Rates & Equilibrium
IGCSE Chemistry Topic 6 15:53 English narration · English + 中文 subtitles burned in
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Milk on the counter spoils in a day.
放在台面上的牛奶一天就变质。
In the fridge, it lasts a week.
放在冰箱里，能放一个星期。
Same milk — but the reaction runs at a different speed.
还是同样的牛奶—— 但反应以不同的速度进行。
First, a quick check: a chemical change makes brand-new substances, like milk turning sour, or wood burning.
先快速区分一下：化学变化生成全新的物质，比如牛奶变酸， 或木头燃烧。
A physical change, like ice melting, makes no new substance.
物理变化，比如冰融化，不生成新物质。
Today we ask two big questions about chemical reactions: how fast do they go, and how far?
今天我们要问关于化学反应的两个大问题： 它们进行得多快，以及进行到多远？
Iron rusting is a chemical change — a new substance forms on the metal.
铁生锈是化学变化——金属上生成了新物质。
Physical changes only alter state or shape, and are usually easy to reverse: melting ice, boiling water, or dissolving sugar.
物理变化只改变状态或形状， 通常容易逆转：冰融化、水沸腾、或糖溶解。
Chemical changes make new substances and are usually hard to reverse: burning, rusting, or cooking an egg.
化学变化生成新物质， 通常难以逆转：燃烧、生锈、或煮鸡蛋。
Signs that a chemical change has happened include a colour change, a gas being given off, an energy change such as heat or light, or the formation of a precipitate — a solid forming in a solution.
化学变化发生的迹象包括颜色改变、 放出气体、能量变化例如热或光、或者沉淀——也就是固体——在溶液中形成。
Keep that list for the exam.
把这份清单记牢，考试会用到。
Welcome to rates and equilibrium.
欢迎来到反应速率与平衡。
We'll see what controls the speed of a reaction, how reactions can go both ways, and what redox means.
我们将了解什么控制反应的速度、反应如何能双向进行， 以及氧化还原是什么意思。
Let's begin.
让我们开始吧。
The rate of reaction tells you how fast the reactants change into products.
反应速率告诉你反应物变成生成物有多快。
A high rate means reactants disappear quickly and products appear quickly.
速率高意味着反应物很快消失、 生成物很快出现。
A low rate means the change is slow.
速率低意味着变化很慢。
Every explanation of rate in this topic starts from the same idea: particles must meet and react.
本专题里每一个关于速率的解释， 都从同一个想法出发：粒子必须相遇并发生反应。
Reactions happen when particles collide.
反应发生在粒子碰撞时。
But not every collision works.
但不是每次碰撞都有效。
For a collision to cause a reaction, two things must be true.
要让碰撞引发反应，必须满足两件事。
The particles must hit each other, and they must hit with enough energy — at least the activation energy.
粒子必须彼此相撞，而且必须以足够的能量相撞——至少达到活化能。
A slow, gentle bump does nothing.
缓慢、轻柔的碰撞什么也做不了。
The more often particles collide, and the harder they collide, the faster the reaction.
粒子碰撞得越频繁、越猛烈，反应就越快。
This is collision theory, and it explains everything about rate.
这就是碰撞理论，它解释了关于速率的一切。
So how do we speed a reaction up?
那么，我们怎样加快反应？
Five ways, all through collision theory.
有五种方法，都通过碰撞理论。
Higher concentration, or higher gas pressure, packs more particles into the same space, so they collide more often.
更高的浓度，或更高的气体压强， 把更多粒子挤进同样的空间，所以它们碰撞得更频繁。
Smaller pieces — a larger surface area — expose more particles to collide.
更小的碎块——更大的表面积—— 让更多粒子暴露出来碰撞。
Higher temperature makes particles move faster, so they collide more often, and harder.
更高的温度让粒子运动更快，所以它们碰撞得更频繁、更猛烈。
And a catalyst gives the reaction an easier path.
而催化剂给反应一条更容易的路径。
Each one raises the number of successful collisions per second.
每一种都提高了每秒成功碰撞的次数。
Look at the two boxes.
看这两个盒子。
The right-hand box is more concentrated — more particles in the same volume.
右边的盒子浓度更高——同样体积里有更多粒子。
Because the particles are closer, they bump into each other more often, so successful collisions happen more often and the rate rises.
因为粒子更近， 它们更常互相碰撞，所以成功碰撞更频繁，速率上升。
Gas pressure works the same way.
气体压强也是同样的道理。
Raising the pressure squeezes the gas particles closer together, so collisions become more frequent.
