Equilibrium
AP Chemistry Topic 7 8:24 English narration · English + 中文 subtitles burned in
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Transcript
A chemical plant at dusk.
黄昏中的一座化工厂。
Inside it, nitrogen from the air is turned into ammonia — the fertiliser that feeds about half the people alive today.
在它内部，空气中的氮被转化成氨——这种化肥养活了今天大约一半的人口。
But the nitrogen and the hydrogen never react completely.
但氮和氢从来不会完全反应。
They reach a balance and stop.
它们达到一种平衡，然后停下。
So chemists learned how to move that balance.
于是化学家学会了如何移动这个平衡。
That is this whole unit.
这就是本单元的全部内容。
Unit Seven, in six steps.
第七单元，分六步。
What equilibrium really is.
平衡到底是什么。
The two numbers that decide which way a reaction runs.
决定反应往哪个方向进行的两个数。
ICE tables, for the arithmetic.
ICE 表，用来算数。
What a very large or very small constant tells you.
常数很大或很小时说明了什么。
Le Chatelier's principle.
勒沙特列原理。
And solubility — the same idea again, in a beaker.
最后是溶解度——同一个思想，只不过换到了烧杯里。
Start with pure reactant in a reversible reaction.
从可逆反应中的纯反应物开始。
At first only the forward reaction can happen, so it is fast.
起初只有正反应能发生，所以它很快。
As product builds up, the reverse reaction speeds up.
随着产物积累，逆反应加快。
Watch the two rates move towards each other.
看这两条速率互相靠拢。
When they meet, the amounts stop changing.
当它们相遇时，各物质的量就不再变化。
But look at the particles along the top: they are still swapping, both ways.
但看看上面那排粒子：它们仍在双向转换。
Nothing is frozen.
什么都没有冻结。
That is why we call it dynamic.
这正是我们称它为动态的原因。
The same story as a graph: rate against time.
把同样的故事画成图：速率对时间。
The forward rate starts high and falls, because the reactant is being used up.
正反应速率一开始很高，然后下降，因为反应物在被消耗。
The reverse rate starts at zero and climbs, because product is being made.
逆反应速率从零开始上升，因为产物在生成。
The two meet, and from then on they stay equal.
两者相遇，从那以后就保持相等。
Everything to the right of that is equilibrium.
那个点右边的一切都是平衡状态。
Equal rates — but the amounts are usually nowhere near equal.
速率相等——但各物质的量通常相差很远。
Now plot the concentrations.
现在改画浓度。
The reactant falls, the product rises, and both flatten at the same moment.
反应物下降，产物上升，两者在同一时刻变平。
Notice two things.
注意两件事。
The lines are level, but not at the same height — at equilibrium the amounts are almost never equal.
线是水平的，但高度并不相同——在平衡时，各物质的量几乎从不相等。
And neither line reaches zero.
而且两条线都没有降到零。
Once they are flat, we can write the number this unit is built on: products over reactants.
一旦它们变平，我们就能写出本单元的核心那个数：产物除以反应物。
Two symbols run this unit.
两个符号贯穿本单元。
The equilibrium constant is products divided by reactants, each raised to its own coefficient — but only at equilibrium.
平衡常数是产物除以反应物，各自按它的系数取幂——但只在平衡时成立。
The reaction quotient is the same expression, worked out from current concentrations at any moment.
反应商是同一个表达式，只是用当前浓度在任何时刻计算。
Pure solids and pure liquids are left out of both.
纯固体和纯液体在两者中都不写。
Now compare.
现在来比较。
Quotient below the constant: the reaction runs forward.
反应商低于常数：反应正向进行。
Above it: backward.
高于常数：逆向进行。
Equal: you are at equilibrium.
相等：你就处于平衡。
Let's write one properly.
我们来正式写一个。
Dinitrogen tetroxide breaks apart into nitrogen dioxide.
四氧化二氮分解成二氧化氮。
The rule never changes: products on top, reactants underneath.
规则永远不变：产物在上，反应物在下。
Now look at the two in front of the product.
现在看产物前面的那个二。
It becomes a power, never a multiplier — that slip costs marks every year.
