Electrolysis
IGCSE Chemistry Topic 4 10:25 English narration · English + 中文 subtitles burned in
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Electricity can do something amazing: it can tear a compound apart.
电能做一件神奇的事：它能把化合物拆开。
Pass a current through a melted or dissolved compound, and it splits into its elements.
让电流通过熔化或溶解的化合物， 它就分解成各种元素。
This is how we extract reactive metals like aluminium from their ores.
这就是我们从矿石中提取像铝这样活泼金属的方法。
It is how chlorine is made for cleaning water.
这就是制造用于净水的氯气的方法。
And it is how a cheap metal spoon gets a shiny, protective coating of silver.
这也是让廉价金属勺子镀上一层闪亮、 保护性的银的方法。
The tool for all of this is called electrolysis.
做到这一切的工具，叫做电解。
Welcome to electrolysis — splitting compounds with electricity.
欢迎来到电解——用电来分解化合物。
We'll meet the cell, the electrodes, and the products.
我们将认识电解池、电极和产物。
Let's begin.
让我们开始吧。
Electrolysis is the breaking down — the decomposition — of an ionic compound using an electric current.
电解是用电流把离子化合物分解。
But there is a condition: the compound must be molten, or dissolved in water.
但有一个条件：化合物必须是熔化的，或溶解在水中。
Why?
为什么？
Because electrolysis needs the ions to move.
因为电解需要离子能够移动。
In a solid, the ions are locked in place and cannot carry charge.
在固体中，离子被锁在原位，无法携带电荷。
Melt it, or dissolve it, and the ions break free — now they can flow, and the current can split the compound apart.
把它熔化，或溶解，离子就获得自由——现在它们能流动，电流就能把化合物拆开。
Look at a simple school demonstration: electrolysing water.
看一个简单的学校演示：电解水。
Two tubes collect gas above the electrodes.
两根管子在电极上方收集气体。
One tube fills about twice as high as the other.
一根管子里的气体 大约是另一根的两倍高。
That larger volume is hydrogen, formed at the cathode.
体积更大的是氢气，在阴极生成。
The smaller volume is oxygen, formed at the anode.
体积更小的是氧气， 在阳极生成。
The two-to-one ratio matches the formula of water: two hydrogen atoms for every oxygen atom.
二比一的比例正好对应水的组成：每有一个氧原子，就有两个氢原子。
So electrolysis does not only split salts — it can also split water itself.
所以电解不只是拆开盐，它也能把水本身拆开。
Here is the set-up: an electrolytic cell.
这就是装置：电解池。
Two electrodes dip into the electrolyte — the molten or dissolved compound.
两个电极浸入电解质——熔化或溶解的化合物。
They are often inert, made of platinum or graphite, so they don't react.
它们通常是惰性的，用铂或石墨制成，所以不参与反应。
One electrode is positive: the anode.
一个电极是正极：阳极。
The other is negative: the cathode.
另一个是负极：阴极。
Now watch the charge flow.
现在看电荷的流动。
Electrons travel through the wires from the power supply.
电子通过导线从电源流出。
Inside the liquid, the ions move: positive ions are pulled to the negative cathode, and negative ions to the positive anode.
在液体内部，离子移动：正离子被拉向负的阴极，负离子被拉向正的阳极。
Here is a real electrolysis cell in the lab.
这是实验室里真实的电解池。
A power supply drives a current through a liquid.
电源驱动电流通过液体。
Two solid electrodes dip into that liquid — the electrolyte.
两根固体电极浸入那液体， 也就是电解质。
The electrodes are often inert, for example platinum or graphite, so they do not react and only conduct the current.
电极通常是惰性的，例如铂或石墨，所以它们不反应，只传导电流。
Remember: the positive electrode is the anode; the negative electrode is the cathode.
记住：正极是阳极；负极是阴极。
Watch how charge moves in molten lead bromide.
看熔化溴化铅中电荷如何移动。
Electrons travel through the external circuit — the wires — from the power supply.
电子通过外电路，也就是导线，从电源流出。
Inside the melt, ions do the carrying: lead two-plus ions move to the cathode, and bromide ions move to the anode.
在熔体内部，由离子来载流：二价铅离子移向阴极，溴离子移向阳极。
At the cathode the lead ions gain electrons and become lead metal.
在阴极，铅离子得到电子，变成铅金属。
At the anode the bromide ions lose electrons and become bromine.
在阳极，溴离子失去电子，变成溴。
Different carriers, one continuous current.
载体不同，但电流是连续的。
What forms at each electrode?
每个电极上生成什么？
There is one simple rule.
有一条简单的规则。
At the cathode, you always get a metal, or hydrogen.
在阴极，你总是得到金属，或氢气。
At the anode, you get a non-metal.
在阳极，你得到非金属。
Here are the three you must know.
这是你必须记住的三个例子。
Molten lead bromide gives silvery lead at the cathode, and red-brown bromine vapour at the anode.
熔化的溴化铅在阴极生成银色的铅， 在阳极生成红棕色的溴蒸气。
Concentrated sodium chloride solution gives hydrogen, and pale-green chlorine.
