Question 32 — Shipwrecks, Corrosion and Conservation (25 marks) Answer parts (a) and (b) of the question on pages 2–4 of the Section II Writing Booklet - HSC - SSCE Chemistry - Question 32 - 2015 - Paper 1

Sacrificial anodes are used as a protective measure against corrosion in metals. The principle lies in galvanic corrosion:

1. Anode and Cathode: A more reactive metal, such as zinc or magnesium, is used as the anode, and it corrodes in place of the iron or steel (cathode).
2. Oxidation Reaction: At the anode, the sacrificial metal oxidizes, releasing electrons: $ext{Zn} \rightarrow ext{Zn}^{2+} + 2e^-$
3. Reduction Reaction: At the cathode (iron), these electrons reduce Fe$^{2+}$ ions to iron: $ext{Fe}^{2+} + 2e^- \rightarrow ext{Fe}$. Therefore, the sacrificial anode effectively protects the iron from corrosion by preferentially corroding itself.

To compare the corrosion rates of iron and stainless steel, the following investigation can be conducted:

1. Preparation: Prepare two metal samples, one made of iron and the other of stainless steel, and label them accordingly.
2. Experimental Setup: Place each sample in separate containers filled with an electrolyte, such as saltwater, to enhance corrosion.
3. Measurement: Measure the mass of each sample before immersion and record their initial weights.
4. Time Interval: Leave the samples in the electrolyte for a specified time period (e.g., several weeks).
5. Final Measurement: After the exposure, remove the samples, clean them, and measure their final weights to determine weight loss due to corrosion. The sample that loses more mass experiences higher corrosion rates.

The percentage composition of carbon in steel directly impacts its properties:

1. Mild Steel: Typically contains about 0.05% to 0.25% carbon. Its properties include being soft and malleable, making it easy to work with and suitable for various constructions (e.g., automotive frames).
2. High Carbon Steel: Contains approximately 0.6% to 1.0% carbon. It is significantly harder and has high tensile strength, ideal for cutting tools and high-stress components. Higher carbon content increases hardness but reduces malleability, demonstrating this relationship effectively.

To draw a scientific diagram of an electrolytic cell:

1. Electrolytic Cell Setup: Include a beaker containing magnesium sulfate solution.
2. Electrodes: Represent the magnesium rod as the anode and the metal spoon as the cathode. Label them appropriately.
3. Electron Flow Direction: Draw arrows to indicate electron flow from anode to cathode.
4. Polarity: Label the anode (+) and cathode (-) correctly, showing the direction of electron transfer clearly.

Davy’s experiments with electrolysis provided significant insights into electron transfer reactions:

1. Electrochemical Processes: Davy demonstrated how electric currents could drive chemical reactions, leading to the isolation of various elements such as potassium and sodium.
2. Understanding Electron Flow: His work established a connection between the flow of electrons and the chemical reactions occurring at the electrodes, providing foundational knowledge for electrochemistry.
3. Large-scale Applications: Davy predicted large-scale applications in using electrolysis for industrial purposes, influencing future technologies in metal extraction and electroplating.

The rusting processes of the two ships differ significantly due to environmental conditions:

1. Shallow Water (60 m): In shallow environments, rusting occurs more swiftly because the water temperature is generally higher, allowing more oxygen to dissolve: $4Fe + 3O_2 + 6H_2O \rightarrow 4Fe(OH)_3$
   Then, Fe(OH)$_3$ dehydrates to form rust (Fe$_2$O$_3$·nH$_2$O).

2. Deep Water (4000 m): In deeper areas, lower temperatures lead to reduced oxygen levels, slowing down rusting rates and thus affecting the rate of corrosion. The same rusting reactions occur, but at a significantly reduced rate due to the limited oxygen presence. The differences in depth and conditions highlight how environmental factors influence rusting.