GCSE Chemistry: Balancing Chemical Equations Step-by-Step — 配平化学方程式：逐步详解

GCSE Chemistry (AQA) – Balancing Chemical Equations: A Step-by-Step Guide | GCSE 化学 (AQA) – 配平化学方程式：逐步指南

What is a Chemical Equation? | 什么是化学方程式？

化学方程式（Chemical Equation）是用化学符号和化学式来表示化学反应的式子。反应物（reactants）写在左边，生成物（products）写在右边，中间用箭头（→）连接。

A chemical equation is a symbolic representation of a chemical reaction. Reactants are written on the left, products on the right, connected by an arrow (→). The equation shows which substances react and what they produce, using chemical formulas to identify each substance.

Why Do We Need to Balance Equations? | 为什么需要配平方程式？

配平化学方程式是 GCSE 化学最基础的技能之一。其背后的原理是质量守恒定律（Law of Conservation of Mass）：在化学反应中，原子不会被创造也不会被消灭 – 它们只是重新排列。因此，反应物中原子的总数必须等于生成物中原子的总数。

Balancing chemical equations is one of the most fundamental skills in GCSE Chemistry. The principle behind it is the Law of Conservation of Mass: in a chemical reaction, atoms are neither created nor destroyed – they are simply rearranged. Therefore, the total number of atoms in the reactants must equal the total number of atoms in the products.

在 AQA GCSE 化学考试中，配平方程式几乎出现在每一份试卷中 – 无论是在选择题、简答题还是计算题里。掌握这项技能不仅能帮你拿到基础分，还能为理解摩尔计算（mole calculations）、产率计算（percentage yield）和滴定（titrations）打下坚实基础。

In AQA GCSE Chemistry exams, balancing equations appears in almost every paper – whether in multiple-choice, short-answer, or calculation questions. Mastering this skill not only secures easy marks but also builds the foundation for understanding mole calculations, percentage yield, and titrations.

The Step-by-Step Method | 逐步配平法

以下是一个系统性的配平方法，适用于 GCSE 阶段遇到的绝大多数化学方程式：

Here is a systematic method that works for the vast majority of equations you will encounter at GCSE level:

Step 1: Write the Unbalanced Equation | 第一步：写出未配平的方程式

首先，写出正确的化学式。确保每种物质的化学式是正确的 – 很多同学在配平前就写错了化学式，结果全盘皆输。例如：

Start by writing the correct chemical formulas. Make sure each substance’s formula is correct – many students get the formulas wrong before they even start balancing, which leads to total failure. For example:

Example: Hydrogen reacts with oxygen to form water | 氢气与氧气反应生成水

H₂ + O₂ → H₂O

这是未配平的方程式 – 左边有 2 个氢原子和 2 个氧原子，右边只有 2 个氢原子和 1 个氧原子。不平衡。

This is the unbalanced equation – the left side has 2 hydrogen atoms and 2 oxygen atoms, while the right side has only 2 hydrogen atoms and 1 oxygen atom. It is unbalanced.

Step 2: Count Atoms on Each Side | 第二步：数两侧的原子数

画一个简单的表格，列出反应物和生成物中每种元素的原子数：

Draw a simple table listing the number of atoms of each element in the reactants and products:

Element / 元素	Reactants / 反应物	Products / 生成物
H	2	2
O	2	1

一目了然：氧原子不平衡。左边比右边多 1 个氧原子。

It is clear at a glance: the oxygen atoms are unbalanced. The left side has one more oxygen atom than the right.

Step 3: Balance One Element at a Time | 第三步：每次配平一种元素

重要规则：只改变化学式前的系数（coefficients），绝不能改变化学式中的下标（subscripts）。改变下标会改变物质本身 – 比如把 H₂O 改成 H₂O₂，这就不是水，而是过氧化氢了。

Important rule: only change the coefficients (the numbers in front of formulas), never change the subscripts within formulas. Changing subscripts changes the substance itself – turning H₂O into H₂O₂ gives you hydrogen peroxide, not water.

