Balancing Chemical Equations | 配平化学方程式

📚 Balancing Chemical Equations | 配平化学方程式

Chemical equations are the language of chemistry. They show how reactants transform into products during a reaction. Understanding how to write and balance them is a core skill in IGCSE science.

化学方程式是化学的语言。它们显示反应物如何在反应中转化为产物。理解如何书写和配平它们是IGCSE科学中的核心技能。

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1. The Law of Conservation of Mass | 质量守恒定律

In any chemical reaction, atoms are neither created nor destroyed. Therefore, the total mass of the reactants must equal the total mass of the products. This is the law of conservation of mass.

在任何化学反应中，原子既不会凭空产生，也不会凭空消失。因此，反应物的总质量必须等于产物的总质量。这就是质量守恒定律。

This law is the fundamental reason why chemical equations must be balanced. If the number of atoms of each element is not the same on both sides, mass would not be conserved, which is impossible in real reactions.

这一定律是化学方程式必须配平的根本原因。如果每种元素的原子数量在两侧不相同，质量就不会守恒，而真实反应中这是不可能的。

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

A chemical equation uses symbols and formulae to represent a reaction. The reactants are written on the left side, and the products are written on the right side, separated by an arrow (→) that shows the direction of the reaction.

化学方程式用符号和化学式来表示反应。反应物写在左侧，产物写在右侧，用箭头(→)表示反应方向。

For example, when hydrogen burns in oxygen, water is formed:

例如，当氢气在氧气中燃烧时，会形成水：

H₂ + O₂ → H₂O

This equation is unbalanced because the left side has two oxygen atoms, while the right side has only one. We need to fix this by adding coefficients.

这个方程式是不平衡的，因为左侧有两个氧原子，而右侧只有一个。我们需要通过添加系数来修正它。

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3. Balancing by Inspection | 观察法配平

Balancing involves placing whole-number coefficients in front of formulae so that the number of atoms of each element is the same on both sides. A coefficient multiplies the entire formula that follows it.

配平是在化学式前面加上整数系数，使每种元素的原子在两侧数量相等。系数乘以它后面的整个化学式。

Start by balancing elements that appear in only one reactant and one product. Leave elements that appear in multiple compounds until the end.

首先配平那些只出现在一个反应物和一个产物中的元素。将出现在多个化合物中的元素留到最后。

It is often helpful to balance metals first, then non-metals, and finally hydrogen and oxygen.

通常先配平金属元素，再配平非金属元素，最后配平氢和氧。

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4. Step-by-Step Example | 逐步示例

Let us balance the equation for the reaction of hydrogen with oxygen to form water.

让我们配平氢气和氧气反应生成水的方程式。

Step 1: Write the unbalanced equation.

步骤1：写出不平衡的方程式。

H₂ + O₂ → H₂O

Step 2: Count the number of atoms of each element on both sides.

步骤2：计算两侧每种元素的原子数量。

Element	Reactants	Products
H	2	2
O	2	1

Step 3: Balance the oxygen atoms by putting a coefficient 2 in front of H₂O.

步骤3：通过在H₂O前加系数2来配平氧原子。

H₂ + O₂ → 2H₂O

Step 4: Now count hydrogen atoms again. The right side has 4 hydrogen atoms, so we place a coefficient 2 in front of H₂ on the left.

步骤4：再次数氢原子。右侧有4个氢原子，所以在左侧H₂前加系数2。

2H₂ + O₂ → 2H₂O

Step 5: Check all atoms. Both sides have 4 hydrogen and 2 oxygen atoms. The equation is balanced.

步骤5：检查所有原子。两侧都有4个氢原子和2个氧原子。方程式已配平。

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5. Common Symbols in Equations | 方程式中的常见符号

Chemical equations use several symbols to provide extra information.

化学方程式使用多种符号来提供额外信息。

• + reads as ‘and’ and separates different reactants or products.

  + 读作’和’，用于分隔不同的反应物或产物。

• → means ‘produces’ or ‘yields’, showing the direction from reactants to products.

  → 表示’生成’或’得到’，表示从反应物到产物的方向。

• ⇌ indicates a reversible reaction, where the reaction can occur in both directions.

  ⇌ 表示可逆反应，即反应能向两个方向进行。

• Δ written above the arrow means heat is supplied to the reaction.

  Δ 写在箭头上方表示反应需要加热。

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6. State Symbols | 状态符号

State symbols indicate the physical state of each substance in a reaction.

状态符号表示反应中各物质的物理状态。

• (s) means solid.

