📚 IB Chemistry: Acid-Base Definitions and Theories | IB化学：酸碱定义与不同理论
The concept of acids and bases is fundamental to chemistry, but its definition has evolved over time. In the IB Chemistry syllabus, students must understand the similarities and differences between the Arrhenius, Brønsted-Lowry, and Lewis theories, and be able to apply them to predict reaction behaviour.
酸碱概念是化学的基础，但它们的定义随着时间不断演变。在IB化学课程中，学生必须理解阿伦尼乌斯、布朗斯特-劳里和路易斯理论之间的异同，并能运用它们预测反应行为。
1. The Arrhenius Theory | 阿伦尼乌斯理论
The Arrhenius theory was proposed by Svante Arrhenius in 1884. It defines an acid as a substance that produces hydrogen ions (H⁺) when dissolved in water, and a base as a substance that produces hydroxide ions (OH⁻) when dissolved in water.
阿伦尼乌斯理论由斯万特·阿伦尼乌斯于1884年提出。该理论将酸定义为溶于水时产生氢离子（H⁺）的物质，将碱定义为溶于水时产生氢氧根离子（OH⁻）的物质。
For example, hydrochloric acid (HCl) ionises in water to give H⁺ and Cl⁻, while sodium hydroxide (NaOH) dissociates to give Na⁺ and OH⁻.
例如，盐酸（HCl）在水中电离产生H⁺和Cl⁻，而氢氧化钠（NaOH）解离产生Na⁺和OH⁻。
HCl → H⁺ + Cl⁻ NaOH → Na⁺ + OH⁻
The main limitation of this theory is that it only works in aqueous solutions. It cannot explain acid-base behaviour in non-aqueous solvents, nor does it account for substances like ammonia (NH₃) that act as bases without containing OH⁻.
该理论的主要局限在于它仅适用于水溶液。它无法解释非水溶剂中的酸碱行为，也无法解释氨（NH₃）等不含OH⁻却表现出碱性的物质。
2. The Brønsted-Lowry Theory | 布朗斯特-劳里理论
The Brønsted-Lowry theory, proposed independently by Johannes Brønsted and Thomas Lowry in 1923, defines an acid as a proton (H⁺) donor and a base as a proton acceptor. This theory is not limited to water.
布朗斯特-劳里理论由约翰内斯·布朗斯特和托马斯·劳里于1923年独立提出。该理论将酸定义为质子（H⁺）供体，将碱定义为质子受体。该理论不受限于水溶液。
In this view, acid-base reactions involve the transfer of a proton from one species to another. For example:
在这种观点下，酸碱反应涉及质子从一种物质转移到另一种物质。例如：
HCl + NH₃ → NH₄⁺ + Cl⁻
Here, HCl donates a proton to NH₃. HCl is the acid, and NH₃ is the base. The products NH₄⁺ and Cl⁻ are related to the reactants through proton transfer.
这里，HCl将质子传递给NH₃。HCl是酸，NH₃是碱。产物NH₄⁺和Cl⁻通过质子转移与反应物相关联。
This theory also works for solvent systems such as liquid ammonia and for gas-phase reactions, making it much broader than the Arrhenius concept.
该理论同样适用于液氨等溶剂体系以及气相反应，因此比阿伦尼乌斯概念适用范围更广。
3. The Lewis Theory | 路易斯理论
The Lewis theory, proposed by Gilbert N. Lewis in 1923, goes further by defining an acid as an electron-pair acceptor and a base as an electron-pair donor. This theory is the most general of the three.
路易斯理论由吉尔伯特·N·路易斯于1923年提出，它更进一步将酸定义为电子对接受体，将碱定义为电子对给予体。这是三种理论中适用范围最广的。
Under the Lewis definition, a proton (H⁺) is a Lewis acid because it accepts an electron pair. A hydroxide ion (OH⁻) is a Lewis base because it donates an electron pair. However, many species that do not contain hydrogen are also Lewis acids, such as boron trifluoride (BF₃) and aluminium chloride (AlCl₃).
