Nitrogen and sulfur
A-Level Chemistry Topic 12 7:40 English narration · English + 中文 subtitles burned in
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Take a breath.
深吸一口气。
Nearly four fifths of what just went into your lungs was nitrogen — and it came straight back out again, completely unchanged.
你刚吸进肺里的气体，将近五分之四是氮气——而它又原封不动地被呼了出来。
Nitrogen is everywhere, and it reacts with almost nothing.
氮气无处不在，却几乎不与任何东西反应。
Yet every protein in your body is built from nitrogen atoms, and a field of wheat can starve without them.
可是你身体里的每一种蛋白质都由氮原子构成， 一整片麦田也会因为缺氮而长不好。
All that nitrogen is locked inside one of the strongest bonds in chemistry.
所有这些氮，都被锁在化学中最强的化学键之一里面。
Today: how it is locked, and what happens when it breaks open inside a hot engine.
今天我们来看：它是怎样被锁住的，以及当它在高温发动机里被打开时会发生什么。
This is nitrogen and sulfur.
这一讲是氮和硫。
We will explain why nitrogen is so unreactive, meet ammonia and the ammonium ion, and then follow the oxides of nitrogen out of a car engine and into smog and acid rain.
我们会解释氮气为什么这么不活泼，认识氨和铵离子， 然后跟着氮氧化物从汽车发动机一路走到烟雾和酸雨。
So why is nitrogen so unreactive?
那么氮气为什么这么不活泼？
There are two reasons, and the exam wants both.
有两个原因，考试两个都要写。
First, the two atoms are held together by a triple bond — three shared pairs of electrons.
第一，两个氮原子之间是三键 ——三对共用电子。
Breaking it costs nine hundred and ninety-four kilojoules for every mole, far more than the double bond in oxygen or the single bond in chlorine.
要断开它，每摩尔需要九百九十四千焦，远远高于氧气里的双键或氯气 里的单键。
It is the strongest bond you will meet at this level.
这是你在这个阶段会遇到的最强的化学键。
Second, the molecule is perfectly symmetrical, so it has no polarity — it is non-polar. There is no positive end and no negative end, so nothing is pulled towards it.
第二，这个分子完全对称，所以没有 极性——它是非极性的：没有正端也没有负端，什么都不会被它吸引。
That strength is exactly why ammonia is so hard to make.
正因为这么牢固，制氨才这么困难。
In the Haber process, nitrogen and hydrogen are forced together over an iron catalyst, at about four hundred and fifty degrees and two hundred atmospheres.
在哈伯法中，氮气和氢气在铁催化剂上被强行结合， 温度约四百五十摄氏度，压强约二百个大气压。
Those hard conditions are the price of the triple bond — and the ammonia they make becomes the fertiliser our crops need.
这些苛刻的条件，就是三键的代价—— 而制得的氨，会变成农作物所需要的氮肥。
Ammonia is the nitrogen compound you must know, and its chemistry comes from one thing: the lone pair of electrons on the nitrogen atom.
氨是你必须掌握的氮的化合物，它的化学性质来自一件事：氮原子上的那对孤对电子。
Nitrogen shares three of its outer electrons with hydrogen atoms; the last two stay behind as a pair that belongs to nitrogen alone.
氮把三个外层电子与氢共用，剩下的两个留下来，成为只属于氮的一对电子。
In the Bronsted and Lowry theory, a base is a proton acceptor — something that takes a hydrogen ion.
按照布朗斯特-劳里理论，碱是质子受体——能接受氢离子的物质。
A hydrogen ion has no electrons of its own, so that lone pair is exactly what it needs.
氢离子自己没有电子， 所以这对孤对电子正是它需要的。
Ammonia gives it a home, and that is why ammonia is a base — its basicity comes from that lone pair.
氨给了它一个位置，这就是氨显碱性的原因—— 它的碱性就来自那对孤对电子。
Watch what that lone pair actually does.
