Reaction kinetics
A-Level Chemistry Topic 8 7:13 English narration · English + 中文 subtitles burned in
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Iron left outside takes years to rust away.
放在户外的铁要好几年才会锈掉。
Strike a match, and almost the same chemistry — oxygen attacking a fuel — is over in less than a second.
划一根火柴，几乎是同一类化学—— 氧气与燃料反应——不到一秒就结束了。
Same chemistry, wildly different speeds.
同样的化学，速度却相差极大。
Why?
为什么？
Because a reaction only happens when particles crash into each other hard enough, and the right way round.
因为只有当粒子以足够大的能量、并且以正确的方向相撞时，反应才会发生。
Control how often that happens, and you control the reaction.
能控制这件事发生的频率，就能控制反应。
That is what this whole topic is about.
这就是本章要讲的全部内容。
This is reaction kinetics: how fast reactions go, and how we control them.
这就是反应动力学：研究反应有多快，以及我们如何控制它。
We will build one idea — collisions — and then use it three times: for concentration, for temperature, and for catalysts.
我们只建立一个核心概念——碰撞——然后把它用三次： 用在浓度上、用在温度上、用在催化剂上。
Watch what a collision must get right.
看看一次碰撞必须做对哪几件事。
First, the particles have to meet.
第一，粒子必须真的相遇。
Second, they must hit hard enough; a gentle bump just bounces off.
第二，它们必须撞得足够猛；轻轻一碰就只会弹开。
Third, they must be lined up the right way round.
第三，它们必须以正确的方向对准。
Miss any one of those, and nothing happens.
这三点缺任何一点，就什么也不会发生。
Now the words the exam wants.
现在来看考试要求的术语。
The rate of reaction is the change in amount, or in concentration, in each unit of time.
反应速率是单位时间内物质的量或浓度的变化。
A collision that leads to a reaction is called an effective collision.
能够引发反应的碰撞叫做有效碰撞。
One that fails is a non-effective collision — too little energy, or the wrong direction.
没能引发反应的叫无效碰撞—— 要么能量不够，要么方向不对。
So the rate depends on the frequency of effective collisions: how many useful collisions happen each second.
所以反应速率取决于有效碰撞的频率： 每秒钟发生多少次有用的碰撞。
Here is our first prediction.
这是我们的第一个推论。
Pack the same particles into a smaller space — that is what raising the concentration of a solution, or the pressure of a gas, does.
把同样多的粒子挤进更小的空间—— 提高溶液的浓度，或提高气体的压强，做的就是这件事。
They are closer together, so they collide more often.
它们靠得更近，所以碰撞得更频繁。
More collisions each second means more effective collisions each second, so the rate rises.
每秒碰撞次数增加， 每秒的有效碰撞次数也增加，速率就上升了。
But notice what does not change: the fraction of particles with enough energy is exactly the same.
但要注意什么没有变：具有足够能量的粒子所占的比例完全没变。
How do we measure a rate?
我们怎么测速率？
Anything that changes as the reaction runs will do.
只要有什么随反应进行而改变，都可以用来测。
Collect the gas given off and read its volume.
收集放出的气体，读出体积。
Stand the flask on a balance and watch the mass fall.
把锥形瓶放在天平上，看质量下降。
Or time how long a colour takes to appear.
或者计时看颜色多久出现。
In a clock reaction we time one fixed change, so the rate is proportional to one over the time.
在"时钟反应"中，我们只对一个固定的变化计时， 所以速率与时间的倒数成正比。
On a graph of volume against time, the rate is the gradient: steepest at the start, then flattening.
在体积对时间的图上，速率就是斜率： 开始时最陡，然后逐渐变平。
Let's do one properly.
我们来完整做一道。
A reaction gives off carbon dioxide.
某反应放出二氧化碳。
In the first thirty seconds, forty-eight cubic centimetres of gas are collected. After two minutes, ninety-six cubic centimetres in total.
前三十秒收集到四十八立方厘米气体； 两分钟后一共收集到九十六立方厘米。
Find the average rate over each period.
求这两段时间内的平均速率。
Average rate is the change divided by the time taken.
平均速率就是变化量除以所用的时间。
Over the first thirty seconds, that is forty-eight divided by thirty — one point six cubic centimetres per second.
前三十秒：四十八除以三十， 等于每秒一点六立方厘米。
Over the whole two minutes, ninety-six divided by one hundred and twenty — zero point eight.
整个两分钟：九十六除以一百二十，等于零点八。
So the rate has halved.
所以速率减半了。
The reactants are being used up, so fewer collisions happen each second.
反应物正在被消耗，因此每秒发生的碰撞变少了。
A collision also needs a minimum amount of energy.
一次碰撞还需要一个最低限度的能量。
We call that the activation energy.
我们把它叫做活化能。
Think of it as a hill the reaction must climb: the particles have to arrive with enough energy to get over the top.
可以把它想成反应必须翻过的一座山：粒子必须带着足够的能量才能越过山顶。
One exam habit here: measure the activation energy from the level of the reactants up to the top of the hill, never from the bottom of the axis.
这里有一个考试习惯：活化能要从反应物的能级量到山顶， 绝不能从坐标轴的底部量起。
The drop from reactants to products is a different quantity — the enthalpy change.
从反应物降到生成物的那一段是另一个量——焓变。
Not every molecule carries the same energy.
