Electrochemistry & Redox
A-Level Chemistry Topic 6 6:34 English narration · English + 中文 subtitles burned in
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Transcript
Two pieces of iron.
两块铁。
One is quietly rusting on a fence.
一块正静静地在栅栏上生锈，另一块在电池里，把电子推过你的手机。
The other sits inside a battery, pushing electrons through your phone.
它们看起来完全不同——一个是腐朽，一个是动力。
They look like completely different things — one is decay, one is power.
但从化学上说，它们是同一个过程：电子转移——电子从一种物质转移到另一种物质。
But chemically they are the same process: electron transfer from one substance to another.
化学家用一个"记账数字"来追踪这些电子。
Chemists keep track of those electrons with one bookkeeping number, and once you can assign it, this whole topic stops being guesswork and becomes arithmetic.
一旦你会给它赋值， 这一整章就不再靠猜，而变成了算术。
Welcome to electrochemistry.
欢迎来到电化学。
We will learn the oxidation number, use it to spot exactly what is oxidised and what is reduced, and then use it to balance the hardest equations on the whole paper.
我们将学习氧化数，用它准确判断什么被氧化、什么被还原， 再用它去配平整张试卷上最难的方程式。
Let's begin.
让我们开始吧。
Start with the tool itself.
先看这个工具本身。
The oxidation number tells you how many electrons an atom has lost or gained, compared with the free element.
氧化数告诉你：与游离态单质相比，一个原子失去或得到了多少电子。
It is not a real charge — it is bookkeeping.
它不是真实电荷——它只是"记账"。
Seven rules give you every value you will ever need.
七条规则能给出你需要的所有数值。
An uncombined element is zero.
未化合的单质是零。
A simple ion equals its charge.
简单离子等于它的电荷。
Group one is always plus one, and group two is always plus two.
第一族永远是正一，第二族永远是正二。
Hydrogen is plus one — except in metal hydrides, where it is minus one.
氢是正一——但在金属氢化物中是负一。
Oxygen is minus two — except in peroxides, where it is minus one.
氧是负二——但在过氧化物中是负一。
Fluorine is always minus one.
氟永远是负一。
And then the last rule does the work: in a neutral compound all the numbers add up to zero, and in an ion they add up to the charge.
最后一条规则最管用：在中性化合物中，所有数字加起来等于零； 在离子中，加起来等于该离子的电荷。
Use that last rule, and you can find any unknown in seconds.
用最后那条规则，你几秒钟就能求出任何未知数。
Take the sulfate ion. Each oxygen is minus two, and there are four of them, so together they contribute minus eight. The whole ion carries a charge of minus two.
以硫酸根离子为例： 每个氧是负二，共有四个，合起来贡献负八；整个离子带负二电荷。
So sulfur, plus minus eight, must equal minus two — so sulfur must be plus six.
所以硫加上负八必须等于负二——因此硫是正六。
Now the nitrate ion. Three oxygens give minus six, and the charge is minus one, so nitrogen minus six equals minus one, and nitrogen is plus five.
再看硝酸根离子： 三个氧给出负六，电荷是负一，所以氮减去六等于负一，氮是正五。
The method never changes: fix everything you know, then solve for the one thing you don't.
方法始终不变：先把已知的都定下来，再解出那个未知的。
Now watch what those numbers are really tracking.
现在来看这些数字究竟在追踪什么。
Drop a strip of zinc into copper sulfate solution.
把一片锌放进硫酸铜溶液里。
Each zinc atom gives up two electrons and goes into the solution as a zinc ion. Each copper ion takes two electrons and plates out as copper metal on the strip.
每个锌原子交出两个电子，以锌离子的形式进入溶液； 每个铜离子接过两个电子，以铜单质的形式镀在锌片上。
Nothing else happened at all — electrons simply moved across.
其他什么也没发生——只是电子转移了过去。
That is a redox reaction, and the oxidation numbers record it perfectly: zinc climbs from zero up to plus two, and copper drops from plus two down to zero.
这就是氧化还原反应， 而氧化数把它记录得清清楚楚：锌从零升到正二，铜从正二降到零。
So here are the two definitions, and one mnemonic that locks them together.
这就是两个定义，还有一个把它们锁在一起的口诀。
Oxidation is the loss of electrons, and the oxidation number goes up. Reduction is the gain of electrons, and the oxidation number goes down.
氧化是失去电子，氧化数升高；还原是得到电子，氧化数降低。
Remember it as OIL RIG: Oxidation Is Loss, Reduction Is Gain. And notice that they always come as a pair.
