Acids and Bases
AP Chemistry Topic 8 8:24 English narration · English + 中文 subtitles burned in
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Two strips of paper — one red, one blue.
两条纸片——一条红色，一条蓝色。
Dip them in a liquid and the colour changes, and that colour tells you whether the liquid is an acid or a base.
把它们浸入液体，颜色就会改变， 而颜色告诉你这液体是酸还是碱。
Nothing else about it looks different.
液体本身看不出别的差别。
The paper is reporting how many hydrogen ions hide in the water.
纸片报告的，是水里藏着多少氢离子。
Unit Eight, in six steps.
第八单元，分六步。
What acids and bases really are, and how pH measures them.
酸和碱到底是什么，以及 pH 如何衡量它们。
Strong acids, where the answer is just the concentration.
强酸，答案就是浓度本身。
Weak acids, where it is not.
弱酸则不是。
Buffers and their equation.
缓冲溶液和它的公式。
Titrations and their curves.
滴定和滴定曲线。
And why structure decides strength.
以及结构为什么决定强弱。
Here is the Brønsted–Lowry definition the exam wants.
这就是考试要的定义。
An acid is a proton donor: it gives a hydrogen ion away.
酸是质子的给予者：它把一个氢离子送出去。
A base is a proton acceptor: it takes one.
碱是质子的接受者：它把氢离子接过来。
What an acid leaves behind can take that proton back, so it is a base — the conjugate base.
酸给出质子后剩下的部分能把质子再拿回去， 所以它是碱——共轭碱。
The two are a conjugate pair, one proton apart.
两者合称共轭酸碱对，只相差一个质子。
Water is amphoteric — it plays either part — so water itself always holds a few ions.
水两种角色都能扮演，所以水本身总是含有少量离子。
Look at pure water.
来看纯水。
Now and then one water molecule passes a proton to another.
时不时地，一个水分子会把一个质子传给另一个水分子。
That leaves a hydronium ion and a hydroxide ion — autoionization, which happens in every water solution.
这样就留下一个水合氢离子和一个氢氧根离子——这就是自偶电离，任何水溶液中都在发生。
Multiply the two ion concentrations and you always get the same number, the ion product of water.
把这两种离子的浓度相乘，总能得到同一个数，也就是水的离子积。
Take the negative logarithm of both sides and out comes the rule this unit runs on: pH plus pOH is fourteen.
对两边取负对数，就得出这一单元赖以运行的规则：pH 加 pOH 等于十四。
Careful — neutral means equal ions, not the number seven.
要小心——中性指的是两种离子相等，而不是数字七。
Warm the water and neutral sits below seven — Kw is temperature dependent.
把水加热，中性就落在七以下。
Here is that scale.
这就是那把尺子。
pH is the negative logarithm of the hydrogen ion concentration, from zero to fourteen.
pH 是氢离子浓度的负对数，从零到十四。
Below seven is acidic, seven neutral, above seven alkaline.
低于七是酸性，七是中性，高于七是碱性。
And it is logarithmic, so every step is a factor of ten: pH three holds a hundred times more hydrogen ions than pH five.
它是对数，所以每走一格就是十倍： pH 为三的溶液，氢离子比 pH 为五的多一百倍。
Two beakers, the same concentration of acid in each.
两只烧杯，酸的浓度相同。
On the left a strong acid: every molecule has given its proton away, so the liquid is all ions and no whole acid is left.
左边是强酸：每个分子都把质子送了出去， 所以液体里全是离子，没有完整的酸。
On the right a weak acid: only partly dissociated — only a few have split, most are still whole.
右边是弱酸：只有少数解离，大多数仍然完整。
Same acid, very different hydrogen ion concentration.
酸相同，氢离子浓度却相差很大。
A strong acid is the easy case: it ionizes completely, so the hydrogen ion concentration is just the concentration you were given.
强酸是最简单的情况：它完全电离，所以氢离子浓度就是题目给的浓度。
Zero point zero one zero molar hydrochloric acid gives a pH of two.