配平顺序的建议（虽然不是绝对的规则，但对 GCSE 非常有效）：

A suggested balancing order (not an absolute rule, but highly effective for GCSE):

1. 先配平金属元素（metals） / Balance metals first
2. 再配平非金属元素（non-metals, except H and O）
3. 最后配平氢（H）和氧（O） – 因为它们经常出现在多种物质中 / Balance H and O last, as they often appear in multiple substances

回到我们的例子 – 氧不平衡。将 H₂O 的系数设为 2：

Back to our example – oxygen is unbalanced. Set the coefficient of H₂O to 2:

H₂ + O₂ → 2H₂O

现在重新数原子：右边有 4 个 H 和 2 个 O。氢也不平衡了！

Now recount atoms: the right side has 4 H and 2 O. Hydrogen is now unbalanced too!

Step 4: Re-check and Adjust | 第四步：重新检查并调整

氧现在已经平衡了（2 = 2），但氢不平衡（左边 2，右边 4）。将 H₂ 的系数设为 2：

Oxygen is now balanced (2 = 2), but hydrogen is not (left 2, right 4). Set the coefficient of H₂ to 2:

2H₂ + O₂ → 2H₂O

Element / 元素	Reactants / 反应物	Products / 生成物
H	4	4 ✓
O	2	2 ✓

配平完成！2H₂ + O₂ → 2H₂O

Balanced! 2H₂ + O₂ → 2H₂O

Worked Examples | 例题详解

Example 1: Combustion of Methane | 例题一：甲烷的燃烧

甲烷（CH₄）在氧气中燃烧生成二氧化碳和水。这是 GCSE 最常见的方程式之一。

Methane (CH₄) burns in oxygen to produce carbon dioxide and water. This is one of the most common equations at GCSE.

Unbalanced / 未配平： CH₄ + O₂ → CO₂ + H₂O

Step 1 – Count atoms / 数原子：C: left 1 / right 1 ✓; H: left 4 / right 2 ✗; O: left 2 / right 3 ✗

Step 2 – Balance H first / 先配平 H：Set H₂O coefficient to 2 → CH₄ + O₂ → CO₂ + 2H₂O. H: left 4 / right 4 ✓. O: left 2 / right 4 ✗.

Step 3 – Balance O / 配平 O：Set O₂ coefficient to 2 → CH₄ + 2O₂ → CO₂ + 2H₂O. O: left 4 / right 4 ✓.

Balanced: CH₄ + 2O₂ → CO₂ + 2H₂O

Example 2: Neutralisation of Sulfuric Acid | 例题二：硫酸的中和反应

硫酸（H₂SO₄）与氢氧化钠（NaOH）反应生成硫酸钠（Na₂SO₄）和水。

Sulfuric acid (H₂SO₄) reacts with sodium hydroxide (NaOH) to produce sodium sulfate (Na₂SO₄) and water.

Unbalanced / 未配平： H₂SO₄ + NaOH → Na₂SO₄ + H₂O

Step 1 – Count atoms / 数原子：Na: left 1 / right 2 ✗; S: left 1 / right 1 ✓; O: left 5 / right 5 ✓; H: left 3 / right 2 ✗

Step 2 – Balance Na (metal) / 配平 Na（金属）：Set NaOH coefficient to 2 → H₂SO₄ + 2NaOH → Na₂SO₄ + H₂O. Na: left 2 / right 2 ✓. H: left 4 / right 2 ✗.

Step 3 – Balance H / 配平 H：Set H₂O coefficient to 2 → H₂SO₄ + 2NaOH → Na₂SO₄ + 2H₂O. H: left 4 / right 4 ✓. O: left 6 / right 6 ✓.

Balanced: H₂SO₄ + 2NaOH → Na₂SO₄ + 2H₂O

Example 3: Displacement (Thermite) Reaction | 例题三：置换（铝热）反应

铝（Al）与氧化铁（Fe₂O₃）发生铝热反应，生成氧化铝（Al₂O₃）和铁（Fe）。

Aluminium (Al) reacts with iron(III) oxide (Fe₂O₃) in the thermite reaction to produce aluminium oxide (Al₂O₃) and iron (Fe).

Unbalanced / 未配平： Al + Fe₂O₃ → Al₂O₃ + Fe

Step 1 – Count atoms / 数原子：Al: left 1 / right 2 ✗; Fe: left 2 / right 1 ✗; O: left 3 / right 3 ✓

Step 2 – Balance Al (metal) / 配平 Al（金属）：Set Al coefficient to 2 → 2Al + Fe₂O₃ → Al₂O₃ + Fe. Al: left 2 / right 2 ✓.

