Kinetics
AP Chemistry Topic 5 8:00 English narration · English + 中文 subtitles burned in
Chapters
Transcript
Look inside a car's exhaust pipe.
看看汽车排气管的内部。
This ceramic honeycomb is a catalytic converter.
这块陶瓷蜂窝是催化转化器。
The gases rushing through it are poisonous.
穿过它的气体有毒。
Left alone they would take years to become harmless.
如果任其自然，它们要好几年才会变得无害。
In here it takes under a second.
而在这里，不到一秒就完成了。
Only the speed changed.
改变的只有速度。
Unit Five is Kinetics, in seven steps.
第五单元，分七步。
What a rate is and what changes it.
什么是速率，以及什么会改变它。
The rate law, and how experiments give us the orders.
速率方程，以及实验如何给出反应级数。
Concentration against time, and half-life.
浓度随时间的变化，还有半衰期。
Collisions, and why almost all of them fail.
碰撞，以及为什么几乎所有碰撞都失败。
Energy profiles.
能量图。
Mechanisms.
反应机理。
And last, catalysts.
最后是催化剂。
Let's start at the beginning.
我们从头开始。
So what is a rate?
那么速率是什么？
A change in concentration divided by the time it took.
就是浓度的变化除以所用的时间。
Concentration of reactants decreases; the minus sign keeps it positive as reactants disappear.
前面的负号让它在反应物减少时保持为正。
Now the small numbers under the deltas: each species is divided by its own coefficient, so one single rate describes the whole reaction.
再看三角形下面的小数字：每种物质都除以自己的系数， 这样整个反应就只用一个速率来描述。
On the right: a gas syringe, and a slope.
右边：一支气体注射器，再取斜率。
Four things speed a reaction up: concentration, surface area, temperature, and a catalyst.
有四件事能加快反应：浓度、表面积、温度，还有催化剂。
More particles in the same space means more collisions every second — and collision theory explains all four.
同样的空间里粒子更多，每秒的碰撞就更多——碰撞理论能解释这四件事。
Here is the rate law: the rate equals a constant times each reactant's concentration raised to a power, and those powers are the orders.
这就是速率方程：速率等于一个常数，乘以每种反应物浓度的某次方，这些次方就是反应级数。
The rule students break most often: the orders come from experiment — experimentally — never from the balanced equation.
学生最常犯的错误是：级数来自实验，绝不能从配平方程里读出来。
Find them by the method of initial rates.
用初始速率法来求。
And the constant carries units set by the overall order.
而速率常数的单位由总级数决定。
Plot the rate against the concentration and every order has its own shape.
把速率对浓度作图，每个级数都有自己的形状。
Zero order is a flat line: the rate ignores the concentration.
零级是一条水平线：速率完全不受浓度影响。
First order is a straight line through the origin: twice the concentration, twice the rate.
一级是过原点的直线：浓度加倍，速率加倍。
Second order curves upward: double it and the rate goes up four times.
二级向上弯：浓度加倍，速率变四倍。
Three experiments, with concentrations and initial rates.
三组实验，给出浓度和初始速率。
Compare experiment one with two: B is fixed, A is doubled, and the rate goes up four times.
比较实验一和实验二：B 不变，A 加倍，速率变成四倍。
Four is two squared, so it is second order in A.
四是二的平方，所以对 A 是二级。
Now compare one with three: A is fixed, B is doubled, and the rate does not move.
再比较实验一和实验三：A 不变，B 加倍，速率完全不动。
So it is zero order in B, and B leaves the rate law.
所以对 B 是零级， B 完全从速率方程里消失。
That gives a rate law of the constant times A squared, overall order two.
于是速率方程是常数乘以 A 的平方，总级数是二。
Last, the constant: divide experiment one's rate by its concentration squared. It comes out as zero point two.
最后求常数：用实验一的速率除以它的浓度的平方，得到零点二。
Always say which experiments you compared.
一定要写清楚你比较了哪两组实验。
Now, concentration against time.
现在换个角度：浓度对时间作图。
That comes from the integrated rate laws, and each order gives one straight-line plot.
这来自积分速率方程，每个级数都有一种图会画成直线。
Zero order: the concentration itself.
零级：直接画浓度。
First order: its natural logarithm.
一级：画它的自然对数。
Second order: one over the concentration.
二级：画浓度的倒数。
Whichever line is straight gives the order, and its slope gives the constant.
哪条线是直的，就说明是几级，而它的斜率给出速率常数。
A first order reaction has a constant half-life.
一级反应有半衰期，而且是恒定的。
After one half-life, half is left.
经过一个半衰期，剩下一半。
After a second of the same length, a quarter is left.
再经过一个同样长的半衰期，剩下四分之一。
After a third, an eighth.
再一个，剩下八分之一。
Look at the three gaps along the bottom: they are equal, and they stay equal whatever you started with.
看底下这三段间隔：它们相等，而且不管起始量是多少都相等。
The half-life is zero point six nine three over the rate constant.
半衰期等于零点六九三除以速率常数。
Why do most collisions do nothing?
为什么大多数碰撞什么也没发生？
Watch.
看。
A slow collision just bounces apart: not enough energy to break the old bonds.
一次慢速碰撞只是弹开：能量根本不足以断开旧键。
The bar on the right compares each collision with the activation energy, the minimum needed.
右边的能量条把每次碰撞的能量与活化能比较，那是所需的最低能量。
Next, plenty of energy, but the orientation is wrong — the atom hits the wrong end.
接下来能量足够了，但原子撞在了错误的一端。
Both conditions must be met.
两个条件都必须满足。
Now temperature.
现在说温度。
Energy is spread among the molecules like this; only the shaded tail on the right can react.