浓氯化钠溶液生成氢气，和淡绿色的氯气。
And dilute sulfuric acid gives hydrogen, and oxygen.
稀硫酸生成氢气，和氧气。
Memorise this products rule until it is automatic.
把这条产物规则背到成为本能。
Metals and hydrogen form at the cathode — the negative electrode.
金属和氢气在阴极生成，也就是负极。
Non-metals form at the anode — the positive electrode.
非金属在阳极生成，也就是正极。
Hydrogen is the only non-metal that appears at the cathode, because it can compete when water is present.
氢气是唯一会出现在阴极的非金属， 因为有水时它能参与竞争。
For every product question, first name the electrode, then apply the rule.
每道产物题，先说出电极，再套规则。
Lock in the three core examples with their observations.
用现象把三个核心例子锁牢。
Molten lead bromide: silvery lead liquid at the cathode, and red-brown bromine vapour at the anode.
熔化的溴化铅：阴极是银色的铅液，阳极是红棕色的溴蒸气。
Concentrated sodium chloride solution: bubbles of hydrogen at the cathode, and pale-green chlorine gas at the anode.
浓氯化钠溶液：阴极冒出氢气泡，阳极冒出淡绿色的氯气。
Dilute sulfuric acid: hydrogen bubbles at the cathode, and oxygen bubbles at the anode.
稀硫酸：阴极是氢气泡， 阳极是氧气泡。
Names plus colours or bubbles earn the observation marks.
既要会说名称，又要会说颜色或气泡，才能拿现象分。
For aqueous solutions, dilute or concentrated can change the product.
对水溶液来说，稀还是浓，会改变产物。
The three core examples already show this: concentrated sodium chloride gives chlorine at the anode, while dilute sulfuric acid gives oxygen.
三个核心例子已经说明这一点：浓氯化钠 在阳极生成氯气，而稀硫酸在阳极生成氧气。
Always read the question carefully.
永远仔细读题。
Is the electrolyte molten, or aqueous?
电解质是熔化的， 还是水溶液？
If aqueous, is it dilute or concentrated?
如果是水溶液，是稀还是浓？
State first, products second.
先看状态，再写产物。
Let's predict some products.
我们来预测一些产物。
Take molten zinc chloride.
以熔化的氯化锌为例。
Because it is molten, only two ions are present: zinc and chloride.
因为它是熔化的，只有两种离子： 锌离子和氯离子。
Zinc is a metal, so it forms at the cathode.
锌是金属，所以它在阴极生成。
Chlorine is a non-metal, so it forms at the anode.
氯是非金属，所以它在阳极生成。
Easy.
很简单。
But watch out with water.
但遇到水就要小心。
An aqueous solution brings water into the competition.
水溶液把水引入了竞争。
That is why concentrated sodium chloride solution gives hydrogen at the cathode — not sodium.
这就是为什么浓氯化钠溶液在阴极生成氢气—— 而不是钠。
Always check the state first: molten, or aqueous?
永远先检查状态：熔化的，还是水溶液？
At the electrodes, electrons are gained and lost.
在电极上，电子被得到和失去。
At the cathode, positive ions gain electrons — this is reduction.
在阴极，正离子得到电子——这是还原。
At the anode, negative ions lose electrons — this is oxidation.
在阳极，负离子失去电子——这是氧化。
Remember it with OIL RIG: oxidation is loss, reduction is gain.
用OIL RIG来记：氧化是失去，还原是得到。
We write a half-equation for each.
我们为每个电极写一个半反应式。
For molten lead bromide: at the cathode, a lead ion plus two electrons becomes lead.
对于熔化的溴化铅：在阴极，一个铅离子加两个电子变成铅。
At the anode, two bromide ions become bromine, plus two electrons.
在阳极，两个溴离子变成溴，加两个电子。
The atoms and the charges must both balance.
原子和电荷都必须配平。
Half-equations must balance both atoms and charges.
半反应式必须同时配平原子和电荷。
At the cathode for molten lead bromide: lead two-plus plus two electrons becomes lead.
熔化溴化铅在阴极：二价铅离子加两个电子变成铅。
Atoms match: one lead on each side.
原子匹配：两边各有一个铅。
Charges match: two-plus on the left cancels with two electrons.
电荷匹配：左边的二价正电荷与两个电子抵消。
At the anode: two bromide ions become one bromine molecule plus two electrons.
在阳极：两个溴离子变成一个溴分子，加两个电子。
Put the electrons on the correct side — gain on the left for reduction, loss on the right for oxidation.
把电子放在正确的一侧： 还原时得到电子写在左边，氧化时失去电子写在右边。
Now copper sulfate solution — a favourite exam topic.
现在看硫酸铜溶液，这是考试最爱考的题。
With inert carbon electrodes, copper forms at the cathode as a brown-red solid.
用惰性碳电极时，铜在阴极以棕红色固体生成。
Oxygen forms at the anode as bubbles.
氧气在阳极以气泡生成。
The blue colour of the solution slowly fades, because copper ions are removed from the solution and not replaced.
