Chemical Formulae and Chemical Equations | 化学式与化学方程式

📚 Chemical Formulae and Chemical Equations | 化学式与化学方程式

Chemical formulae and chemical equations form the language of chemistry. They allow us to describe substances, represent reactions quantitatively, and calculate amounts of reactants and products. Mastery of writing, interpreting, and manipulating these symbolic representations is essential for success in Cambridge A-Level Chemistry and underpins all quantitative topics from stoichiometry to energetics.

化学式与化学方程式构成了化学的语言。它们使我们能够描述物质，定量表示反应，并计算反应物和生成物的量。熟练掌握这些符号表示法的书写、解读和运用，对于在剑桥A-Level化学中取得成功至关重要，也是从化学计量到能量学等所有定量课题的基础。

1. Types of Chemical Formulae | 化学式的类型

The empirical formula shows the simplest whole-number ratio of atoms of each element in a compound. For example, glucose has the molecular formula C₆H₁₂O₆, but its empirical formula is CH₂O. This is obtained by dividing each subscript by the greatest common factor, 6.

经验式（最简式）表示化合物中各元素原子的最简整数比。例如，葡萄糖的分子式为 C₆H₁₂O₆，但其经验式为 CH₂O。这是通过将每个下标除以最大公约数 6 得到的。

The molecular formula gives the actual number of atoms of each element in a molecule. It is a whole-number multiple of the empirical formula. For ethanoic acid, the molecular formula is C₂H₄O₂, while its empirical formula is CH₂O.

分子式给出分子中各元素原子的实际数目。它是经验式的整数倍。例如乙酸的分子式为 C₂H₄O₂，而经验式为 CH₂O。

The structural formula shows the arrangement of atoms and bonds in a molecule, often using condensed notation. For example, ethanol can be written as CH₃CH₂OH, indicating a chain with a hydroxyl group. Branched groups are shown in parentheses, such as CH₃CH(CH₃)CH₃ for methylpropane.

结构式显示分子中原子的排布和成键方式，常采用简写形式。例如乙醇可写作 CH₃CH₂OH，表明碳链上连有羟基。支链基团用括号表示，如甲基丙烷写作 CH₃CH(CH₃)CH₃。

The displayed formula shows every atom and every bond in the molecule, with lines representing covalent bonds. Ethene, for instance, is drawn as H₂C=CH₂, clearly showing the carbon–carbon double bond and all C–H bonds.

显示式展示分子中所有原子和所有键，用线表示共价键。例如乙烯画作 H₂C=CH₂，清晰显示碳–碳双键及所有 C–H 键。

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2. Writing Chemical Formulae: Valency and Ionic Compounds | 书写化学式：化合价与离子化合物

For ionic compounds, the chemical formula is built by balancing the total positive and negative charges so that the overall charge is zero. The charges (valencies) of common ions must be memorised. A list of key ions is shown below.

对于离子化合物，化学式是通过平衡正负总电荷使整体呈电中性来构建的。必须熟记常见离子的电荷（化合价）。一些关键离子如下表所示。

• Na⁺ (sodium ion) – 钠离子
• K⁺ (potassium ion) – 钾离子
• Ca²⁺ (calcium ion) – 钙离子
• Mg²⁺ (magnesium ion) – 镁离子
• Al³⁺ (aluminium ion) – 铝离子
• Cl⁻ (chloride ion) – 氯离子
• O²⁻ (oxide ion) – 氧离子
• OH⁻ (hydroxide ion) – 氢氧根离子
• NO₃⁻ (nitrate ion) – 硝酸根离子
• SO₄²⁻ (sulfate ion) – 硫酸根离子
• CO₃²⁻ (carbonate ion) – 碳酸根离子
• NH₄⁺ (ammonium ion) – 铵根离子

The ‘cross-over’ method is used to determine the formula: the magnitude of the cation charge becomes the subscript of the anion, and vice versa, then simplified if needed. For aluminium oxide, Al³⁺ and O²⁻ cross to give Al₂O₃. For ammonium sulfate, NH₄⁺ and SO₄²⁻ give (NH₄)₂SO₄ – parentheses are used to keep the polyatomic ion intact.

