Acids & Bases: Strong vs Weak, Strong vs Weak | 强酸弱酸与强碱弱碱

📚 Acids & Bases: Strong vs Weak, Strong vs Weak | 强酸弱酸与强碱弱碱

In IB Chemistry (and CIE A-Level alike), one of the most fundamental distinctions is between strong and weak acids, and strong and weak bases. This distinction governs pH calculations, titration curves, buffer behaviour, and salt hydrolysis. Let us build a clear, exam-ready understanding of these concepts from first principles.

在IB化学（以及CIE A-Level）中，最基础的区别之一就是强酸与弱酸、强碱与弱碱。这个区别决定了pH计算、滴定曲线、缓冲行为以及盐水解。让我们从基本原理出发，建立清晰且贴合考点的理解。

1. Definitions: Arrhenius vs Brønsted–Lowry | 定义：阿伦尼乌斯 vs 布朗斯特–劳里

According to the Arrhenius theory, an acid is a substance that produces H⁺(aq) ions in water, while a base produces OH⁻(aq) ions. This works well for aqueous solutions but is limited. The Brønsted–Lowry theory extends the definitions: an acid is a proton (H⁺) donor, and a base is a proton acceptor. This broader definition covers reactions in non-aqueous solvents and emphasises the role of conjugate acid–base pairs.

根据阿伦尼乌斯理论，酸是能在水中产生H⁺(aq)离子的物质，碱是能产生OH⁻(aq)离子的物质。该理论适用于水溶液，但有其局限性。布朗斯特–劳里理论扩展了定义：酸是质子供体，碱是质子受体。这个更广泛的定义涵盖了非水溶剂中的反应，并强调了共轭酸碱对的作用。

For IB, you must be comfortable with both. In the Brønsted–Lowry sense, every acid–base reaction involves two conjugate pairs. For example, in the reaction

对于IB课程，你必须熟练掌握这两种理论。在布朗斯特–劳里框架下，每个酸碱反应都涉及两对共轭酸碱对。例如，在反应中

CH₃COOH + H₂O ⇌ CH₃COO⁻ + H₃O⁺

acetic acid donates a proton to water. The conjugate pairs are CH₃COOH/CH₃COO⁻ and H₂O/H₃O⁺.

乙酸将一个质子传递给水。共轭对为CH₃COOH/CH₃COO⁻和H₂O/H₃O⁺。

2. Strong vs Weak Acids: The Core Idea | 强酸与弱酸：核心概念

A strong acid is one that completely dissociates (ionises) in aqueous solution. Examples include HCl, HNO₃, H₂SO₄ (first proton), and HBr. In a 0.1 mol dm⁻³ HCl solution, essentially all HCl molecules exist as H₃O⁺ and Cl⁻.

强酸是在水溶液中完全解离（电离）的酸。例子包括HCl、HNO₃、H₂SO₄（第一个质子）和HBr。在0.1 mol dm⁻³的HCl溶液中，几乎所有HCl分子都以H₃O⁺和Cl⁻的形式存在。

A weak acid, by contrast, only partially dissociates. Acetic acid (CH₃COOH), hydrofluoric acid (HF), and carbonic acid (H₂CO₃) are classic examples. At equilibrium, the undissociated acid molecule coexists with its ions. For a typical weak acid HA in water:

相比之下，弱酸仅部分解离。乙酸（CH₃COOH）、氢氟酸（HF）和碳酸（H₂CO₃）是典型例子。在平衡状态下，未解离的酸分子与其离子共存。对于典型弱酸HA在水中：

HA(aq) ⇌ H⁺(aq) + A⁻(aq)

The position of this equilibrium lies far to the left. The degree of dissociation is often less than 5% for a 0.1 mol dm⁻³ solution of a weak acid.

该平衡的位置远偏向左。对于0.1 mol dm⁻³的弱酸溶液，解离度通常小于5%。

3. Strong vs Weak Bases: The Same Logic | 强碱与弱碱：同样的逻辑

A strong base dissociates completely in water. Group 1 hydroxides (NaOH, KOH) and Ba(OH)₂ are strong bases. They provide OH⁻ directly and in full stoichiometric amount.

