📚 Electron Transfer Reactions | 电子转移反应
Electron transfer reactions, known as redox reactions, are fundamental to chemical change. They underpin processes from combustion and respiration to corrosion and industrial electrolysis. In IB Chemistry R3.2, we explore how electrons move between species, how to track oxidation states, and how to harness these transfers in electrochemical cells.
电子转移反应，即氧化还原反应，是化学变化的基础。它们支撑着从燃烧、呼吸到腐蚀和工业电解等过程。在IB化学R3.2中，我们探索电子如何在物种间移动，如何追踪氧化态，以及如何利用这些转移在电化学电池中。
1. Defining Oxidation and Reduction | 定义氧化与还原
Oxidation is the loss of electrons; reduction is the gain of electrons. This electron‑transfer definition applies regardless of the presence of oxygen. A classic mnemonic is 'OIL RIG' – Oxidation Is Loss, Reduction Is Gain.
氧化是失去电子；还原是得到电子。这种电子转移的定义无论氧气是否存在都适用。经典的记忆法是'OIL RIG'——氧化是失电子，还原是得电子。
²⁺ + 2e⁻, magnesium is oxidised. In the same reaction, O₂ + 4e⁻ → 2O²⁻, oxygen is reduced.
当镁燃烧时：Mg → Mg²⁺ + 2e⁻，镁被氧化。在同一反应中，O₂ + 4e⁻ → 2O²⁻，氧被还原。
Oxidation and reduction always occur simultaneously. The total number of electrons lost must equal the total number gained.
氧化和还原总是同时发生。失去的电子总数必须等于得到的电子总数。
2. Oxidation States: Rules and Assignments | 氧化态：规则与确定
Oxidation state (or oxidation number) is a bookkeeping tool that tracks electrons in a compound or ion. It indicates the degree of oxidation of an atom. Assigning oxidation states follows a set of hierarchical rules:
氧化态（或氧化数）是一种记录工具，用于追踪化合物或离子中的电子。它指示原子被氧化的程度。确定氧化态遵循一系列层次规则：
- Free elements have oxidation state 0 (e.g., Na, Cl₂, O₂). 游离态元素氧化态为0（如 Na, Cl₂, O₂）。
- For a simple ion, the oxidation state equals the charge (e.g., Na⁺ is +1, Cl⁻ is -1). 对于简单离子，氧化态等于电荷数（如 Na⁺为+1，Cl⁻为-1）。
- Oxygen is usually -2 in compounds, except in peroxides (-1) or with fluorine. 氧在化合物中通常为-2，过氧化物中为-1，或与氟结合时例外。
- Hydrogen is +1 when bonded to nonmetals, -1 when bonded to metals. 氢与非金属结合时为+1，与金属结合时为-1。
- The sum of oxidation states in a neutral compound is zero; in a polyatomic ion, it equals the ion's charge. 中性化合物中氧化态总和为零；多原子离子中等于离子电荷。
- In a compound, the more electronegative element is assigned the negative oxidation state. 在化合物中，电负性更大的元素被赋予负氧化态。
Assign oxidation states to confirm redox: if an atom's oxidation state increases, it is oxidised; if it decreases, it is reduced.
通过确定氧化态来确认氧化还原：如果原子的氧化态升高，它被氧化；如果降低，它被还原。
3. Writing Half-Equations | 书写半反应方程式
Half-equations separate the oxidation and reduction processes, showing electron transfer explicitly. They are essential for balancing redox reactions, especially in aqueous solutions where H⁺, OH⁻ and H₂O may be involved.
半反应方程式将氧化和还原过程分开，明确显示电子转移。它们对配平氧化还原反应至关重要，尤其是在涉及 H⁺、OH⁻ 和 H₂O 的水溶液中。
Steps to write a half-equation:
书写半反应方程式的步骤：
- Balance atoms other than O and H. 配平除O和H以外的原子。
- Add H₂O to balance O atoms. 添加 H₂O 以配平 O 原子。
- Add H⁺ to balance H atoms (acidic conditions). 添加 H⁺ 以配平 H 原子（酸性条件）。
- Add electrons (e⁻) to balance charge. 添加电子 (e⁻) 以配平电荷。
³⁺: Fe²⁺ → Fe³⁺ + e⁻. Reduction of MnO₄⁻ to Mn²⁺ in acid: MnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂O.
例如：Fe²⁺ 氧化为 Fe³⁺：Fe²⁺ → Fe³⁺ + e⁻。酸性条件下 MnO₄⁻ 还原为 Mn²⁺：MnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂O。
In basic solutions, add OH⁻ to both sides to neutralise H⁺, forming H₂O. Always combine half-equations ensuring electrons cancel.
在碱性溶液中，在
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