结构、成键与有机化学入门

1.1

From atoms to molecules that react

A lump of sodium metal and a flask of chlorine gas sit a whole periodic table apart. Unit 1 explains what makes them different — shells, bonding, structure — and then makes them react: the arithmetic of the mole tells you exactly how much salt you get. This is the unit where chemistry's two languages, the equation and the diagram, are both learned properly.

WCH11 Structure, Bonding and Introduction to Organic Chemistry is the first IAS paper: 1 hour 30 minutes, 80 marks, all compulsory. Its Section A is 20 one-mark multiple-choice questions — the fastest marks on any paper if the definitions are exact — followed by structured questions on Topics 1-5.

1.1

化学式、方程式与物质的量（专题1）

教学大纲

主题 1（规格页码 pp.20-21）。书写带状态符号的化学式和配平方程式；根据信息构建方程式；摩尔和阿伏伽德罗常数；摩尔计算包括反应质量、气体体积（标准室温下摩尔体积 24 dm3；pV=nRT）；溶液浓度和滴定计算；根据质量组成和燃烧数据推导实验式和分子式；产率百分比和质量原子经济性；实验工作的危险与风险用语。该主题在每个WCH11试卷中持续评估（通常占选择题 10-14 题及结构化部分的开头）。

来源：Cambridge International 教学大纲

One mole is $6.02 \times 10^{23}$ particles; one molar mass in grams contains that many. Every amount-of-substance question is three moves:

$$n = \frac{m}{M_r} \qquad n = \frac{V}{24\ \mathrm{dm^3}} \qquad n = cV$$

Worked check. 0.12 mol dm⁻³ sulfur dioxide, 50 cm³: $n = 0.12 \times 0.050 = 6.0 \times 10^{-3}$ mol. Balanced equations then act as exchange rates between substances: multiply by the coefficient ratio, never by the mass. Atom economy and percentage yield complete the story — atom economy 原子经济 asks how much of the reactant mass ends up in the WANTED product:

$$\text{atom economy} = \frac{M_r(\text{desired product})}{\sum M_r(\text{all products})} \times 100\%$$

Empirical formulae come from dividing each mass by its $A_r$ and ratio-ing the results; scale to the molecular formula with the measured $M_r$.

1.2

原子结构与周期表（专题2）

教学大纲

主题 2（规格页码 pp.22-23）。亚原子粒子、质量数与同位素；根据同位素丰度计算相对原子质量；电子排布s/p/d标记法包括过渡金属离子；电离能及其连续跃变作为电子层和亚层的证据；第 2 周期第一电离能的趋势及族内向下趋势中的铍-硼和氮-氧异常；质谱仪（电离、加速、偏转、检测）及光谱解读包括双原子氯模式；根据电离能数据预测所在族。

来源：Cambridge International 教学大纲

Strip electrons one at a time and record each energy: a big jump 大跳跃 marks the start of a new shell. Across Period 3 the first ionisation energy rises — nuclear charge grows, shells shrink — with two famous dips:

First ionisation energies across Period 3 with the magnesium-to-aluminium subshell drop and the phosphorus-to-sulfur pairing dip annotated.

Mg → Al: the outer electron moves from 3s to 3p — a slightly higher subshell, slightly further out, easier to remove.
P → S: sulfur's fourth 3p electron is the first PAIRED one; the pairing repulsion makes it easier to remove.

The mass spectrometer weighs atoms: vaporise, ionise, accelerate, deflect (light and low-charge bend most), detect. Chlorine's two isotopes give peaks at m/z 35 and 37 with the 3:1 ratio, plus molecular peaks at 70, 72 and 74 in the 9:6:1 pattern.

1.3

化学键与结构（专题3）

教学大纲

主题 3（规格页码 pp.24-25）。离子键与点叉图；晶格结构及基于离子电荷与半径的熔点推理；共价键与配位共价键；通过VSEPR理论判断分子形状与键角（从直线形到八面体形，包括孤对电子对NH3和H2O的影响）；电负性与键极性；极性分子测试（偶极矩相加或抵消）；金属键与导电性；巨型共价晶格（金刚石、石墨）与分子晶体；阳离子的极化力与共价特征。基于键合性质的题目在每次考试中都会出现。

来源：Cambridge International 教学大纲

Bonding	Particles	Melting point	Conducts?
Ionic 离子	ions in lattice	high	molten/dissolved only
Covalent molecular	small molecules	low	never
Covalent giant	atoms in network	very high	graphite only
Metallic 金属	cations + delocalised electrons	varied	solid and liquid

Shapes by VSEPR: count bonding pairs AND lone pairs, arrange to minimise repulsion (lone pairs push harder). Water's 104.5° from two lone pairs, ammonia's 107° from one. Polarity 分子极性 needs BOTH polar bonds AND dipoles that do not cancel: CO₂ is linear and non-polar; H₂O is bent and polar. A small, highly charged cation polarises an anion and gives ionic bonds covalent character — that is why MgCl₂ has more covalent character than NaCl.

