Chemical Energetics
A-Level Chemistry Topic 5 8:21 English narration · English + 中文 subtitles burned in
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A Bunsen burner gets hot enough to soften glass.
本生灯能热到把玻璃烧软。
An instant cold pack, squeezed once, drops close to freezing.
而一只速冷冰袋，捏一下就降到接近冰点。
Both are just chemical reactions — one in a flame, one inside a plastic bag — and the only real difference between them is the sign of a single number.
两者都只是化学反应——一个在火焰里，一个在塑料袋里—— 而它们之间真正的区别，只是一个数字的正负号。
Learn to read that number, and you can say in advance whether a reaction will warm the room or chill it.
学会读懂这个数字，你就能事先说出一个反应会让房间变暖还是变冷。
Welcome to chemical energetics.
欢迎来到化学能量学。
We will define the enthalpy change, calculate it in two completely different ways, and then use Hess's law to reach the values you cannot measure at all.
我们将给焓变下定义，用两种完全不同的方法计算它， 然后用盖斯定律求出那些根本无法直接测量的数值。
Let's begin.
让我们开始吧。
Every chemical reaction takes in or gives out energy.
每一个化学反应都会吸收或放出能量。
Measured at constant pressure, that energy change is called the enthalpy change, and its symbol is delta H.
在恒压下测得的这个能量变化叫做焓变， 符号是德尔塔 H。
If the reaction gives out heat, the surroundings warm up, the products end up holding less energy than the reactants, and delta H is negative.
如果反应放出热量，周围环境升温，产物所含的能量比反应物少， 焓变为负——这叫放热反应。
We call that exothermic. If the reaction takes heat in, the surroundings cool down, the products hold more energy than the reactants, and delta H is positive. That one is endothermic.
如果反应吸收热量，周围环境降温， 产物所含的能量比反应物多，焓变为正——这叫吸热反应。
Burning is exothermic. The cold pack is endothermic.
燃烧是放热的， 冰袋是吸热的。
Get the sign right, and half the marks in this topic are already yours.
把正负号弄对，这一章一半的分数就已经是你的了。
We draw all of this on a reaction pathway diagram.
我们把这一切画在反应路径图上。
Energy runs up the vertical axis, and the reaction runs from left to right.
纵轴表示能量，反应从左往右进行。
The reactants start at one level, the products finish at another, and delta H is simply the gap between them.
反应物从一个能级出发，产物停在另一个能级，而焓变就是两者之间的差距。
In an exothermic reaction the products finish lower, so the arrow points down. In an endothermic reaction they finish higher, so it points up.
放热反应中产物更低，所以箭头向下；吸热反应中产物更高，所以箭头向上。
And in both cases there is a hill in between.
而两种情况中间都有一座"能量山"。
The height of that hill, measured up from the reactants, is the activation energy — the least energy the particles need before they can react at all.
这座山从反应物量起的高度就是活化能—— 粒子必须先获得的最低能量，否则根本无法反应。
To compare values fairly, we fix the conditions.
为了公平比较数值，我们把条件固定下来。
Standard conditions are two hundred and ninety-eight kelvin and one hundred and one kilopascals, with every substance in its normal physical state. That is what the little circle on the symbol means.
标准条件是二百九十八开尔文、 一百零一千帕，且每种物质都处于其正常的物理状态——符号上那个小圆圈就是这个意思。
Then there are four definitions to learn, and the exam wants them almost word for word.
接着有四个定义要背，考试几乎要求一字不差。
Enthalpy change of reaction: for the amounts shown in the equation.
反应焓变：按方程式中所示的物质的量。
Of formation: one mole of a compound made from its elements.
生成焓变：由元素单质生成一摩尔化合物。
Of combustion: one mole of a substance burned completely in oxygen.
燃烧焓变：一摩尔物质在氧气中完全燃烧。
And of neutralisation: one mole of water made from an acid and an alkali.
中和焓变：由酸和碱生成一摩尔水。
So where does the energy actually come from?
