HKDSE Chemistry Core
Section 9: Rate of Chemical Reaction | 化學反應速率
Billy Sir’s Smart Notes: Master Reaction Rates, Collision Theory, and Catalysts.
由資深導師 Billy Sir 編寫，助你極速掌握反應速率、碰撞理論及催化劑。
1. Definition & Finding Rates from Graphs | 定義與從圖表求速率
The rate of reaction is defined as the change in concentration (or mass/volume) of a reactant or product per unit time. 反應速率定義為單位時間內反應物或生成物濃度（或質量/體積）的變化。
📈 Rates from a Concentration-Time Graph | 從濃度-時間圖求速率
- Average Rate (平均速率): The change in concentration over a specific time interval. Found by calculating the slope of the line connecting two points on the curve. 特定時間段內的濃度變化。透過計算曲線上兩點連線的斜率求得。
- Instantaneous Rate (瞬時速率): The rate at a specific instant in time. Found by drawing a tangent to the curve at that time \( t \) and calculating its slope. 特定時刻的速率。透過在該時刻 \( t \) 畫一條切線並計算其斜率求得。
- Initial Rate (初速): The instantaneous rate at \( t = 0 \). Found by drawing a tangent at the origin. It is usually the maximum rate. 在 \( t = 0 \) 時的瞬時速率。透過在原點畫切線求得，通常是最大速率。
2. Methods to Monitor Reaction Rate | 監測反應速率的方法
We can monitor the progress of a reaction by measuring physical or chemical changes over time. 我們可以透過測量隨時間發生的物理或化學變化來監測反應進度。
| Method (方法) | When to use? (何時使用？) | Example (例子) |
|---|---|---|
| Physical: Volume of Gas 物理：氣體體積 |
When a gas is produced. 當有氣體生成時。 |
\( \text{Mg}(s) + 2\text{HCl}(aq) \rightarrow \text{MgCl}_2(aq) + \text{H}_2(g) \) (Using a gas syringe / 使用氣筒) |
| Physical: Mass Loss 物理：質量減少 |
When a heavy gas (e.g., \( \text{CO}_2 \)) escapes from an open flask. 當重氣體從敞口燒瓶逸出時。 |
\( \text{CaCO}_3(s) + 2\text{HCl}(aq) \rightarrow \text{CaCl}_2(aq) + \text{H}_2\text{O}(l) + \text{CO}_2(g) \) |
| Physical: Colorimetry 物理：比色法 |
When there is a change in color intensity (absorbance). 當顏色深淺（吸光度）發生變化時。 |
Reactions involving colored ions like \( \text{I}_2(aq) \) or \( \text{MnO}_4^-(aq) \). |
| Chemical: Titrimetric Analysis 化學：滴定分析 |
When a reactant/product can be titrated. Requires quenching (stopping the reaction suddenly, e.g., by cooling). 當反應物/生成物可被滴定時。需要「淬滅」（例如透過冷卻突然停止反應）。 |
Titrating acid or iodine concentration at different time intervals. |
3. Clock Reactions | 鐘反應
A clock reaction is used to determine the initial rate of a reaction. We measure the time (\( t \)) taken for a sudden, visible change (like a color change) to occur. 鐘反應主要用於測定反應初速。我們測量發生突然可見變化（如變色）所需的時間 (\( t \))。
Example: The Iodine Clock Reaction, where the solution suddenly turns dark blue-black when a specific amount of iodine is produced and reacts with starch. 例子：碘鐘反應，當生成特定量的碘並與澱粉反應時，溶液會突然變成深藍黑色。
4. Simple Collision Theory & Factors Affecting Rate | 簡單碰撞理論與影響速率的因素
⚠️ Core Concept: Effective Collisions | 核心概念：有效碰撞
For a reaction to occur, particles must collide with: 要發生反應，粒子碰撞時必須具備：
- Energy greater than or equal to the Activation Energy (\( E_a \)). 能量大於或等於活化能 (\( E_a \))。
- The correct orientation. 正確的碰撞方向。
The rate of reaction depends on the frequency of effective collisions. 反應速率取決於有效碰撞的頻率。
| Factor (因素) | Explanation based on Collision Theory (基於碰撞理論的解釋) |
|---|---|
| Concentration / Pressure 濃度 / 壓力 (氣體) |
Higher concentration/pressure means more particles per unit volume. This increases the collision frequency, leading to a higher frequency of effective collisions. 較高濃度/壓力意味著單位體積內有更多粒子。這增加了碰撞頻率，從而提高有效碰撞頻率。 |
| Surface Area (Solid) 表面積 (固體) |
Smaller pieces (e.g., powder) have a larger total surface area. More particles are exposed to react, increasing the collision frequency. 較小的碎塊（如粉末）總表面積較大。更多粒子暴露於反應中，增加碰撞頻率。 |
| Temperature 溫度 |
Higher temperature increases the kinetic energy of particles. A much larger fraction of particles possesses energy \( \ge E_a \). This drastically increases the frequency of effective collisions. 溫度升高會增加粒子的動能。擁有能量 \( \ge E_a \) 的粒子比例大幅增加，極大地提高了有效碰撞頻率。 |
5. Catalysts | 催化劑
A catalyst is a substance that increases the rate of a chemical reaction without being chemically changed or consumed at the end of the reaction. 催化劑是一種能加快化學反應速率，但在反應結束時其化學性質和質量保持不變的物質。
- How it works: It provides an alternative reaction pathway with a lower activation energy (\( E_a \)). 運作原理：它提供了一條活化能 (\( E_a \)) 較低的替代反應途徑。
- Because \( E_a \) is lower, a greater fraction of particles have enough energy to react upon collision, increasing the frequency of effective collisions. 由於 \( E_a \) 降低，更多粒子在碰撞時擁有足夠能量進行反應，增加了有效碰撞頻率。
- Note: A catalyst does NOT change the enthalpy change (\( \Delta H \)) or the yield of the reaction. 注意：催化劑不會改變反應的焓變 (\( \Delta H \)) 或產率。
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