Core Section 10: Chemical Equilibrium

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HKDSE Chemistry – Chemical Equilibrium

HKDSE Chemistry Core
Section 10: Chemical Equilibrium | 化學平衡

Billy Sir’s Smart Notes: Master Reversible Reactions, Equilibrium Constants, and Le Chatelier’s Principle.
由資深導師 Billy Sir 編寫，助你極速掌握可逆反應、平衡常數及勒沙得利爾原則。

1. Reversible Reactions & Dynamic Equilibrium | 可逆反應與動態平衡

Unlike irreversible reactions (which go to completion, like combustion), reversible reactions can proceed in both the forward and backward directions, denoted by the symbol \( \rightleftharpoons \). 與不可逆反應（如燃燒，會完全反應）不同，可逆反應可以同時向正向和逆向進行，以符號 \( \rightleftharpoons \) 表示。

⚖️ Dynamic Nature of Equilibrium | 平衡的動態性質

A system reaches dynamic equilibrium in a closed system when: 當封閉系統達到動態平衡時：

  • The rate of the forward reaction equals the rate of the backward reaction. 正向反應速率等於逆向反應速率。
  • The macroscopic properties (e.g., concentration, color, pressure) remain constant. 宏觀性質（如濃度、顏色、壓力）保持不變。
  • The reaction has NOT stopped; both forward and backward reactions are still occurring at the microscopic level. 反應並未停止；在微觀層面上，正逆反應仍在持續進行。

2. Equilibrium Constant (\( K_c \)) | 平衡常數

For a general reversible reaction: \( aA + bB \rightleftharpoons cC + dD \), the equilibrium constant \( K_c \) is given by: 對於一般可逆反應，平衡常數 \( K_c \) 的公式為：

$$ K_c = \frac{[C]^c [D]^d}{[A]^a [B]^b} $$

Note: \( [X] \) represents the equilibrium concentration of substance X in \( \text{mol dm}^{-3} \). 注意：\( [X] \) 代表物質 X 的平衡濃度。

⚠️ Exam Trap: Should Water be Included in \( K_c \)? | 考試陷阱：水應否計算在 \( K_c \) 內？

  • DO NOT include water if the reaction occurs in an aqueous solution (water is the solvent). Its concentration is so large that it remains practically constant. 如果反應在水溶液中進行（水是溶劑），則不包括水。因為其濃度極大，實際上保持不變。
  • DO include water if it is a reactant or product in a non-aqueous system (e.g., esterification, gas-phase reactions). Here, water’s concentration changes significantly. 如果水是非水系統中的反應物或生成物（例如酯化反應、氣相反應），則必須包括水。此時水的濃度變化顯著。

3. Investigating Esterification via Titration | 透過滴定法研究酯化反應

Esterification is a classic reversible reaction used to study equilibrium: 酯化反應是用來研究平衡的經典可逆反應：

$$ \text{CH}_3\text{COOH}(l) + \text{C}_2\text{H}_5\text{OH}(l) \rightleftharpoons \text{CH}_3\text{COOC}_2\text{H}_5(l) + \text{H}_2\text{O}(l) $$

Because this is a liquid-phase reaction without water as a solvent, water MUST be included in the \( K_c \) expression. 由於這是液相反應，水不是溶劑，因此水必須包含在 \( K_c \) 表達式中。

Step (步驟) Explanation (解釋)
1. Setup
設置		 Mix known amounts of ethanoic acid and ethanol with a few drops of concentrated \( \text{H}_2\text{SO}_4 \) (catalyst). Leave the mixture in a stoppered flask for several days to reach equilibrium.
將已知份量的乙酸和乙醇與幾滴濃硫酸（催化劑）混合。將混合物放在加塞的燒瓶中數天以達到平衡。
2. Quenching
淬滅		 Withdraw a sample and add it to a large volume of cold water. This quenches (stops) the reaction immediately so the equilibrium position doesn’t shift during titration.
抽取樣本並加入大量冷水中。這會立即「淬滅」（停止）反應，確保滴定過程中平衡位置不會移動。
3. Titration
滴定		 Titrate the quenched mixture with standard NaOH(aq) using phenolphthalein indicator to find the total amount of acid remaining (ethanoic acid + sulfuric acid catalyst).
使用標準 NaOH(aq) 和酚酞指示劑滴定淬滅後的混合物，以找出剩餘的總酸量（乙酸 + 硫酸催化劑）。
4. Calculation
計算		 Subtract the known amount of catalyst acid to find the equilibrium amount of ethanoic acid. Use an ICE (Initial, Change, Equilibrium) table to find all other concentrations and calculate \( K_c \).
減去已知的催化劑酸量，求出乙酸的平衡量。使用 ICE 表格找出所有其他濃度並計算 \( K_c \)。

4. Factors Affecting Equilibrium Position | 影響平衡位置的因素

Le Chatelier’s Principle (勒沙得利爾原則): If a change is imposed on a system at equilibrium, the system will shift its equilibrium position to counteract the change. 如果對處於平衡狀態的系統施加改變，系統會移動其平衡位置以抵消該改變。

Factor (因素) Effect on Equilibrium Position (對平衡位置的影響) Effect on \( K_c \) (對 \( K_c \) 的影響)
Concentration
濃度		 Adding a reactant shifts equilibrium to the right (products). Removing a product also shifts it to the right.
增加反應物會使平衡向右（生成物）移動。移除生成物也會使平衡向右移動。		 No change
不變
Pressure (Gases only)
壓力 (僅限氣體)		 Increasing pressure shifts equilibrium to the side with fewer moles of gas.
增加壓力會使平衡向氣體摩爾數較少的一側移動。		 No change
不變
Temperature
溫度		 Increasing temperature shifts equilibrium in the endothermic direction (\( \Delta H > 0 \)).
升高溫度會使平衡向吸熱方向移動。		 Changes! (Only temperature affects \( K_c \))
會改變！（只有溫度會影響 \( K_c \)）
Catalyst
催化劑		 No shift. It speeds up both forward and backward reactions equally. Equilibrium is reached faster.
不移動。它同等程度地加快正逆反應速率，僅使系統更快達到平衡。		 No change
不變

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