增大压强把气体粒子挤得更近，碰撞就更频繁。
Same idea: more particles per unit of space means a faster reaction.
同一个想法：单位空间内粒子越多， 反应就越快。
A big lump of solid only lets acid touch its outer face.
一大块固体只让酸接触到它的外表面。
Crush the same solid into a powder, and acid can reach far more particles at once.
把同样的固体压成粉末，酸就能一次接触到 多得多的粒子。
Breaking a solid into smaller pieces increases its surface area, so more particles are exposed for collisions.
把固体打碎成更小的块会增大表面积，所以有更多粒子暴露出来碰撞。
That is why powdered marble reacts with acid much faster than a single large chip of the same mass.
这就是为什么粉末状大理石与酸反应，比同样质量的一大块要快得多。
Temperature does two things at once.
温度同时做两件事。
Particles gain kinetic energy and move faster, so they collide more often.
粒子获得动能并运动得更快，所以碰撞更频繁。
And a larger share of those collisions now have enough energy to clear the activation energy barrier.
而且这些碰撞中 有更大比例现在拥有足够的能量去越过活化能屏障。
So temperature raises both the frequency of collisions and the fraction that succeed.
所以温度既提高碰撞频率， 也提高成功碰撞的比例。
That is why warming a reaction often speeds it up dramatically — more meetings, and more of them hard enough to react.
这就是为什么加热反应往往会显著加快——更多相遇， 而且更多相遇猛烈到足以反应。
A catalyst is special.
催化剂很特别。
It speeds up a reaction, but is not used up — it comes out unchanged at the end, ready to work again.
它加快反应，但不被消耗——反应结束时它原样出来，随时可以再次工作。
How does it work?
它是怎么起作用的？
Look at the energy profile.
看能量进程图。
The catalyst provides a lower path over the hill — it lowers the activation energy.
催化剂提供一条越过小山的更低的路径——它降低活化能。
With a smaller barrier to climb, far more collisions now have enough energy to react.
要翻越的屏障变小了，现在有多得多的碰撞拥有足够的能量去反应。
So the reaction speeds up, without any extra heat.
所以反应加快了， 而不需要额外的热。
Enzymes are the catalysts inside living things.
酶就是生物体内的催化剂。
Look carefully at both pathways.
仔细看两条路径。
The catalyst lowers the activation energy, so the hill is shorter.
催化剂降低了活化能，所以小山更矮。
But the start and end energy levels are the same.
但起点和终点的能量水平相同。
That means the overall energy change of the reaction — delta H — is unchanged.
这意味着反应的总能量变化——焓变——没有改变。
A catalyst never makes a reaction more or less exothermic; it only makes it faster.
催化剂绝不会让反应更放热或更吸热； 它只让反应更快。
The products end up with the same energy either way.
无论有没有催化剂，生成物最终能量都一样。
How do we measure a rate?
我们怎样测量速率？
We watch something change over time.
我们观察某样东西随时间的变化。
If a gas is made, we collect it and measure its volume.
如果生成气体，我们收集它并测量它的体积。
Or, if gas escapes, we put the flask on a balance and watch the mass fall.
或者，如果气体逸出，我们把烧瓶放在天平上，看质量下降。
Then we plot the amount against time.
然后我们画出量对时间的图。
The graph is steepest at the start, when the reaction is fastest and reactants are plentiful.
图在开始时最陡，那时反应最快、反应物充足。
It gradually flattens as reactants run out, and goes flat when the reaction is finished.
随着反应物耗尽，图逐渐变平， 反应结束时变成水平。
Here is the gas syringe method in the lab.
这是实验室里的气体注射器法。
A conical flask holds the reaction mixture.
锥形瓶装着反应混合物。
A delivery tube carries the gas into a gas syringe.
导气管把气体送进气体注射器。
You read the volume of gas at regular times — every ten or twenty seconds, for example — and then plot volume against time.
你按固定时间读取气体体积——例如每十秒或二十秒——然后画体积对时间的图。
The steeper the graph at any moment, the faster the rate at that moment.
图在任一时刻越陡，那一时刻的速率就越快。
Always start timing as soon as the reactants mix.
一定要在反应物一混合就开始计时。
A third way uses a precipitate.
第三种方法利用沉淀。
Some reactions turn a clear mixture cloudy as a solid forms.
有些反应会随着固体生成，使澄清混合物变浑浊。
Place a mark under the flask, and time how long it takes for the mark to disappear when you look down through the liquid.
在烧瓶下放一个标记，从上方透过液体观察，计时看标记多久消失。
A shorter time means a faster rate.
