HKDSE Chemistry Core
Section 3: Metals | 金屬
Billy Sir’s Smart Notes: Master Metal Reactivity, Extraction, Displacement, and Rusting.
由資深導師 Billy Sir 編寫，助你極速掌握金屬活潑性、提取方法、置換反應及鐵的生銹。
1. Discovery & Extraction of Metals | 金屬的發現與提取
The method used to extract a metal from its ore depends on its position in the reactivity series. The more reactive the metal, the harder it is to extract, and the later it was discovered in history. 從礦石中提取金屬的方法取決於其在活潑性序中的位置。金屬越活潑，提取越困難，在歷史上被發現的時間也越晚。
| Metals (金屬) | Reactivity (活潑性) | Extraction Method (提取方法) | Discovery (發現時期) |
|---|---|---|---|
| K, Na, Ca, Mg, Al | High (高) | Electrolysis of molten ore 電解熔融礦石 |
Modern times (現代) |
| Zn, Fe, Pb, Cu | Moderate (中) | Heating ore with Carbon 與碳共熱 |
Middle Ages (中古時期) |
| Hg, Ag | Low (低) | Heating ore alone 單獨加熱礦石 |
Ancient times (古代) |
| Au, Pt | Very Low (極低) | Found free in nature (Physical panning) 以游離態存在 (物理淘洗) |
Prehistoric (史前) |
2. Metal Reactivity Series | 金屬活潑性序
Metals are arranged in order of their reactivity: K > Na > Ca > Mg > Al > Zn > Fe > Pb > (H) > Cu > Hg > Ag > Pt > Au. 金屬按其活潑性排列。
🔥 Reactions with Oxygen, Water, and Acid | 與氧、水及酸的反應
- With Oxygen (與氧反應): Most metals react to form metal oxides. K and Na burn vigorously; Cu only forms a black layer of CuO on the surface when heated. (多數金屬形成金屬氧化物。鉀鈉劇烈燃燒；銅加熱只在表面形成黑色氧化銅。)
- With Water/Steam (與水/水蒸氣反應):
- K, Na, Ca: React with cold water to form metal hydroxide + \( \text{H}_2 \). (與冷水反應生成金屬氫氧化物和氫氣)
- Mg, Al, Zn, Fe: React with steam to form metal oxide + \( \text{H}_2 \). (與水蒸氣反應生成金屬氧化物和氫氣)
- With Dilute Acid (與稀酸反應): Metals above Hydrogen (K to Pb) react with dilute HCl or \( \text{H}_2\text{SO}_4 \) to form salt and \( \text{H}_2 \) gas. Cu, Ag, Au do NOT react with dilute acids. (氫以上的金屬與稀酸反應生成鹽和氫氣。銅、銀、金不與稀酸反應。)
3. Displacement Reactions | 置換反應
A more reactive metal can displace a less reactive metal from its salt solution. 較活潑的金屬能將較不活潑的金屬從其鹽溶液中置換出來。
📝 Classic Example: Zinc and Copper(II) Sulphate | 經典例子：鋅與硫酸銅(II)
If you put a Zinc strip into a blue Copper(II) sulphate solution, Zinc dissolves, and reddish-brown Copper solid forms. The blue color of the solution fades. (將鋅片放入藍色硫酸銅溶液中，鋅會溶解，並生成紅棕色銅固體。溶液的藍色會變淡。)
Reason: Zinc is more reactive than Copper, so it loses electrons more readily to form \( \text{Zn}^{2+} \) ions, forcing \( \text{Cu}^{2+} \) ions to accept electrons and become Cu atoms. (原因：鋅比銅活潑，更容易失去電子，迫使銅離子接收電子變成銅原子。)
4. Rusting of Iron vs. Aluminium | 鐵的生銹 vs 鋁的抗腐蝕性
| Feature (特徵) | Iron (Fe) | 鐵 | Aluminium (Al) | 鋁 |
|---|---|---|
| Corrosion Name | Rusting (生銹) | Corrosion (腐蝕) |
| Conditions Needed | Both Oxygen (\( \text{O}_2 \)) and Water (\( \text{H}_2\text{O} \)). 必須同時有氧氣和水。 |
Oxygen (\( \text{O}_2 \)). 氧氣。 |
| Nature of Oxide Layer | Rust (\( \text{Fe}_2\text{O}_3 \cdot x\text{H}_2\text{O} \)) is permeable and flaky. It falls off, exposing fresh iron to further rusting. 鐵銹是疏鬆且具滲透性的，會剝落，導致內層鐵繼續生銹。 |
Aluminium oxide (\( \text{Al}_2\text{O}_3 \)) is tough and impermeable. It sticks to the surface, protecting inner Al from further attack. 氧化鋁層堅硬且不透水，能緊附表面，保護內層鋁免受進一步腐蝕。 |
| Prevention/Enhancement | Painting, oiling, galvanizing (Zinc coating), sacrificial protection. 塗漆、塗油、鍍鋅、犧牲性保護。 |
Anodization (陽極電鍍): Artificially thickening the oxide layer to increase corrosion resistance and allow dyeing. 人工加厚氧化層以增強抗腐蝕性及便於染色。 |
💡 Exam Tip: Why is Aluminium used for window frames? | 考試提示：為何用鋁做窗框？
Even though Aluminium is more reactive than Iron (higher in the reactivity series), it is widely used for outdoor structures because of its self-protecting \( \text{Al}_2\text{O}_3 \) layer. It is also less dense (lighter) than iron! 雖然鋁比鐵更活潑，但由於其自我保護的氧化鋁層，它被廣泛用於戶外結構。而且它的密度比鐵低（較輕）！
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