细胞、氧化还原比和预测极限
引入
词汇
| English | 释义 | 拼音 |
|---|---|---|
| standard electrode potential/ˈstændəd ɪˈlektrəʊd pəˈtenʃl/ | 标准电极电势 | biāo zhǔn diàn jí diàn shì |
| potential/pəˈtenʃl/ | 电势 | diàn shì |
| feasibility/ˌfiːzɪˈbɪlɪti/ | 可行性 | kě xíng xìng |
| activation barrier | 活化能垒 | huó huà néng lěi |
English
Which way do electrons move?
- In a spontaneous zinc–copper cell, zinc loses electrons and copper(II) ions gain them. The negative electrode is zinc.
- Electrons move through the external wire. Salt-bridge ions carry charge inside the connecting bridge; electrons do not flow through the solutions.
练习
盐桥通过移动_____来传导电荷。
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离子通过盐桥移动。电子通过外部导体移动。
English
Compare standard reductions
- A standard electrode potential 标准电极电势 is measured relative to the standard hydrogen electrode at 298 K, 100 kPa gas pressure and 1.00 mol per cubic decimetre solutions.
- With reduction potentials +0.80 and +0.34 V, the more positive couple is reduced. Cell voltage is 0.80 − 0.34 = +0.46 V.
练习
由+0.54 V和−0.25 V还原电势构成的自发电池中，哪一对被还原？
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更正的还原电势对应正极还原反应。
练习
根据+0.54 V和−0.25 V还原电势计算标准电池电压。
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0.54 − (−0.25) = 0.79 V.
English
Balance electrons without scaling voltage
- Copper oxidation gives two electrons; each silver ion needs one. The net equation is Cu + 2Ag⁺ → Cu²⁺ + 2Ag.
- Multiplying a half-equation changes amount and total transferred charge together. It does not multiply the voltage.
Example
Electrons move in the wire, while ions carry current in the salt bridge.
练习
将还原半方程式系数加倍会使其标准电极电位加倍。
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电位是单位电荷的量；增加物质的量不会改变它。
English
Use the actual redox ratio
- Acidified manganate(VII) gains five electrons per MnO₄⁻, so its ratio with one-electron Fe²⁺ is 1:5.
- 22.40 cubic centimetres of 0.0200 moles per cubic decimetre titrant is 0.000448 mol. It reacts with 0.00224 mol Fe²⁺.
English
State the prediction limits
- Positive standard voltage predicts thermodynamic feasibility 可行性 under standard conditions. It does not prove an observable reaction rate.
- A high activation barrier 活化能垒 can make reaction slow. Non-standard concentrations can change the potential 电势 and the predicted direction.
Watch out
At standard conditions, a positive cell voltage is compatible with a very slow reaction.
Key idea
Balance electron transfer before using a titration ratio. Compare standard electrode potentials for a standard prediction, then state concentration and rate limits.
练习
仅基于标准电势进行预测时，哪两个条件构成限制？
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动力学能垒和浓度变化限制了标准热力学计算所能预测的内容。