Thermochemistry
AP Chemistry Topic 6 8:53 English narration · English + 中文 subtitles burned in
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Squeeze this pack and in seconds it turns icy cold.
捏一下这个袋子，几秒钟内它就变得冰凉。
Nothing is burning; nothing is cooling it from outside.
没有东西在燃烧，也没有外面的东西在给它降温。
A solid is simply dissolving in the water inside, and that takes energy in.
只是里面的一种固体在水里溶解，而这需要吸收能量。
The pack pulls it out of your hand, so your hand feels cold.
袋子把能量从你手上取走，所以你的手感觉冷。
Every chemical change moves energy like this.
每一个化学变化都像这样搬运能量。
Today we learn to put a number on it.
今天我们就来学会用数字描述它。
Thermochemistry tracks energy in reactions.
热化学追踪反应中的能量。
Unit Six, in seven steps.
第六单元，分七步。
Hot and cold, and which way the energy flows.
冷与热，以及能量流向哪边。
Energy diagrams, and the enthalpy change.
能量图，还有焓变。
Calorimetry, which puts a number on the heat.
量热法，它把热量变成一个数字。
Phase changes, and their flat steps.
相变，以及它们的水平段。
Bond enthalpies, and the sign trap in them.
键焓，还有藏在里面的正负号陷阱。
Formation enthalpies, straight from a table.
生成焓，直接查表得到。
And last, Hess's law.
最后是盖斯定律。
Let's begin.
我们开始吧。
First, the two words this whole unit rests on.
首先是支撑整个单元的两个词。
The reacting chemicals are the system; everything around them is the surroundings.
参加反应的物质叫做系统；它们周围的一切叫做环境。
An exothermic change pushes heat out, so the surroundings get hotter, and the enthalpy change is negative.
放热变化把热推出去，所以环境变热，焓变是负的。
An endothermic change pulls heat in, so the surroundings get colder, and the change is positive.
吸热变化把热拉进来，所以环境变冷，焓变是正的。
And here is the trap: the thermometer sits in the surroundings, so a rising temperature means the reaction lost energy.
这里有个陷阱：温度计放在环境里，所以温度上升意味着反应本身失去了能量。
Now draw that as a picture.
现在把它画成图。
Energy goes up the page, and the reaction runs left to right.
能量沿着纵轴向上，反应从左向右进行。
In the first diagram the products end up lower than the reactants.
第一张图里，生成物比反应物低。
Energy was released, so this one is exothermic.
能量被放出，所以这是放热反应。
In the second, the products end up higher.
第二张图里，生成物更高。
Energy was absorbed, so this one is endothermic.
能量被吸收，所以这是吸热反应。
The gap between the two levels is the enthalpy change.
两条能级之间的差就是焓变。
The hill in the middle is the activation energy: it changes the speed, never the sign.
中间的那座山是活化能：它改变快慢，从不改变正负号。
But what is temperature, really?
那么温度到底是什么？
It is the average kinetic energy of the particles.
它是粒子平均动能的量度。
In the hot block the particles move fast.
在热的一块里，粒子运动得快。
In the cold block they move slowly.
在冷的一块里，它们运动得慢。
Put them in contact and the particles collide across the boundary.
让两块接触，粒子就会在边界上碰撞。
Every collision hands a little energy from a fast particle to a slow one.
每一次碰撞都把一点能量从快粒子交给慢粒子。
So the hot side slows down, the cold side speeds up, and the exchange stops only when the two averages are equal.
于是热的一侧慢下来，冷的一侧快起来，只有当两个平均值相等时交换才停止。
That is thermal equilibrium, and it is why heat only ever flows from hot to cold.
这就是热平衡，也是热量只会从热流向冷的原因。
So how do we measure that heat?
那么我们怎么测量这些热量呢？
With a calorimeter, and this one is only a coffee cup.
用量热计，而这一个只是一个纸杯。
The insulated cup keeps the energy inside.
隔热的杯子把能量留在里面。
The lid stops it escaping upward.
杯盖挡住能量从上面跑掉。
The thermometer reads the solution, never the reaction itself.
温度计读的是溶液，而不是反应本身。
Water is the perfect partner here: its specific heat is high, so it needs a lot of energy for a small rise in temperature.
水是这里最合适的搭档：它的比热容很高，所以温度升高一点点也需要很多能量。
And the whole method rests on one sentence. The heat the water gains is the heat the reaction lost.