强碱在水中完全解离。第1族氢氧化物（NaOH、KOH）和Ba(OH)₂是强碱。它们直接且按化学计量比完全提供OH⁻。

A weak base only partially accepts protons from water. Ammonia (NH₃) is the classic example. The equilibrium is:

弱碱仅部分从水中接受质子。氨（NH₃）是典型例子。其平衡为：

NH₃(aq) + H₂O(l) ⇌ NH₄⁺(aq) + OH⁻(aq)

Other weak bases include amines (e.g., CH₃NH₂) and the carbonate ion (CO₃²⁻). The key point is that the position of equilibrium determines basic strength, not the solubility of the base.

其他弱碱包括胺类（如CH₃NH₂）和碳酸根离子（CO₃²⁻）。关键点是平衡位置决定碱的强度，而不是碱的溶解度。

4. The Equilibrium Constants Ka and Kb | 平衡常数Ka和Kb

The strength of a weak acid is quantified by its acid dissociation constant, Ka. For HA ⇌ H⁺ + A⁻:

弱酸的强度由其酸解离常数Ka量化。对于HA ⇌ H⁺ + A⁻：

Ka = [H⁺][A⁻] / [HA]

The larger the Ka, the stronger the acid (at equal concentration). Values of Ka are typically very small for weak acids, so pKa = –log₁₀ Ka is often used. A smaller pKa means a stronger acid.

Ka越大，酸越强（在相同浓度下）。弱酸的Ka值通常非常小，因此常使用pKa = –log₁₀ Ka。pKa越小，酸越强。

Similarly, for a weak base B: B + H₂O ⇌ BH⁺ + OH⁻, the base dissociation constant is:

类似地，对于弱碱B：B + H₂O ⇌ BH⁺ + OH⁻，碱解离常数为：

Kb = [BH⁺][OH⁻] / [B]

You should also remember the relationship for a conjugate acid–base pair:

你还应记住共轭酸碱对的关系：

Ka × Kb = Kw = 1.0 × 10⁻¹⁴ at 25 °C

5. pH Calculations: Strong vs Weak | pH计算：强酸与弱酸

For a strong acid, since dissociation is complete, [H⁺] equals the initial concentration directly. For 0.01 mol dm⁻³ HCl, pH = –log₁₀(0.01) = 2.00.

对于强酸，由于解离完全，[H⁺]直接等于初始浓度。对于0.01 mol dm⁻³ HCl，pH = –log₁₀(0.01) = 2.00。

For a weak acid, you cannot take [H⁺] equal to the initial concentration. Instead, you use the ICE table approach. For HA of initial concentration c and Ka:

对于弱酸，不能将[H⁺]直接等于初始浓度。应使用ICE表方法。对于初始浓度为c、Ka为的HA：

Ka = x² / (c – x) and [H⁺] = x

When Ka is very small and c is not too low, we approximate c – x ≈ c, giving the simplified form [H⁺] = √(Ka × c). Always state this approximation in your working.

当Ka非常小且c不太低时，可近似c – x ≈ c，得到简化形式[H⁺] = √(Ka × c)。在答题中务必说明这一近似。

For a 0.1 mol dm⁻³ CH₃COOH with Ka = 1.8 × 10⁻⁵, [H⁺] = √(1.8 × 10⁻⁵ × 0.1) ≈ 1.34 × 10⁻³ mol dm⁻³, so pH ≈ 2.87. Note the same concentration of a strong acid would have pH = 1.00.

对于0.1 mol dm⁻³ CH₃COOH，Ka = 1.8 × 10⁻⁵，[H⁺] = √(1.8 × 10⁻⁵ × 0.1) ≈ 1.34 × 10⁻³ mol dm⁻³，因此pH ≈ 2.87。注意相同浓度的强酸pH = 1.00。

6. pH of Strong and Weak Bases | 强碱与弱碱的pH

For a strong base, [OH⁻] equals the base concentration. Using pOH = –log₁₀[OH⁻] and pH + pOH = 14 (at 25 °C), you can easily find pH. For 0.05 mol dm⁻³ NaOH, pOH = –log₁₀(0.05) ≈ 1.30, so pH ≈ 12.70.