那么能量究竟从哪里来？
During a reaction, old bonds break and new bonds form.
在反应过程中，旧的键断裂，新的键生成。
Breaking a bond always needs energy put in, so breaking is endothermic. Making a bond always releases energy, so making is exothermic.
断键总是需要吸收能量，所以断键是吸热的；成键总是放出能量，所以成键是放热的。
The enthalpy change of the whole reaction is just the difference between the two: the sum of the bonds broken, minus the sum of the bonds made.
整个反应的焓变就是两者之差：断键所需能量之和，减去成键放出能量之和。
If more energy comes out than went in, delta H is negative and the reaction is exothermic.
如果放出的比吸收的多，焓变为负，反应就是放热的。
And note — a bond energy is always a positive number, because it measures breaking.
还要注意—— 键能永远是正数，因为它衡量的是断键。
Let's use it.
我们来用一用。
Hydrogen plus chlorine gives two molecules of hydrogen chloride.
氢气加氯气生成两个氯化氢分子。
The bond energies are: hydrogen to hydrogen, four hundred and thirty-six; chlorine to chlorine, two hundred and forty-two; and hydrogen to chlorine, four hundred and thirty-one.
键能是： 氢—氢四百三十六，氯—氯二百四十二，氢—氯四百三十一。
Bonds broken: one hydrogen–hydrogen and one chlorine–chlorine, which is six hundred and seventy-eight.
断裂的键：一个氢—氢和一个氯—氯，共六百七十八。
Bonds made: two hydrogen–chlorine bonds, which is eight hundred and sixty-two.
生成的键：两个氢—氯键，共八百六十二。
Subtract, and delta H is minus one hundred and eighty-four kilojoules per mole.
相减，焓变等于负一百八十四千焦每摩尔。
Negative — exothermic, exactly as burning should be.
是负值——放热，正如燃烧应有的那样。
Before we leave bond energies, one thing the exam asks straight after that calculation: why does your answer not quite match the real value?
在离开键能之前，讲一件考试常常紧接着问的事： 为什么你算出来的答案和真实值对不上？
Here is why.
原因在这里。
Take the same C to H bond in four different molecules — methane, ethane, methanol, chloroform.
看同一个碳氢键在四种不同分子里的情况——甲烷、乙烷、甲醇、三氯甲烷。
Every one of them is a carbon-hydrogen bond, and every one takes a slightly different amount of energy to break, because the rest of the molecule around it is different.
它们每一个都是碳氢键，而每一个断裂时所需的能量都略有不同， 因为它周围的分子环境不一样。
So the data book prints one number for C to H, and that number is an average taken over many compounds — not the exact value in the molecule you were asked about.
所以数据手册上碳氢键只印一个数字，而那个数字是在许多化合物上取的平均值—— 并不是你被问到的那个分子里的确切值。
That is why a delta H worked out from bond energies is only an estimate.
这就是为什么用键能算出来的焓变只是一个估计值。
Two more things follow.
还有两点随之而来。
Bond energies apply to gases, so the calculation quietly assumes every species is gaseous, and it will be further out if your reactants or products are liquids or solids.
键能是对气体而言的， 所以这个计算默默假设了所有物种都是气态； 如果你的反应物或产物是液体或固体，偏差还会更大。
And a few bond energies are exact rather than averaged — the ones for a diatomic molecule like hydrogen or chlorine, where there is only one such bond in existence.
另外，有少数键能是精确值而不是平均值—— 比如氢气或氯气这类双原子分子的键，因为世界上这种键只有一种。
So when a question asks why your value differs, name it: average bond energies.
所以当题目问你为什么数值有差异时，把它点名说出来：平均键能。
You can also measure an enthalpy change directly, in the lab.
你也可以在实验室里直接测量焓变。
Run the reaction in an insulated cup, and watch the temperature of the water or the solution.
把反应放在绝热杯中进行， 观察水或溶液的温度变化。
The heat transferred is the mass, times the specific heat capacity, times the temperature change.