常使用“交叉”法确定化学式：阳离子电荷的绝对值成为阴离子的下标，反之亦然，必要时化简。对于氧化铝，Al³⁺与 O²⁻ 交叉得 Al₂O₃。对于硫酸铵，NH₄⁺ 与 SO₄²⁻ 交叉得 (NH₄)₂SO₄，其中使用括号保持多原子离子的完整性。

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3. Balancing Chemical Equations | 配平化学方程式

At the heart of every chemical equation lies the law of conservation of mass: atoms cannot be created or destroyed. A balanced equation ensures there are equal numbers of each type of atom on both the reactant and product sides.

每个化学方程式的核心是质量守恒定律：原子不可创造或毁灭。配平后的方程式确保反应物和生成物两边每种原子的数目相等。

To balance an equation, you adjust the coefficients (large numbers) placed in front of chemical formulae. You must never change the subscripts inside a formula, as that would alter the identity of the substance. A systematic approach starts with elements that appear in only one reactant and one product, leaving oxygen and hydrogen often until later.

配平方程式时，需要调整化学式前面的系数（大数字）。绝不能改变化学式内部的下标，因为那会改变物质的本质。系统性的方法是先从只出现在一种反应物和一种生成物中的元素入手，通常将氧和氢元素留到后面。

Consider the combustion of methane: CH₄ + O₂ → CO₂ + H₂O. Begin with carbon: one CO₂ on the right balances carbon. Next, hydrogen: four H in CH₄ requires a coefficient of 2 in front of H₂O, giving 4 H on each side. Finally, oxygen: we now have 2 O in CO₂ plus 2 O in 2H₂O, total 4 O atoms on the right; placing a coefficient 2 before O₂ balances the equation: CH₄ + 2O₂ → CO₂ + 2H₂O.

以甲烷燃烧为例：CH₄ + O₂ → CO₂ + H₂O。先从碳开始：右边一个 CO₂ 平衡碳。然后是氢：CH₄ 中有 4 个氢，需要在 H₂O 前放系数 2，使右边也有 4 个氢。最后是氧：现在 CO₂ 中有 2 个氧，2H₂O 中有 2 个氧，右边共 4 个氧；在 O₂ 前放系数 2 即平衡：CH₄ + 2O₂ → CO₂ + 2H₂O。

A balanced equation for the reaction of aluminium with oxygen can be written with fractions at first and then multiplied through: Al + O₂ → Al₂O₃. With fractional coefficients, 2Al + 1.5O₂ → Al₂O₃, then multiply by 2 to remove the fraction: 4Al + 3O₂ → 2Al₂O₃.

铝与氧反应的方程式可先使用分数系数，然后整体乘以分母消除分数：Al + O₂ → Al₂O₃。用分数系数：2Al + 1.5O₂ → Al₂O₃，再乘以 2 得 4Al + 3O₂ → 2Al₂O₃。

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4. State Symbols and Reaction Conditions | 状态符号与反应条件

State symbols provide essential information about the physical state of each species under the reaction conditions: (s) for solid, (l) for liquid, (g) for gas, and (aq) for an aqueous solution (dissolved in water). A correctly written A-Level equation almost always requires these symbols.

状态符号提供了在反应条件下每种物质物理状态的重要信息：(s) 表示固体，(l) 表示液体，(g) 表示气体，(aq) 表示水溶液（溶解于水）。规范书写的 A-Level 方程式几乎总是需要这些符号。

For instance, the reaction of calcium carbonate with hydrochloric acid is written as: CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + CO₂(g) + H₂O(l). Here the solid carbonate reacts with aqueous acid to produce an aqueous salt, carbon dioxide gas, and liquid water.

