Purpose
To demonstrate the reactivity series and its practical application in preventing corrosion using zinc as a sacrificial anode.
Materials Needed
- Two 8-cm crystallization dishes (or 2 100-ml beakers)
- Two 6-penny nails
- One 2 x 8-cm zinc strip
- 10 cm of 22-gauge copper wire
- Overhead projector
- 200 ml of 0.25 M HCl
- 7 ml of 3% H2O2
- 1 mL of 14 M KSCN
Procedure
- Preparation:
- Thoroughly clean both nails using steel wool.
- Wind one end of the copper wire several times around the top of one of the nails.
- Solder the wire to the nail.
- Assembly:
- Wrap the other end of the wire several times around the zinc strip, approximately 1 cm from the end.
- Fold the zinc strip over so that the wire is in the crease, ensuring a firm attachment to both the nail and the zinc.
- Create a loop in the wire so that the nail and zinc strip are about 3 cm apart.
- Reagent Mixing:
- Mix the remaining reagents (0.25 M HCl, 3% H2O2, and 14 M KSCN) in a 250-ml beaker.
- Experiment Setup:
- Simultaneously place one nail in one dish and the nail-zinc assembly in the other dish.
- Observation:
- A faint pink color will form in both dishes due to the formation of the iron thiocyanate complex.
- The color in the dish containing only the nail will continue to increase in intensity.
- A vigorous evolution of gas will be observed around the nail connected to the zinc strip, with no additional increase in color intensity in that dish.
- Final Step:
- After 30 seconds, cut the wire connecting the zinc strip to the nail.
Chemical Reactions
The following reactions occur during the demonstration:
| Reaction | Type | Equation |
|---|---|---|
| Zn(s) → Zn²⁺(aq) + 2 e⁻ | Oxidation | Zn(s) → Zn²⁺(aq) + 2 e⁻ |
| Fe²⁺(aq) + 2 SCN⁻(aq) → [Fe(SCN)]²⁺(aq) | Complex Formation | Fe²⁺(aq) + 2 SCN⁻(aq) → [Fe(SCN)]²⁺(aq) |
| 2 H⁺(aq) + 2 e⁻ → H₂(g) | Reduction | 2 H⁺(aq) + 2 e⁻ → H₂(g) |
Conclusion
The formation of the colored iron complex indicates that iron is being oxidized to Fe³⁺. This process is prevented as long as the nail is connected to the zinc strip, which undergoes oxidation instead. The demonstration effectively showcases the principle of cathodic protection in preventing corrosion.
Safety Note
Be careful not to get the alligator clip into the solution, as it may give a false positive.
Additional Information
For more details, visit: Journal of Chemical Education