整个方法都建立在一句话上：水得到的热量，就是反应失去的热量。
Let's put numbers on that.
我们来代入数字。
A student dissolves point zero one zero moles of a salt in one hundred grams of water.
一名学生把零点零一零摩尔的某种盐溶解在一百克水里。
The temperature climbs by five point zero degrees, and the specific heat of water is four point one eight.
温度上升了五点零度，水的比热容是四点一八。
Find the enthalpy change per mole.
求每摩尔的焓变。
Pause here and try it.
先暂停，自己试一试。
Step one: the heat gained by the water is mass, times specific heat, times temperature change.
第一步：水得到的热量等于质量乘以比热容乘以温度变化。
One hundred, times four point one eight, times five, is two thousand and ninety joules.
一百乘以四点一八再乘以五，等于二千零九十焦耳。
Step two is the sign.
第二步是符号。
The water gained that heat, so the reaction released it: minus two point zero nine kilojoules.
水得到了这些热量，所以反应放出了它：负二点零九千焦。
Step three: divide by the moles you actually reacted.
第三步：除以你实际反应掉的摩尔数。
That gives minus two hundred and nine kilojoules per mole, and it is negative because the water warmed up.
得到负二百零九千焦每摩尔， 而它是负的，因为水升温了。
Heat a solid steadily and watch.
稳定地加热一块固体，仔细看。
The temperature climbs, then holds flat while the solid melts, climbs again, then holds flat again while the liquid boils.
温度上升，然后在固体熔化时保持水平， 再次上升，然后在液体沸腾时又一次保持水平。
Look at those flat steps.
看这些水平段。
Energy is still going in, but it is pulling the particles apart against the forces between them, not making them move faster, so the temperature cannot rise.
能量仍在不断进入，但它是在克服粒子之间的作用力把粒子拉开，而不是让它们运动得更快， 所以温度不能上升。
The heat needed is the number of moles times the enthalpy of fusion for melting, or of vaporization for boiling.
所需的能量等于摩尔数乘以熔化焓，沸腾时则乘以汽化焓。
Vaporization always costs more, because boiling separates the particles completely.
汽化总是要花更多能量，因为沸腾把粒子彻底分开。
So what exactly is that enthalpy change?
那么焓变究竟是什么？
It is the heat a reaction gives out or takes in at constant pressure, and constant pressure just means an open beaker on the bench.
它是反应在恒定压强下放出或吸收的热量， 而恒定压强其实就是台面上一个敞口的烧杯。
Products lower than the reactants: negative, exothermic.
生成物比反应物低：负值，放热。
Products higher: positive, endothermic.
生成物更高：正值，吸热。
And one more rule the exam loves.
还有一条考试很爱考的规则。
Enthalpy is extensive.
焓是广延量。
It belongs to the equation as written, so double every coefficient and you double the enthalpy change.
它属于所写出的那个方程， 所以把每个系数加倍，焓变也加倍。
Where does that energy come from in the first place?
这些能量最初到底来自哪里？
From bonds.
来自化学键。
Breaking a bond always costs energy: you are pulling atoms apart.
断开一个键总是要消耗能量：你在把原子拉开。
Making a bond always gives energy back.
形成一个键总是把能量还回来。
So take hydrogen and chlorine up to the top of this diagram, where every atom is separate.
所以先把氢和氯一路带到这张图的顶端，那里每个原子都是分开的。
Breaking those two bonds costs six hundred and seventy-eight kilojoules.
断开这两个键要花六百七十八千焦。
Now let the atoms pair up into two hydrogen chloride bonds: that releases eight hundred and sixty-two.
现在让原子两两配对，形成两个氯化氢键： 这会放出八百六十二。
More came out than went in, so the enthalpy change is negative.
放出的比吸收的多，所以焓变是负的。
Bonds broken minus bonds made, in that order.
断键减去成键，顺序不能反。
There is a second route to the same number, and it needs no bond data at all, just a table.
还有第二条路可以得到同样的数字，而且完全不需要键能数据，只要一张表。
The standard enthalpy of formation is the enthalpy change when one mole of a compound is made from its elements.
标准生成焓，是由元素单质生成一摩尔化合物时的焓变。
An element already in its standard state is made from itself, so its value is zero.
已经处于标准状态的单质是由它自己生成的，所以它的值是零。
Remember that one, it saves you every time.
