iGCSE Chemistry 0620 Extended - 9.5 Corrosion of metals- Exam Style Questions Paper 4- New Syllabus

(a)(i) Metallic bonding comprises a regular lattice array of positive metal cations anchored inside a surrounding “sea” of mobile, delocalised valence electrons. Strong, multi-directional electrostatic forces hold these components tightly together.
Valence electrons release from their parent metal atoms and form a shared mobile grid that keeps the regular layers of positive metallic cores strongly bound.

(a)(ii) The delocalised electrons are free to move and flow through the entire lattice structure to carry an electrical current.
Even when locked into a fixed solid shape, the mobile valence electron cloud remains unattached and can drift smoothly across the structure when a voltage is applied.

(b)(i) 1. It reacts with oxygen to release vital thermal energy (acting as fuel).
2. It reacts with carbon dioxide to generate carbon monoxide (the reducing agent).
Coke burns within the blast furnace to maintain the intense temperatures needed while simultaneously creating carbon monoxide gas to remove oxygen from the iron ore.

(b)(ii) Limestone undergoes thermal decomposition to produce basic calcium oxide ($CaO$). This basic calcium oxide then undergoes a neutralisation reaction with acidic silicon(IV) oxide ($SiO_2$) impurities to form liquid slag (calcium silicate).
Sandy impurities cannot melt on their own at standard furnace settings. Combining them with basic lime converts them into low-density molten calcium silicate ($CaSiO_3$) that can be easily tapped away.

(b)(iii) $\text{Fe}_2\text{O}_3 + 3\text{CO} \rightarrow 2\text{Fe} + 3\text{CO}_2$ (or $\text{Fe}_2\text{O}_3 + 3\text{C} \rightarrow 2\text{Fe} + 3\text{CO}$)
Iron(III) oxide loses its bound oxygen atoms via gas-phase reduction by carbon monoxide, producing pure liquid iron metal and carbon dioxide gas.

(b)(iv) The working temperature near the base of the blast furnace is much higher than the melting point of iron.
The furnace operating temperature exceeds 1500 °C at its base, causing the newly isolated iron metal to melt into a liquid that drops down into the crucible.

(c) Alloys contain added foreign atoms of significantly different sizes, which alters the regular geometry of the metallic lattice. This disruption prevents the atomic layers from sliding smoothly over or past one another under stress.
Pure metals possess uniform rows that slip easily when struck. Mixing in atoms of differing sizes distorts this alignment, creating an interlocking atomic structure that blocks sliding and increases hardness.

(d)(i) oxygen AND water
Iron requires simultaneous exposure to both atmospheric oxygen gas and liquid moisture/water to convert into hydrated iron(III) oxide (rust).

(d)(ii) galvanising
The specific technical process of coating a steel or iron piece with a protective sacrificial layer of zinc metal is called galvanising.

(d)(iii) It forms a continuous physical barrier that blocks oxygen and water from reaching the iron.
The intact external layer of zinc completely seals off the underlying iron surface from external air and moisture, preventing chemical corrosion.

(d)(iv) Zinc is more reactive than iron, so it undergoes preferential oxidation by losing electrons and forming zinc ions ($Zn^{2+}$) instead of the iron.
Even if the outer coat is scratched open, the zinc metal corrodes sacrificially because it sheds electrons more readily than iron, protecting the newly exposed iron from rusting.