溶液的蓝色慢慢变淡，因为铜离子从溶液中被取走，又没有补充。
Inert electrodes only conduct; they do not dissolve.
惰性电极只导电；它们不溶解。
Change the electrodes to copper, and the story changes.
把电极换成铜，情况就变了。
Copper still forms at the cathode, coating it.
铜仍然在阴极生成，镀在电极上。
But now the copper anode itself dissolves into the solution, replacing the copper ions that plate out.
但现在铜阳极自己 溶解进入溶液，补充那些镀出来的铜离子。
The blue colour stays the same.
蓝色保持不变。
This is how impure copper is purified: pure copper builds up on the cathode, while impurities from the anode fall away or stay in solution.
这就是提纯不纯铜的方法： 纯铜在阴极上堆积，阳极上的杂质掉落或留在溶液中。
One very useful job for electrolysis: electroplating.
电解有一个非常有用的用途：电镀。
This coats a metal object with a thin layer of another metal, to make it look better and resist corrosion.
它给金属物体镀上一层薄薄的另一种金属， 让它更好看、更耐腐蚀。
The recipe is simple.
方法很简单。
Make the object you want to coat the cathode.
把你想镀的物体作为阴极。
Make the plating metal the anode.
把镀层金属作为阳极。
And use a solution containing ions of the plating metal.
使用含有镀层金属离子的溶液。
The plating metal dissolves from the anode, travels through the solution, and coats the object at the cathode.
镀层金属从阳极溶解，穿过溶液，在阴极镀到物体上。
Look at the electroplating set-up carefully.
仔细看电镀装置。
The object to coat sits as the cathode.
要镀的物体作为阴极。
A bar of the plating metal is the anode.
一根镀层金属棒作为阳极。
The liquid is a solution of ions of that same plating metal — often copper sulfate when plating with copper.
液体是同一种 镀层金属离子的溶液，镀铜时常用硫酸铜。
Metal dissolves from the anode, travels as ions, and plates onto the object.
金属从阳极溶解，以离子形式迁移， 再镀到物体上。
Appearance improves, and corrosion resistance improves too.
外观变好，抗腐蚀性也变好。
Finally, a clever relative: the hydrogen-oxygen fuel cell.
最后，一个巧妙的亲戚：氢氧燃料电池。
It combines hydrogen and oxygen to make electricity directly, and the only product is water.
它把氢和氧结合，直接产生电， 而唯一的产物是水。
Compare it to a petrol engine.
把它和汽油发动机比较。
The fuel cell is clean — only water comes out, while petrol makes carbon dioxide and pollutants.
燃料电池很清洁——只排出水， 而汽油会产生二氧化碳和污染物。
But hydrogen is hard and dangerous to store, and making it can still use fossil fuels.
但氢很难储存，也很危险， 而且制造氢仍然可能使用化石燃料。
Clean at the point of use, but not perfect.
使用时清洁，但并不完美。
A hydrogen-oxygen fuel cell is shown here.
这里画出了氢氧燃料电池。
Hydrogen is fed to one electrode, oxygen to the other.
氢气送入一个电极，氧气送入另一个电极。
An external circuit can run a motor.
外电路可以驱动电机。
The overall reaction is two hydrogen molecules plus one oxygen molecule making two water molecules.
总反应是两个氢分子加一个氧分子生成两个水分子。
Chemical energy becomes electricity directly, and the only chemical product leaving the cell is water.
化学能直接变成电能， 离开电池的唯一化学产物是水。
Compare a fuel cell with a petrol engine.
把燃料电池和汽油发动机比较。
The fuel cell's main product is water only, so it is clean at the vehicle.
燃料电池的主要产物只有水，所以在车辆处很清洁。
A petrol engine makes carbon dioxide and other pollutants.
汽油发动机会产生二氧化碳和其他污染物。
Petrol is easy to store; hydrogen is flammable, needs high pressure, and is hard and dangerous to store.
汽油容易储存；氢易燃，需要高压， 又难又危险。
Hydrogen may still be made using fossil fuels, so the whole chain is not always green.
氢仍可能用化石燃料来制造，所以整条链条并不总是绿色的。
Advantages: clean at use, and efficient.
优点：使用时清洁，而且效率高。
Disadvantages: storage and how the hydrogen was made.
缺点：储存，以及氢是怎么造出来的。
Three marks to lock in.
锁住三个分。
First: at the cathode, negative, you get a metal or hydrogen; at the anode, positive, a non-metal.
第一：在阴极（负极），你得到金属或氢气；在阳极（正极），得到非金属。
Positive ions go to the cathode, negative ions to the anode.
正离子去阴极，负离子去阳极。
Second: a molten compound gives its two elements, but an aqueous solution can give something different — concentrated sodium chloride gives hydrogen, not sodium.
第二：熔化的化合物生成它的两种元素， 但水溶液可能生成不同的东西——浓氯化钠生成氢气，而不是钠。
Third: reduction is gain of electrons at the cathode; oxidation is loss at the anode — OIL RIG.
第三：还原是在阴极得到电子； 氧化是在阳极失去电子——OIL RIG。
Master these, and this topic is yours.
掌握这些，这个专题就是你的了。