HKDSE Chemistry Core
Section 4: Acids and Alkalis | 酸和鹼
Billy Sir’s Smart Notes: Master Acid-Base Reactions, pH Scale, Neutralization, and Titration.
由資深導師 Billy Sir 編寫，助你極速掌握酸鹼反應、pH標度、中和作用及滴定。
1. Definitions & Basicity | 定義與鹽基度
Acids (酸): Substances that ionize in water to produce hydrogen ions, \( \text{H}^+\text{(aq)} \), as the only positive ions. 在水中電離並產生氫離子作為唯一陽離子的物質。
Alkalis (鹼): Soluble bases that ionize in water to produce hydroxide ions, \( \text{OH}^-\text{(aq)} \). 可溶性鹽基，在水中電離產生氫氧離子。
🔬 Basicity of an Acid | 酸的鹽基度
Basicity is the maximum number of ionizable hydrogen atoms per acid molecule. 鹽基度是指每個酸分子最多能電離出的氫原子數目。
- Monobasic (一元酸): \( \text{HCl} \), \( \text{HNO}_3 \), \( \text{CH}_3\text{COOH} \) (Only 1 ionizable H).
- Dibasic (二元酸): \( \text{H}_2\text{SO}_4 \), \( \text{H}_2\text{CO}_3 \) (2 ionizable H).
- Tribasic (三元酸): \( \text{H}_3\text{PO}_4 \) (3 ionizable H).
2. Chemical Reactions of Acids | 酸的化學反應
Acids have three main characteristic reactions. Memorize these general word equations! 酸有三個主要的特徵反應。請熟記這些通式！
| Reaction Type (反應類型) | General Equation (通式) | Example (例子) |
|---|---|---|
| Acid + Metal 酸 + 金屬 |
Acid + Metal \( \rightarrow \) Salt + Hydrogen 酸 + 金屬 \( \rightarrow \) 鹽 + 氫氣 |
\( \text{Mg(s)} + 2\text{HCl(aq)} \rightarrow \text{MgCl}_2\text{(aq)} + \text{H}_2\text{(g)} \) |
| Acid + Base (Oxide/Hydroxide) 酸 + 鹽基 (氧化物/氫氧化物) |
Acid + Base \( \rightarrow \) Salt + Water 酸 + 鹽基 \( \rightarrow \) 鹽 + 水 |
\( \text{CuO(s)} + \text{H}_2\text{SO}_4\text{(aq)} \rightarrow \text{CuSO}_4\text{(aq)} + \text{H}_2\text{O(l)} \) |
| Acid + Carbonate / Hydrogencarbonate 酸 + 碳酸鹽 / 碳酸氫鹽 |
Acid + Carbonate \( \rightarrow \) Salt + Water + Carbon Dioxide 酸 + 碳酸鹽 \( \rightarrow \) 鹽 + 水 + 二氧化碳 |
\( \text{CaCO}_3\text{(s)} + 2\text{HCl(aq)} \rightarrow \text{CaCl}_2\text{(aq)} + \text{H}_2\text{O(l)} + \text{CO}_2\text{(g)} \) |
3. Strength vs. Concentration & pH Scale | 強度、濃度與 pH 標度
⚠️ Exam Trap: Strong vs. Concentrated | 考試陷阱：強酸與濃酸的分別
Strength (強度) refers to the extent of ionization. A strong acid (e.g., \( \text{HCl} \)) completely ionizes in water, while a weak acid (e.g., \( \text{CH}_3\text{COOH} \)) only partially ionizes. 強度指電離的程度。強酸在水中完全電離，弱酸只部分電離。
Concentration (濃度) refers to the amount of solute dissolved in a given volume of solution. You can have a dilute solution of a strong acid or a concentrated solution of a weak acid! 濃度指溶解在特定體積溶液中的溶質量。你可以有稀的強酸，或濃的弱酸！
pH Scale | pH 標度
The pH scale measures the hydrogen ion concentration in a solution. It typically ranges from 0 to 14. pH 標度量度溶液中氫離子的濃度。範圍通常由 0 至 14。
- pH < 7: Acidic (The lower the pH, the higher the \( \text{H}^+ \) concentration). 酸性 (pH 越低，氫離子濃度越高)
- pH = 7: Neutral (e.g., pure water). 中性 (例如：純水)
- pH > 7: Alkaline (The higher the pH, the higher the \( \text{OH}^- \) concentration). 鹼性 (pH 越高，氫氧離子濃度越高)
4. Neutralization & Titration | 中和作用與滴定
Neutralization is the reaction between an acid and a base to form a salt and water. The ionic equation for all acid-alkali neutralizations is: 中和作用是酸與鹽基反應生成鹽和水的過程。所有酸鹼中和的離子方程式皆為：
This reaction is exothermic (releases heat). 此反應為放熱反應。
🧪 Volumetric Analysis (Titration) | 容量分析 (滴定)
Titration is an experimental technique used to determine the unknown concentration of a solution by reacting it with a solution of known concentration (standard solution). 滴定是一種實驗技術，透過讓未知濃度的溶液與已知濃度的標準溶液反應，來求出未知濃度。
- Burette (滴定管): Delivers variable volumes of solution accurately. 準確地釋出可變體積的溶液。
- Pipette (移液管): Delivers a fixed, accurate volume of solution (e.g., 25.0 cm³) into a conical flask. 準確地轉移固定體積的溶液至錐形瓶。
- Indicator (指示劑): Used to find the end point.
- Methyl Orange (甲基橙): Red in acid, Yellow in alkali. End point is orange. (酸中紅色，鹼中黃色，終點為橙色)
- Phenolphthalein (酚酞): Colourless in acid, Pink in alkali. (酸中無色，鹼中粉紅色)
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