看看这对孤对电子究竟做了什么。
A hydrogen ion is a bare proton, with an empty orbital and no electrons at all.
氢离子就是一个裸露的质子，只有一个空轨道， 完全没有电子。
The lone pair moves in and becomes the bonding pair, so both of the shared electrons came from the nitrogen.
孤对电子移过去，变成成键电子对——共用的两个电子都来自氮。
A bond made that way has its own name: a coordinate bond, also called a dative bond.
这样形成的键有自己的名字：配位键，也叫配位共价键。
Now the part students miss. Once the ion has formed, all four bonds are identical — you cannot tell which one the lone pair made — and the ammonium ion is a perfect tetrahedron.
下面是学生最容易忽略的一点： 铵离子一旦形成，四个键完全相同，你分不出哪一个是孤对电子形成的， 而且它是一个规则的四面体。
Now a reaction you will be asked to write out.
接下来是一个必须会写的反应。
Warm an ammonium salt with sodium hydroxide and ammonia gas is driven off — a displacement — because hydroxide is the stronger base and takes the proton back.
把铵盐与氢氧化钠一起加热，就会放出氨气——这是置换—— 因为氢氧根是更强的碱，把质子夺了回去。
The ionic equation is the one worth learning, because it works for every ammonium salt.
真正值得记的是离子方程式， 因为它对任何铵盐都适用。
You detect the gas two ways: the sharp smell, and damp red litmus paper turning blue.
检验这种气体有两种方法：刺激性气味， 以及湿润的红色石蕊试纸变蓝。
Say damp.
一定要说"湿润"。
Dry paper shows you nothing, because the ammonia has to dissolve in the water before it can act as a base.
干的试纸什么也看不到， 因为氨必须先溶在水里，才能表现出碱性。
Let's use all of that.
我们来用一用。
A white solid is warmed with aqueous sodium hydroxide, and a gas comes off that turns damp red litmus blue.
把一种白色固体与氢氧化钠溶液一起加热， 放出的气体使湿润的红色石蕊试纸变蓝。
Name the gas, name the ion in the solid, and explain the reaction.
说出这种气体、固体中的离子，并解释这个反应。
Pause the video and try it.
先暂停视频，自己试一试。
Ready?
好了吗？
Only one common gas turns damp red litmus blue, and that gas is ammonia.
能使湿润红色石蕊变蓝的常见气体只有一种，就是氨。
So the white solid must contain the ammonium ion.
所以这种白色固体一定含有铵离子。
Hydroxide is the stronger base, so it takes the proton from the ammonium ion, giving ammonia and water.
氢氧根是更强的碱，它从铵离子那里夺走质子， 生成氨和水。
Write that ionic equation, say the paper was damp, and the marks are yours.
写出这个离子方程式，说明试纸是湿润的，分数就到手了。
Nitrogen will react, but only when something gives it enough energy — and two things do.
氮确实会反应，但前提是有足够的能量——有两件事能做到。
Lightning heats the air so fiercely that nitrogen and oxygen combine directly; that is the natural source.
闪电把空气加热到极高的温度， 让氮气和氧气直接化合，这是天然来源。
The man-made source is the important one: inside an internal combustion engine the burning fuel goes past two thousand degrees, and the same reaction happens there.
人为来源更重要：内燃机内部燃料燃烧时 温度超过两千摄氏度，同样的反应就在那里发生。
Both make nitrogen monoxide.
两者都生成一氧化氮。
Out in the air, nitrogen monoxide meets more oxygen and is quickly oxidised to nitrogen dioxide, a choking brown gas.
到了空气中， 一氧化氮再遇到氧气，很快被氧化成二氧化氮——一种呛人的棕色气体。
So we clean them up before they leave the car.
所以我们要在废气离开汽车之前把它们处理掉。
A catalytic converter sits in the exhaust pipe: a ceramic honeycomb coated with platinum and rhodium, giving an enormous surface area.