记成"OIL RIG"：氧化是失去，还原是得到。
The electrons that one species loses are exactly the electrons another species gains, so you can never have oxidation without reduction happening somewhere in the same equation.
还要注意，它们总是成对出现： 一种物质失去的电子，正好就是另一种物质得到的电子。 所以在同一个方程式里，绝不可能只有氧化而没有还原。
Which brings us to agents — and this is exactly where students lose marks.
这就说到"剂"了——学生恰恰在这里丢分。
An oxidising agent takes electrons away from something else. But by taking them, it is itself reduced.
氧化剂从别的物质那里夺走电子； 但正因为夺走了电子，它自己被还原。
A reducing agent gives electrons to something else, so it is itself oxidised.
还原剂把电子给别的物质， 所以它自己被氧化。
Read that twice, because the agent always does the opposite of what its own name suggests.
这句话要读两遍，因为"剂"的作用总是与它的名字相反。
Back to zinc and copper: the zinc gives electrons away, so zinc is the reducing agent; the copper ion takes them, so the copper ion is the oxidising agent.
回到锌和铜：锌给出电子，所以锌是还原剂；铜离子夺取电子，所以铜离子是氧化剂。
Oxidation numbers do one more job, and it is the one worth the most marks: they balance equations that otherwise look impossible.
氧化数还有一个用途，而且是最值分的一个：配平那些看起来根本配不平的方程式。
There are three steps.
分三步。
First, find the element whose number rises — that one is oxidised — and the element whose number falls — that one is reduced.
第一，找出氧化数升高的元素——它被氧化——和氧化数降低的元素——它被还原。
Second, make the total rise equal the total fall, because every electron lost has to be gained somewhere.
第二，让总升高量等于总降低量，因为每个失去的电子都必须在某处被得到。
That single condition fixes the ratio for you.
仅这一个条件就替你定下了系数比。
And finally, balance the oxygen with water molecules, and the hydrogen with hydrogen ions.
最后，用水分子配平氧，用氢离子配平氢。
Let's do the classic one. Manganate seven reacting with iron two, in acid solution.
我们来做那道经典题：高锰酸根在酸性溶液中与亚铁离子反应。
Manganese falls from plus seven all the way down to plus two — a fall of five. Iron rises from plus two to plus three — a rise of only one.
锰从正七一路降到正二——降了五；铁从正二升到正三——只升了一。
So we need five iron ions for every single manganate ion, and now the totals match: five up, five down.
所以每一个高锰酸根离子需要五个亚铁离子，这样总量就对上了：升五、降五。
Balance the oxygen with four water molecules, then balance the hydrogen with eight hydrogen ions.
再用四个水分子配平氧，用八个氢离子配平氢。
Check the charges on both sides agree, and the equation is finished.
检查两边电荷相等，方程式就完成了。
One special case has a name of its own, and it appears often. Disproportionation is a redox reaction in which the same element is oxidised and reduced at the same time. Chlorine in cold water does exactly this.
有一种特殊情况有自己的名字，而且经常出现：歧化反应， 指同一种元素在同一个反应中同时被氧化和被还原。
The chlorine starts at zero. Some of it goes up to plus one, in the chlorate one ion. The rest goes down to minus one, as chloride.
氯在冷水中正是如此： 氯原来是零，一部分升到正一，生成次氯酸根；另一部分降到负一，成为氯离子。
One element, moving in both directions at once.
同一种元素，同时向两个方向变化。
If a question asks you to explain it, show both oxidation-number changes — quoting only one of them will not score the mark.
如果题目要求解释， 两处氧化数的变化都要写出来——只写一处是拿不到分的。
Three marks students throw away.
三个学生常丢的分。
First, the two exceptions. Oxygen is minus two except in peroxides, and hydrogen is plus one except in metal hydrides — examiners test those two deliberately, every year.
第一，两个例外：氧是负二，但过氧化物中是负一； 氢是正一，但金属氢化物中是负一——考官每年都故意考这两点。
Second, an oxidising agent is itself reduced.
第二，氧化剂自身被还原。
Name what is oxidised, name what is reduced, and name which species is the agent — all three, or you lose part of the mark.
要写出什么被氧化、什么被还原、哪一种是"剂"—— 三样都要写，否则会扣分。
Third, when you balance by oxidation number, state clearly that the total increase equals the total decrease, then balance the oxygen with water and the hydrogen with hydrogen ions.
第三，用氧化数配平时， 要明确写出总升高量等于总降低量，然后用水配平氧、用氢离子配平氢。