  (s) 表示固态。

• (l) means liquid.

  (l) 表示液态。

• (g) means gas.

  (g) 表示气态。

• (aq) means aqueous, i.e. dissolved in water.

  (aq) 表示水溶液，即溶解在水中。

For example, the reaction between magnesium and hydrochloric acid is written as:

例如，镁与盐酸的反应写成：

Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g)

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7. Ionic Equations | 离子方程式

In aqueous reactions, some ions do not take part in the reaction. These are called spectator ions. Removing them gives a net ionic equation.

在水溶液反应中，有些离子不参与反应，它们被称为旁观离子。移除它们后可得到净离子方程式。

For example, when silver nitrate reacts with sodium chloride, the full equation is:

例如，硝酸银与氯化钠反应时，完整方程式为：

AgNO₃(aq) + NaCl(aq) → AgCl(s) + NaNO₃(aq)

In ionic form, we show all soluble salts as separated ions:

在离子形式中，我们将所有可溶性盐拆写为离子：

Ag⁺(aq) + NO₃⁻(aq) + Na⁺(aq) + Cl⁻(aq) → AgCl(s) + Na⁺(aq) + NO₃⁻(aq)

The sodium and nitrate ions appear on both sides, so they are spectator ions. Removing them gives:

钠离子和硝酸根离子出现在两侧，因此它们是旁观离子。移去它们得到：

Ag⁺(aq) + Cl⁻(aq) → AgCl(s)

This net ionic equation shows the actual chemical change taking place.

这个净离子方程式显示了实际发生的化学变化。

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8. Practice Examples | 练习示例

Let us balance a few more equations step by step.

让我们逐步配平更多方程式。

Example 1: Methane burns in oxygen to form carbon dioxide and water.

示例1：甲烷在氧气中燃烧生成二氧化碳和水。

CH₄ + O₂ → CO₂ + H₂O

Balance carbon first: already balanced. Next balance hydrogen: left has 4, right has 2, so put 2 in front of H₂O. Then balance oxygen: right now has 2 + 2 = 4 oxygen atoms, left needs 2 in front of O₂. The balanced equation is:

先配平碳：已经平衡。接着配平氢：左侧有4个，右侧有2个，所以在H₂O前加2。然后配平氧：右侧现在有2+2=4个氧原子，左侧需要在O₂前加2。配平后的方程式为：

CH₄ + 2O₂ → CO₂ + 2H₂O

Example 2: Iron reacts with oxygen to form iron(III) oxide.

示例2：铁与氧气反应生成氧化铁(III)。

Fe + O₂ → Fe₂O₃

Balance iron by putting 4 in front of Fe. Balance oxygen by putting 3 in front of O₂. Results in:

在Fe前加4配平铁，在O₂前加3配平氧。得到：

4Fe + 3O₂ → 2Fe₂O₃

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9. Common Mistakes | 常见错误

Students often make the following mistakes when balancing equations.

学生在配平方程式时常犯以下错误。

• Changing subscripts: You must never change the small numbers in a chemical formula to balance an equation. Only coefficients can be changed.

  改变下标：绝不能在配平时改动化学式中的小数字，只能改变系数。

• Adding coefficients to every compound: Only add a coefficient where it is needed. Do not add the same coefficient to all formulas.

  给每个化合物都加系数：只在需要的地方加系数，不要给所有化学式加相同的系数。

• Forgetting to multiply the coefficient through the entire formula: A coefficient before a formula multiplies every atom in that formula.

  忘记将系数乘以整个化学式：化学式前的系数会乘以该化学式中的每个原子。

• Using fractions instead of whole numbers: In IGCSE, you should balance equations using the smallest whole-number coefficients.

  使用分数而不是整数：在IGCSE中，你应该用最小的整数系数来配平方程式。

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10. Summary | 总结

Balancing chemical equations is a vital skill that applies the law of conservation of mass. Start with elements that appear once, then adjust coefficients step by step, and always check your final atom counts.

配平化学方程式是一项重要技能，它应用了质量守恒定律。从只出现一次的元素开始，逐步调整系数，并始终检查最终的原子数量。

Practise with many examples until the process becomes automatic. Once you master this skill, you will find chemistry much easier to understand.

通过大量练习直到这个过程变得自动。一旦你掌握了这项技能，你会发现化学更容易理解。

Remember: never change subscripts, use whole-number coefficients, and include state symbols where required.

记住：不要改变下标，使用整数系数，并在需要时加上状态符号。

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