HKDSE Chemistry Core
Section 7: Redox Reactions, Chemical Cells & Electrolysis | 氧化還原反應、化學電池與電解
Billy Sir’s Smart Notes: Master Oxidation Numbers, Voltaic Cells, and Electrolytic Processes.
由資深導師 Billy Sir 編寫，助你極速掌握氧化數、化學電池及電解過程。
1. Definition of Redox Reactions | 氧化還原反應的定義
A redox reaction involves the simultaneous occurrence of oxidation and reduction. 氧化還原反應涉及氧化和還原同時發生。
| Concept (概念) | Oxidation (氧化) | Reduction (還原) |
|---|---|---|
| Oxygen (氧) | Gain of oxygen (得氧) | Loss of oxygen (失氧) |
| Hydrogen (氫) | Loss of hydrogen (失氫) | Gain of hydrogen (得氫) |
| Electrons (電子) | Loss of electrons (失電子) | Gain of electrons (得電子) |
| Oxidation Number (氧化數) | Increase in oxidation number (氧化數增加) | Decrease in oxidation number (氧化數減少) |
💡 Mnemonic: OIL RIG | 記憶口訣
Oxidation Is Loss of electrons, Reduction Is Gain of electrons. 氧化是失去電子，還原是獲得電子。
- Oxidizing Agent (氧化劑): Causes oxidation of another substance, while it is reduced itself. (導致其他物質氧化，自身被還原。)
- Reducing Agent (還原劑): Causes reduction of another substance, while it is oxidized itself. (導致其他物質還原，自身被氧化。)
2. Simple Chemical Cell (Voltaic Cell) | 簡單化學電池
A chemical cell converts chemical energy into electrical energy through spontaneous redox reactions. 化學電池透過自發的氧化還原反應將化學能轉化為電能。
- Anode (陽極): The electrode where oxidation occurs. It is the negative pole (-) because it supplies electrons to the external circuit. (發生氧化的電極。它是負極，因為它向外電路提供電子。)
- Cathode (陰極): The electrode where reduction occurs. It is the positive pole (+) because it receives electrons from the external circuit. (發生還原的電極。它是正極，因為它從外電路接收電子。)
🌉 The Salt Bridge | 鹽橋的作用
A salt bridge (e.g., filter paper soaked in \( \text{KNO}_3 \) solution) is essential in a cell with two half-cells. 在具有兩個半電池的系統中，鹽橋（例如浸有硝酸鉀溶液的濾紙）是必不可少的。
- It completes the circuit by allowing ions to flow. (允許離子流動以閉合電路。)
- It maintains electrical neutrality in the half-cells (Cations move to cathode, anions move to anode). (維持半電池的電中性：陽離子移向陰極，陰離子移向陽極。)
3. Simple Electrolytic Cell | 簡單電解池
Electrolysis is the decomposition of an electrolyte by passing an electric current through it. It converts electrical energy into chemical energy (non-spontaneous reaction). 電解是透過通電使電解質分解的過程。它將電能轉化為化學能（非自發反應）。
| Feature (特徵) | Chemical Cell (化學電池) | Electrolytic Cell (電解池) |
|---|---|---|
| Energy Conversion (能量轉換) | Chemical \( \rightarrow \) Electrical 化學能 \( \rightarrow \) 電能 |
Electrical \( \rightarrow \) Chemical 電能 \( \rightarrow \) 化學能 |
| Anode (陽極 – Oxidation) | Negative Pole (-) 負極 | Positive Pole (+) 正極 |
| Cathode (陰極 – Reduction) | Positive Pole (+) 正極 | Negative Pole (-) 負極 |
⚡ Preferential Discharge of Ions | 離子的優先放電
During electrolysis of aqueous solutions, multiple ions compete to discharge at the electrodes. The outcome depends on three factors: 在水溶液的電解過程中，多種離子會競爭在電極上放電。結果取決於三個因素：
- Position in Electrochemical Series (電化學序中的位置): Ions lower in the ECS are preferentially discharged. (e.g., \( \text{Cu}^{2+} \) discharges before \( \text{H}^+ \); \( \text{OH}^- \) discharges before \( \text{SO}_4^{2-} \)). (電化學序較低的離子優先放電。)
- Concentration Effect (濃度效應): A higher concentration of a halide ion (e.g., concentrated \( \text{Cl}^- \)) can override the ECS rule and discharge preferentially over \( \text{OH}^- \). (高濃度的鹵素離子可凌駕電化學序規則，優先於氫氧離子放電。)
- Nature of Electrode (電極性質): Reactive electrodes (e.g., Copper anode) will dissolve (oxidize) instead of anions discharging. (活潑電極如銅陽極會自身溶解氧化，而非讓陰離子放電。)
4. Applications of Electrolysis | 電解的應用
💍 Electroplating | 電鍍
Coating a metal object with a thin layer of another metal to prevent corrosion or improve appearance. 在金屬物件表面鍍上一層薄薄的其他金屬，以防止腐蝕或改善外觀。
- Anode (+): The plating metal (e.g., pure Silver). (陽極：電鍍金屬，如純銀)
- Cathode (-): The object to be plated (e.g., an iron spoon). (陰極：被鍍物件，如鐵匙)
- Electrolyte: A solution containing ions of the plating metal (e.g., \( \text{AgNO}_3 \) solution). (電解質：含有電鍍金屬離子的溶液)
🏭 Purification of Copper | 銅的提純
Used to purify impure copper extracted from ores. 用於提純從礦石中提取的不純銅。
- Anode (+): Impure copper (Dissolves: \( \text{Cu} \rightarrow \text{Cu}^{2+} + 2e^- \)). (陽極：不純銅。溶解成銅離子)
- Cathode (-): Pure copper (Grows: \( \text{Cu}^{2+} + 2e^- \rightarrow \text{Cu} \)). (陰極：純銅。銅離子還原成銅原子並沉積)
- Electrolyte: Copper(II) sulphate solution (\( \text{CuSO}_4 \)). (電解質：硫酸銅溶液)
- Note: Impurities fall to the bottom as “anode sludge” (陽極泥).
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