催化转化器装在排气管里： 一块陶瓷蜂窝，表面镀有铂和铑，具有极大的表面积。
Nitrogen monoxide and carbon monoxide meet on that surface and react with each other.
一氧化氮和一氧化碳在这个表面上 相遇并互相反应。
The nitrogen monoxide is reduced back to ordinary nitrogen; the carbon monoxide is oxidised to carbon dioxide.
一氧化氮被还原成普通的氮气，一氧化碳被氧化成二氧化碳。
Two poisons go in, two harmless gases come out, and the catalyst itself is unchanged.
两种有毒气体进去，两种无害气体出来，而催化剂本身没有变化。
Not everything is caught.
并不是所有的都被处理掉了。
In strong sunlight, the oxides of nitrogen attack unburned hydrocarbons from the same exhaust, and one of the products is peroxyacetyl nitrate, PAN for short.
在强烈的阳光下，氮氧化物会与同样来自尾气的未燃烃反应， 产物之一是过氧乙酰硝酸酯，简称 PAN。
PAN is a major part of photochemical smog: the brown haze that sits over a busy city on a hot, still day.
PAN 是光化学烟雾的主要成分： 在炎热无风的日子里笼罩在繁忙城市上空的棕色雾霾。
It needs three things together — traffic fumes, strong sunshine, and air that is not moving.
它需要三样东西同时出现—— 交通尾气、强烈日照，以及不流动的空气。
The same gases also make acid rain, and they do it by two different roads.
同样这些气体还会造成酸雨，而且有两条不同的途径。
The direct road is simple: nitrogen dioxide dissolves in rain water and, with oxygen, forms nitric acid.
直接的一条很简单： 二氧化氮溶在雨水里，再与氧气一起生成硝酸。
The second road belongs to sulfur.
第二条属于硫。
Coal and oil contain sulfur, and burning them puts sulfur dioxide into the air.
煤和石油中含有硫， 燃烧时把二氧化硫排到空气中。
That sulfur dioxide is oxidised to sulfur trioxide, which dissolves in rain as sulfuric acid — and nitrogen dioxide speeds that oxidation up.
二氧化硫被氧化成三氧化硫，溶在雨水里就是硫酸—— 而二氧化氮会加快这个氧化过程。
Saying it speeds things up is worth a mark, so look at how.
说它"加快"是可以得分的，但要说清楚是怎么加快的。
In the first step, nitrogen dioxide hands an oxygen atom to sulfur dioxide: sulfur trioxide is made, and the nitrogen dioxide is used up, becoming nitrogen monoxide.
第一步，二氧化氮把一个氧原子 交给二氧化硫：生成三氧化硫，二氧化氮被消耗，变成一氧化氮。
In the second step, that nitrogen monoxide meets oxygen in the air and turns straight back into nitrogen dioxide.
第二步， 这个一氧化氮在空气中遇到氧气，又直接变回二氧化氮。
Used in one step, made again in a later one.
在一步中被消耗， 在后一步中又被生成。
Add the two steps together and the nitrogen dioxide cancels out; it never appears in the overall equation.
把两步相加，二氧化氮被抵消掉，它根本不出现在总方程式里。
That is exactly what a catalyst does.
这正是催化剂的作用。
Three marks students throw away.
三个学生常丢的分。
First, when you explain why nitrogen is unreactive, give both reasons: the strong triple bond with its very high bond energy, and the non-polar molecule.
第一，解释氮气为什么不活泼时，两个原因都要给： 键能很高的牢固三键，以及非极性的分子。
One reason on its own is half the marks.
只写一个原因，只能拿一半分。
Second, trace everything ammonia does back to the lone pair — it is why ammonia is a base, and why the ammonium ion contains a coordinate bond.
第二，把氨的一切行为都追溯到那对孤对电子——它是氨显碱性的原因， 也是铵离子中存在配位键的原因。
Third, in the test for an ammonium salt, write the word damp, and give the ionic equation, not just a description.
第三，检验铵盐时，一定要写上"湿润"， 并给出离子方程式，而不只是文字描述。