Properties of Substances and Mixtures
AP Chemistry Topic 3 10:54 English narration · English + 中文 subtitles burned in
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Transcript
Boiling water.
沸腾的水。
To make steam you must reach one hundred degrees Celsius.
要把它变成水蒸气，必须加热到一百摄氏度。
Methane has almost the same mass, and boils at minus one hundred and sixty-two.
甲烷分子的质量几乎相同，沸点却是零下一百六十二度。
Nothing inside the molecules explains a gap that big.
分子内部的结构无法解释这么大的差距。
The answer is the forces between them.
答案在于分子之间的作用力。
These are the intermolecular and interparticle forces — attractions between molecules, not inside them.
分子间作用力是分子与分子之间的吸引，不在分子内部。
Here is the ladder.
这就是它们的强弱阶梯。
Dispersion exists in every molecule, and grows with the electron cloud; its full name is London dispersion forces, and it comes from temporary, fluctuating dipoles.
色散力存在于每一个分子中，电子云越大，色散力越强； 它的全名是伦敦色散力，来自瞬时的、不断涨落的偶极。
Dipole-dipole is extra, and only polar molecules have it — its strength depends on the size of the dipoles and their relative orientation, a delta-plus end lining up with a neighbour's delta-minus.
偶极-偶极作用是额外多出来的，只有极性分子才有—— 它的强弱取决于偶极的大小，以及它们的相对取向： 一端的 δ+ 正好对上邻居的 δ−。
Hydrogen bonding is much stronger, but only when hydrogen is bonded to nitrogen, oxygen or fluorine.
氢键强得多，但只在氢与氮、氧或氟相连时才出现。
Ion-dipole is the strongest here.
离子-偶极作用在这里最强。
You judge the whole ladder qualitatively, by looking at the sign of the partial charges: a larger or better-aligned partial charge, or a full ionic charge, pulls harder.
整个阶梯可以定性地判断， 只要看部分电荷的正负与大小：部分电荷越大、排列得越好，或者带整数电荷，吸引就越强。
Now the evidence.
现在看证据。
Fluorine and hydrogen chloride weigh almost the same, yet the polar one boils a hundred degrees higher.
氟气和氯化氢的质量几乎一样，但有极性的那个沸点高出一百度。
Water is the lightest, and boils highest.
水是三者中最轻的，沸点却最高。
Start at the bottom of the ladder.
先从阶梯的最下面开始。
In any molecule the electrons keep moving, so for an instant more of them sit on one side: a temporary dipole.
任何分子里的电子都在不停运动， 所以某一瞬间会有更多电子偏到一侧，这就形成了瞬时偶极。
It pulls the neighbour's electrons and induces a dipole there too, so the two attract.
它拉动邻近分子的电子，在那里也诱导出一个偶极，两者相互吸引。
Bigger molecules hold more electrons in a softer cloud, so the force grows.
分子越大，电子越多，电子云越松软，这种力也就越强。
Now the top of the ladder.
再看阶梯的最上面。
When hydrogen is bonded to nitrogen, oxygen or fluorine, that atom pulls the shared electrons hard, and the hydrogen is left very positive.
当氢与氮、氧或氟相连时， 那个原子把共用电子拉得很紧，氢就带上很强的正电性。
It is then strongly attracted to a lone pair on the next molecule.
于是它被邻近分子上的一对孤对电子强烈吸引。
The dashed line is the hydrogen bond: between molecules, never inside one.
图中的虚线就是氢键：它是分子之间的作用力，绝不是分子内部的化学键。
A real exam question.
这是一道真实的考题。
Tetrabromomethane boils at four hundred and sixty-three kelvin; the mixed halogen compound at two hundred and sixty-nine.
四溴甲烷的沸点是四百六十三开尔文， 而那个混合卤代化合物只有二百六十九开尔文。
Why?
为什么？
First, list the forces.
第一步，列出各自的作用力。
The first molecule is symmetrical, so it is nonpolar: dispersion only.
第一个分子是对称的，所以没有极性，只有色散力。
The second one is polar, so it also has dipole-dipole.
第二个分子有极性，所以还多了偶极-偶极作用。
Choose the polar one and you fall into the trap.