零点零一零摩尔每升的盐酸，pH 等于二。
For a strong base, work in pOH first: the same sodium hydroxide gives a pOH of two, so the pH is twelve.
对强碱，先算 pOH： 同样的氢氧化钠给出 pOH 等于二，所以 pH 是十二。
And a group two hydroxide releases two hydroxide ions, so double it first.
还有，第二族的氢氧化物会放出两个氢氧根离子，要先把浓度乘以二。
Now a weak acid, properly.
现在正式做一个弱酸。
Zero point one zero molar ethanoic acid, ionization constant one point eight times ten to the minus five.
零点一零摩尔每升的乙酸，电离常数是一点八乘以十的负五次方。
Set it out as an ICE table: the acid alone, no ions.
用 ICE 表来写：起始只有酸，没有离子。
Let x be the amount that ionizes, so the acid loses x and both ions gain x.
设 x 是电离的量， 那么酸减少 x，两种离子各增加 x。
Now write the constant: x squared over zero point one zero minus x.
现在写出常数：x 的平方，除以零点一零减 x。
A weak acid barely ionizes, so x is tiny beside zero point one zero — drop it.
弱酸几乎不电离，所以 x 与零点一零相比非常小——把它舍去。
Then x is one point three times ten to the minus three, and the pH is two point nine.
于是 x 等于一点三乘以十的负三次方，pH 就是二点九。
Last, check the shortcut: x is one point three percent of the start.
最后检查这个近似：x 只占起始浓度的百分之一点三。
Where does strength come from?
强弱从何而来？
An acid is strong when the conjugate base it leaves behind is stable.
当酸留下的共轭碱很稳定时，这个酸就强。
Down a group the bond to hydrogen gets weaker, so hydrogen iodide beats hydrogen fluoride.
在同一族里往下走，与氢的键变弱，所以碘化氢比氟化氢强。
In an oxyacid every extra oxygen pulls charge away, so more oxygens means a stronger acid.
在含氧酸里，每多一个氧就把电荷拉走一些，所以氧越多酸越强。
We score it with pKa: smaller pKa, stronger acid.
我们用 pKa 来衡量：pKa 越小，酸越强。
For a conjugate pair the two constants multiply to the ion product of water, so pKa and pKb add to fourteen.
对共轭酸碱对，两个常数相乘等于水的离子积， 所以 pKa 与 pKb 相加等于十四。
And compare pH with pKa to see which form wins — below it the acid, above it the conjugate base.
把 pH 和 pKa 相比就知道哪种形式占优—— 低于它是酸式，高于它是共轭碱。
A buffer holds its pH almost steady.
缓冲溶液能让 pH 几乎保持不变。
You need two partners together: a weak acid, and its conjugate base.
需要两个搭档同时存在：一种弱酸，以及它的共轭碱。
Then whatever arrives, one of them is waiting.
这样不管来的是什么，总有一个在等着。
Add acid, and the conjugate base mops up the hydrogen ions and turns back into the weak acid.
加入酸，共轭碱就把氢离子吸收掉，变回弱酸。
Add base, and the weak acid hands over a proton.
加入碱，弱酸就交出一个质子。
Either way, the pH barely changes.
无论哪种情况，pH 都几乎不变。
Watch what that buys you.
看看这样做能换来什么。
Both flasks get the same strong base, added steadily.
两个瓶子都以同样的速度加入同样的强碱。
Without a buffer the pH shoots straight up.
没有缓冲溶液时，pH 立刻飙升。
With a buffer the line stays almost flat, because the base is used up as fast as it arrives.
有缓冲溶液时，曲线几乎是平的， 因为碱一加进来就被消耗掉。
Then the buffer breaks: once one partner is gone the pH jumps.
然后缓冲失效了：一旦其中一个搭档用完，pH 就跳升。
That limit is the buffer capacity.
这个极限就是缓冲容量。
What pH does a buffer sit at?
缓冲溶液的 pH 到底是多少？
The Henderson-Hasselbalch equation answers that: pH equals pKa plus the logarithm of the base-to-acid ratio.
