1. Purpose of the Experiment
- to know how to measure a reaction rate.
- to know what affects the reaction rate.
- to know how to prevent rusting
2. Chemistry of Rusting Iron.
Rust is a general term for iron oxides formed by the reaction of iron with oxygen. Several forms of rust are distinguishable visually, and form under different circumstances. The chemical composition of rust is typically hydrated iron(III) oxide (Fe2O3.nH2O), and under wet conditions may include iron(III) oxide-hydroxide (FeO(OH)). Rusting is the common term for corrosion of iron and its alloys, such as steel. Although oxidation of other metals is equivalent, these oxides are not commonly called rust.
Pure, solid iron oxidizes in water:
Fe(s) -> Fe2+(aq) + 2e-
These electrons will quickly react with the disassociated hydrogen ions (in H3O+(aq) form) and the dissolved oxygen in the water (O2(aq)):
4e-(aq) + 4H3O+(aq) + O2(aq) -> 6H2O(l)
Therefore, as seen from the above equation, the more acidic the water, the greater will be the rate of corrosion (since the concentration of H3O+(aq) will be greater.) At extremely low pH’s, the hydrogen ions will react with the electrons producing hydrogen gas instead:
2H+(aq) + 2e-(aq) -> H2(g)
Thus, as seen from the above equations, the pH of the solution (whether it is pure water or water containing electrolytes) rises. This leads to the formation of OH- ions (in cases where the body of water is significantly large, the pH does not rise as sharply, but this is of no consequence since OH- ions are always present, even in pure water.) The cations then react with the OH- or even the H+ ions and dissolved oxygen to form a variety of compounds, which constitute rust:
Fe2+(aq) + 2OH-(aq) -> Fe(OH)2(s)
4Fe2+(aq) + 4H+(aq) + O2(aq) -> 4Fe3+(aq) + 2H2O(l)
Fe3+(aq) + 3OH-(aq) -> Fe(OH)3(s)
From the above equations, it is seen that the pH and amount of dissolved oxygen can affect the outcome of the reactions. In water with limited dissolved oxygen Fe3O4(s) is formed, which is a black solid and commonly called lodestone:
6Fe2+(aq) + O2(aq) + 12OH-(aq) -> 2Fe3O4(s) + 6H2O(l)
The porous Fe(OH)3 rust can slowly disintegrate into a crystallized form, which is the familiar red-brown rust:
2Fe(OH)3(s) -> Fe2O3•H2O(s) + 2H2O(l)
3. Rust Prevention => Finishes
| Name |
Photo |
Feature |
| Common nail |
 |
Bright common nails have no finish. They can cause rust streaks if they are used in siding or decking. Hydrated rust is permeable to air and water, allowing the metal to continue to corrode - internally - even after a surface layer of rust has formed. Given sufficient hydration, the iron mass can eventually convert entirely to rust and disintegrate. |
| Galvanized nail |
 |
A common way of making nails corrosion-resistant is to coat them with zinc. Hot-dipped (H.D.) nails have been galvanized by dipping them in molten zinc. Electrogalvanized nails are plated with zinc, and are not as corrosion-resistant as hot-dipped nails. A third process peens zinc onto the nail. By roughening the nail's surface, all these treatments–but especially hot-dipping–increase the holding power of the nail. These rely on the zinc oxides protecting the once-scratched surface rather than oxidizing as a sacrificial anode. In this picture, the silver color is zinc. The rust of zinc is white. |
| Vinyl coated nail |
 |
Corrosion control can be done using a coating to isolate the metal from the environment such as air(oxygen) and water. Paint, wax, vinyl and cement are commonly used substances for coating. Vinyl coating also provides greater holding power. In this picture, the yellow color is caused by the vinyl coating. However, the state of coating was not even resulting in uneven rusting. |
4. Reaction Rate
Reaction rate is a measure of how fast a chemical reaction takes place. We usually express the reaction rate in terms of how fast a product is produced or how fast a reactant is consumed. The reaction rate of a general chemical reaction, aA + bB → pP + qQ, can be defined as:
For rusting, it is difficult to write an equation due to the complexity of the reaction. Thus, we can express the reaction like the following.
Iron + Oygen → Iron oxides (rust)
4Fe + 3O2 → 2 Fe2O3
According to the law of conservation of mass, the increased mass of nail and rust is the amount of oxygen.
Rusting rates = [the increased amount of product] / [time]
= [the consumed mass of oygen] / [time]
= [Nail's Change in mass] / [time in the unit of days]
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