传递的热量等于质量乘以比热容再乘以温度变化。
Two traps live here, and between them they cost more marks than anything else.
这里有两个陷阱，它们加起来比其他任何地方都更容易失分。
Use the mass of the water or the solution — never the mass of the fuel you burned.
要用水或溶液的质量——绝不是你烧掉的燃料的质量。
And at the end, divide by the number of moles, then put a minus sign in front, so a temperature rise correctly gives a negative delta H.
最后再除以物质的量，并在前面加负号，这样温度升高才会正确地给出负的焓变。
Try it.
试一试。
Burning zero point five zero grams of methanol raises the temperature of one hundred grams of water by eighteen degrees.
燃烧零点五零克甲醇，使一百克水升温十八度。
Methanol's relative molecular mass is thirty-two, and the specific heat capacity of water is four point one eight.
甲醇的相对分子质量是三十二，水的比热容是四点一八。
First the heat: one hundred, times four point one eight, times eighteen, gives seven thousand five hundred and twenty joules.
先求热量： 一百乘以四点一八再乘以十八，得到七千五百二十焦耳。
Then the moles: zero point five zero divided by thirty-two is zero point zero one five six moles.
再求物质的量： 零点五零除以三十二等于零点零一五六摩尔。
Divide, add the minus sign, and the enthalpy change of combustion is about minus four hundred and eighty kilojoules per mole.
相除，加上负号， 燃烧焓变约为负四百八十千焦每摩尔。
One problem is left. Some enthalpy changes simply cannot be measured directly — the reaction may be far too slow, or it may make several products at once. Hess's law solves that.
还剩下一个问题：有些焓变根本无法直接测量——反应可能太慢， 也可能同时生成好几种产物。
It says the total enthalpy change is the same whichever route you take from the reactants to the products, because energy is conserved.
盖斯定律解决了这个问题。 它指出：无论你从反应物到产物走哪条路线，总的焓变都相同，因为能量守恒。
So watch: draw the direct route, then draw an indirect route through some intermediate. Both must add up to exactly the same total.
所以看：画出直接路线，再经由某个中间物画出间接路线， 两条路线加起来必须完全相等。
That gives you a cycle you can actually solve.
这样你就得到一个真正可解的循环。
Put the reactants and the products along the top, joined by the enthalpy change you want to find.
把反应物和产物放在上面一行， 用你要求的那个焓变连起来。
Then send both down to the same intermediate — usually the elements, if the question gives you formation data, or the combustion products, if it gives you combustion data.
然后让两者都向下指向同一个中间物—— 如果题目给的是生成焓数据，通常就是各元素单质；如果给的是燃烧焓数据， 就是燃烧产物。
Now follow the arrows around: the direct route equals the indirect route.
现在沿着箭头绕一圈：直接路线等于间接路线。
Rearrange, and the value you could not measure falls straight out.
变形一下，那个测不出来的数值就直接出来了。
Two rules keep you safe. Draw every arrow the correct way round. And if you travel against an arrow, change the sign.
有两条规则保你不出错： 每个箭头的方向都要画对；如果你逆着箭头走，就要改变正负号。
Three marks students throw away.
三个学生常丢的分。
First, the sign convention in bond energy questions: bonds broken minus bonds made.
第一，键能题的符号约定：断键减去成键。
Getting that the wrong way round is the classic error of this whole topic, and it flips the sign of every answer.
把它反过来是这一整章最经典的错误，而且会让每个答案的正负号都颠倒。
Second, in the heat equation use the mass of the water or the solution, not the mass of the fuel — then divide by moles before you quote delta H.
第二，热量公式中要用水或溶液的质量，而不是燃料的质量—— 然后在写出焓变之前先除以物质的量。
Third, define each enthalpy change with its exact standard conditions: combustion means one mole burned completely in excess oxygen.
第三，每个焓变都要写出其确切的标准条件： 燃烧是指一摩尔物质在过量氧气中完全燃烧。
And always give a sign and units.
并且永远要写上正负号和单位。