1.4 1.5

有机化学入门与烷烃（专题4）

教学大纲

主题 4（指定页码26-27）。官能团与命名法（IUPAC规则应用于支链及环状化合物）；同系物与结构异构；分子式类型（展开式、结构式、键线式）；石油分馏与裂解制取烷烃；完全与不完全燃烧方程式；卤素自由基取代反应（引发、增长、终止）及弯曲箭头机理；燃烧产物的温室气体与污染化学；危险与风险评估用语。

主题 5（指定页码28-30）。双键作为σ+π键；几何（E/Z）异构与Cahn-Ingold-Prelog次序规则；亲电加成机理（含HBr、Br2及酸性高锰酸根），包括马氏规则预测及溴鎓离子解释的反式加成；加聚反应及聚合物处置（焚烧、回收、填埋的权衡）；溴水检验不饱和度；氢化反应。基于碳正离子稳定性的不对称烯烃加成是结构化试题的核心。

来源：Cambridge International 教学大纲

A homologous series 同系列 shares a formula and a chemistry; structural isomers share a molecular formula only. Learn the prefixes (meth-, eth-, prop-, but-, pent-, hex-) and the suffix map (-ane, -ene, -ol, -al, -one, -oic acid). Count from the end giving the LOWEST locants.

Alkanes: unreactive skeletons. Radical substitution with chlorine — initiation 始发 (Cl₂ → 2Cl·, UV light), propagation 传递 (Cl· + alkane → HCl + alkyl·; alkyl· + Cl₂ → chloroalkane + Cl·), termination 终止 (any two radicals combine). The mechanism explains the mixture of products: any radical can meet any radical.

Alkenes: the π bond is an exposed electron cloud above and below the σ bond, so it attracts electrophiles. Electrophilic addition with HBr: Markovnikov's rule — H goes where more hydrogens already are — because that route passes through the MORE STABLE secondary carbocation. Bromine water decolourises: the test for unsaturation 不饱和. E/Z isomerism needs two DIFFERENT groups on each double-bond carbon; apply CIP precedence to name them.

1.4 1.5

Rates and equilibria — first taste

Reactions happen when collisions carry at least the activation energy $E_a$. Heat the mixture and the Maxwell-Boltzmann distribution flattens rightward — the shaded tail beyond $E_a$ grows dramatically:

Maxwell-Boltzmann distributions at two temperatures with the activation energy marked; the shaded tails are the fractions able to react.

A catalyst offers a lower $E_a$ without being consumed. At equilibrium 平衡, forward and backward rates are equal in a closed system; Le Chatelier's principle predicts the response to any change. Concentration, pressure and temperature shift position; only TEMPERATURE changes the equilibrium constant itself.

1.4 1.5

自我检测

Calculate the mass of sodium carbonate needed to make 250 cm³ of 0.100 mol dm⁻³ solution. ($M_r$ = 106)
Why is the second ionisation energy of sodium nearly ten times its first?
Draw the shape of and bond angle in NH₄⁺ and in BF₃.
Why does SF₆ have zero dipole moment while SF₄ is polar?
Write the two propagation steps for the chlorination of methane.
Why does HBr add to propene the "wrong way round" (2-bromopropane major)?
What does a catalyst change on the Maxwell-Boltzmann diagram, and what does it not change?
State two changes that shift an equilibrium but leave K unchanged.

Answers: 1 $0.100 \times 0.250 \times 106 = 2.65$ g; 2 the second electron comes from the inner 2p shell, much closer to the nucleus; 3 NH₄⁺ tetrahedral 109.5°, BF₃ trigonal planar 120°; 4 SF₆ is octahedral - the six bond dipoles cancel; SF₄ has a lone pair giving a see-saw shape whose dipoles do not cancel; 5 Cl· + CH₄ → HCl + CH₃·; CH₃· + Cl₂ → CH₃Cl + Cl·; 6 the secondary carbocation intermediate is more stable than the primary, so the route through it dominates; 7 lowers the position of Eₐ (more of the distribution exceeds it) - it does not change the distribution itself; 8 concentration changes and pressure changes (temperature changes K).