Pearson Edexcel · International A-Level · 化学

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:

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:

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

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:

• 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.

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.

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.

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:

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.