Energetics
IGCSE Chemistry Topic 5 5:51 English narration · English + 中文 subtitles burned in
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Snap a glow stick, and it lights up.
掰一根荧光棒，它会发光。
Hold a hand-warmer, and it gets hot.
握住一个暖手宝，它会变热。
Open an instant cold pack, and it turns icy.
打开一个速冷冰袋，它会变得冰凉。
In every case, a chemical reaction is moving energy.
每一种情况下，都是化学反应在转移能量。
Some reactions give energy out, and warm their surroundings.
有些反应放出能量，使周围变暖。
Others take energy in, and cool them down.
有些反应吸收能量，使周围变冷。
Today we learn to tell them apart, and to track exactly where the energy goes.
今天我们要学会区分它们，并追踪能量究竟去了哪里。
Welcome to energetics — the energy of reactions.
欢迎来到能量学——反应中的能量。
Exothermic and endothermic, enthalpy change, and bonds.
放热与吸热、焓变，还有化学键。
Let's begin.
让我们开始吧。
Every reaction is one of two kinds.
每个反应都属于两类之一。
An exothermic reaction gives out thermal energy to its surroundings, so the temperature around it goes up.
放热反应向周围放出热能，所以周围的温度升高。
Combustion — burning a fuel — and neutralisation — an acid reacting with an alkali — are both exothermic.
燃烧（烧燃料）和中和（酸与碱反应）都是放热反应。
An endothermic reaction takes in thermal energy from its surroundings, so the temperature goes down.
吸热反应从周围吸收热能，所以温度下降。
Thermal decomposition, breaking a compound apart with heat, is endothermic.
热分解——用热把化合物分解开——是吸热反应。
We measure the energy transferred as the enthalpy change, written delta H.
我们把转移的能量称为焓变，写作ΔH。
For exothermic, delta H is negative — energy leaves.
对放热反应，ΔH是负的——能量离开。
For endothermic, it is positive — energy is taken in.
对吸热反应，它是正的——能量被吸收。
Here is how we picture the energy of a reaction.
我们是这样描绘反应能量的。
The vertical axis is energy; the horizontal axis is the progress of the reaction, from left to right.
纵轴是能量；横轴是反应的进程，从左到右。
The reactants start at one energy level, and the products end at another.
反应物从一个能量水平开始，生成物在另一个水平结束。
Watch the line rise over a hill in the middle, then settle at the product level.
看这条线在中间越过一座小山， 然后停在生成物的水平。
That hill, and where the line ends up, tell us everything about the reaction's energy.
那座小山，以及线最终停在哪里，告诉我们关于这个反应能量的一切。
Why the hill?
为什么会有小山？
Because particles do not react every time they meet.
因为粒子并不是每次相遇都会反应。
To react, colliding particles need a minimum amount of energy — the activation energy.
要反应，碰撞的粒子需要一个最小的能量—— 活化能。
A gentle collision just bounces off; nothing happens.
轻柔的碰撞只会弹开；什么都不会发生。
Only a collision with enough energy, at least the activation energy, can break bonds and start the reaction.
只有能量足够的碰撞，至少达到活化能， 才能断键并引发反应。
Think of it as a barrier the particles must climb over before they can react.
把它想象成粒子在反应之前必须翻越的一道屏障。
Now the full picture: reaction pathway diagrams.
现在看完整的图景：反应进程图。
For an exothermic reaction, the products end up lower than the reactants.
对放热反应，生成物最终比反应物低。
Energy is given out, so delta H is negative.
能量放出，所以ΔH是负的。
For an endothermic reaction, the products end up higher than the reactants.
对吸热反应，生成物最终比反应物高。
Energy is taken in, so delta H is positive.
能量吸收， 所以ΔH是正的。
On both diagrams, the height of the hill is the activation energy, and the gap between the reactant and product levels is delta H.
在两个图上，小山的高度都是活化能，反应物和生成物水平之间的差距是ΔH。
Just remember: exothermic ends low, endothermic ends high.
只要记住：放热终点低，吸热终点高。
Where does this energy come from?