记住这一条，每次都能帮你省事。
Below it is the enthalpy of combustion, a different quantity: one mole burned completely in oxygen.
下面一行是燃烧焓，那是另一个量：一摩尔物质在氧气中完全燃烧。
Let's burn methane.
我们来烧甲烷。
Methane plus two oxygen give carbon dioxide plus two water.
甲烷加两个氧气，生成二氧化碳加两个水。
The formation values are given: methane, minus seventy-five.
生成焓已经给出：甲烷，负七十五。
Carbon dioxide, minus three hundred and ninety-four.
二氧化碳，负三百九十四。
Water, minus two hundred and eighty-six.
水，负二百八十六。
Oxygen is an element in its standard state, so it is zero.
氧气是处于标准状态的单质，所以是零。
Now the rule: products minus reactants.
现在用规则：生成物减反应物。
The products add up to minus nine hundred and sixty-six, and watch that two, because two moles of water are made.
生成物加起来是负九百六十六，注意那个二，因为生成了两摩尔水。
The reactants are just minus seventy-five.
反应物只有负七十五。
Subtract, and the enthalpy change is minus eight hundred and ninety-one kilojoules.
相减，焓变就是负八百九十一千焦。
A big release, exactly what you expect when a fuel burns.
放出很多能量，这正是燃料燃烧时你会预期的。
Some reactions can never be measured directly.
有些反应永远无法直接测量。
Hess's law rescues you.
盖斯定律救了你。
Enthalpy is a state function: the enthalpy change depends only on where you start and where you finish, never on the route between them.
焓是状态函数：焓变只取决于你从哪里开始、在哪里结束，而与中间走哪条路无关。
So the direct arrow across the top must equal the two arrows that go the long way round.
所以顶上那条直接的箭头，必须等于绕远路的那两条箭头之和。
Add up the steps you can measure, and you have the answer you could not.
把你能测的步骤加起来，就得到了你测不出的那个答案。
Here is how that looks in an exam.
在考试里它是这样的。
You want carbon plus half an oxygen molecule making carbon monoxide.
你想求碳加半个氧分子生成一氧化碳。
You cannot measure it, because some of the carbon monoxide always burns further.
你无法直接测量，因为总有一部分一氧化碳会继续燃烧。
But you are given two reactions you can measure: carbon burning to carbon dioxide, minus three hundred and ninety-four, and carbon monoxide burning to carbon dioxide, minus two hundred and eighty-three.
但题目给了你两个可以测量的反应：碳燃烧生成二氧化碳，负三百九十四； 一氧化碳燃烧生成二氧化碳，负二百八十三。
The second points the wrong way, so turn it round, and reversing flips the sign to plus two hundred and eighty-three.
第二个方向反了，所以把它掉个头，而反转会把符号翻成正二百八十三。
Now add the two lines.
把两行相加。
Carbon dioxide now appears on both sides, so it cancels.
二氧化碳现在出现在两边，所以它被抵消。
The oxygen tidies up too, leaving one half on the left.
氧气也要整理一下，左边只剩下二分之一个。
And what is left is exactly the reaction you wanted.
剩下的正好是你要的反应。
Minus three hundred and ninety-four plus two hundred and eighty-three is minus one hundred and eleven kilojoules.
负三百九十四加二百八十三，等于负一百一十一千焦。
Always write the reversed step out in full: that is where marks get lost.
一定要把反转后的那一步完整写出来，因为分数常常就丢在这里。
Four marks students throw away.
四个学生常丢的分。
One: every enthalpy change needs a sign and a unit.
第一：每个焓变都需要符号和单位。
Minus means exothermic, and a bare number scores nothing.
负号表示放热，只写一个光秃秃的数字得不到分。
Two: in the heat equation, use the mass of the water or the solution, never the mass of the solid you dropped into it.
第二：在热量公式里，用水或溶液的质量，绝不要用你放进去的固体的质量。
Three: bonds broken minus bonds made, in that order, because breaking takes energy in and making gives it out.
第三：断键减去成键，顺序不能反，因为断键吸收能量、成键放出能量。
Four: in a Hess cycle, reverse a step and you flip the sign; double a step and you double its enthalpy change.
第四：在盖斯循环里，把一步反转就要翻符号；把一步加倍，它的焓变也加倍。
Get those right and Unit Six is yours.
把这四点做对，第六单元就是你的了。