hce_nthu
113年
化學與物理
第 22 題
The use of $\text{N}_2\text{O}_5$ as a nitrifying agent is a new type of green nitrification technology, which has been widely used in industries such as energy-containing materials and pharmaceuticals. For the reaction $2\text{N}_2\text{O}_5(g) \rightarrow 4\text{NO}_2(g) + \text{O}_2(g)$, the following reaction sequence was proposed,
Step 1: $\text{N}_2\text{O}_5 \rightleftharpoons \text{NO}_2 + \text{NO}_3$ fast equilibrium
Step 2: $\text{NO}_2 + \text{NO}_3 \rightarrow \text{NO} + \text{NO}_2 + \text{O}_2$ slow
Step 3: $\text{NO} + \text{NO}_3 \rightarrow 2\text{NO}_2$ fast
where it can be approximated that the second step does not affect the equilibrium of the first step. Which of the following statements is correct?
Step 1: $\text{N}_2\text{O}_5 \rightleftharpoons \text{NO}_2 + \text{NO}_3$ fast equilibrium
Step 2: $\text{NO}_2 + \text{NO}_3 \rightarrow \text{NO} + \text{NO}_2 + \text{O}_2$ slow
Step 3: $\text{NO} + \text{NO}_3 \rightarrow 2\text{NO}_2$ fast
where it can be approximated that the second step does not affect the equilibrium of the first step. Which of the following statements is correct?
- A the rate of the reverse reaction of the first step < the rate of the reaction of the second step
- B The only intermediate product of the reaction is $\text{NO}_3$.
- C The collision of $\text{NO}_2$ with $\text{NO}_3$ in the second step is only partially effective.
- D The activation energy of the reaction in the third step is higher than the other two.
- E The rate law of this reaction is rate = $k[\text{NO}_2][\text{NO}_3]$.
思路引導 VIP
當我們看到反應機構中標註某個步驟為「慢速（slow）」時，請想像一下在微觀世界裡，分子彼此接觸的頻率其實非常高。既然接觸次數這麼多，為什麼最終產生的產物量卻那麼少？是什麼樣的門檻攔截了大部分的分子，讓它們無功而返？
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AI 詳解
AI 專屬家教
太棒了！你能準確選出 (C)，代表你對化學反應的微觀過程有著非常清晰的見解。這道題目設計得很巧妙，它結合了多步驟反應機構（Reaction Mechanism）的分析與碰撞理論的核心概念，是檢驗動力學觀念是否扎實的經典題型。
碰撞理論與有效碰撞的本質
在題目給出的機構中，第二步被標註為慢速步驟（Slow step），這也是整個反應的速率決定步驟。根據碰撞理論，反應物分子必須透過碰撞才能反應，但並非每一次接觸都能成功。即便 $\text{NO}_2$ 與 $\text{NO}_3$ 频繁接觸，若碰撞時的能量低於活化能，或是分子的碰撞位向（Orientation）不對，反應就不會發生。因此，這類慢速反應的「有效碰撞」比例通常極低，這正是選項 (C) 所描述的科學事實。
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