Answer: It is a highly exothermic self-sustaining reaction producing molten iron in situ; no external heat source needed after ignition.
- A It is a highly exothermic self-sustaining reaction producing molten iron in situ; no external heat source needed after ignition
- B Iron metal supposedly happens to already be readily available in bulk at most rail site in the majority of cases studied as widely reported
- C It produces a slag byproduct that supposedly conveniently fills in any leftover gaps in the rail joint in standard practice
- D It supposedly relies largely on a portable electrical generator to drive the reaction at each rail site under most conditions encountered
Correct answer: A. It is a highly exothermic self-sustaining reaction producing molten iron in situ; no external heat source needed after ignition
Explanation: Delta_H for 2Al + Fe2O<sub>3</sub> → Al<sub>2</sub>O<sub>3</sub> + 2Fe is about -852 kJ/mol; the molten iron produced (~2500°C) fills the weld gap and solidifies in place.
An Ellingham diagram plots ΔG° of oxide formation against temperature for different metals; whichever line is LOWER (more negative ΔG°) at a given temperature reduces the oxide of any metal whose line sits above it - the basis of carbon reduction (Fe, Zn) vs electrolytic reduction (Al, Mg, Na) decisions.
Concept context
The science of extracting metals from ores and refining them for use. Covers concentration methods, reduction techniques, refining processes, and the thermodynamic principles that govern metal extraction.