Step 3 – Balance Fe / 配平 Fe： Set Fe coefficient to 2 → 2Al + Fe₂O₃ → Al₂O₃ + 2Fe. Fe: left 2 / right 2 ✓.

Balanced: 2Al + Fe₂O₃ → Al₂O₃ + 2Fe

Example 4: AQA Exam-Style — With State Symbols | 例题四：AQA 考试题型—含状态符号

AQA 考试常常要求写出带有状态符号（state symbols）的配平方程式：(s) = 固体，(l) = 液体，(g) = 气体，(aq) = 水溶液。

AQA exams often require writing balanced equations with state symbols: (s) = solid, (l) = liquid, (g) = gas, (aq) = aqueous solution.

Calcium carbonate reacts with hydrochloric acid / 碳酸钙与盐酸反应：

CaCO₃(s) + HCl(aq) → CaCl₂(aq) + H₂O(l) + CO₂(g)

注意这里生成了三种产物：氯化钙、水和二氧化碳。逐步配平：

Note that three products are formed: calcium chloride, water, and carbon dioxide. Step-by-step:

• Ca: left 1 / right 1 ✓
• C: left 1 / right 1 ✓ (appears in both CaCO₃ and CO₂)
• O: left 3 / right 3 ✓ (1 in H₂O, 2 in CO₂)
• H: left 1 / right 2 ✗
• Cl: left 1 / right 2 ✗

将 HCl 系数设为 2 → CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + H₂O(l) + CO₂(g)。H: left 2 / right 2 ✓。Cl: left 2 / right 2 ✓。

Set HCl coefficient to 2 → CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + H₂O(l) + CO₂(g). H: left 2 / right 2 ✓. Cl: left 2 / right 2 ✓.

Balanced: CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + H₂O(l) + CO₂(g)

Common Pitfalls to Avoid | 常见易错点

1. Changing Subscripts Instead of Coefficients | 改变下标而非系数

这是最常见的错误。记住：H₂O 永远是 H₂O，不能为了配平改成 H₄O₂。只能改变前面的系数。

This is the most common mistake. Remember: H₂O is always H₂O – you cannot change it to H₄O₂ to balance. Only change the coefficients in front.

2. Forgetting Diatomic Elements | 忘记双原子分子

某些元素在自然界中以双原子分子形式存在。记住 BrINClHOF（发音：”brinkle-hoff”）：Br₂、I₂、N₂、Cl₂、H₂、O₂、F₂。如果方程式涉及这些元素，请确保以双原子形式书写。

Certain elements exist naturally as diatomic molecules. Remember BrINClHOF (pronounced “brinkle-hoff”): Br₂, I₂, N₂, Cl₂, H₂, O₂, F₂. If an equation involves these elements, make sure to write them as diatomic molecules.

3. Not Dealing with Polyatomic Ions as a Group | 不把多原子离子当整体处理

对于像 SO₄²⁻、NO₃⁻、CO₃²⁻、OH⁻ 这样的离子，如果它们在反应前后保持不变，可以把它们当作一个”整体”来处理。例如在中和反应中，H₂SO₄ + 2NaOH → Na₂SO₄ + 2H₂O，SO₄ 作为一个整体保持不变。

For ions like SO₄²⁻, NO₃⁻, CO₃²⁻, OH⁻, if they remain unchanged through the reaction, treat them as a “unit.” For example, in neutralisation: H₂SO₄ + 2NaOH → Na₂SO₄ + 2H₂O – SO₄ remains unchanged as a unit.

4. Losing Track During Multi-Step Balancing | 多步配平中失去追踪

在每一步之后都重新计算所有原子数。不要凭直觉猜测 – 使用表格来追踪变化。

Recount all atoms after every step. Do not rely on intuition – use a table to track changes.