它变成指数，绝不是乘数——每年都有人在这一步丢分。
Then the measured values go in: zero point two zero for the reactant, zero point one zero for the product.
然后代入测得的数值：反应物是零点二零，产物是零点一零。
The answer comes out as zero point zero five.
答案是零点零五。
Well below one, so this flask holds mostly reactant.
远小于一，所以这个瓶里主要是反应物。
The size of the constant tells you where the balance sits.
常数的大小告诉你平衡偏向哪一边。
Much bigger than one: products win, and the reaction nearly finishes.
远大于一：产物占优，反应几乎完成。
Much smaller than one: barely anything reacts.
远小于一：几乎不反应。
Close to one: a real mixture.
接近一：得到真正的混合物。
On the right, three rules you need.
右边是你需要的三条规则。
Reverse a reaction and the constant flips upside down — it inverts.
把反应倒过来写，常数取倒数——倒过来。
Multiply the coefficients by a number and the constant takes that power.
把系数乘以一个数，常数就取那个次方。
Add two reactions and their constants multiply.
把两个反应相加，常数相乘。
Given the constant, and asked for concentrations?
给了常数、要你求浓度？
Build an ICE table.
就列一张 ICE 表。
Three rows, and the letters are the instructions.
三行，字母本身就是操作说明。
I is what you poured in.
I 是你一开始加入的量。
C is the change, written as x, with the coefficient in front.
C 是变化量，写成 x，前面带上系数。
E is initial plus change — that is the line that goes into the expression.
E 是初始量加变化量——这一行才是代进表达式的那一行。
Then solve for x.
然后解出 x。
When the constant is tiny, treat x as far smaller than the starting amount and skip the quadratic.
当常数很小时，可以把 x 看作远小于起始量，从而免去解二次方程。
Now a full one.
现在做一道完整的。
Hydrogen and iodine make hydrogen iodide, and the constant is fifty.
氢和碘生成碘化氢，常数是五十。
We start with zero point one zero zero of each, and no product.
我们从每种零点一零零开始，没有产物。
Fill the table in.
把表填好。
Both reactants lose x.
两种反应物都减少 x。
The product gains two x, because of the two in the equation.
产物增加二 x，因为方程里有个二。
So the bottom row reads zero point one zero zero minus x, twice, and two x.
于是最后一行是零点一零零减 x，两次，以及二 x。
Substitute that bottom row into the constant.
把最后一行代入常数式。
Look: top and bottom are both squares, so we take the square root of both sides.
看：上下都是平方，所以两边同时开平方。
That is easy to solve, and x is zero point zero seven eight.
这样就很好解，x 等于零点零七八。
The product is two x — zero point one five six moles per litre.
产物是二 x——零点一五六摩尔每升。
So how do we move an equilibrium?
那么我们怎样移动一个平衡呢？
Watch.
看。
The system is balanced.
系统处于平衡。
Now we pour in more of the substance on the left, and its bar jumps up.
现在我们往左边那种物质里再加一些， 它的柱子跳了上去。
The system answers by converting some of it across, until it settles again.
系统的回应是把其中一部分转化过去，直到重新稳定下来。
It did not cancel what we did — it shifted to partly counteract the change.
它并没有抵消我们所做的——它只是部分抵消了这个变化。
We end with more of both.
最后两边都变多了。
That is Le Chatelier's principle in one picture.
这就是勒沙特列原理，一幅图就说清楚了。
Three stresses to know.
要知道三种干扰。
Add more of a substance and the equilibrium shifts away from it — add reactant, get more product.
加入更多某种物质，平衡就朝远离它的方向移动——加入反应物，就得到更多产物。
Squeeze the container to raise the pressure, and it shifts to the side with fewer gas molecules.
压缩容器提高压强，平衡移向气体分子数较少的一边。
Raise the temperature, and it shifts in the endothermic direction, the one that takes heat in.
升高温度，平衡朝吸热方向移动，也就是吸收热量的那个方向。
And the bottom line is examined every year: a catalyst does not shift the position of equilibrium — it only speeds both directions equally.