亨德森—哈塞尔巴尔赫方程给出答案： pH 等于 pKa 加上碱与酸浓度之比的对数。
Take zero point two zero molar ethanoic acid, pKa four point seven four, with zero point three zero molar ethanoate.
取零点二零摩尔每升的乙酸， pKa 为四点七四，再加零点三零摩尔每升的乙酸根。
Put the numbers in: the ratio is one point five, and its logarithm is zero point one eight.
代入数字：比值是一点五，它的对数是零点一八。
So the pH is four point nine two.
所以 pH 等于四点九二。
Notice where that lands: more base than acid, so it sits just above the pKa.
注意它落在哪里：碱比酸多，所以它就略高于 pKa。
If the two were similar — equal — the logarithm would be zero and the pH would equal the pKa.
如果两者相等，对数为零，pH 就正好等于 pKa。
Buffer capacity is greater when both partners are concentrated. So pick an acid whose pKa is near the pH you want.
所以要挑一个 pKa 接近目标 pH 的酸。
A titration follows the pH while you add measured amounts of one solution to the other.
滴定是一边把一种溶液按量加入另一种，一边跟踪 pH。
Watch the curve build.
看曲线是怎么长出来的。
At first it creeps up slowly, because the acid still dominates.
开始时它上升很慢，因为酸仍然占优。
Then, within a fraction of a millilitre, it leaps almost vertically.
接着，在不到一毫升之内，它几乎垂直地跃升。
That leap is the equivalence point: the base added now matches the acid you started with.
这一跃就是等当点：此刻加入的碱，正好等于你起始的酸。
Now two titrations on one graph.
现在把两条滴定曲线放在一张图上。
Blue is a strong acid, orange a weak acid.
蓝色是强酸，橙色是弱酸。
The weak one starts higher and rises gently, because a buffer forms as soon as it is part-neutralized.
弱酸起点更高，上升也更平缓，因为它一被部分中和就形成了缓冲溶液。
Halfway to the equivalence point, half the acid has been converted, so acid and conjugate base are equal, and the pH equals the pKa.
到等当点的一半时，一半的酸被转化，酸和共轭碱相等，pH 就等于 pKa。
At its equivalence point the weak acid finishes above seven, because the conjugate base reacts with water.
在它的等当点，弱酸结束在七以上，因为共轭碱会与水反应。
The shaded band is an indicator's colour range: pick one inside the steep jump.
阴影带是指示剂的变色范围：要挑一个落在陡跃之内的。
One last idea: solubility.
最后一个想法：溶解度。
Calcium carbonate is almost insoluble — a little dissolves into calcium ions and carbonate ions, and the rest stays solid.
碳酸钙几乎不溶——只有少量溶解成钙离子和碳酸根离子， 其余仍是固体。
Now add acid.
现在加入酸。
Hydrogen ions grab the carbonate ions, because carbonate is a weak base.
因为碳酸根是弱碱，氢离子会把碳酸根抓走。
A product has been removed, so more solid dissolves to replace it.
一种产物被移走了，于是更多固体溶解来补上。
Lower the pH and the salt gets more soluble.
降低 pH，这种盐就更容易溶解。
The test is simple: does the negative ion react with acid?
判断方法很简单：这个负离子会不会与酸反应？
Carbonate, fluoride and hydroxide do; chloride and nitrate do not.
碳酸根、氟离子和氢氧根会； 氯离子和硝酸根不会。
Four marks students throw away.
四个学生常丢的分。
One: neutral means equal hydronium and hydroxide, not the number seven — warm water is neutral below seven.
第一：中性指的是水合氢离子和氢氧根相等，而不是数字七—— 温水是中性的，pH 却在七以下。
Two: pH is a logarithm, so one unit is a factor of ten.
第二：pH 是对数，所以一个单位就是十倍。
Three: a buffer needs both partners present; a weak acid alone is not a buffer.
第三：缓冲溶液必须两个搭档都在；单独一种弱酸不是缓冲溶液。
Four: the half-equivalence pH is the pKa, not the equivalence point.
第四：一半处的 pH 才是 pKa，不是等当点。
Get those right and this unit is yours.
把这些做对，这个单元就是你的了。