5. Fractions in Coefficients | 系数中出现分数

在 GCSE 层面，最终答案中的系数应为最简整数比。如果得到了分数（如 1/2），将所有系数乘以分母以消去分数：

At GCSE level, coefficients in the final answer should be in the simplest whole-number ratio. If you get a fraction (e.g., 1/2), multiply all coefficients by the denominator to eliminate the fraction:

C₂H₆ + 7/2 O₂ → 2CO₂ + 3H₂O → multiply by 2 → 2C₂H₆ + 7O₂ → 4CO₂ + 6H₂O

From Balancing to Moles: Connecting the Dots | 从配平到摩尔：建立联系

在 GCSE 化学中，配平方程式不仅仅是一个独立技能 – 它是通往摩尔计算（mole calculations）的桥梁。配平方程式的系数（coefficients）直接给出了反应中各物质的摩尔比（mole ratio）。以甲烷燃烧为例：

In GCSE Chemistry, balancing equations is not just a standalone skill – it is the bridge to mole calculations. The coefficients in a balanced equation directly give you the mole ratio between substances. Take the combustion of methane:

CH₄ + 2O₂ → CO₂ + 2H₂O

这个方程式告诉我们：1 摩尔甲烷与 2 摩尔氧气反应，生成 1 摩尔二氧化碳和 2 摩尔水。摩尔比（mole ratio）为 CH₄ : O₂ : CO₂ : H₂O = 1 : 2 : 1 : 2。

This equation tells us: 1 mole of methane reacts with 2 moles of oxygen to produce 1 mole of carbon dioxide and 2 moles of water. The mole ratio is CH₄ : O₂ : CO₂ : H₂O = 1 : 2 : 1 : 2.

在 AQA GCSE 化学考试中，你经常会被问到这样的问题：”如果 0.5 摩尔甲烷完全燃烧，会产生多少摩尔二氧化碳？” 答案就是直接用摩尔比：0.5 摩尔 CO₂。这就是为什么正确配平方程式如此关键 – 错误配平的方程式会导致错误的摩尔比，从而连锁导致所有后续计算错误。

In AQA GCSE Chemistry exams, you will often be asked questions like: “If 0.5 moles of methane are completely burned, how many moles of carbon dioxide are produced?” The answer comes directly from the mole ratio: 0.5 moles of CO₂. This is why correct balancing is so critical – an incorrectly balanced equation leads to a wrong mole ratio, which cascades into errors in all subsequent calculations.

Balancing Ionic Equations (Half-Equations) | 配平离子方程式（半反应）

AQA GCSE 化学也要求你能够配平离子方程式（ionic equations）和半反应方程式（half-equations），这在电解（electrolysis）和氧化还原反应（redox）中尤其重要。

AQA GCSE Chemistry also requires you to balance ionic equations and half-equations, which are especially important in electrolysis and redox reactions.

What is an Ionic Equation? | 什么是离子方程式？

离子方程式只显示实际参与反应的离子，省略了旁观离子（spectator ions） – 那些在反应前后没有变化的离子。

An ionic equation shows only the ions that actually participate in the reaction, omitting spectator ions – ions that remain unchanged throughout the reaction.

例如，盐酸与氢氧化钠的中和反应：

For example, the neutralisation of hydrochloric acid with sodium hydroxide:

Full equation / 完整方程式： HCl + NaOH → NaCl + H₂O

Ionic equation / 离子方程式： H⁺ + OH⁻ → H₂O

Na⁺ 和 Cl⁻ 是旁观离子（spectator ions），它们在反应前后没有变化，因此在离子方程式中被省略。

Na⁺ and Cl⁻ are spectator ions – they remain unchanged before and after the reaction, so they are omitted from the ionic equation.

Half-Equations at Electrodes | 电极处的半反应方程式

在电解过程中，我们需要分别写出阳极（anode）和阴极（cathode）的半反应方程式。以下是配平半反应的关键规则：

During electrolysis, we need to write separate half-equations for the anode and cathode. Here are the key rules for balancing half-equations:

1. 写出涉及的物质 / Write the substances involved
2. 配平原子（除 H 和 O 以外的原子优先）/ Balance atoms (non-H and non-O first)
3. 配平 O 原子 – 加水（H₂O）/ Balance O atoms by adding water (H₂O)
4. 配平 H 原子 – 加氢离子（H⁺）/ Balance H atoms by adding hydrogen ions (H⁺)
5. 配平电荷 – 加电子（e⁻）/ Balance charge by adding electrons (e⁻)

Example: Electrolysis of molten lead(II) bromide / 电解熔融溴化铅：

Cathode / 阴极（还原 / reduction）：Pb²⁺ + 2e⁻ → Pb

Anode / 阳极（氧化 / oxidation）：2Br⁻ → Br₂ + 2e⁻

注意每个半反应中的原子和电荷都已配平。将两个半反应相加可以得到完整的氧化还原方程式。

Note that atoms and charge are balanced in each half-equation. Adding the two half-equations together gives the full redox equation.

