HKDSE Chemistry – Chemical Reactions and Energy

HKDSE Chemistry Core
Section 8: Chemical Reactions and Energy | 化學反應與能量

Billy Sir’s Smart Notes: Master Enthalpy Changes, Calorimetry, and Hess’s Law.
由資深導師 Billy Sir 編寫，助你極速掌握焓變、量熱法及赫斯定律。

1. Conservation of Energy & Enthalpy Change | 能量守恆與焓變

The Law of Conservation of Energy states that energy can neither be created nor destroyed, but can only be changed from one form to another. 能量守恆定律指出，能量不能被創造或消滅，只能由一種形式轉換為另一種形式。

🔥 Exothermic vs Endothermic | 放熱與吸熱反應

Exothermic Reaction (放熱反應): Heat is released to the surroundings. Temperature of surroundings increases. \( \Delta H < 0 \) (Negative). 釋放熱能到環境中。環境溫度上升。\( \Delta H \) 為負值。
Endothermic Reaction (吸熱反應): Heat is absorbed from the surroundings. Temperature of surroundings decreases. \( \Delta H > 0 \) (Positive). 從環境中吸收熱能。環境溫度下降。\( \Delta H \) 為正值。

\Delta H = H_{\text{products}} – H_{\text{reactants}}

2. Standard Enthalpy Changes | 標準焓變

Standard conditions (\( \ominus \)): 298 K (25°C) and 1 atm pressure. 標準條件：298 K (25°C) 及 1 大氣壓。

Type (種類)	Definition (定義)	Sign of \( \Delta H \)
Standard Enthalpy Change of Combustion (\( \Delta H_c^\ominus \)) 標準燃燒焓變	The enthalpy change when 1 mole of a substance is completely burned in oxygen under standard conditions. 在標準條件下，1摩爾物質在氧氣中完全燃燒時的焓變。	Always Negative (-) 恆為負值
Standard Enthalpy Change of Neutralization (\( \Delta H_n^\ominus \)) 標準中和焓變	The enthalpy change when an acid and an alkali react to form 1 mole of water under standard conditions. 在標準條件下，酸與鹼反應生成1摩爾水時的焓變。	Always Negative (-) 恆為負值
Standard Enthalpy Change of Formation (\( \Delta H_f^\ominus \)) 標準生成焓變	The enthalpy change when 1 mole of a compound is formed from its constituent elements in their standard states. 在標準條件下，由處於標準狀態的成分元素生成1摩爾化合物時的焓變。	Positive (+) or Negative (-) 正或負值

Note: The standard enthalpy change of formation of any element in its standard state is zero (e.g., \( \Delta H_f^\ominus [\text{O}_2(g)] = 0 \)). 注意：任何處於標準狀態的元素的標準生成焓變均為零。

3. Experimental Determination of Enthalpy Change | 實驗測定焓變

Calorimetry is used to measure the heat change during a chemical reaction. 量熱法用於測量化學反應過程中的熱量變化。

⚠️ Exam Trap: Heat Transferred vs Enthalpy Change | 考試陷阱：轉移熱量 vs 焓變

Do not confuse heat transferred (\( q \)) with enthalpy change (\( \Delta H \)). Enthalpy change must include the sign (+ or -) and is usually expressed per mole (\( \text{kJ mol}^{-1} \)). 不要混淆轉移熱量 (\( q \)) 與焓變 (\( \Delta H \))。焓變必須包含正負號，且通常以每摩爾 (\( \text{kJ mol}^{-1} \)) 表示。

q = mc\Delta T $$ $$ \Delta H = \frac{-q}{n}

\( m \) = mass of the solution/water (質量)
\( c \) = specific heat capacity (比熱容量, usually \( 4.2 \text{ J g}^{-1}\text{K}^{-1} \))
\( \Delta T \) = maximum temperature change (最大溫度變化)
\( n \) = number of moles of limiting reactant (限量反應物的摩爾數)

Major Source of Error: Heat loss to the surroundings. 主要誤差來源：熱量散失到環境中。

4. Hess’s Law | 赫斯定律

Hess’s Law states that the total enthalpy change of a chemical reaction is independent of the pathway taken, provided the initial and final states are the same. 赫斯定律指出，只要初始狀態和最終狀態相同，化學反應的總焓變與反應途徑無關。

🔄 Enthalpy Cycles & Calculations | 焓循環與計算

Hess’s Law is extremely useful for calculating enthalpy changes that cannot be measured directly by experiments (e.g., formation of methane). 赫斯定律對於計算無法透過實驗直接測量的焓變（例如甲烷的生成）非常有用。

\Delta H_{\text{reaction}} = \sum \Delta H_f^\ominus (\text{Products}) – \sum \Delta H_f^\ominus (\text{Reactants})

\Delta H_{\text{reaction}} = \sum \Delta H_c^\ominus (\text{Reactants}) – \sum \Delta H_c^\ominus (\text{Products})

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