对于强碱，[OH⁻]等于碱的浓度。使用pOH = –log₁₀[OH⁻]和pH + pOH = 14（25 °C下），可以轻松求得pH。对于0.05 mol dm⁻³ NaOH，pOH = –log₁₀(0.05) ≈ 1.30，所以pH ≈ 12.70。

For a weak base such as NH₃ (Kb = 1.8 × 10⁻⁵), use the ICE approach. For a 0.10 mol dm⁻³ solution:

对于弱碱如NH₃（Kb = 1.8 × 10⁻⁵），使用ICE方法。对于0.10 mol dm⁻³溶液：

[OH⁻] = √(Kb × c) = √(1.8 × 10⁻⁵ × 0.10) ≈ 1.34 × 10⁻³ mol dm⁻³

Then pOH ≈ 2.87, and pH ≈ 14 – 2.87 = 11.13. The same concentration of a strong base would give pH ≈ 13.00.

然后pOH ≈ 2.87，pH ≈ 14 – 2.87 = 11.13。相同浓度的强碱pH ≈ 13.00。

7. Conjugate Acid–Base Pairs and Their Strength | 共轭酸碱对及其强度

There is an inverse relationship between the strength of an acid and the strength of its conjugate base. A strong acid like HCl has a very weak conjugate base (Cl⁻), which shows essentially no tendency to accept a proton in water. A weak acid like CH₃COOH has a moderately weak conjugate base (CH₃COO⁻), which is basic enough to affect pH in salt solutions.

酸的强度与其共轭碱的强度之间存在反比关系。像HCl这样的强酸具有非常弱的共轭碱（Cl⁻），在水中几乎不表现出接受质子的倾向。像CH₃COOH这样的弱酸具有中等强度的共轭碱（CH₃COO⁻），其碱性足以在盐溶液中影响pH。

This concept is essential for predicting whether a salt is acidic, basic, or neutral. For example, sodium ethanoate (CH₃COONa) is a basic salt because the acetate ion hydrolyses:

这一概念对于预测盐呈酸性、碱性还是中性至关重要。例如，乙酸钠（CH₃COONa）是碱性盐，因为乙酸根离子发生水解：

CH₃COO⁻(aq) + H₂O(l) ⇌ CH₃COOH(aq) + OH⁻(aq)

8. Conductivity and Rate of Reaction: Experimental Evidence | 导电性与反应速率：实验证据

One experimental way to distinguish strong and weak acids is to measure electrical conductivity at the same concentration. A strong acid, being fully dissociated, produces many more ions and hence conducts electricity far better than a weak acid at equal molarity.

区分强酸和弱酸的实验方法之一是测量相同浓度下的电导率。强酸完全解离，产生更多离子，因此在相同摩尔浓度下导电能力远强于弱酸。

Another evidence is the rate of reaction with a metal (e.g., magnesium). Equal concentrations of a strong acid react faster (more vigorous bubbling) than a weak acid, because the [H⁺] is higher for the strong acid. This is a common IB multiple-choice question.

另一个证据是与金属（如镁）的反应速率。相同浓度的强酸反应更快（气泡更剧烈），因为强酸的[H⁺]更高。这是IB常见的选择题考点。

Property 性质	Strong Acid (e.g., HCl) 强酸（如HCl）	Weak Acid (e.g., CH₃COOH) 弱酸（如CH₃COOH）
Dissociation 解离程度	100%	Partial, <5% (typical) 部分，通常小于5%
[H⁺] at equal c 等浓度时[H⁺]	= c	<< c
pH at 0.1 mol dm⁻³ 0.1 mol dm⁻³时pH	1.00	~2.87
Conductivity 导电性	High 高	Low 低
Ka	Very large 非常大	Small (e.g., 1.8 × 10⁻⁵) 小（如1.8 × 10⁻⁵）

9. Titration Curves and Indicator Selection | 滴定曲线与指示剂选择

Titration curves differ markedly between strong–strong, strong–weak, and weak–strong combinations. For a strong acid titrated with a strong base, the equivalence point pH is exactly 7, and the vertical section of the curve spans roughly pH 3–11. Methyl orange and phenolphthalein both work.

强酸–强碱、强酸–弱碱、弱酸–强碱等不同组合的滴定曲线显著不同。强酸滴定强碱时，等当点pH正好为7，曲线垂直段大约跨越pH 3–11。甲基橙和酚酞都适用。

For a weak acid titrated with a strong base, the equivalence point is basic (pH > 7) because the conjugate base of the weak acid hydrolyses. Phenolphthalein (colour change pH 8.2–10.0) is suitable, but methyl orange is not, because its colour change occurs at pH 3.1–4.4, too early. For a weak base titrated with a strong acid, the equivalence point is acidic (pH < 7), and methyl orange is the better choice.