这条曲线表示能量在分子中的分布；只有右边阴影部分的尾巴能够反应。
Heat the mixture and the curve spreads out: the peak drops, but the tail grows sharply — more particles above the activation energy.
加热混合物，整条曲线就会展开：峰变低，但尾巴显著变大。
That is why a small rise in temperature can double the rate.
这就是为什么温度稍微升高，速率就能翻倍。
A catalyst instead moves the barrier left.
催化剂则不同，它把能垒往左移。
The energy along the reaction path: reactants climb to a peak, then drop to products.
能量沿反应路径的变化：反应物爬上一个峰，然后下降到生成物。
That peak is the transition state.
这个峰就是过渡态。
The climb from the reactants to the peak is the activation energy.
从反应物爬到峰顶的高度就是活化能。
The gap between reactants and products is the enthalpy change, and the products sit lower here, so this reaction is exothermic.
反应物与生成物之间的差值是焓变，这里生成物更低，所以这个反应是放热的。
The Arrhenius equation ties it together: bigger barrier, smaller rate constant.
阿伦尼乌斯方程把它们联系起来：能垒越大，速率常数越小。
A balanced equation says what goes in and out, never how.
配平方程只说什么进去、什么出来，从不说怎么发生。
The real route is the mechanism: a list of elementary steps, each one a single collision.
真正的路径是反应机理： 一串基元步骤，每一步都是一次碰撞。
Molecularity counts how many particles collide in a step. A species made in one step and used up later is an intermediate; it never appears in the overall equation.
在某一步生成、又在后面某一步被消耗的物种叫中间体； 它不会出现在总方程里。
The steps must add up to that overall equation.
各步相加必须等于总方程。
And for an elementary step, and only then, the rate law comes straight from the stoichiometry.
而对于基元步骤——也只有基元步骤——速率方程可以直接从计量数写出来。
Here is a two-step reaction drawn as one profile.
这是一个两步反应画成的一张能量图。
Two peaks, one per step, with a valley between.
两个峰，每步一个，中间有一个谷。
That valley is the intermediate: real, but short-lived.
那个谷就是中间体：真实存在，但寿命很短。
The first hill is far taller, so the first step is the slow one, the rate-determining step.
第一个山峰高得多， 所以第一步是慢步，也就是决速步骤。
Like a road with two bridges: the narrow one decides the traffic.
就像一条路上有两座桥：窄的那座决定车流。
Overall, nitrogen dioxide plus carbon monoxide give nitrogen monoxide and carbon dioxide.
总反应是二氧化氮加一氧化碳，生成一氧化氮和二氧化碳。
Step one is slow: two nitrogen dioxide molecules collide.
第一步慢：两个二氧化氮分子相撞。
Step two is fast, and uses the carbon monoxide.
第二步快，用掉一氧化碳。
First check: add them.
第一项检查：把两步相加。
Nitrogen trioxide is made in step one and used in step two, so it cancels — the intermediate.
三氧化氮在第一步生成、在第二步被消耗，所以抵消； 那就是中间体。
What is left is exactly the overall equation.
剩下的正好是总方程。
Second check: the slow step is two of them colliding, so the rate is the constant times nitrogen dioxide squared.
第二项检查：慢步是两个二氧化氮分子相撞， 所以速率等于常数乘以二氧化氮浓度的平方。
Carbon monoxide is in the overall equation but nowhere in the rate law; it joins after the slow step.
一氧化碳出现在总方程里， 却完全不在速率方程里；它是在慢步之后才参与的。
What if the fast step comes first?
如果快步在前面呢？
Step one is a fast equilibrium; step two, the slow one, is rate-determining.
第一步是快速平衡； 第二步是慢步，也就是决速步。
Its rate law contains the intermediate, and a rate law may only contain measurable species.
它的速率方程里含有中间体， 而速率方程只能含有可以测量的物种。
The fix is the pre-equilibrium: forward and backward rates in the fast step are equal, so the intermediate can be written using the reactants.
解决办法是预平衡： 快步中正反应和逆反应的速率相等，所以中间体可以用反应物表示。
Substitute that in, gather the constants, and the intermediate is gone.
代入、合并常数，中间体就消失了。
Now every term is measurable.
现在每一项都是可以测量的。
The orange curve is the reaction alone; the green curve is the same reaction with a catalyst.
橙色曲线是没有催化剂的反应；绿色曲线是同一个反应加了催化剂。
A catalyst does not push anything over the hill; it offers a different route with a lower barrier, often by binding the reactants in a better position.
催化剂并不把粒子推过山头；它提供另一条能垒更低的路径， 常常是把反应物结合住，让它们摆到更合适的位置。
Now look at what did not change.
现在看没有改变的东西。
Reactants and products sit at the same levels, so the enthalpy change is identical.
反应物和生成物都在同样的高度，所以焓变完全一样。
And it is not used up.
而且它本身不会被消耗。
A catalyst changes how fast, never how far: the position of equilibrium does not move.
催化剂只改变反应有多快，不改变反应最终能进行到什么程度：平衡的位置不会移动。
Four marks students throw away.
四个学生常丢的分。
One: orders come from experiment, never from the coefficients.
第一：级数来自实验，绝不来自系数。
Two: on an energy profile the highest hill is the activation energy, the reactant-to-product gap the enthalpy change.
第二：在能量图上，山峰的高度是活化能，反应物到生成物的差值是焓变。
Three: temperature works mainly by pushing more particles past the barrier, not just by adding collisions.
第三：温度主要靠把更多粒子推过能垒，而不只是增加碰撞次数。
Four: a catalyst lowers the activation energy and nothing else.
第四：催化剂只降低活化能，别的什么都不改变。
Get those right.
把这四点做对。