而最下面那行每年都会考：催化剂不移动平衡的位置——它只是让正逆两个方向同等加快。
Le Chatelier is a shortcut.
勒沙特列原理是一条捷径。
Here is the real reason, and it earns the extra marks.
真正的原因在这里，而它能拿到额外的分。
Put the constant on a line.
把常数放在一条数轴上。
At equilibrium the quotient sits exactly on it.
平衡时，反应商正好落在它上面。
Now add more reactant.
现在加入更多反应物。
Reactant is on the bottom of the fraction, so the quotient drops below it.
反应物在分数的下面，所以反应商掉到它下面去了。
The system is no longer at equilibrium, so it runs forward until the quotient climbs back onto the constant.
系统不再处于平衡， 于是它正向进行，直到反应商重新爬回常数上。
New equilibrium, same constant.
新的平衡，同样的常数。
But change the temperature and the constant itself moves.
但如果改变温度，常数本身就会移动。
The same rules decide what dissolves.
同样的规则决定什么能溶解。
A barely soluble salt sits in equilibrium with its own ions, and its constant has a special name: the solubility product.
一种几乎不溶的盐与它自己的离子处于平衡， 它的常数有个专门的名字：溶度积。
The solid is left out, as always, so for silver chloride it is just silver ion times chloride ion.
固体照旧不写，所以对氯化银来说， 就是银离子乘以氯离子。
Here is the trick the exam loves. If the water already contains one of those ions, the common-ion effect pushes dissolving backwards and lowers solubility.
考试最爱考的窍门在这里： 如果水里已经含有其中一种离子，同离子效应就把溶解推回去，并降低溶解度。
Look at the drop in the corner.
看角落里那个下降。
Numbers on that.
给它代上数字。
Silver chloride dissolves only a little: its solubility product is one point eight times ten to the minus ten.
氯化银只能溶解一点点：它的溶度积是一点八乘以十的负十次方。
Call the amount that dissolves s.
把溶解的量记作 s。
Each unit gives one silver ion and one chloride ion, so the product is s squared.
每一个单元给出一个银离子和一个氯离子，所以乘积是 s 的平方。
Take the square root: one point three times ten to the minus five.
开平方：一点三乘以十的负五次方。
Now dissolve it in salty water instead — one tenth of a mole of chloride per litre.
现在改成溶解在含盐的水里——每升有零点一摩尔的氯离子。
That chloride is already there, so s is the constant divided by nought point one: one point eight times ten to the minus nine.
那些氯离子本来就在，所以 s 等于常数除以零点一：一点八乘以十的负九次方。
Seven thousand times less dissolves, and the constant never moved.
溶解的量少了七千倍，而常数从未改变。
Four marks students give away.
四个学生常送掉的分。
One: at equilibrium the rates are equal, the amounts are not — never write that the concentrations become the same.
第一：平衡时速率相等，量并不相等—— 千万不要写成浓度变得相同。
Two: leave pure solids and pure liquids out of every expression.
第二：每个表达式都不写纯固体和纯液体。
Three: only a change in temperature changes the constant itself; a substance or a squeeze moves the position, and a catalyst moves neither.
第三：只有温度的改变才会改变常数本身；加入物质或压缩容器改变的是平衡的位置， 而催化剂两者都不改变。
Four: use the quotient to choose the direction, and an ICE table for the numbers.
第四：用反应商判断方向，用 ICE 表算出数值。
Four things to carry out of Unit Seven.
从第七单元要带走四件事。
Equilibrium is dynamic: the rates are equal, not the amounts.
平衡是动态的：速率相等，而不是量相等。
Compare the quotient with the constant for the direction, and an ICE table for the numbers.
用反应商与常数比较来判断方向，用 ICE 表算出数值。
Le Chatelier tells you which way a stress pushes the position, and only temperature changes the constant.
勒沙特列原理告诉你干扰把平衡的位置推向哪一边，而只有温度会改变常数。
And solubility is equilibrium again, with a common ion pushing dissolving backwards.
溶解度则是平衡的又一次重演，同离子会把溶解推回去。