Chemical energetics (A2)
A-Level Chemistry Topic 23 8:02 English narration · English + 中文 subtitles burned in
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Sodium is a soft metal that burns in air. Chlorine is a choking, poisonous gas.
钠是一种在空气中会燃烧的软金属，氯是一种令人窒息的有毒气体。
Put them together and you get table salt.
把它们放在一起，得到的却是食盐。
Making one mole of it gives out a great deal of energy: four hundred and eleven kilojoules.
生成一摩尔食盐会放出大量能量：四百一十一千焦。 那么这些能量从哪里来？
So where does it come from? One step in that reaction gives out far more than all the rest.
这个反应中有一步放出的能量，比其余所有步骤加起来还多。
Welcome back to chemical energetics.
欢迎回到化学能量学。
This time we go deeper: an energy nobody can measure, what happens when a salt dissolves, entropy, and the equation that decides whether a reaction can happen at all.
这一次我们走得更深：一种谁也无法直接测量的能量、 盐溶解时到底发生了什么、全新的一个量——熵， 以及那条决定反应到底能不能发生的方程。
Two definitions, word for word.
两个定义，要一字不差。
The lattice energy is the enthalpy change when one mole of a solid ionic lattice forms from its gaseous ions.
晶格能是指一摩尔固体离子晶格由其气态离子形成时的焓变， 它永远是负值。
It is always negative. The enthalpy change of atomisation is the energy needed to make one mole of gaseous atoms from the element, and it is always positive.
原子化焓变是指由元素生成一摩尔气态原子所需的能量，它永远是正值。
Two phrases carry the marks: one mole, and gaseous.
两个定义的得分点都在两个词上：一摩尔，以及气态。
And nobody can measure a lattice energy directly — you cannot bottle gaseous ions.
而没有人能直接测量晶格能——你无法把气态离子装进瓶子里。
Now the definition students get wrong.
接下来是学生最容易写错的定义。
The first electron affinity is the enthalpy change when one mole of gaseous atoms each gain one electron.
第一电子亲和能是指一摩尔气态原子各得到一个电子时的焓变。
For chlorine it gives out three hundred and forty-nine kilojoules.
对氯来说，这个过程放出三百四十九千焦。
The second electron affinity is the opposite: you push an electron onto an ion that is already negative, they repel, and so it costs energy.
第二电子亲和能正好相反： 你把一个电子推到一个已经带负电的离子上，两者互相排斥，因此要消耗能量。
And down Group sixteen or seventeen the atom gets bigger, so it holds the electron less strongly.
而沿第十六族或第十七族往下，原子变大，对这个电子的束缚就更弱。
Watch the Born-Haber cycle build.
来看玻恩–哈伯循环（Born–Haber cycle）是怎么搭起来的。
Start with sodium metal and chlorine gas.
从钠金属和氯气开始。
Climb up: make gaseous atoms, then pull an electron off the sodium.
先往上走：生成气态原子， 再从钠原子上拿走一个电子。
Come down: the chlorine atom takes that electron, and the ions crash together into the solid.
然后往下走：氯原子接受这个电子，两种气态离子相遇， 结合成固体。
Every step can be measured in a laboratory except the last.
除了最后一步，其余每一步都可以在实验室里测出来。
The journey must add up to the enthalpy change of formation, so the lattice energy is whatever is left over.
而由于整条路线加起来必须等于生成焓变，晶格能就是剩下的那一部分。
So how do you build one?
那么该怎么搭这个循环？
There are two routes from the elements to the ionic solid.
从元素到离子固体有两条路线。
The short way is a single step, the enthalpy change of formation.
短的一条只有一步：生成焓变。
The long way climbs through atomisation, ionisation and electron affinity, then drops as the lattice forms.
长的一条要经过原子化、电离、电子亲和，最后随着晶格形成而下降。
By Hess's law the two must be equal, so the missing value falls out.
根据盖斯定律，两条路线必须相等，于是你不知道的那个值就直接解出来了。
Against an arrow, change the sign.
逆着箭头走，就要变号。
And watch the multiplier: two chloride ions mean two atomisation steps and two electron affinities.