根据路易斯定义，质子（H⁺）是路易斯酸，因为它接受电子对。氢氧根离子（OH⁻）是路易斯碱，因为它提供电子对。然而，许多不含氢的物质也是路易斯酸，例如三氟化硼（BF₃）和氯化铝（AlCl₃）。
For example, when BF₃ reacts with NH₃, the lone pair on nitrogen is donated to the empty orbital of boron:
例如，当BF₃与NH₃反应时，氮上的孤对电子被提供给硼的空轨道：
BF₃ + NH₃ → BF₃·NH₃
This reaction is an acid-base reaction under the Lewis theory, even though no proton is transferred. In contrast, the Brønsted-Lowry theory would not classify it as an acid-base reaction.
根据路易斯理论，这个反应是酸碱反应，尽管没有质子转移。相反，布朗斯特-劳里理论不会将其归类为酸碱反应。
4. Conjugate Acid-Base Pairs | 共轭酸碱对
In the Brønsted-Lowry theory, a conjugate acid-base pair consists of two species that differ by a single proton (H⁺). When an acid donates a proton, what remains is its conjugate base. When a base accepts a proton, it forms its conjugate acid.
在布朗斯特-劳里理论中，共轭酸碱对由两个相差一个质子（H⁺）的物种组成。当酸失去一个质子时，剩余部分就是它的共轭碱。当碱接受一个质子时，就形成它的共轭酸。
Consider the generic equation:
考虑通用方程式：
HA + B ⇌ A⁻ + HB⁺
Here, HA and A⁻ are a conjugate pair, and B and HB⁺ are another conjugate pair. HA is the acid, A⁻ is its conjugate base; B is the base, HB⁺ is its conjugate acid.
这里，HA和A⁻是一对共轭对，B和HB⁺是另一对共轭对。HA是酸，A⁻是它的共轭碱；B是碱，HB⁺是它的共轭酸。
In water, acetic acid (CH₃COOH) partially ionises:
在水中，乙酸（CH₃COOH）部分电离：
CH₃COOH + H₂O ⇌ CH₃COO⁻ + H₃O⁺
CH₃COOH and CH₃COO⁻ form one conjugate pair, while H₂O and H₃O⁺ form another. The hydronium ion (H₃O⁺) is the conjugate acid of water.
CH₃COOH和CH₃COO⁻构成一对共轭对，H₂O和H₃O⁺构成另一对。水合氢离子（H₃O⁺）是水的共轭酸。
5. Acid and Base Strength | 酸碱强度
The strength of an acid or base is determined by its tendency to donate or accept protons. A strong acid donates its proton completely in water, while a weak acid only partially ionises. The same principle applies to bases.
酸碱的强度由其给出或接受质子的趋势决定。强酸在水中完全给出质子，而弱酸仅部分电离。碱的原理相同。
Strong acids commonly encountered include HCl, HNO₃, H₂SO₄ and HBr. Strong bases include NaOH and KOH. Weak acids include ethanoic acid (CH₃COOH) and carbonic acid (H₂CO₃). Weak bases include ammonia (NH₃) and amines.
常见的强酸包括HCl、HNO₃、H₂SO₄和HBr。强碱包括NaOH和KOH。弱酸包括乙酸（CH₃COOH）和碳酸（H₂CO₃）。弱碱包括氨（NH₃）和胺。
For a weak acid HA in water, the equilibrium constant is denoted Kₐ:
对于水中的弱酸HA，其平衡常数表示为Kₐ：
Kₐ = [H₃O⁺][A⁻] / [HA]
A larger Kₐ indicates a stronger acid. Similarly, Kb is the base dissociation constant for a weak base.
Kₐ越大表示酸越强。类似地，Kb是弱碱的碱解离常数。
6. The Role of the Solvent | 溶剂的作用
The solvent plays a critical role in acid-base chemistry. Water is amphiprotic, meaning it can act as both an acid and a base. In the presence of a stronger acid, water acts as a base; in the presence of a stronger base, water acts as an acid.
溶剂在酸碱化学中起着关键作用。水是两性的，既可作为酸也可作为碱。当遇到更强的酸时，水作为碱；当遇到更强的碱时，水作为酸。
For example, when HCl dissolves in water, water accepts a proton to form H₃O⁺:
例如，当HCl溶于水时，水接受质子形成H₃O⁺：
HCl + H₂O → H₃O⁺ + Cl⁻
Here, water acts as a base. But when ammonia reacts with water, water donates a proton:
这里，水作为碱。但当氨与水反应时，水给出质子：
NH₃ + H₂O ⇌ NH₄⁺ + OH⁻
In this reaction, water acts as an acid. The autoionisation of water can be written as:
在这个反应中，水作为酸。水的自偶电离可以写作：
2H₂O ⇌ H₃O⁺ + OH⁻
The ion product of water is Kw = [H₃O⁺][OH⁻] = 1.0 × 10⁻¹⁴ at 25°C. This relationship is essential for pH calculations.
