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

In Cambridge A Level Chemistry, the ability to write and interpret chemical formulae and chemical equations is a foundational skill. It underpins stoichiometry, energetics, equilibrium, and nearly every quantitative topic. This article reviews the types of formulae, the rules for writing ionic compounds, and the systematic method for balancing chemical and ionic equations.

在剑桥 A Level 化学中，书写与解读化学式和化学方程式是一项基础技能。它支撑着化学计量、能量学、平衡以及几乎所有定量主题。本文回顾化学式的类型、离子化合物的书写规则，以及配平化学方程式和离子方程式的系统方法。

1. What Is a Chemical Formula? | 什么是化学式？

A chemical formula uses element symbols and numerical subscripts to show the elements present in a substance and the ratio in which their atoms combine. For example, H₂O indicates two hydrogen atoms and one oxygen atom per molecule. The subscript after an element symbol applies only to the element immediately before it.

化学式使用元素符号和数字下标来表示物质中所含元素及其原子结合的比例。例如 H₂O 表示每个分子中有两个氢原子和一个氧原子。元素符号后的下标只适用于紧邻其前方的那个元素。

In ionic compounds, the formula represents the simplest whole-number ratio of ions in the giant lattice, not discrete molecules. In covalent substances, the molecular formula gives the exact number of atoms in one molecule.

在离子化合物中，化学式表示巨大晶格中离子的最简整数比，而不是独立的分子。在共价物质中，分子式给出一个分子中准确的原子数目。

2. Empirical Formula | 实验式（最简式）

The empirical formula is the simplest whole-number ratio of atoms of each element in a compound. It is obtained from experimental data, often percentage composition by mass. For example, glucose has the molecular formula C₆H₁₂O₆ but its empirical formula is CH₂O because the ratio 6:12:6 simplifies to 1:2:1.

实验式是化合物中各元素原子的最简整数比。它通常由实验数据（如质量百分比组成）求得。例如葡萄糖的分子式为 C₆H₁₂O₆，但其实验式为 CH₂O，因为比例 6:12:6 可简化为 1:2:1。

To calculate an empirical formula, divide the mass or percentage of each element by its relative atomic mass, then divide each result by the smallest value obtained. If necessary, multiply all ratios by a small integer to remove fractions.

计算实验式时，先将各元素的质量或百分比除以相对原子质量，再将每个结果除以所得数值中的最小值。如有必要，将所有比例乘以一个小整数以消除分数。

3. Molecular Formula | 分子式

The molecular formula shows the actual number of atoms of each element in one molecule of a covalent compound. It is a whole-number multiple of the empirical formula. The multiplier is found by dividing the relative molecular mass by the empirical formula mass.

分子式表示一个共价化合物分子中各元素原子的实际数目。它是实验式的整数倍。倍数可通过将相对分子质量除以实验式质量得到。

For example, if the empirical formula of a hydrocarbon is CH₂ and its relative molecular mass is 56, the empirical mass is 14, so the multiplier is 56 ÷ 14 = 4. The molecular formula is therefore C₄H₈.

例如，若某烃的实验式为 CH₂，相对分子质量为 56，则实验式质量为 14，倍数即为 56 ÷ 14 = 4。因此分子式为 C₄H₈。

4. Structural and Displayed Formulae | 结构式与显示式

A structural formula shows how atoms are arranged and bonded without drawing every bond. Condensed forms such as CH₃CH₂OH or CH₃COOH are common. A displayed formula shows every atom and every covalent bond in the molecule, for example H–C–C–O–H with all bonds drawn.

结构式显示原子如何排列和成键，但不必画出每一个键。常用的简写形式如 CH₃CH₂OH 或 CH₃COOH。显示式则画出分子中的每个原子和每条共价键，例如乙醇的 H–C–C–O–H 全部键均画出。

In Cambridge exams, candidates must be able to convert between molecular, structural, displayed, and skeletal formulae. Skeletal formulae omit hydrogen atoms attached to carbon and use lines to represent carbon-carbon bonds.

在剑桥考试中，考生必须能够在分子式、结构式、显示式和骨架式之间转换。骨架式省略与碳相连的氢原子，并用线段表示碳碳键。

5. Ionic Formulae | 离子式

Ionic compounds are electrically neutral, so the total positive charge from cations must balance the total negative charge from anions. The formula is written with the cation first, using subscripts to achieve charge balance. For example, magnesium chloride contains Mg²⁺ and Cl⁻, so the formula is MgCl₂.

离子化合物呈电中性，因此阳离子的总正电荷必须平衡阴离子的总负电荷。书写时阳离子在前，使用下标使电荷平衡。例如氯化镁含 Mg²⁺ 和 Cl⁻，因此化学式为 MgCl₂。

When polyatomic ions are present, parentheses are used if more than one polyatomic ion is needed. For example, calcium nitrate is Ca(NO₃)₂, and aluminium sulfate is Al₂(SO₄)₃. Common polyatomic ions include NO₃⁻, SO₄²⁻, CO₃²⁻, OH⁻, NH₄⁺, and PO₄³⁻.

当存在多原子离子时，若需要多个多原子离子则使用括号。例如硝酸钙为 Ca(NO₃)₂，硫酸铝为 Al₂(SO₄)₃。常见的多原子离子包括 NO₃⁻、SO₄²⁻、CO₃²⁻、OH⁻、NH₄⁺ 和 PO₄³⁻。

6. Writing Chemical Equations | 书写化学方程式

A chemical equation uses formulae to represent a chemical change. Reactants are written on the left, products on the right, and an arrow → separates them. A word equation gives names, while a symbol equation gives formulae, for example: hydrogen + oxygen → water becomes 2H₂ + O₂ → 2H₂O.

化学方程式用化学式表示化学变化。反应物写在左侧，生成物写在右侧，用箭头 → 分隔。文字方程式给出物质名称，符号方程式给出化学式，例如：氢气 + 氧气 → 水 写为 2H₂ + O₂ → 2H

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