还要当心系数：两个氯离子就意味着两步原子化和两个电子亲和能。
Let's do one properly.
我们来认真做一道。
Find the lattice energy of magnesium chloride from these six values.
用这六个数据求氯化镁的晶格能。
First the multipliers: there are two chloride ions, so double both chlorine steps, and magnesium loses two electrons, so use both ionisation energies.
先看系数：这里有两个氯离子， 所以两个与氯有关的步骤都要乘以二；镁失去两个电子，所以第一和第二电离能都要用上。
Now add up everything except the lattice energy: plus one thousand eight hundred and eighty-three.
接着把除晶格能以外的所有步骤加起来，得到正一千八百八十三。
Hess's law closes the cycle, and the lattice energy is minus two thousand five hundred and twenty-four kilojoules per mole.
盖斯定律闭合这个循环，于是晶格能等于负六百四十一减去这个总和， 也就是负二千五百二十四千焦每摩尔。
What makes a lattice energy large?
什么会让晶格能变大？
Only two things.
只有两件事。
First, the ionic charge: sodium chloride, from singly charged ions, is minus seven hundred and eighty-seven, while magnesium oxide, from doubly charged ions, is minus three thousand seven hundred and ninety-one.
第一，离子电荷：氯化钠由带一个电荷的离子构成， 是负七百八十七；氧化镁由带两个电荷的离子构成，是负三千七百九十一。
Second, the ionic radius: sodium fluoride beats potassium fluoride, because the smaller ion sits closer.
第二，离子半径：氟化钠强过氟化钾，因为较小的离子靠得更近。
Put them together and you have charge density.
把两者合起来，就是电荷密度。
Now dissolve that solid.
现在把这种固体溶到水里。
Think of it as two steps.
可以把它看成两步。
First you pull the lattice apart into gaseous ions, the reverse of the lattice energy, which costs a lot.
第一步，把晶格拆散成气态离子， 这是晶格能的逆过程，要消耗很多能量。
Then water surrounds each ion and holds it: the enthalpy change of hydration, always exothermic.
第二步，水把每个离子包围住并抓牢， 这就是水合焓变，它总是放热的。
Small, highly charged ions are hydrated most strongly.
半径小、电荷高的离子水合得最强。
The arrow straight across is what you actually measure, the enthalpy change of solution.
底下那条横着的箭头才是你真正能测到的量：溶解焓变。
Add the two steps and the cycle is done.
把这两步加起来，循环就完成了。
A real exam question: find the enthalpy change of solution of magnesium fluoride.
这是一道真题：求氟化镁的溶解焓变。
Pause the video and try it. Ready? The trap is the formula.
先暂停视频，自己试一试。
There are two fluoride ions, so you need two lots of hydration for the fluoride, and one for the magnesium: minus two thousand nine hundred and thirty-six.
好了吗？ 陷阱藏在化学式里：这里有两个氟离子，所以氟的水合焓要算两份，再加上镁的一份， 合计为负二千九百三十六。
Then reverse the lattice energy, because we are breaking the solid: plus two thousand nine hundred and fifty-seven.
然后把晶格能反过来，因为我们是在拆开固体， 于是得到正二千九百五十七。
Add them: plus twenty-one kilojoules per mole.
两者相加：正二十一千焦每摩尔。
A four-mark favourite: why does solubility change down a group?
这是一道四分的常客：为什么沿着族往下走，溶解度会变化？
The metal ion gets bigger, so the lattice energy and the hydration enthalpy weaken together — both of them become less exothermic.
往下走，金属离子变大， 于是晶格能和水合焓一起变弱——两者都变得不那么放热。
With a small anion, a fluoride or a hydroxide, the lattice energy changes more, so solubility rises down the group.
如果阴离子很小， 比如氟离子或氢氧根，晶格能变化更大，所以往下走化合物更易溶。
With a large anion, a sulfate, hydration wins instead, and solubility falls.
如果阴离子很大，比如硫酸根，水合焓占了上风，溶解度就下降。
The mark is never for saying both get weaker; it is for saying which one changes more.
得分点从来不是"两者都变弱"，而是说清哪一项变化更大。
Now something new.