例如，碳酸钙与盐酸的反应写作：CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + CO₂(g) + H₂O(l)。此处固体碳酸盐与酸溶液反应生成盐溶液、二氧化碳气体和液态水。

In precipitation reactions, the solid product is marked with (s), such as in: Pb(NO₃)₂(aq) + 2KI(aq) → PbI₂(s) + 2KNO₃(aq). The state symbols help identify the precipitate and the spectator ions.

在沉淀反应中，固体产物用 (s) 标注，例如：Pb(NO₃)₂(aq) + 2KI(aq) → PbI₂(s) + 2KNO₃(aq)。状态符号有助于识别沉淀和旁观离子。

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5. Ionic Equations and Spectator Ions | 离子方程式与旁观离子

An ionic equation simplifies a reaction by showing only the ions or molecules that actually undergo a chemical change. Ions that appear unchanged on both sides of the equation are called spectator ions and are omitted from the net ionic equation.

离子方程式通过只显示实际发生化学变化的离子或分子来简化反应。在方程式两边未发生变化的离子称为旁观离子，在净离子方程式中被省略。

To write an ionic equation, start with the full balanced equation including state symbols. Then split all aqueous strong electrolytes into their constituent ions. Solids, liquids, and gases remain as molecules or formula units. Finally, cancel identical ions that appear on both sides.

书写离子方程式时，先写出包含状态符号的完整配平方程式。然后将所有强电解质溶液拆分为组成离子。固体、液体和气体仍保留分子或化学式形式。最后消去两边相同的离子。

For the reaction of silver nitrate with sodium chloride, the full equation is AgNO₃(aq) + NaCl(aq) → AgCl(s) + NaNO₃(aq). Splitting aqueous species: Ag⁺(aq) + NO₃⁻(aq) + Na⁺(aq) + Cl⁻(aq) → AgCl(s) + Na⁺(aq) + NO₃⁻(aq). Cancelling Na⁺ and NO₃⁻ leaves the net ionic equation: Ag⁺(aq) + Cl⁻(aq) → AgCl(s).

对于硝酸银与氯化钠的反应，完整方程式为 AgNO₃(aq) + NaCl(aq) → AgCl(s) + NaNO₃(aq)。拆分溶液物种：Ag⁺(aq) + NO₃⁻(aq) + Na⁺(aq) + Cl⁻(aq) → AgCl(s) + Na⁺(aq) + NO₃⁻(aq)。消去 Na⁺ 和 NO₃⁻ 后得到净离子方程式：Ag⁺(aq) + Cl⁻(aq) → AgCl(s)。

This representation highlights that the reaction is simply the combination of silver ions and chloride ions to form solid silver chloride, irrespective of the source of the ions.

这种表示法突出了反应的本质仅仅是银离子与氯离子结合形成氯化银固体，而与离子的来源无关。

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6. The Mole Concept and Stoichiometry | 摩尔概念与化学计量

The mole is the central unit in quantitative chemistry. One mole of any substance contains exactly 6.02 × 10²³ elementary entities (Avogadro constant) and has a mass equal to its relative formula mass in grams. The molar mass M has units g mol⁻¹.

摩尔是定量化学中的核心单位。一摩尔任何物质恰好包含 6.02 × 10²³ 个基本单元（阿伏伽德罗常数），并以克为单位具有与其相对式量相等的质量。摩尔质量 M 的单位为 g mol⁻¹。

The fundamental relationship linking mass, moles, and molar mass is:

n = m ÷ M

其中 n 为物质的量（单位 mol），m 为质量（g），M 为摩尔质量（g mol⁻¹）。

A balanced equation provides the mole ratio of reactants and products. For the synthesis of ammonia, N₂(g) + 3H₂(g) ⇌ 2NH₃(g), the mole ratio N₂ : H₂ : NH₃ is 1 : 3 : 2. This means 1 mole of nitrogen reacts with 3 moles of hydrogen to produce 2 moles of ammonia. Stoichiometric calculations use this ratio to convert between substances.

配平方程式提供了反应物与生成物之间的摩尔比。对于合成氨反应 N₂(g) + 3H₂(g) ⇌

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