弱酸滴定强碱时，等当点呈碱性（pH > 7），因为弱酸的共轭碱发生水解。酚酞（变色范围pH 8.2–10.0）适用，但甲基橙不适用，因为其变色范围在pH 3.1–4.4，太早。弱碱滴定强酸时，等当点呈酸性（pH < 7），选择甲基橙更合适。

10. Buffer Solutions: The Weak Acid/Base Connection | 缓冲溶液：弱酸/弱碱的关联

A buffer solution resists changes in pH when small amounts of acid or base are added. Acidic buffers consist of a weak acid and its conjugate base (e.g., CH₃COOH and CH₃COONa). Basic buffers consist of a weak base and its conjugate acid (e.g., NH₃ and NH₄Cl).

缓冲溶液在加入少量酸或碱时能抵抗pH变化。酸性缓冲液由弱酸及其共轭碱组成（如CH₃COOH和CH₃COONa）。碱性缓冲液由弱碱及其共轭酸组成（如NH₃和NH₄Cl）。

The Henderson–Hasselbalch equation is often used for buffer pH calculations:

亨德森–哈塞尔巴尔赫方程常用于缓冲液pH计算：

pH = pKa + log₁₀([A⁻] / [HA])

When [A⁻] = [HA], the buffer pH equals pKa. This explains why the most effective buffers are chosen with pKa close to the desired pH.

当[A⁻] = [HA]时，缓冲液pH等于pKa。这解释了为什么最有效的缓冲液要选择pKa接近目标pH的弱酸。

11. Hydrolysis of Salts: Acidic, Basic, Neutral | 盐水解：酸性、碱性、中性

Salt hydrolysis refers to the reaction of a salt ion with water to form H⁺ or OH⁻. You can predict the pH of a salt solution by considering the parent acid and base:

盐水解是指盐的离子与水反应生成H⁺或OH⁻。通过考虑盐的来源酸和碱，可以预测盐溶液的pH：

Salt of strong acid + strong base (e.g., NaCl): No hydrolysis; neutral pH = 7.

强酸和强碱的盐（如NaCl）：不发生水解；中性，pH = 7。
Salt of weak acid + strong base (e.g., CH₃COONa): Anion hydrolyses; basic pH > 7.

弱酸和强碱的盐（如CH₃COONa）：阴离子水解；碱性，pH > 7。
Salt of strong acid + weak base (e.g., NH₄Cl): Cation hydrolyses; acidic pH < 7.

强酸和弱碱的盐（如NH₄Cl）：阳离子水解；酸性，pH < 7。
Salt of weak acid + weak base (e.g., CH₃COONH₄): pH depends on relative Ka and Kb.

弱酸和弱碱的盐（如CH₃COONH₄）：pH取决于Ka和Kb的相对大小。

This systematic approach is directly examined in IB Paper 1 and Paper 2 questions.

这种系统的方法在IB Paper 1和Paper 2中直接考查。

12. Common Mistakes and Exam Tips | 常见错误与考试提示

Students frequently make these errors:

学生常犯以下错误：

Assuming that the pH of a strong acid is always very low (like 0 or 1). In fact, pH depends on concentration. 1 × 10⁻⁴ mol dm⁻³ HCl has pH = 4.00, which is still strong.

认为强酸的pH总是很低（如0或1）。实际上pH取决于浓度。1 × 10⁻⁴ mol dm⁻³ HCl的pH = 4.00，但它仍是强酸。
Forgetting that pH of a weak acid requires Ka. Do not use the acid concentration directly for [H⁺].

忘记弱酸的pH需要用Ka计算。不要直接将酸浓度作为[H⁺]。
Confusing ‘strong’ with ‘concentrated’. Strength is a property of the acid (degree of dissociation), while concentration is how much is dissolved.

混淆“强”与“浓度高”。强度是酸的性质（解离程度），浓度是溶解量。
For buffer calculations, always check that the approximation c – x ≈ c is valid; if pH is close to pKa, the buffer is working.

做缓冲液计算时，务必检查c – x ≈ c近似是否成立；如果pH接近pKa，缓冲液正在发挥作用。

Finally, internalise these key equations and their conditions. They are your toolkit for any acid–base equilibrium question.

最后，熟记这些关键方程及其适用条件。它们是你解决任何酸碱平衡问题的工具箱。

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Acids & Bases: Strong vs Weak, Strong vs Weak | 强酸弱酸与强碱弱碱