时间越短， 速率越快。
This method is simple, but it only gives one timing for the whole reaction, not a full graph of rate over time.
这种方法简单，但只给出整个反应的一个计时，而不是速率随时间的完整图。
On a graph of product against time, the line is steepest at the start — that is the fastest rate.
在生成物对时间的图上，线在开始时最陡——那是最快的速率。
As reactants are used up, collisions become less frequent, so the curve becomes less steep.
随着反应物被消耗， 碰撞变少，曲线变得不那么陡。
When the reaction finishes, the line goes flat.
反应结束时，线变成水平。
Notice the two curves: a faster run and a slower run can still reach the same final amount of product if the same total of reactants was used.
注意两条曲线： 一次较快的和一次较慢的，如果用了同样总量的反应物，仍可达到相同的最终产物量。
Rate is about speed, not about how much product you finally get.
速率关乎速度，不关乎你最终得到多少产物。
Some reactions are reversible: they can go both forwards and backwards, shown by a double arrow.
有些反应是可逆的：它们可以正向和逆向进行，用双箭头表示。
Heat blue copper sulfate crystals and they turn white, losing their water; add water back and the blue returns.
加热蓝色的硫酸铜晶体， 它们会变白，失去水分；再加水，蓝色又回来。
In a closed container, a reversible reaction reaches equilibrium.
在封闭容器中，可逆反应会达到平衡。
At equilibrium, the forward and backward reactions happen at exactly the same rate, so the amounts of reactants and products stop changing.
在平衡时，正反应和逆反应以完全相同的速率进行，所以反应物和生成物的量停止变化。
It looks still, but both reactions are still running.
它看起来静止，但两个反应仍在进行。
Two classic colour-change examples sit in every textbook.
每个课本里都有两个经典的变色例子。
Blue hydrated copper two sulfate loses water when heated and becomes white anhydrous copper two sulfate; adding water turns it blue again.
蓝色的水合硫酸铜加热时失去水分， 变成白色的无水硫酸铜；加水又变回蓝色。
Pink hydrated cobalt two chloride loses water when heated and becomes blue anhydrous cobalt two chloride; adding water turns it pink again.
粉色的水合氯化钴加热时失去水分， 变成蓝色的无水氯化钴；加水又变回粉色。
Adding water to either anhydrous solid and watching the colour return is used as a simple test for water.
向任一无水固体加水并观察颜色恢复， 被用作检验水的简单方法。
Watch the rates meet.
看这两条速率相遇。
Early on, plenty of reactant means a fast forward reaction, and almost no reverse reaction.
开始时反应物很多，正反应很快，几乎没有逆反应。
As product builds up, the reverse rate climbs while the forward rate falls.
随着产物积累，逆反应速率上升，正反应速率下降。
When the two rates become equal, the system is at equilibrium.
当两个速率相等时， 系统就处于平衡。
Concentrations stop changing — constant, not necessarily equal.
浓度停止变化——是不变，不一定是相等。
And the system must be closed: if products escape, the reverse reaction can never catch up.
而且系统必须封闭： 如果产物能逃逸，逆反应就永远追不上。
You can push an equilibrium one way or the other by changing the conditions.
你可以通过改变条件把平衡推向一边或另一边。
Increase the temperature, and the equilibrium shifts to take that heat in.
升高温度，平衡就移动以吸收那些热。
Increase the pressure, and it shifts to the side with fewer gas molecules.
增大压强，它就移向气体分子较少的一边。
Increase the concentration of a reactant, and it shifts to make more product.
增大反应物的浓度，它就移动以生成更多产物。
The system always responds to oppose the change you made.
系统总是做出反应来对抗你所做的改变。
This is how industry gets more of the product it wants.
工业就是这样获得更多它想要的产物的。
Be precise with temperature.
对温度要说得精确。
Heating moves the equilibrium in the endothermic direction — the direction that takes heat in.
加热使平衡向吸热方向移动——也就是吸收热量的方向。
Cooling moves it in the exothermic direction — the direction that gives heat out.
冷却使它向放热方向移动——也就是放出热量的方向。
For pressure, count the gas molecules on each side of the equation; the equilibrium shifts to the side with fewer gas molecules when pressure rises.
对于压强，数一数方程两边的 气体分子；压强升高时，平衡移向气体分子较少的一边。
Adding a substance shifts the equilibrium away from that substance, to use some of it up.
加入某种物质会使平衡 移离该物质，以消耗掉一部分。
And a catalyst does not move the position of equilibrium at all — it only helps the system reach equilibrium faster.
