Example: Rusting Iron

Published 2010-03-18 | Author: Jason Waskiewicz

A “jazzy” picture of the stages of rust (presented for a high school level). Iron is red, oxygen is blue and electrons are black. At the left you see iron atoms with two oxygen molecules, then the electrons go to the oxygen molecules leaving oxygen ions and iron ions which then disolve.

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Rusting Iron

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% Rusting Iron
% Author: Jason Waskiewicz
% A "jazzy" picture of the stages of rust (presented for a high school level).
% Iron is red, oxygen is blue and electrons are black. At the left you see
% iron atoms with two oxygen molecules, then the electrons go to the oxygen molecules
% leaving oxygen ions and iron ions which then disolve.
\documentclass{article}
\usepackage{tikz}
\begin{document}
    \begin{tikzpicture}[
        >=stealth,
        iron/.style={shade, ball color=red},
        electron/.style={shade, ball color=black},
        oxygen/.style={shade, ball color=blue},
        droplet/.style={ball color=blue!20, opacity=0.4},
    ]

    %Draw the iron atoms
    \foreach \x in {1,1.5,2,2.5,3,3.5,4,4.5,5,8}
            \draw [iron] (\x,1,-0.5) circle (0.25cm);
    \foreach \x in {1.25,1.75,2.25,2.75,3.25,3.75,4.25,4.75,5.25,5.75,6.25,6.75,7.25,7.75}
            \draw [iron] (\x,0.55,-0.5) circle (0.25cm);

    %Draw the iron electrons; this isn't totally realistic for illustrating Fe+3 ions
    \foreach \x in {1.25,1.75,2.25,2.75}
            \draw [electron] (\x,1.25,-0.5) circle (0.1cm);
    \foreach \x in {1,1.5,2,2.5,3,3.5,4,4.5,5,5.5,6,6.5,7,7.5,8}
            \draw [electron] (\x,0.75,-0.5) circle (0.1cm);

    %Draw the O2 molecules
    \draw [oxygen] (1.5,1.8,-0.5) circle (0.15cm);
    \draw [oxygen] (1.6,2.0,-0.5) circle (0.15cm);

    \draw [oxygen] (1.8,1.6,-0.5) circle (0.15cm);
    \draw [oxygen] (2.05,1.6,-0.5) circle (0.15cm);

    %Draw the arrows showing the electrons going to the O2 molecules
    \draw (3.45,1.35) -- (3.75,2.25) [->,thick];
    \draw (3.95,1.35) -- (3.95,2.25) [->,thick];
    \draw (4.45,1.35) -- (4.35,1.85) [->,thick];
    \draw (4.95,1.35) -- (4.5,1.85) [->,thick];

    %Draw O-2 ions with (-) labels
    \shadedraw [oxygen] (3.75,2.4,-0.5) circle (0.15cm) node [above=3pt,right=2pt] {\small{2-}};
    \shadedraw [oxygen] (4.15,2.0,-0.5) circle (0.15cm) node [above=3pt,right=2pt] {\small{2-}};

    %Draw the dissolved Fe+3 ions and O-2 ions
    \shadedraw [iron] (6.25,2.5,-0.5) circle (0.25cm) node [above=3pt,right=4pt] {\small{3+}};
    \shadedraw [iron] (6.85,2.0,-0.5) circle (0.25cm) node [above=3pt,right=4pt] {\small{3+}};

    \shadedraw [oxygen] (6.95,1.5,-0.5) circle (0.15cm) node [above=3pt,right=2pt] {\small{2-}};
    \shadedraw [oxygen] (6.15,2.0,-0.5) circle (0.15cm) node [above=3pt,right=2pt] {\small{2-}};
    \shadedraw [oxygen] (7.05,2.5,-0.5) circle (0.15cm) node [above=3pt,right=2pt] {\small{2-}};

    %Draw the time arrows
    \draw (2.2,2.5) -- (3.35,2.5) [->,very thick];
    \draw (4.8,2.5) -- (6.15,2.5) [->,very thick];

    %Draw the water droplet
    \begin{scope}
            \clip (1,1) rectangle (8.5,5);
            \draw[droplet] (4.5,1,-0.5) circle (3.5cm);
    \end{scope}
    \end{tikzpicture}
\end{document}

Comments

  • #1 C V Horie, September 21, 2010 at 6:06 p.m.

    Sorry, not very accurate. The Fe(II) ions are created in the middle of the drop, losing electrons. The O2 molecules lose 4e at the edge, react with H2O and react with the Fe(II) to form insoluble Fe(III).

    Now if you created that, complete with electron flow, that would be useful.

    Thanks anyway

  • #2 Jason Waskiewicz, November 7, 2010 at 5:09 a.m.

    I don't know if you'll see this comment, but I want to respond. Please forgive the long response: I want to respond to all of your points.

    The illustration is meant to represent some of the steps of rust formation, not what occurs in certain spots or the droplet. It could well be that I should have portrayed the water as a field of blue rather than as a droplet.

    You are correct about the behavior at the edge of the droplet. The picture would be improved had I shown the formation of insoluble Fe2O3 as a next step. This is described in my lesson, but not in the picture. Perhaps I should include a pile of this molecule on the side of the droplet? I also chose not to illustrate the cathode and anode in the reaction.

    I did avoid the reaction of oxygen with water to form hydroxide ions. I was torn between the need for completeness and my goal of illustrating a process my (high school) students typically have trouble visualizing. With high school students it's sometimes hard to find the balance between what they have the background to understand and "telling the full story".

    If you might improve on this illustration, I would love to see it. As I noted, I was conscious of some of its limitations. I'm not sure if I could accurately show all of the reactions that are part of rust in a picture of this sort. Most pictures of this sort that I found online and my own library omitted parts of the rusting process. The best descriptions I found were written, which brought me right back to the original problem: my students were not able to visualize the process.

    Nevertheless, if you can create or recommend a better picture (suitable for my audience), I would be grateful. My specialties are Physics and mathematics, so I recognize my limitations when I teach other science courses.

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