水的离子积为Kw = [H₃O⁺][OH⁻] = 1.0 × 10⁻¹⁴（25°C）。这个关系对pH计算至关重要。
7. Comparison of the Three Theories | 三种理论的比较
Each theory has its own scope and limitations. The following table summarises the key differences:
每种理论都有其适用范围和局限性。下表总结了主要差异：
| Theory | Acid definition | Base definition | Scope |
| Arrhenius | Produces H⁺ in water | Produces OH⁻ in water | Aqueous solutions only |
| Brønsted-Lowry | Proton donor | Proton acceptor | Any solvent with proton transfer |
| Lewis | Electron-pair acceptor | Electron-pair donor | All reactions involving lone pairs |
In general, every Arrhenius acid-base reaction is also a Brønsted-Lowry reaction, and every Brønsted-Lowry reaction is also a Lewis reaction. The reverse is not true.
通常，每个阿伦尼乌斯酸碱反应也是布朗斯特-劳里反应，每个布朗斯特-劳里反应也是路易斯反应。反之则不然。
8. Amphiprotic and Amphoteric Species | 两性物种
An amphiprotic species is one that can either donate or accept a proton, depending on its environment. Water, hydrogen carbonate ion (HCO₃⁻), and hydrogen sulfate ion (HSO₄⁻) are common examples.
两性物种是指根据环境不同既能给出质子也能接受质子的物种。水、碳酸氢根离子（HCO₃⁻）和硫酸氢根离子（HSO₄⁻）是常见例子。
For example, HCO₃⁻ can act as an acid:
例如，HCO₃⁻可作为酸：
HCO₃⁻ + OH⁻ → CO₃²⁻ + H₂O
It can also act as a base:
它也可以作为碱：
HCO₃⁻ + H₃O⁺ → H₂CO₃ + H₂O
The term "amphoteric" is broader and includes species that can react with both acids and bases, even without proton transfer. Aluminium oxide and zinc oxide are amphoteric oxides.
“两性”（amphoteric）一词更广泛，包括能与酸和碱反应但不一定发生质子转移的物种。氧化铝和氧化锌是两性氧化物。
9. Lewis Acids and Bases in Coordination Chemistry | 配位化学中的路易斯酸碱
In coordination chemistry, metal cations act as Lewis acids because they accept electron pairs from ligands. Ligands such as water, ammonia and cyanide are Lewis bases because they donate lone pairs.
在配位化学中，金属阳离子作为路易斯酸，因为它们接受来自配体的电子对。水、氨和氰化物等配体是路易斯碱，因为它们提供孤对电子。
Consider the formation of the tetraamminecopper(II) complex:
考虑四氨合铜(II)配合物的形成：
Cu²⁺ + 4NH₃ → [Cu(NH₃)₄]²⁺
Cu²⁺ accepts lone pairs from four NH₃ molecules, making it a Lewis acid. Each NH₃ donates a lone pair, making it a Lewis base.
Cu²⁺接受四个NH₃分子的孤对电子，因此是路易斯酸。每个NH₃提供一对孤对电子，因此是路易斯碱。
This example shows how the Lewis theory is indispensable for understanding reactions that do not involve protons, such as complex ion formation and many organic reactions.
这个例子表明，路易斯理论对于理解不涉及质子的反应（如配合离子形成和许多有机反应）是不可或缺的。
10. IB Exam Relevance | IB考试要点
In the IB Chemistry syllabus, students should be able to identify acids and bases according to all three theories and classify conjugate pairs correctly. They must also be able to distinguish between strong and weak acids in terms of dissociation and pH.
在IB化学课程中，学生应能根据三种理论识别酸碱，正确区分共轭对。还必须能从解离和pH角度区分强酸和弱酸。
Common exam questions ask students to write equations for acid-base reactions, identify the conjugate acid or base, or predict whether a species is amphiprotic. Practice with both molecular and ionic equations is essential.
