Chemical Reactions
AP Chemistry Topic 4 7:42 English narration · English + 中文 subtitles burned in
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Transcript
A strip of magnesium metal.
一条镁带。
Touch it with a flame and it burns — combustion — with a light too bright to look at.
用火焰一点，它就燃烧起来——这就是燃烧反应——发出刺眼得无法直视的白光。
What is left is not magnesium any more. It is a white powder.
烧完以后剩下的已经不是镁了，而是一种白色粉末。
Something new was made — and this unit is about reading that, writing it, and measuring it.
有新的物质生成了——这一单元要学的，就是读懂它、写出它、算出它。
Here is the plan.
这是我们的计划。
How to tell a reaction happened, and how to write it so no atom is lost.
怎样判断反应发生了，以及怎样写方程式才不会丢掉任何原子。
Then net ionic equations.
然后是净离子方程式。
Then stoichiometry, and the reactant that runs out first.
然后是化学计量，以及最先用完的那种反应物。
Then titration.
接着是滴定。
And last, the reaction families the exam asks you to name.
最后是考试要求你辨认的几大类反应。
How do you know a reaction happened?
怎么知道反应发生了呢？
Look for evidence.
要找证据。
A color change.
颜色改变。
A gas bubbling off.
有气体冒出。
A solid appearing in a clear liquid — we call that a precipitate.
澄清的液体里出现固体—— 我们把它叫做沉淀。
Or heat and light, given out or taken in.
或者放出、吸收热和光。
But they are evidence, not proof.
但它们只是证据，不是证明。
Steam leaving a kettle is a gas, and nothing has reacted.
水壶冒出的水蒸气也是气体，可什么反应也没发生。
Now the line the exam cares about.
现在说考试真正在意的那条界线。
In a physical change the substance keeps its identity. Melting ice is still water; only the forces between particles change.
在物理变化中，物质仍然是它自己： 冰融化以后还是水，改变的只是粒子之间的作用力。
In a chemical change, bonds inside the particles break and new bonds form, so new substances appear.
在化学变化中， 粒子内部的化学键断裂、新的化学键形成，于是有新物质出现。
One case sits on the fence: dissolving salt breaks ionic bonds, yet the ions survive.
有一种情况介于两者之间：盐溶解时离子键被打断，但离子依然存在。
Here is that burn as an equation.
把刚才的燃烧写成方程式。
Two magnesium atoms and one oxygen molecule go in; two units of magnesium oxide come out.
两个镁原子和一个氧分子进去，两个单位的氧化镁出来。
Two magnesium and two oxygen on each side.
两边各有两个镁、两个氧。
Atoms are conserved, and so is charge.
原子守恒，电荷也守恒。
You are seeing one reaction three ways — particulate particles, symbolic symbols, and the macroscopic beaker.
你正在用三种方式看同一个反应：微观粒子、化学符号，还有宏观的烧杯现象。
Now mix two clear solutions.
现在把两种澄清溶液混在一起。
The liquid turns cloudy and a solid settles out — a precipitate, a product that will not dissolve.
液体变浑浊，有固体沉下来——这就是沉淀， 一种不会溶解的生成物。
But look at what really happened.
但看看真正发生了什么。
Most of the ions in those beakers did nothing. They started dissolved and they ended dissolved.
烧杯里大多数离子什么也没做： 一开始是溶解的，最后还是溶解的。
Only two of them joined up.
真正结合起来的只有两种。
So we strip it down, in three steps.
所以我们要把它化简，分三步。
Start with the molecular equation: silver nitrate plus sodium chloride gives silver chloride and sodium nitrate.
先写分子方程式：硝酸银加氯化钠，生成氯化银和硝酸钠。
Next write the complete ionic equation, splitting every dissolved compound into its ions.
接着写全离子方程式，把每一种溶解的离子化合物都拆成离子。
Now find the ions unchanged on both sides.
