Preventing rust

Acknowledgement

This activity was developed based on a procedure developed by the Royal Society of Chemistry together with the Nuffield foundation.

Introduction

Some redox reactions are useful, such as the ones in our cells providing the Gibbs energy to make ATP, or those charging a battery and providing electricity from a charged battery. Many steps in making desired products, from aluminum to medications, involve redox reactions. Some redox reactions, however, are undesirably and very costly. The prime example is corrosion or rust-formation, a reaction of metals (in case of rust, specifically iron and steel) with oxygen. As you know, oxygen is about 20% of the gases in air. Oxygen also dissolves in water, which allows fish to breath. Typical conditions that promote rust are the presence of water and salt - ship wreaks in the ocean rust quickly, providing an opportunity for marine life to settle in.

Rust is a complex substance containing iron(III), but initially \(\ce{Fe^2+(aq)}\) forms. We will detect the formation of this ion with an indicator that forms an intensely blue-colored substance called Prussian blue when it combines with iron(II).

\[\ce{Fe(s) -> Fe^2+(aq) + 2 e-}\]

\[\ce{Fe^2+(aq) + indicator -> Prussian\ blue}\]

Today's experiments explore how we can avoid formation of rust. We will use an indicator (potassium hexacyanoferrate) to see the formation iron(II) even at low concentrations, and use gelatin to prevent mixing, to see where the redox reaction occurs. In this way, we don't have to wait days to see the rust itself.

Part 1: setup

The reactions will be done in small test tubes provided. You will work in groups of two, preparing a total of 5 test tubes. Add the following to separate tubes:

1. an untreated iron nail
2. an iron nail with some magnesium wire (or band) wrapped around it
3. an iron nail with some copper wire wrapped around it

For test-tube 4 and 5, every bench will try a different treatment to prevent rusting

"Shower bench"

• wrap an iron nail in saran wrap and place it into the 4th test tube
• use a stainless steel nail (these are more expensive because it contains other elements such as chromium and nickel) in the 5th test tube

"Middle bench"

• paint an iron nail, let it dry, and place it into the 4th test tube
• take a galvanized nail (coated with zinc), score it with the provided handsaw in two places, and place it into the 5th test tube

"Fire bench"

• cover an iron nail in oil and place it into the 4th test tube
• cover an iron nail in butter and place it into the 5th test tube

Once the 5 test tubes contain nails, pour hot gelatin solution (containing the indicator for iron(II)) into the test tube to a level that covers the nail completely. This will set after a couple of minutes. Once set, the redox reactions will continue, but convectional mixing will no longer occur, making it easier to see where exactly the rust forms.

Part 2

Formulate the half reactions and net redox reactions for test tube 2 and 3 on your data sheet. Here is an example for the reaction of the redox pairs \(\ce{OH^-/O2}\) and \(\ce{H2/H+}\):

\[\ce{2H+(aq) + 2e- -> H2(g)}\ \ \ E^\circ = 0 V\]

\[\ce{O2(g) + 2H2O(l) + 4e- -> 4OH^- (aq)}\ \ \ E^\circ = 0.4 V\]

The direction of the reaction is determined by the reduction potentials. In this case, the top reaction will go in the reverse direction (oxidation), and the bottom reaction as written, giving a positive cell potential. The combined reaction (after doubling the top reaction so that electrons cancel) will be:

\[\ce{O2(g) + 2H2O(l) + 2H2(g) -> 4OH^- (aq) + 4 H+}\ \ \ E^\circ = 0.4 V\]

Relevant half reactions and reduction potentials are available in your textbook.

Part 3

Go to one of the stations and explore the electrolysis of water. Pay attention to how the color changes at the graphite electrodes correlate with the poles of the battery. When you are done, mix the solution so that the pH is equal to 7 throughout again.

Part 4

Write down your observations from the reactions set up in part 1. Compare your observations for test tubes 4 and 5 with the other students at your bench, and plan a short presentation that includes projecting your results using the document camera in the lab. While the other groups are presenting, take notes.

Part 5

Check out if there is time. If there is no time, make an appointment for checkout next week.