如果你选了极性的那个，就掉进了陷阱。
The first has many more electrons in a much larger, softer cloud, so its dispersion alone beats both forces in the other.
第一个分子的电子多得多，电子云更大也更松软， 仅色散力就强过另一个分子的两种力之和。
Compare the totals, never one force.
要比较的是总和，绝不是单看某一种力。
Now solids.
现在看固体。
Match properties to structure.
把性质和结构对应起来。
Giant ionic: strong attraction between ions, high melting point, brittle, conducting only when the ions can move.
巨型离子晶体：离子之间吸引很强，熔点高，性脆， 只有当离子能够移动时才导电。
Simple molecular, like ice: weak forces between the molecules, so it melts low.
简单分子晶体，如冰： 分子之间的力很弱，所以熔点低。
Giant covalent, like diamond — the AP name is covalent-network: melting means breaking covalent bonds, so it melts very high.
巨型共价晶体，如金刚石，AP 里叫共价网络固体：熔化它就要打断共价键，所以熔点非常高。
Graphite is the exception worth knowing: also covalent-network, also high-melting, but soft, because its flat layers slide over one another.
石墨是值得记住的例外：同样是共价网络，同样熔点很高，却很软， 因为它的平面层可以彼此滑动。
Metallic: free valence electrons, so it conducts heat and electricity, and it is malleable and ductile — the ions slide past each other without breaking the bonding.
金属晶体：有自由价电子，所以导电导热，而且可锻、可延展—— 金属离子可以互相滑过，而不破坏金属键。
One last split, across all four: a solid is either crystalline, with its particles in a regular repeating three-dimensional arrangement, or amorphous, with no long-range order at all, like glass.
最后还有一个贯穿四类的划分：固体要么是晶体，粒子在三维空间里规则重复排列； 要么是非晶体，完全没有长程有序，比如玻璃。
Now warm it up again.
现在再加热。
In a solid the particles touch and stay in place; they only vibrate.
固体中的粒子彼此接触、位置固定，只能在原地振动。
Add energy and they break the pattern: in a liquid they still touch, but slide past each other.
加入能量后，它们打破了规则排列：在液体中仍然彼此接触，却能相互滑动。
Add more and they fly apart.
再加能量，它们就四散飞开。
A gas is mostly empty space, so it has no fixed shape or volume.
气体中大部分是空隙， 所以既没有固定形状，也没有固定体积。
For gases, one equation ties it all together: pressure times volume equals moles, times the gas constant, times temperature.
对气体来说，一个方程把一切联系起来：压强乘体积， 等于物质的量乘气体常数再乘温度。
Three rules.
三条规则。
Temperature is always in kelvin.
温度一律用开尔文。
The gas constant must match your units.
气体常数必须与你的压强和体积单位相匹配。
When the gas is sealed in, pressure times volume over temperature stays constant.
当气体被密封时，压强乘体积再除以温度保持不变。
And in a mixture, each gas adds its own partial pressure.
而在混合气体中，每种气体各自贡献自己的分压。
A released question.
再看一道真题。
Hydrogen chloride fills a rigid six litre container at seven point four five atmospheres and two hundred and ninety-six kelvin.
氯化氢气体装在一个刚性的六升容器里， 压强为七点四五个大气压，温度为二百九十六开尔文。
Part one: how many moles?
第一问：有多少摩尔？
Rearrange, so moles equals pressure times volume, over the gas constant times temperature.
变形一下，物质的量等于压强乘体积， 再除以气体常数乘温度。
That gives one point eight four moles.
这就得到一点八四摩尔。
Part two: the gas is cooled to two hundred and seventy-one kelvin.
第二问：气体被冷却到二百七十一开尔文。
Because the container is rigid and sealed, volume and moles are fixed, so pressure over temperature is constant.
因为容器是刚性且密封的，体积和物质的量都不能改变， 所以压强除以温度保持不变。
Six point eight two atmospheres.
六点八二个大气压。
Why does that equation work?
这个方程为什么成立？
Kinetic molecular theory.
因为分子运动论。
Gas particles are tiny compared with the space between them, they move constantly and randomly, they bounce without losing energy, and they do not attract.