而催化剂完全不移动平衡位置——它只帮助系统 更快达到平衡。
The Haber process shows this in action.
哈伯法展示了这一点的实际应用。
It makes ammonia for fertilisers, from nitrogen in the air and hydrogen from natural gas.
它用空气中的氮气和天然气中的氢气制造氨，用于化肥。
The reaction is reversible, so the conditions are a careful compromise: about four hundred fifty degrees, and a very high pressure of two hundred atmospheres, with an iron catalyst.
这个反应是可逆的，所以条件是一个精心权衡的折中：大约四百五十度， 以及二百个大气压的很高压强，用铁做催化剂。
The high pressure pushes the equilibrium towards ammonia; the catalyst and temperature keep the rate fast enough to be useful.
高压把平衡推向氨； 催化剂和温度让速率保持足够快以便有用。
Write the Haber equation carefully.
仔细写出哈伯法的方程。
One nitrogen molecule reacts with three hydrogen molecules to give two ammonia molecules, all as gases, with a reversible arrow.
一个氮分子与三个氢分子反应，生成两个氨分子， 全部是气体，用可逆箭头。
The hydrogen comes from methane — natural gas.
氢气来自甲烷——天然气。
The nitrogen comes from the air.
氮气来自空气。
Four gas molecules on the left become two on the right, so high pressure favours ammonia.
左边四个气体分子变成右边两个，所以高压有利于氨。
The iron catalyst speeds both directions equally and does not change how much ammonia you would get at true equilibrium — it just gets you there sooner.
铁催化剂同样加快两个方向， 不会改变在真正平衡时你能得到多少氨——它只是让你更快到达平衡。
Why these conditions?
为什么是这些条件？
Higher pressure would give more product, but very high pressure is dangerous and expensive, so about two hundred atmospheres is a compromise.
更高的压强会给出更多产物，但极高压强危险又昂贵， 所以大约二百个大气压是折中。
A lower temperature would favour ammonia because the forward reaction is exothermic, but the reaction would be too slow, so about four hundred fifty degrees keeps a useful rate.
更低的温度会有利于氨，因为正反应是放热的， 但反应会太慢，所以大约四百五十度保持有用的速率。
The catalyst speeds the reaction without changing the position of equilibrium, which lowers cost.
催化剂加快反应而不改变 平衡位置，从而降低成本。
Industry always balances yield against rate and safety.
工业总是在产率、速率和安全之间取得平衡。
The Contact process is industry's other classic equilibrium.
接触法是工业上另一个经典的平衡过程。
It turns sulfur dioxide and oxygen into sulfur trioxide, which is used to make sulfuric acid.
它把二氧化硫和氧气变成三氧化硫， 用来制造硫酸。
Two sulfur dioxide molecules plus one oxygen molecule give two sulfur trioxide molecules, all as gases, reversible.
两个二氧化硫分子加一个氧分子生成两个三氧化硫分子， 全部是气体，可逆。
The sulfur dioxide comes from burning sulfur, or from roasting sulfide ores; the oxygen comes from the air.
二氧化硫来自燃烧硫，或焙烧硫化物矿石；氧气来自空气。
Typical conditions: about four hundred fifty degrees, a modest pressure of about two atmospheres, and a vanadium five oxide catalyst.
典型条件：大约四百五十度、大约两个大气压的适中压强，以及五氧化二钒催化剂。
The same compromise logic applies.
同样的折中逻辑也适用。
Three gas molecules become two, so higher pressure would favour sulfur trioxide — but the yield is already high at only about two atmospheres, so industry avoids costly extreme pressure.
三个气体分子变成两个，所以更高的压强会有利于三氧化硫—— 但在仅约两个大气压时产率已经很高，所以工业避免昂贵的极高压强。
The forward reaction is exothermic, so a lower temperature would give more product, yet the rate would suffer; four hundred fifty degrees is again the rate–yield compromise.
正反应是放热的， 所以更低温度会给出更多产物，但速率会受损；四百五十度再次是速率与产率的折中。
The catalyst does not shift the equilibrium; it only reaches it faster and cheaper.
催化剂不移动平衡；它只让平衡更快、更便宜地到达。
Finally, redox — reactions where oxidation and reduction happen together.
最后，氧化还原——氧化和还原同时发生的反应。
The simplest view: oxidation is gaining oxygen, and reduction is losing oxygen.
最简单的看法：氧化是得到氧，还原是失去氧。
The deeper view is about electrons: oxidation is loss of electrons, reduction is gain — remember OIL RIG.
更深入的看法是关于电子：氧化是失去电子，还原是得到——记住氧化是失、还原是得。
When one substance is oxidised, another must be reduced.