现在找出两边都原样出现的离子。
Those are the spectator ions — cross them out.
它们就是旁观离子——把它们划掉。
What is left is the net ionic equation: a silver ion plus a chloride ion gives solid silver chloride.
剩下的就是净离子方程式：一个银离子加一个氯离子，生成固体氯化银。
Now the counting.
现在来算。
A balanced equation is a recipe in moles, never in grams, so every calculation follows one road. Grams, to moles, across the mole ratio, to moles, back to grams.
配平的方程式是一份用摩尔写的配方，绝不是用克写的， 所以每一道计算都走同一条路线：克，到摩尔，跨过摩尔比，再到摩尔，回到克。
The mole ratio is just the coefficients of the balanced equation.
摩尔比就是配平方程式里的系数。
And never cross from grams straight to grams — that is the most common lost mark here.
而且绝不要从克直接跳到克—— 这是这里最常见的失分点。
Let's walk it.
我们来走一遍这条路线。
Four grams of hydrogen burn in excess oxygen — what mass of water forms?
四克氢气在过量的氧气中燃烧，生成多少克水？
Step one, grams to moles: hydrogen has a molar mass of two, so four divided by two is two moles.
第一步，克换成摩尔：氢的摩尔质量是二，所以四除以二等于两摩尔。
Step two, the mole ratio: two hydrogen give two water, one to one, so two moles of water.
第二步，摩尔比：两个氢气生成两个水，是一比一，所以水也是两摩尔。
Step three, moles back to grams: water has a molar mass of eighteen, so the answer is thirty six grams.
第三步，摩尔换回克：水的摩尔质量是十八，所以答案是三十六克。
Oxygen never entered the calculation.
氧气从头到尾没有进入计算。
Real reactions rarely come in perfect proportions.
真实的反应很少按完美的比例配好。
Here you start with four hydrogen molecules and one oxygen molecule.
这里一开始有四个氢分子和一个氧分子。
The recipe needs two hydrogen for every oxygen, so the oxygen is gone after two waters, and two hydrogen are left over.
配方要求每一个氧配两个氢，所以生成两个水以后氧就用完了，还剩下两个氢分子。
Oxygen is the limiting reactant.
氧是限量反应物。
It runs out first, so it caps the product — that cap is the theoretical yield.
它最先用完，因此它决定了产物的上限——这个上限就是理论产量。
Real mass over theoretical yield is the percent yield.
实际得到的质量除以理论产量，就是产率。
Sometimes what you want is a concentration, and you measure it by reaction.
有时候你想求的是浓度，而测它的办法是让它反应。
That is a titration.
这就是滴定。
The burette holds the solution you know exactly.
滴定管里装的是你已经确切知道浓度的溶液。
The flask holds the one you do not, plus an indicator.
锥形瓶里装的是你不知道的那一种， 再加上指示剂。
You add drop by drop until the reaction is just complete.
你一滴一滴地加，直到反应刚好完全。
That moment is the equivalence point.
那一刻就是等当点。
How do you know you have reached it?
怎么知道已经到了呢？
Watch the pH.
看酸碱度。
Add base to acid and at first the pH barely moves.
往酸里加碱，一开始酸碱度几乎不动。
Then, within a drop or two, it leaps almost vertically, straight through the equivalence point, and levels off high.
然后，只差一两滴，它就几乎垂直地猛跳上去，正好穿过等当点，然后在高处趋于平缓。
An indicator changes color inside that jump.
指示剂就在这段跳跃里变色。
That visible moment is the endpoint.
这个看得见的时刻，叫做终点。
Now the exam's calculation.
现在做考试的计算。
Twenty five millilitres of hydrochloric acid needs exactly thirty millilitres of zero point one molar sodium hydroxide.
二十五毫升盐酸恰好被三十毫升、浓度为零点一摩尔每升的 氢氧化钠中和。
Find the acid's concentration.
求这份盐酸的浓度。
Always start on the side you know everything about.