气体粒子与它们之间的空隙相比非常小，它们不停地做无规则运动， 碰撞时不损失能量，彼此之间也没有吸引。
Pressure is simply particles hitting the walls, and the kelvin temperature is just their average kinetic energy.
压强不过是粒子撞击容器壁的结果，而开尔文温度就是它们的平均动能。
They do not all move at the same speed.
粒子并不是都以相同的速率运动。
This curve shows the spread: a peak near a typical speed, and a long tail of fast ones.
这条曲线画出了速率的分布： 在某个典型速率附近有一个峰，右边拖着一条长长的高速尾巴。
Heating shifts the whole curve right and flattens it.
加热后，整条曲线向右移动并且变平。
At one temperature every gas has the same average kinetic energy, so a lighter molecule moves faster.
在同一温度下，所有气体的平均动能相同，所以越轻的分子必然运动得越快。
No gas is truly ideal: two of those assumptions break.
没有真正理想的气体：其中两条假设并不成立。
Molecules do attract each other, so one leaving the wall is pulled back and hits a little softer, and the real pressure is lower.
分子之间确实互相吸引，所以一个离开器壁的分子会被拉回来； 它撞得稍轻一些，实际压强就更低。
And molecules do take up space, so the free volume is less than the container, which pushes pressure up.
分子也确实占据体积，可用的空间比容器小，这又会把压强推高。
Gases stray most at high pressure and low temperature.
气体在高压和低温下偏离得最厉害。
A solution is a mixture that is the same everywhere: solute dissolved in solvent.
溶液是各处都相同的混合物：溶质溶解在溶剂中。
We measure it with molarity — moles of solute per litre of solution.
我们用摩尔浓度来衡量它——每升溶液中溶质的摩尔数。
Dilution: adding water changes the volume but not the moles, so the first molarity times the first volume equals the second.
稀释：加水改变的是体积，不是摩尔数， 所以第一个浓度乘第一个体积，等于第二个。
Solubility is how much solute will dissolve, and it follows one phrase: like dissolves like.
溶解度就是能溶解多少溶质，它只看一句话：相似相溶。
Polar and ionic solutes dissolve in polar solvents, nonpolar in nonpolar — dissolving happens when the new solute-solvent attractions are about as strong as the ones being broken.
极性和离子型溶质溶于极性溶剂，非极性溶于非极性—— 当新形成的溶质-溶剂吸引与被破坏的吸引强度相当时，溶解就会发生。
Put a number through that, because dilution questions hide a trap.
给它代个数，因为稀释题里藏着一个陷阱。
You have fifty millilitres of six molar hydrochloric acid, and you want two molar.
你有五十毫升六摩尔每升的盐酸，想把它变成两摩尔每升。
Start from the idea, not the formula: the moles of hydrogen chloride never change when you add water, so the first molarity times the first volume equals the second molarity times the second volume.
从道理出发，而不是从公式出发：加水时盐酸的摩尔数不会变， 所以第一个浓度乘第一个体积，等于第二个浓度乘第二个体积。
Rearrange for the second volume: six times fifty, over two.
解出第二个体积：六乘五十，除以二。
So the final volume is one hundred and fifty millilitres.
所以最终体积是一百五十毫升。
Now the trap.
现在说陷阱。
The question asked how much WATER to add, and the equation did not give you that — it gave you the volume you finish with.
题目问的是要加多少水，而这个方程并没有给你这个数—— 它给的是你最后得到的体积。
So subtract the fifty you started with: you add one hundred millilitres of water.
所以要减掉你一开始的五十： 你加的是一百毫升水。
Read the last line of a dilution question twice; final volume and added volume are different numbers.
稀释题的最后一句话要读两遍；最终体积和加入的体积，是两个不同的数。
Now the representation the exam wants: a particulate diagram.
再看考试要你画的那种微粒图。
Put an ionic solid into water and it does not stay whole: it separates into free ions, fully dissociated.
把离子固体放进水里，它不会保持完整， 它会解离成自由的离子。
Each positive ion is surrounded by water molecules turning their negative oxygen ends inwards, and each negative ion by the positive hydrogen ends.
