Equilibria
A-Level Chemistry Topic 7 10:12 English narration · English + 中文 subtitles burned in
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Some reversible reactions never finish.
有些反应永远不会结束。
You mix the reactants, the colour changes, and then everything just stops — halfway.
你把反应物混合，颜色变了，然后一切就停在了半路。
Leave it for a week and it looks exactly the same.
放一个星期，看上去还是一模一样。
But nothing has actually stopped.
但其实什么都没有停下来。
Underneath that still surface, both reactions are still running, at full speed, in opposite directions.
在这片静止的表面之下，两个方向的反应都仍在全速进行。
Understand that idea, and you can predict and control the outcome of almost any industrial reaction.
理解了这个概念，你几乎可以预测并控制任何工业反应的结果。
Welcome to equilibria.
欢迎来到化学平衡。
We will define dynamic equilibrium, learn to shift it on demand, put a number on it, and then use all of that on acids and bases.
我们将给动态平衡下定义，学会按需移动平衡， 再给它赋上一个数值，最后把这一切用到酸和碱上。
Let's begin.
让我们开始吧。
Watch the two rates.
看这两条速率曲线。
At the start there is plenty of reactant, so the forward reaction is fast; there is no product yet, so the reverse reaction is zero.
开始时反应物很多，正反应很快；还没有产物， 所以逆反应速率为零。
As reactant is used up the forward rate falls, and as product builds up the reverse rate climbs.
随着反应物被消耗，正反应速率下降； 随着产物积累，逆反应速率上升。
Sooner or later the two curves meet.
迟早两条曲线会相遇。
From that moment on, every molecule converted forwards is matched by one converted back — so the concentrations stop changing.
从那一刻起， 每有一个分子正向转化，就有一个分子逆向转化回来——所以浓度不再改变。
So here is the definition, and the exam wants all three parts.
这就是定义，考试三点都要写。
First, the forward and reverse rates are equal.
第一，正反应和逆反应的速率相等。
Second, the concentrations stay constant — constant, not equal; that is a favourite trap.
第二，各物质的浓度保持不变——是"不变"，不是"相等"，这是最常见的陷阱。
And third, the system must be closed, because if the products can escape, the reverse reaction can never catch up and equilibrium is never reached.
第三，系统必须是封闭的，因为如果产物能跑掉，逆反应就永远追不上， 也就永远达不到平衡。
It is called dynamic precisely because both reactions are still happening — they simply cancel out.
它之所以叫"动态"，正是因为两个反应仍在进行—— 只是彼此抵消了。
Now push the system and see what it does.
现在推一推这个系统，看它怎么反应。
Le Chatelier's principle says: if you change a system that is at equilibrium, the position of equilibrium moves in the direction that opposes your change.
勒夏特列原理指出： 如果你改变一个已达平衡的系统，平衡的位置就会向抵消这个改变的方向移动。
Add more reactant, and it consumes some of it. Squeeze the system, and it moves to take up less room.
加入更多反应物，它就消耗掉一部分；压缩系统，它就往占据更小体积的方向移动。
The system always pushes back.
系统总是会"顶回来"。
Four changes, four answers, and you should know them cold.
四种改变，四个答案，必须烂熟于心。
Raise the concentration of a reactant, and the equilibrium shifts towards the products.
提高某种反应物的浓度，平衡向生成物方向移动。
Raise the pressure, and it shifts towards whichever side has fewer gas molecules.
提高压强，平衡向气体分子数较少的一侧移动。
Raise the temperature, and it shifts in the endothermic direction — the direction that absorbs the extra heat.
升高温度，平衡向吸热方向移动—— 也就是吸收这份多余热量的方向。
And if you add a catalyst, nothing shifts at all. A catalyst speeds up the forward and reverse reactions by exactly the same amount, so equilibrium arrives sooner, but in the very same place.
而如果加入催化剂，平衡根本不会移动： 催化剂把正反应和逆反应加快相同的倍数，所以平衡来得更快，但位置完全不变。
Now let's put a number on the position of equilibrium.
现在给平衡的位置赋上一个数值。
The equilibrium constant is written products over reactants, with each concentration raised to its balancing number from the equation.
平衡常数写成"生成物比反应物"， 每一项浓度都要按方程式中的配平系数取幂。
Square brackets mean concentration in moles per cubic decimetre.
方括号表示浓度， 单位是摩尔每立方分米。
A large value means the equilibrium lies well over towards the products; a small value means it barely moves at all.
数值很大，说明平衡强烈地偏向生成物一侧； 数值很小，说明平衡几乎不移动。
One warning: the units are not automatic. Work them out each time from the powers — sometimes they cancel completely and there are no units.
一个提醒：单位不是自动的， 每次都要根据指数算出来——有时它们完全抵消，就没有单位。
Here is the standard question.
这是标准题型。
One mole each of hydrogen and iodine are sealed in a one cubic decimetre flask. At equilibrium, zero point two zero moles of hydrogen remain.