Advanced Worked Example: Combustion of Propane | 进阶例题：丙烷的燃烧

Example 5: Combustion of Propane | 例题五：丙烷的燃烧

丙烷（C₃H₈）是一种常见的燃料气体，其完全燃烧方程式的配平稍显复杂，但对考试非常有代表性。

Propane (C₃H₈) is a common fuel gas. Balancing its complete combustion equation is slightly more complex and highly representative of exam questions.

Unbalanced / 未配平： C₃H₈ + O₂ → CO₂ + H₂O

Step 1 – Count atoms / 数原子： C: left 3 / right 1 ✗; H: left 8 / right 2 ✗; O: left 2 / right 3 ✗

三种元素都不平衡。按照”金属先配，H 和 O 最后”的原则，先配平 C：

All three elements are unbalanced. Following “metals first, H and O last,” balance C first:

Step 2 – Balance C / 配平 C： Set CO₂ coefficient to 3 → C₃H₈ + O₂ → 3CO₂ + H₂O. C: left 3 / right 3 ✓.

Step 3 – Balance H / 配平 H： Set H₂O coefficient to 4 → C₃H₈ + O₂ → 3CO₂ + 4H₂O. H: left 8 / right 8 ✓.

现在数 O：右边有 3×2 + 4×1 = 10 个 O 原子，左边只有 2 个。需要左边有 10 个 O 原子，所以 O₂ 的系数应该是 5：

Now count O: the right side has 3×2 + 4×1 = 10 O atoms, while the left has only 2. We need 10 O atoms on the left, so the coefficient of O₂ should be 5:

Step 4 – Balance O / 配平 O： Set O₂ coefficient to 5 → C₃H₈ + 5O₂ → 3CO₂ + 4H₂O. O: left 10 / right 10 ✓.

Balanced: C₃H₈ + 5O₂ → 3CO₂ + 4H₂O

检查小窍门：完全燃烧的碳氢化合物方程式有一个模式。对于 CₓHᵧ，CO₂ 的系数 = x，H₂O 的系数 = y/2，O₂ 的系数 = x + y/4。对于丙烷（C₃H₈）：CO₂ 系数 = 3，H₂O 系数 = 4，O₂ 系数 = 3 + 8/4 = 5。匹配！

A quick tip: complete combustion of hydrocarbons follows a pattern. For CₓHᵧ, coefficient of CO₂ = x, coefficient of H₂O = y/2, coefficient of O₂ = x + y/4. For propane (C₃H₈): CO₂ coeff = 3, H₂O coeff = 4, O₂ coeff = 3 + 8/4 = 5. It matches!

How Balancing Appears in AQA Exam Questions | AQA 考试中的配平题型

在 AQA GCSE 化学试卷中，配平方程式经常出现在以下类型的题目中：

In AQA GCSE Chemistry papers, balancing equations frequently appears in the following question types:

1. Direct balancing questions / 直接配平题（1-2 分）： 试卷会给出未配平的方程式，要求你写出正确的系数。例如：”Balance this equation: __ Na + __ H₂O → __ NaOH + __ H₂”

1. Direct balancing questions (1-2 marks): The paper gives you an unbalanced equation and asks for the correct coefficients. Example: “Balance this equation: __ Na + __ H₂O → __ NaOH + __ H₂”

2. Reacting masses questions / 反应质量题（4-6 分）： 这类题目需要你先配平方程式，然后用摩尔比来计算反应物或生成物的质量。配平错误将导致整个计算题全部失分。

2. Reacting masses questions (4-6 marks): These questions require you to first balance the equation, then use the mole ratio to calculate the mass of a reactant or product. A balancing error will cause you to lose all marks for the entire calculation.

3. Required Practical write-ups / 必需实践描述题（3-4 分）： 在描述电解、中和滴定或制备盐类的实验时，通常需要写出配平的化学方程式。

3. Required Practical write-ups (3-4 marks): When describing experiments on electrolysis, neutralisation titrations, or salt preparation, you are typically required to write a balanced chemical equation.