常见的考试问题要求学生写出酸碱反应方程式、识别共轭酸或碱，或判断某物种是否两性。练习分子方程式和离子方程式都至关重要。
Another frequent topic is the relationship between Kₐ, pH and concentration. For example, given the pH and initial concentration of a weak acid, one can calculate Kₐ using the ICE table method.
另一个常见主题是Kₐ、pH与浓度之间的关系。例如，给定弱酸的pH和初始浓度，可以用ICE表法计算Kₐ。
11. Practical Examples and Problem Solving | 实际例子与解题
Example 1: Identify the conjugate acid of water in the reaction NH₃ + H₂O ⇌ NH₄⁺ + OH⁻.
例1：在反应NH₃ + H₂O ⇌ NH₄⁺ + OH⁻中，识别水的共轭酸。
Water donates a proton to NH₃, forming OH⁻. The conjugate acid of water is therefore H₃O⁺, not NH₄⁺. In this particular reaction, water acts as an acid and its conjugate base is OH⁻.
水向NH₃提供一个质子，形成OH⁻。因此水的共轭酸是H₃O⁺，而不是NH₄⁺。在这个特定反应中，水作为酸，其共轭碱是OH⁻。
Example 2: Classify BF₃ according to all three theories.
例2：根据三种理论对BF₃进行分类。
BF₃ does not contain H⁺ or OH⁻, so it is not an Arrhenius acid. It cannot donate a proton, so it is not a Brønsted-Lowry acid. However, it can accept an electron pair, so it is a Lewis acid.
BF₃不含有H⁺或OH⁻，因此不是阿伦尼乌斯酸。它不能给出质子，因此不是布朗斯特-劳里酸。但它可以接受电子对，因此是路易斯酸。
Example 3: The pH of a 0.10 mol dm⁻³ solution of methanoic acid is 2.38. Calculate its Kₐ.
例3：0.10 mol dm⁻³甲酸溶液的pH为2.38。计算其Kₐ。
⁸ ≈ 4.2 × 10⁻³ mol dm⁻³. Since the acid is weak, [A⁻] ≈ [H₃O⁺] and [HA] ≈ 0.10. Therefore:
首先，[H₃O⁺] = 10⁻²·³⁸ ≈ 4.2 × 10⁻³ mol dm⁻³。由于酸是弱酸，[A⁻] ≈ [H₃O⁺]，[HA] ≈ 0.10。因此：
Kₐ = (4.2 × 10⁻³)² / 0.10 ≈ 1.76 × 10⁻⁴ mol dm⁻³
Always remember to check that the approximation is valid: the percentage ionisation is only about 4.2%, which is small enough.
始终记得验证近似是否合理：电离百分比约为4.2%，足够小。
12. Common Mistakes and Tips | 常见错误与提示
One common mistake is confusing Brønsted-Lowry acids with Arrhenius acids. Remember that all Arrhenius acids are Brønsted-Lowry acids, but the reverse is not always true.
一个常见错误是混淆布朗斯特-劳里酸与阿伦尼乌斯酸。记住，所有阿伦尼乌斯酸都是布朗斯特-劳里酸，但反过来并不总是成立。
Another mistake is misidentifying the conjugate pair. The conjugate base of an acid is formed by removing one proton, not by adding a hydroxide ion. For example, the conjugate base of HSO₄⁻ is SO₄²⁻, not H₂SO₄.
另一个错误是错误识别共轭对。酸的共轭碱是通过去掉一个质子形成的，而不是通过加氢氧根离子。例如，HSO₄⁻的共轭碱是SO₄²⁻，而不是H₂SO₄。
When writing equations, always include state symbols where appropriate and show the proton transfer explicitly. In multiple-choice questions, check whether the reaction involves a proton or an electron pair before choosing the theory.
写方程式时，在适当情况下始终包含状态符号，并明确显示质子转移。在选择题中，先判断反应涉及质子还是电子对，再选择理论。
Finally, do not forget that water can act as both acid and base. Many students assume water is always neutral, but in reactions with strong acids or bases, water behaves amphiprotically.
最后，不要忘记水既可作为酸也可作为碱。许多学生假设水总是中性的，但在与强酸或强碱的反应中，水表现为两性。
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