现在来看一个全新的概念。
Entropy is the number of ways the particles and their energy can be arranged in a system.
熵是指在一个体系中，粒子及其能量可以被排列的方式数目。
A solid is locked in a lattice, so there are few ways; a liquid has more, a gas far more.
固体被锁在晶格里，排列方式很少；液体多一些，气体则多得多。
Melting, boiling, dissolving and heating all raise it.
所以熔化、沸腾、溶解，或者仅仅是加热，都会让熵变大。
In the exam, the quickest check is the number of gas molecules. More gas, positive; fewer gas molecules, negative.
考试中最快的判断方法是看气体分子的数目：生成的气体变多，熵变为正； 气体分子变少，熵变为负。
And given standard entropies, take products minus reactants.
而当题目给出标准熵时，就用产物减去反应物。
Neither enthalpy nor entropy alone can tell you whether a reaction will happen.
单靠焓变无法告诉你反应会不会发生，单靠熵变也不行。
One equation puts them together: the Gibbs free energy change equals the enthalpy change, minus the temperature in kelvin times the entropy change.
有一条方程把它们结合起来： 吉布斯自由能变等于焓变，减去以开尔文为单位的温度乘以熵变。
Negative or zero means feasible — that is feasibility.
结果为负或为零，反应就可行——这就是可行性。
Four cases: entropy rising and exothermic works always; rising and endothermic only when hot; falling and exothermic only when cold; falling and endothermic, never.
一共有四种情况：放热且熵增，在任何温度下都可行；吸热且熵增，只有温度足够高才可行； 放热且熵减，只有低温才可行；吸热且熵减，则永远不可行。
One warning: check the units.
计算之前有一个提醒：先检查单位。
Limestone is heated to make quicklime for cement, taking in one hundred and seventy-eight kilojoules per mole.
把石灰石加热制生石灰——水泥的原料——这个反应要吸收一百七十八千焦每摩尔。
Start with the entropy change: products minus reactants gives plus one hundred and sixty point four, positive because a gas is made.
先求熵变：产物减反应物，得到正一百六十点四，是正值，因为有气体生成。
At room temperature, in kilojoules, the Gibbs free energy change is plus one hundred and thirty — nothing happens.
接着在室温下、以千焦为单位计算：吉布斯自由能变是正一百三十，所以什么也不会发生。
Set it to zero and the changeover temperature is eleven hundred and ten kelvin.
再把它设为零，转变温度就是一千一百一十开尔文。
That is why a lime kiln runs red hot.
这正是石灰窑要烧得通红的原因。
Look again at the cold pack you met at AS.
再看看你在 AS 阶段见过的速冷冰袋。
Squeeze the bag and it drops close to freezing.
捏一下袋子，它就降到接近冰点。
Ammonium nitrate is dissolving, and that takes energy in: the enthalpy change of solution is plus twenty-six.
里面是硝酸铵在溶解，而这个过程要吸收能量：溶解焓变是正二十六。
On enthalpy alone that should never happen.
只看焓变，这件事根本不该发生。
But an ordered lattice becomes free ions in water, so the entropy change is large and positive, and it more than pays.
但整齐有序的晶格变成了水中自由移动的离子， 所以熵变是一个很大的正值，足以抵偿还有余。
The Gibbs free energy change comes out negative.
吉布斯自由能变算出来是负的。
Entropy is driving this, not enthalpy.
推动它的是熵，不是焓。
Three marks students throw away.
三个学生常丢的分。
First, the multipliers: if the formula holds two of an ion, every step for that ion is doubled, and one slip ruins the whole calculation.
第一，系数：如果化学式里有两个某种离子， 与它有关的每一步都要乘以二，这里错一次，整道计算就全毁了。
Second, the units: entropy comes in joules, enthalpy in kilojoules, so divide by one thousand before you subtract.
第二，单位：熵的单位是焦每开，而焓是千焦，所以相减之前先除以一千。
Third, in a solubility question, say which term changes more.
第三，任何溶解度题目，都要说清哪一项变化更大。
Get those right, and this topic is yours.
把这三点做对，这个专题就是你的了。