把各一摩尔的氢气和碘蒸气密封在一立方分米的容器里， 平衡时还剩零点二零摩尔氢气。
Set out an ICE table: initial, change, equilibrium.
列一张 ICE 表：初始、变化、平衡。
Initially one, one and zero.
初始分别是一、一和零。
The change line does the work: hydrogen fell by zero point eight zero, so iodine also fell by zero point eight zero, and because the equation makes two of them, hydrogen iodide rose by one point six zero.
"变化"这一行最关键：氢气减少了零点八零， 所以碘也减少零点八零；而由于方程式生成两个，碘化氢增加了一点六零。
Put the equilibrium row into the expression — so the constant is sixty-four, with no units.
把平衡这一行代入表达式——平衡常数就是六十四，没有单位。
For gases we work in pressures instead.
对于气体，我们通常改用压强。
The partial pressure of a gas is the share of the total pressure that this gas provides, and you find it from the mole fraction — the fraction of all the moles that are that gas — times the total pressure.
某气体的分压，就是它在总压强中所占的那一份， 可以用摩尔分数——它的物质的量占总量的比例——乘以总压强求得。
Two moles of nitrogen with six of hydrogen at two hundred kilopascals: nitrogen is two eighths of the mixture, so fifty kilopascals; hydrogen is six eighths, so one hundred and fifty.
两摩尔氮气和六摩尔氢气，总压二百千帕：氮气占八分之二，所以是五十千帕； 氢气占八分之六，所以是一百五十千帕。
Now a numeric Kp.
再看一个 Kp 的算例。
For nitrogen plus three hydrogen giving two ammonia, with partial pressures of twenty, forty and ten kilopascals, Kp is ten squared, over twenty times forty cubed.
对于氮气加三倍氢气生成两倍氨气，若各气体的分压分别是二十、四十和十千帕， Kp 就是十的平方除以二十乘四十的立方。
That works out at seven point eight times ten to the minus five, with units from the powers: per kilopascal squared.
算出来大约是七点八乘十的负五次方， 单位由指数决定，是每千帕平方分之一。
And the fact examiners love: only temperature changes the value of the constant.
还有考官最爱考的一点： 只有温度会改变平衡常数的数值。
Concentration and pressure shift the position, and a catalyst changes neither.
浓度和压强只移动平衡的位置， 而催化剂两者都不改变。
Now use all of that on a real factory.
现在把这些用到真实的工厂里。
The Haber process makes ammonia from nitrogen and hydrogen, and it is exothermic with fewer molecules on the right.
哈伯法用氮气和氢气合成氨， 该反应放热，且右侧的气体分子数更少。
What does the equilibrium want?
平衡"想要"什么？
A low temperature, because the forward reaction is exothermic, and a high pressure, because the product side has fewer molecules.
想要低温， 因为正反应放热；还想要高压，因为生成物一侧分子数更少。
But industry cannot wait: at a low temperature the rate would be hopelessly slow, and very high pressures need enormously expensive equipment.
但工业等不起：温度太低，速率会慢得没法用；而极高的压强需要极其昂贵的设备。
So the answer is a compromise — about four hundred and fifty degrees, about two hundred atmospheres, and an iron catalyst to recover the lost speed.
所以答案是一个折中——大约四百五十摄氏度、大约二百个大气压， 再加上铁催化剂来把损失的速率补回来。
The syllabus names a second industrial equilibrium, and it makes a much better exam question than Haber, because it looks the same and is not.
考纲还点名了第二个工业平衡，而它其实比哈伯法更适合出题， 因为它看起来一样，实际却不一样。
The Contact process turns sulfur dioxide and oxygen into sulfur trioxide, on the way to sulfuric acid.
接触法把二氧化硫和氧气变成三氧化硫，再进一步制硫酸。
Like the Haber reaction it is exothermic, and like Haber it has fewer gas molecules on the right — three become two.
和哈伯反应一样，它是放热的；也和哈伯一样，右边的气体分子更少——三变二。
So the equilibrium wants exactly the same things: cool, and squeezed.
所以平衡想要的完全是同样两件事：低温，加压。
And the conditions look almost the same at first: about four hundred and fifty degrees, the same compromise between yield and rate, with vanadium five oxide as the catalyst instead of iron.
而且条件乍看也差不多：大约四百五十摄氏度， 同样是产率与速率之间的折中，只是催化剂用五氧化二钒，而不是铁。
But look at the pressure.
但看压强。
Haber runs at two hundred atmospheres.
哈伯法在两百个大气压下运行。
The Contact process runs at one to two — barely above atmospheric.
接触法只用一到二个大气压——几乎就是常压。
Why, when Le Chatelier says pressure would help?
勒夏特列原理明明说加压有利，为什么不加？
Because at one atmosphere the yield of sulfur trioxide is already about ninety-six percent.
因为在一个大气压下，三氧化硫的产率已经大约是百分之九十六了。
There is almost nothing left to gain, and compressing gas to two hundred atmospheres is enormously expensive.
几乎没有什么可再争取的，而把气体压到两百个大气压极其昂贵。
So the plant does not buy it.
