.2.  A solution of vinegar, which is about 4% acetic acid (the Vinegar Institute, What Is Vinegar?), should be around pH 2.  So it seems reasonable that rusting should be faster in vinegar than in soda, although there are other variables that might have affected your results.

I have written a little background on acids and bases, designed for teachers but I think you could learn from it, too.  Find it HERE.

An explanation of what's going on in rusting and how it is promoted by acid:

Rusting is a good example of what is called an oxidation/reduction reaction.  In such a reaction, electrons are transferred from one kind of atom to another kind.  In rusting, electrons are transferred from iron atoms to oxygen atoms.  

Rusting is interesting because it is somewhat more complicated than many other such reactions.  First, the electrons cannot be directly transferred from iron to oxygen, they have to move through water.  (Have you noticed that iron has to be wet in order to rust?)  Second - and here is where the acid comes in - certain substances make water a better conductor of electrons.  These include salt and acid.  (This is because both salt and acid come apart (dissociate) into charged atoms called ions, which carry the electrical charge.  And, as you might expect, the higher the concentration of ions in the water, the faster rusting will take place.  So the more acidic a solution, the faster rusting takes place.

Acid also promotes rusting in another way.  Rusting makes the water more basic (the opposite of acidic).  The process goes faster and farther if the base is neutralized by acid.

Here are the chemical reactions involved in rusting:

Electrons leave the iron:
Fe  -->   Fe2+  +  2e-
(e-  represent an electron;  Fe2+ is an iron ion.)
The electrons migrate through the water and combine with oxygen:
O2  + 4H+ +  4e- -->  2 H2O
(without the H+ the product would be OH-)
The Fe2+ ions combine with more oxygen and water to give the reddish-brown stuff we call rust:
4 Fe2+  + O2  +  (4+x) H2-->  2 Fe2O3.xH2O
This Fe2O3.xH2O is iron oxide with some water attached, called a hydrate.  The x indicates that the amount of attached water is variable.

References:  Most general chemistry books cover this topic.  A particularly nice treatment of rusting  is found in
Chemistry: The Central Science, 5th Edition, by Brown, LeMay, and Bursten  (Prentice Hall, 1991).  Library of Congress catalog QD31.2.B78

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