永远从你什么都知道的那一边开始。
Moles equal concentration times volume in litres: zero point one times zero point zero three gives three thousandths of a mole.
摩尔数等于浓度乘以以升为单位的体积：零点一乘以零点零三，得到千分之三摩尔。
One to one, so the acid has the same.
比例是一比一，所以盐酸也一样。
Divide by the acid's volume, and you get zero point one two zero molar.
再除以盐酸的体积， 就得到零点一二零摩尔每升。
Check it — less acid was needed, so it must be more concentrated.
检查一下——用掉的酸更少，所以它一定更浓。
Step back.
退一步看。
Most reactions belong to a few families, and naming the family is often a mark by itself.
大多数反应都属于少数几个家族，而说出家族的名字本身常常就是一分。
Synthesis: two things join into one.
化合：两种东西合成一种。
Decomposition: one splits apart.
分解：一种裂成两种。
Precipitation: an insoluble solid forms.
沉淀：生成不溶的固体。
Acid-base: a proton moves.
酸碱：质子发生转移。
Redox: electrons move.
氧化还原：电子发生转移。
The last three are the ones the exam names.
最后三种是考试点名要求辨认的。
An acid-base reaction is a proton transfer.
酸碱反应就是质子的转移。
In Bronsted-Lowry terms the acid is the proton donor and the base is the proton acceptor.
按布朗斯特-劳里的说法，酸是质子的给出者，碱是质子的接受者。
Hydrogen chloride hands a proton to water, making a hydronium ion and a chloride ion.
氯化氢把一个质子交给水，生成水合氢离子和氯离子。
An acid and what is left of it differ by one proton.
一种酸和它给出质子后剩下的那部分，正好差一个质子。
That is a conjugate pair.
这就是共轭酸碱对。
Water acts either way, so it turns up on both sides so often.
水两边都能当，所以它才那么经常同时出现在方程式的两边。
The other transfer is electrons — that is redox.
另一种转移的是电子，这就是氧化还原反应。
Oil rig: oxidation is loss, reduction is gain.
记住 OIL RIG：氧化是失去电子，还原是得到电子。
Zinc gives up two electrons and becomes a zinc ion, so its oxidation number climbs from zero to plus two.
锌交出两个电子，变成锌离子，它的氧化数从零升到正二。
Those electrons land on a copper ion, which falls from plus two to zero and plates out as copper.
这同样的电子落到铜离子上，铜离子从正二降到零，析出成铜金属。
Write the halves as half-reactions, and the electrons must balance.
把两半分别写成半反应，电子必须平衡。
How do you find an oxidation number?
那么怎样求氧化数呢？
Use the rules.
用规则。
In permanganate, potassium is plus one and each oxygen is minus two.
在高锰酸根里，钾是正一，每个氧是负二。
There are four oxygens, so that is minus eight.
氧有四个，所以一共是负八。
The whole formula is neutral, so the parts must add to zero.
整个化学式是电中性的，所以各部分加起来必须等于零。
Plus one, plus manganese, minus eight is zero, so manganese is plus seven.
正一，加上锰，再减八，等于零，所以锰是正七。
It has no electrons left to lose, so it can only gain them.
它没有电子可以再失去，所以它只能得到电子。
Four marks students lose here.
这里学生最常丢的四分。
One: in a net ionic equation, cross out every spectator ion, and never split a solid, a gas, or a weak acid.
第一：写净离子方程式时，把每一个旁观离子都划掉， 而固体、气体和弱酸绝不能拆开。
Two: always go through moles.
第二：永远要经过摩尔。
Grams straight to grams scores nothing.
从克直接算到克，得不到分。
Three: find the limiting reactant by comparing moles with the coefficients, not masses.
第三：找限量反应物，要用摩尔和系数比较，而不是比较质量。
Four: check that the electrons lost equal the electrons gained.
第四：检查失去的电子和得到的电子是否相等。