每个正离子周围都是水分子， 把带负电的氧端转向内侧；每个负离子周围则是带正电的氢端朝内。
Those ion-dipole attractions are what dissolving is.
这些离子-偶极吸引，正是溶解的本质。
So draw the ions apart, the water turned towards them, and count the particles.
所以要把离子分开画，把水分子朝向它们画，并且数清粒子的个数。
Each part of a mixture keeps its own properties, so separation can be done by physical methods.
混合物中每一种成分都保留着自己的性质，所以我们可以用物理方法把它们分开。
Chromatography uses intermolecular forces: a component held tightly by the paper hardly moves, one that prefers the solvent travels far.
色谱法利用的是分子间作用力：被纸吸附得很牢的成分几乎不移动， 而更喜欢溶剂的成分则跑得很远。
Distillation uses boiling point, set by those same forces.
蒸馏利用的是沸点，而沸点同样由这些力决定。
Filtration only sorts by size.
过滤只能按颗粒大小来分离。
Light lets us study molecules without touching them, and that study is spectroscopy.
光让我们不用接触就能研究分子，这门研究就叫光谱学。
A photon's energy is Planck's constant times its frequency, so shorter wavelength means higher energy.
光子的能量等于普朗克常量乘频率， 所以波长越短，能量越高。
A molecule absorbs a photon only when it matches an energy gap.
只有当能量正好等于某个能级差时，分子才会吸收它。
So different regions of the electromagnetic spectrum probe different changes. Microwaves match rotation, infrared the vibration of bonds, ultraviolet and visible light the jumps of electrons.
所以电磁波谱的不同区域探测的是不同的变化： 微波对应转动，红外对应化学键的振动，紫外和可见光则对应电子的跃迁。
Chemists use that every day, through the Beer-Lambert law.
化学家每天都在用它，也就是比尔-朗伯定律。
Absorbance equals the molar absorptivity, times the path length, times the concentration.
吸光度等于摩尔吸光系数，乘光程，再乘浓度。
Keep the wavelength and the cuvette the same, and absorbance is proportional to concentration.
只要波长和比色皿保持不变，吸光度就与浓度成正比。
Plot absorbance against concentration for a few standards, and read your unknown off the straight line.
用几个标准溶液画出吸光度对浓度的图，再从这条直线上读出未知溶液的浓度。
One number, to make that real.
代一个数，把它变具体。
A dye has a molar absorptivity of two thousand litres per mole per centimetre.
有一种染料，摩尔吸光系数是每摩尔每厘米两千升。
In a one centimetre cell, a sample reads an absorbance of nought point four zero.
在一厘米的比色皿中，一个样品测得的吸光度是零点四零。
Find its concentration.
求它的浓度。
Rearrange for concentration: absorbance divided by the molar absorptivity times the path length.
解出浓度：吸光度除以摩尔吸光系数乘光程。
Put the numbers in — nought point four zero, over two thousand times one — and the concentration is two times ten to the minus four molar.
代入数字——零点四零，除以两千乘一——浓度就是二乘十的负四次方摩尔每升。
Now see why the calibration curve works.
现在就能看出标准曲线为什么有效。
Absorbance is proportional to concentration, so double the concentration and the absorbance doubles too, to nought point eight zero.
吸光度与浓度成正比， 所以浓度加倍，吸光度也加倍，变成零点八零。
That proportionality is the straight line you plot, and it is why one cell and one wavelength must be used for every standard.
这个正比关系就是你画出的那条直线， 也正因为如此，每个标准溶液都必须用同一个比色皿、同一个波长。
Three marks students throw away.
三个学生常丢的分。
First, when you compare boiling points, list every force in each substance and compare the totals — a big nonpolar molecule can beat a small polar one.
第一，比较沸点时， 要把每种物质中所有的作用力都列出来，再比较它们的总和—— 一个大的非极性分子完全可能胜过一个小的极性分子。
Second, boiling breaks the forces between molecules, never the bonds inside them.
第二，沸腾破坏的是分子之间的力，绝不会打断分子内部的键。
Third, in any gas calculation, convert to kelvin first.
第三，做任何气体计算，先把温度换算成开尔文。