所以工厂根本不买这笔账。
That is the real lesson of both processes: Le Chatelier tells you which direction helps, but economics decides how far it is worth going.
这才是两个流程共同的真正教训： 勒夏特列告诉你哪个方向有利，但经济账决定值不值得走那么远。
Acids and bases are equilibria too, so this belongs here.
酸和碱本身也是平衡问题，所以它属于这一章。
A proton is just a hydrogen ion.
质子就是氢离子。
An acid is a proton donor: it gives a hydrogen ion away.
酸是质子供体：它把氢离子给出去。
A base is a proton acceptor: it takes one.
碱是质子受体：它把氢离子接过来。
Hydrogen chloride donates its proton to water, making a hydroxonium ion and a chloride ion.
氯化氢把质子给了水，生成水合氢离子和氯离子。
Now notice the pattern. Each side has one species that has the proton and one that does not — those are conjugate acid-base pairs, and they differ by exactly one hydrogen ion.
现在注意这个规律： 每一侧都有一个"带着质子"的和一个"不带质子"的物种—— 它们就是共轭酸碱对，彼此正好相差一个氢离子。
To identify them, just look for the hydrogen ion that moved.
要找出它们， 只要看那个移动了的氢离子就行。
And learn the common ones by name: hydrochloric, sulfuric, nitric and ethanoic acid; and for the alkalis — the bases that dissolve in water — sodium hydroxide, potassium hydroxide and ammonia.
还要记住常见的酸和碱的名称： 盐酸、硫酸、硝酸和乙酸；碱方面——也就是能溶于水的碱—— 有氢氧化钠、氢氧化钾和氨。
Strong or weak describes how fully an acid splits up in water.
"强"和"弱"描述的是酸在水中电离得有多完全。
A strong acid is fully dissociated — almost every molecule has broken into ions.
强酸完全电离—— 几乎每个分子都拆成了离子。
A weak acid is only partly dissociated, so most molecules are still whole, sitting in their own little equilibrium.
弱酸只部分电离，所以大多数分子仍是完整的， 处在它们自己的小平衡之中。
This is not the same thing as concentrated or dilute, and mixing those two up is the most common error in the whole topic.
这与"浓"和"稀"完全不是一回事， 把这两者混为一谈，是整章里最常见的错误。
You can tell them apart three ways: at the same concentration, the strong acid has the lower pH on universal indicator, fizzes faster in reaction with a reactive metal, and has higher electrical conductivity — because it has more ions.
可以用三种方法区分它们： 在相同浓度下，强酸的 pH 更低、与金属反应冒泡更快、导电性也更好—— 因为它有更多离子。
Put an acid and a base together and you get neutralisation: the hydrogen ions from the acid meet the hydroxide ions from the alkali and a salt is also formed.
把酸和碱放在一起，就发生中和：酸里的氢离子遇上碱里的氢氧根离子， 同时还生成一种盐。
Follow the pH as you add one to the other. You get a titration curve.
一边滴加一边跟踪 pH，就得到一条滴定曲线。
It starts almost flat, then rises through a steep, nearly vertical jump, then flattens off again.
它开始几乎是平的，然后经过一段陡峭、近乎垂直的跃升，之后又变平。
That steep section is the end point — the moment the acid has just been used up.
那段陡峭的部分就是终点——酸刚好被消耗完的那一刻。
A tiny extra drop swings the pH by several units.
再多滴一滴，pH 就会跳好几个单位。
Which tells you how to choose an indicator.
这就告诉你该怎么选指示剂。
Every indicator changes colour over its own narrow pH range.
每种指示剂都在它自己一个很窄的 pH 范围内变色。
For the colour change to mark the end point sharply, that range must fall inside the steep jump of the curve.
要让变色恰好标出终点，这个范围必须落在曲线陡峭跃升的区间之内。
A strong acid with a strong base gives a huge jump, so almost any indicator works.
强酸与强碱的跃升很大，所以几乎任何指示剂都能用。
But the weak-acid jump is shorter and sits higher up the pH scale, so you need an indicator that changes in that higher range — phenolphthalein, not methyl orange.
但弱酸的跃升更短，而且位置更靠上，所以你需要在那个较高范围变色的指示剂—— 用酚酞，而不是甲基橙。
Three marks students throw away.
三个学生常丢的分。
First, when you define dynamic equilibrium, give all three parts: rates equal, concentrations constant, closed system.
第一，给动态平衡下定义时，三点都要写： 速率相等、浓度不变、封闭系统。
Second, a catalyst never shifts the position of equilibrium — it only gets you there faster, by speeding both directions equally.
第二，催化剂绝不移动平衡的位置—— 它只是让你更快到达，因为它把两个方向同等地加快。
Third, when you justify the Haber or Contact conditions, call the conditions a compromise and say what is being traded: yield against rate against cost.
第三，说明哈伯法或接触法的条件时，要说这是"折中"， 并写出在权衡什么：产率、速率与成本。
Quoting the numbers alone earns nothing.
只背出数字是拿不到分的。