18.10
电解
在原电池（伽凡尼电池）中，由于自发氧化还原反应产生的电子通过外部电路传输，氧化还原系统在其周围完成电气工作。 或者，外部电路确实可以在氧化还原系统上工作，方法是施加足够的电压，以在称为电解的过程中驱动非自发反应。 例如，为电池充电需要使用外部电源以反向驱动自发 (放电) 电池反应，从而在一定程度上恢…
伏打电池中的自发氧化还原反应 将化学能转化为电能。或者，外部电能会在电解池中 驱动非自发氧化还原反应，这一过程称为电解。电解池包括用于氧化的阳极 和用于还原的阴极。但是，与伏打电池不同，在伏打电池中，带负电的阳极是电子源，而电解池则像电池一样，有一个外部电子源。电池从连接到正极端子的 阳极吸取电子，并驱动电子流向 连接到负极端子的阴极。因此，电子从外部电源的 正极流向负极。电解用来把离子化合物分离成 它们的组成元素。在熔融或水溶液状态下，它们的组成离子可以自由导电。请考虑溴化钾在 734℃下熔化。这里，溴化物和钾离子 分别以它们的最低或最高氧化态 存在。通电时，溴离子 在阳极被氧化为溴气，钾离子在阴极被还原为 固态钾。在像碘化钠和溴化钾这样的 熔融离子混合物中，标准电极电位 用于预测哪种物质会发生反应。一般来说，具有较低电极电位的 阴离子被氧化，而具有较高正电极电位 的阳离子被还原。有趣的是，由于可能存在水的电解，含水溴化钾的电解会从熔融盐中 产生不同的产物。因此，在每个电极上可能发生两个反应；溴离子或水在阳极 氧化，钾离子或水在阴极 还原。由于电极电位较低，有利于溴离子氧化为溴，而水则在较高的正电极电位 驱动下还原为氢气。半反应的化学计量 可用来计算电解过程中 产生或消耗的物质的量。例如，熔融溴化钾的电解 利用一个电子形成一个钾原子。因此，每摩尔电子或 96, 485 库仑电荷 通过电池，将产生 1 摩尔或约 39 克固态钾。
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Q1: How does electrolysis differ from a voltaic cell?
Electrolysis uses external electrical energy to drive nonspontaneous redox reactions, while voltaic cells convert spontaneous chemical reactions into electrical energy. In electrolysis, an external power source like a battery supplies electrons to the cathode and draws them from the anode. This reverses the natural electron flow, enabling separation of ionic compounds into their constituent elements.
Q2: What determines which ions are oxidized or reduced during electrolysis?
Standard electrode potentials predict which substance reacts during electrolysis. Anions with lower electrode potentials are oxidized at the anode, while cations with more positive electrode potentials are reduced at the cathode. In aqueous solutions, water may also participate, competing with dissolved ions based on their relative electrode potentials.
Q3: Why does electrolysis of aqueous potassium bromide produce different products than molten potassium bromide?
In molten potassium bromide, only bromide and potassium ions are present. In aqueous solution, water molecules can also be oxidized or reduced. Bromide oxidation to bromine is favored at the anode due to lower electrode potential, while water reduction to hydrogen gas is favored at the cathode, producing different products than the molten salt.
Q4: How is electrical current related to the amount of substance produced in electrolysis?
Electrical current measures the rate of electron flow in amperes (coulombs per second). The total charge transferred equals current multiplied by time. Using stoichiometry of half-reactions and Faraday's constant (96,485 coulombs per mole of electrons), you can calculate the moles of substance produced or consumed during electrolysis.
Q5: What is the role of the anode and cathode in an electrolytic cell?
The anode is connected to the positive terminal of the battery and is where oxidation occurs. The cathode is connected to the negative terminal and is where reduction occurs. The external power source draws electrons from the anode and drives them toward the cathode, forcing nonspontaneous redox reactions to proceed.
Q6: How much potassium metal is produced from electrolysis of molten potassium bromide?
One mole of electrons produces one mole of potassium atoms through reduction at the cathode. Since one mole of electrons equals 96,485 coulombs of charge, passing this amount of charge through the cell produces approximately 39 grams of solid potassium metal.
Q7: What are practical applications of electrolysis?
Electrolysis is used to separate ionic compounds into constituent elements, refine metallic ores, manufacture commodity chemicals, and electroplate metallic coatings onto objects like jewelry and utensils. Battery recharging also uses electrolysis, driving the discharge reaction in reverse to restore the battery's composition and voltage.