当一种物质被氧化，另一种一定被还原。
The substance that takes electrons is the oxidising agent; the one that gives them away is the reducing agent.
夺取电子的物质是氧化剂；给出电子的是还原剂。
Electrons always move from one substance to another.
电子总是从一种物质转移到另一种物质。
The substance that loses electrons is oxidised.
失去电子的物质被氧化。
The substance that gains those electrons is reduced.
得到那些电子的 物质被还原。
They always happen together — you cannot have oxidation without reduction.
它们总是一起发生——没有还原就不可能有氧化。
There is also a third way to talk about redox: oxidation numbers.
还有第三种谈论 氧化还原的方式：氧化数。
In oxidation the oxidation number goes up; in reduction it goes down.
在氧化中氧化数升高；在还原中氧化数降低。
Roman numerals in names, like iron two and iron three, are oxidation numbers written into the name.
名称里的 罗马数字，例如铁二价和铁三价，就是写进名称的氧化数。
Learn four rules for oxidation numbers.
记住氧化数的四条规则。
An uncombined element has oxidation number zero.
未化合的元素氧化数为零。
A single-atom ion has an oxidation number equal to its charge — so sodium plus is plus one.
单原子离子的氧化数等于它的电荷—— 所以钠正离子是正一。
In a compound the oxidation numbers add up to zero.
在化合物中氧化数加起来等于零。
In an ion they add up to the charge on the ion.
在离子中它们加起来等于 离子的电荷。
Oxygen is usually minus two.
氧通常是负二。
Hold those rules — the next step is a worked example.
记住这些规则——下一步是一道例题。
Find the oxidation number of manganese in the manganate seven ion — M n O four minus.
求锰酸根七价离子——也就是高锰酸根——中锰的氧化数。
Each oxygen is minus two, and four oxygens give minus eight.
每个氧是负二， 四个氧合计负八。
The oxidation numbers in an ion must add up to the ion charge, which here is minus one — not zero.
离子中的氧化数必须加到离子电荷，这里是负一——不是零。
Zero is only for a neutral compound.
零只适用于中性化合物。
So if manganese is x, then x plus minus eight equals minus one, and manganese is plus seven — exactly what the seven in the name already told you.
所以如果锰是未知数，那么未知数加负八等于负一， 锰就是正七——正好是名称里的七已经告诉你的。
An oxidising agent oxidises another substance and is itself reduced.
氧化剂氧化另一种物质，自身被还原。
A reducing agent reduces another substance and is itself oxidised.
还原剂还原另一种物质，自身被氧化。
Some redox reactions show clear colour changes.
有些氧化还原反应有明显的颜色变化。
Acidified potassium manganate seven is purple; when it acts as an oxidising agent it is reduced, and the purple fades to colourless.
酸化的高锰酸钾是紫色的；当它作为氧化剂时 被还原，紫色褪成无色。
Potassium iodide is colourless; when it is oxidised, red-brown iodine forms.
碘化钾是无色的；当它被氧化时，形成红棕色的碘。
Colour is often your clue that electrons have moved.
颜色常常是电子已经转移的线索。
Three marks to lock in.
锁住三个分。
First: explain every rate change with collision theory — more frequent, or more energetic, successful collisions.
第一：用碰撞理论解释每一个速率变化——更频繁、或更有能量的成功碰撞。
Second: a catalyst speeds up a reaction and is not used up; it works by lowering the activation energy.
第二：催化剂加快反应且不被消耗；它通过降低活化能起作用。
Third: at equilibrium the forward and reverse rates are equal, and the amounts stop changing — but both reactions keep going.
第三：在平衡时正反应和逆反应速率相等， 量停止变化——但两个反应都在继续。
Master these, and this topic is yours.
掌握这些，这个专题就是你的了。
Three more traps.
再记住三个陷阱。
Equilibrium does not mean the amounts of reactant and product are equal — only that they stop changing.
平衡并不意味着反应物和产物的量相等——只是它们停止变化。
Use Le Chatelier carefully: raising temperature favours the endothermic direction; raising pressure favours fewer gas molecules; adding a substance shifts the equilibrium away from it.
仔细使用勒夏特列原理：升高温度有利于吸热方向；增大压强有利于气体分子更少的一边； 加入某种物质使平衡移离它。
And for redox: Oxidation Is Loss, Reduction Is Gain of electrons — they always happen together, and an oxidising agent is itself reduced.
对于氧化还原：氧化是失电子，还原是得电子—— 它们总是一起发生，而且氧化剂自身被还原。