这些能量从哪里来？
From bonds.
来自化学键。
A reaction breaks the bonds in the reactants, and makes new bonds in the products.
一个反应断开反应物中的键，并在生成物中形成新的键。
Breaking bonds always takes energy in — it is endothermic.
断键总是吸收能量——是吸热的。
Making bonds always gives energy out — it is exothermic.
成键总是放出能量——是放热的。
So the enthalpy change is the energy to break all the bonds, minus the energy released making all the new bonds.
所以焓变等于断开所有键所需的能量，减去形成所有新键放出的能量。
If making bonds gives out more than breaking took in, the reaction is exothermic.
如果成键放出的比断键吸收的多，反应就是放热的。
Before we put numbers in, be exact about the word, because the exam asks for it.
在代入数字之前，先把这个词说准确，因为考试会考它的定义。
A bond energy is the energy needed to break one mole of a particular bond, and it is measured in kilojoules per mole.
键能是断开一摩尔某一种化学键所需要的能量，单位是千焦每摩尔。
So a bond energy is a single number attached to one kind of bond: breaking that bond costs that much energy, and making the same bond gives back exactly the same amount.
所以键能就是附在某一种键上的一个数字： 断开这个键要花掉这么多能量，而形成同一个键会原样还回来同样多。
That symmetry is what makes the whole calculation work.
正是这种对称，让整个计算成立。
And one relief: you never learn these values.
还有一个好消息：这些数值你永远不用背。
They are always given to you in the question, in a small table — your job is to use them in the right direction.
它们总会在题目里以一张小表给你——你要做的，是把方向用对。
Let's calculate one.
我们来算一个。
Hydrogen plus chlorine makes hydrogen chloride.
氢加氯生成氯化氢。
We break one hydrogen bond and one chlorine bond: four hundred thirty-six, plus two hundred forty-two, is six hundred seventy-eight taken in.
我们断开一个氢键和一个氯键：四百三十六， 加二百四十二，是吸收六百七十八。
We make two hydrogen-chloride bonds: two times four hundred thirty-one, is eight hundred sixty-two given out.
我们形成两个氢氯键：二乘四百三十一，是放出八百六十二。
The enthalpy change is six hundred seventy-eight minus eight hundred sixty-two, which is minus one hundred eighty-four.
焓变是六百七十八减八百六十二，等于负一百八十四。
It's negative, so the reaction is exothermic.
它是负的，所以反应是放热的。
Three marks to lock in.
锁住三个分。
First: exothermic gives out heat, so the surroundings warm and delta H is negative; endothermic takes heat in, so the surroundings cool and delta H is positive.
第一：放热放出热，所以周围变暖，ΔH是负的；吸热吸收热， 所以周围变冷，ΔH是正的。
Second: bond breaking takes energy in, bond making gives energy out — enthalpy change is break minus make.
第二：断键吸收能量，成键放出能量——焓变等于断键减成键。
Third: on a pathway diagram, an exothermic reaction ends lower, an endothermic one ends higher, and the hill is the activation energy.
第三：在进程图上，放热反应终点较低，吸热反应终点较高，小山就是活化能。
Master these, and this topic is yours.
掌握这些，这个专题就是你的了。
Five leftover facts.
还剩五件事实。
First, combustion and neutralisation are exothermic; thermal decomposition of a compound is endothermic.
第一，燃烧和中和是放热的；化合物的热分解是吸热的。
Second, on a pathway diagram energy is the vertical axis and progress of the reaction is the horizontal one.
第二，进程图上能量是纵轴，反应进程是横轴。
Third, bond energy is the energy needed to break one mole of a particular bond.
第三，键能是断开一摩尔某种键所需的能量。
Fourth, if more energy is given out making bonds than is taken in breaking them, the reaction is exothermic.
第四，如果成键放出的能量多于断键吸收的，反应就是放热的。
Fifth, always check the sign of the final enthalpy change — negative means heat left the chemicals.
第五，一定要核对最终焓变的正负——负号表示热离开了那些化学物质。