并不是每个分子都带有相同的能量。
At any moment, a few are crawling, most are somewhere in the middle, and a few are moving very fast.
在任何时刻，少数分子慢吞吞， 多数分子处在中间，还有少数跑得非常快。
This spread of energies is called the Boltzmann distribution.
这种能量的分布叫做玻尔兹曼分布。
Learn to draw this curve, because the marks are in the details.
一定要学会画这条曲线，因为分数就藏在细节里。
Energy along the bottom, number of molecules up the side.
横轴是能量，纵轴是分子数目。
It starts at the origin.
它从原点开始。
It rises to a single peak — the most common energy.
它上升到唯一的一个峰——最常见的能量。
Then it falls away in a long tail that never touches the axis.
然后沿着一条长长的尾巴下降，这条尾巴永远不与横轴相交。
The whole area under the curve is the total number of molecules.
曲线下的总面积就是分子的总数。
Now mark the activation energy and shade everything to its right.
现在标出活化能，并把它右边全部涂上阴影。
Only those molecules can react, and it is a small slice.
只有这部分分子能够反应，而且这只是很小的一部分。
Now heat it up.
现在给它加热。
The curve flattens and spreads to the right, and its peak drops.
曲线变平并向右延展，峰值下降。
The area stays the same, because you still have the same molecules.
面积保持不变，因为分子总数还是那么多。
Leave the activation energy line exactly where it was — heating does not change it.
活化能那条线要留在原来的位置——加热并不会改变它。
Look at the shading now: far more molecules sit beyond the line.
现在看阴影部分：越过这条线的分子多了很多。
Hotter particles do collide more often, but that effect is small.
温度高的粒子确实碰撞得更频繁，但这个效应很小。
The big one is a much greater proportion of molecules now exceed the activation energy.
真正起作用的，是现在有大得多的分子比例越过了活化能。
That is why a rise of ten degrees can roughly double the rate.
这就是为什么温度只升高十度，速率大约就能翻一倍。
There is a second way to get more molecules over the barrier.
要让更多分子越过能垒，还有第二种办法。
Instead of lifting the molecules, lower the barrier itself.
与其把分子抬高，不如把能垒本身降低。
That is exactly what a catalyst does.
催化剂做的正是这件事。
Watch the hill on the reaction pathway diagram shrink.
看反应路径图上的那座山变矮。
A catalyst speeds up a reaction and is not used up by it; the action is called catalysis.
催化剂加快反应，而它本身不被消耗；这种作用叫做催化作用。
Here is the sentence the mark scheme wants: a catalyst provides a different mechanism, with a lower activation energy.
这就是评分标准想要的那句话：催化剂提供了一条活化能更低的、不同的反应机理。
Different route, smaller hill.
路线不同，山更矮。
Two things it does not do.
有两件事它做不到。
It does not change the enthalpy change — reactants and products stay at exactly the same energies.
它不改变反应的焓变—— 反应物和生成物的能量完全不变。
And it does not shift an equilibrium; it only gets you there sooner.
它也不会移动平衡的位置， 只是让你更快到达那里。
Now put a catalyst on the Boltzmann curve.
现在把催化剂放到玻尔兹曼曲线上。
This is where students go wrong.
学生就是在这里出错的。
A catalyst does not warm anything up, so the curve does not move at all.
催化剂并没有加热任何东西，所以曲线完全不动。
What moves is the activation energy line: it slides to the left.
移动的是活化能那条线：它向左滑。
With the line further left, a much larger fraction of molecules lie beyond it.
线往左移之后， 越过它的分子所占的比例就大得多。
So remember the pair: heating moves the curve; a catalyst moves the line.
所以记住这一对： 加热移动曲线，催化剂移动那条线。
Either way, more molecules can react.
两种方式都让更多分子能够反应。
Finally, two kinds of catalyst, and the difference is simply the physical state.
最后，催化剂分两类，区别就在于物理状态。
A homogeneous catalyst is in the same state as the reactants — concentrated sulfuric acid in a mixture of liquids, for example, when an ester is made.
均相催化剂与反应物处于相同的状态——例如制取酯的时候， 混在液体混合物里的少量浓硫酸。
A heterogeneous catalyst is in a different state — solid iron speeding up gases in the Haber process, or the honeycomb inside a car's catalytic converter.
多相催化剂则处于不同的状态—— 哈伯法中加快气体反应的固体铁，或者汽车催化转化器里的蜂窝状固体。
Different states, same job: a new route with a lower activation energy.
状态不同，作用相同：提供一条活化能更低的新路线。
Three marks students throw away.
三个学生白白丢掉的分。
First, never say only that the particles move faster.
第一，不要只说"粒子运动得更快"。
Explain the change using collision theory: more frequent effective collisions.
要用碰撞理论来解释：有效碰撞更频繁。
Second, sketch the Boltzmann curve properly — from the origin, mark the activation energy, shade to the right, and never let the tail touch the axis.
第二，玻尔兹曼曲线要画规范—— 从原点出发，标出活化能，把右边涂上阴影，尾巴永远不碰横轴。
Third, a catalyst gives a different route with a lower activation energy, and it never changes the enthalpy change.
第三，催化剂提供的是一条活化能更低的不同路线， 而且它绝不改变反应的焓变。