4. Multiple choice trap questions / 选择题陷阱题（1 分）： AQA 有时会在选择题中放入一个看似正确但系数不对的方程式，测试你是否认真检查了原子数。

4. Multiple choice trap questions (1 mark): AQA sometimes includes what appears to be a correct equation in a multiple-choice question, but the coefficients are wrong – testing whether you actually checked the atom count.

5. Yield and atom economy / 产率和原子经济性（3-4 分）： 计算百分比产率（percentage yield）和原子经济性（atom economy）要求使用配平方程式的系数和摩尔比。

5. Yield and atom economy (3-4 marks): Calculating percentage yield and atom economy requires using coefficients and mole ratios from the balanced equation.

AQA GCSE Exam Tips | AQA GCSE 考试技巧

展示过程（Show your working）：AQA 评分方案通常会给中间步骤打分。即使最终答案错误，展示配平过程也可能获得部分分数。

Show your working: AQA mark schemes often award marks for intermediate steps. Even if the final answer is wrong, showing your balancing process can earn partial credit.

检查状态符号（Check state symbols）：如果题目要求写状态符号，缺少它们可能扣分。记住常见模式：酸和碱通常是 (aq)，金属是 (s)，水是 (l)，常见气体（CO₂、O₂、H₂、Cl₂ 等）是 (g)。

Check state symbols: If the question asks for state symbols, missing them can lose marks. Remember common patterns: acids and alkalis are usually (aq), metals are (s), water is (l), and common gases (CO₂, O₂, H₂, Cl₂, etc.) are (g).

必需实践（Required Practicals）：在 AQA 试卷中，配平方程式常出现在与 Required Practicals 相关的题目中。确保你能配平电解、中和、燃烧和置换反应的方程式。

Required Practicals: In AQA papers, balanced equation questions often appear in Required Practical contexts. Make sure you can balance equations for electrolysis, neutralisation, combustion, and displacement reactions.

反向练习（Practice backward）：给定一个配平的方程式，尝试用文字描述该反应。这有助于你在考试中识别题目描述的究竟是哪种反应。

Practice backward: Given a balanced equation, try to describe the reaction in words. This helps you recognise which reaction a question is describing in the exam.

数两次（Count twice）：配平完成后，从左到右再数一遍每个原子。在考试压力下，很容易犯简单的计数错误。

Count everything twice: After balancing, count every atom again from left to right. Under exam pressure, it is easy to make simple counting errors.

Quick Practice Questions | 快速练习题

试试配平以下方程式（答案在底部）：

Try balancing these equations (answers at the bottom):

1. Mg + O₂ → MgO
2. N₂ + H₂ → NH₃
3. C₃H₈ + O₂ → CO₂ + H₂O
4. Fe + Cl₂ → FeCl₃
5. NaOH + H₂SO₄ → Na₂SO₄ + H₂O

Answers / 答案

1. 2Mg + O₂ → 2MgO
2. N₂ + 3H₂ → 2NH₃
3. C₃H₈ + 5O₂ → 3CO₂ + 4H₂O
4. 2Fe + 3Cl₂ → 2FeCl₃
5. 2NaOH + H₂SO₄ → Na₂SO₄ + 2H₂O

Summary / 总结

配平化学方程式是 GCSE 化学的核心技能，也是通往高分的基础。记住关键原则：

Balancing chemical equations is a core GCSE Chemistry skill and the foundation for achieving high marks. Remember the key principles:

• 质量守恒：原子不会被创造或消灭 / Conservation of mass: atoms are neither created nor destroyed
• 只改系数，不改下标 / Only change coefficients, never subscripts
• 先平衡金属，最后平衡 H 和 O / Balance metals first, then H and O last
• 用表格追踪每一步的原子数 / Use a table to track atom counts at each step
• 最终系数必须是最简整数比 / Final coefficients must be in the simplest whole-number ratio

通过系统性的方法、大量的练习以及对常见错误的警觉，你可以完全掌握配平化学方程式这项技能。加油！

With a systematic approach, plenty of practice, and awareness of common pitfalls, you can fully master balancing chemical equations. Good luck!