📚 Acids and Bases: A-Level Key Points Review | A-Level 科学：酸与碱 考点精讲
Acids and bases are central to the A-Level chemistry curriculum, linking topics from atomic structure to organic synthesis. A thorough understanding of proton transfer, equilibrium constants and pH calculations is essential for tackling both quantitative and qualitative exam questions. This article covers the key concepts, common misconceptions and calculation techniques you need to master.
酸与碱是A-Level化学课程的核心，连接着从原子结构到有机合成的众多主题。透彻理解质子转移、平衡常数和pH计算对于解答定量与定性考题至关重要。本文涵盖你需要掌握的关键概念、常见误区以及计算技巧。
1. Bronsted-Lowry Acids and Bases | 布朗斯特-劳里酸碱定义
According to the Bronsted-Lowry theory, an acid is a proton (H⁺) donor and a base is a proton acceptor. This definition applies to all proton-transfer reactions, whether in aqueous solution or in the gas phase.
根据布朗斯特-劳里理论，酸是质子（H⁺）给体，碱是质子受体。该定义适用于所有质子转移反应，无论是在水溶液中还是气相中。
The theory extends the older Arrhenius picture by including bases like NH₃ that do not contain hydroxide ions. In a reaction, the acid donates a proton to the base, forming the conjugate base of the acid and the conjugate acid of the base.
这一定义扩展了早期的阿伦尼乌斯图像，将氨等不含氢氧根离子的碱也纳入其中。在反应中，酸将质子提供给碱，同时形成酸的共轭碱和碱的共轭酸。
HA + B ⇌ A⁻ + HB⁺
2. Conjugate Acid-Base Pairs | 共轭酸碱对
A conjugate acid-base pair consists of two species that differ by exactly one proton. For example, in the reaction HCl + H₂O → H₃O⁺ + Cl⁻, the pairs are HCl/Cl⁻ and H₃O⁺/H₂O.
共轭酸碱对由两种仅相差一个质子的物种组成。例如，在反应 HCl + H₂O → H₃O⁺ + Cl⁻ 中，共轭酸碱对是 HCl/Cl⁻ 和 H₃O⁺/H₂O。
The strength of an acid is inversely related to the strength of its conjugate base. A strong acid such as HCl forms a very weak conjugate base, Cl⁻, which has negligible tendency to recombine with a proton.
酸的强度与其共轭碱的强度成反比。强酸（如HCl）会形成非常弱的共轭碱 Cl⁻，它几乎没有重新结合质子的趋势。
When writing acid-base equations, always identify the two conjugate pairs. This helps you predict the direction of equilibrium: the reaction favours the weaker acid and weaker base side.
书写酸碱方程式时，一定要标出两个共轭对。这有助于预测平衡方向：反应倾向于弱酸和弱碱的一侧。
3. Strong and Weak Acids and Bases | 强酸弱酸与强碱弱碱
Strong acids, such as HCl, H₂SO₄ (first dissociation) and HNO₃, are fully dissociated in dilute aqueous solution. The concentration of H⁺ aq equals the initial concentration of the acid.
强酸（如盐酸、硫酸的第一步解离和硝酸）在稀水溶液中完全电离。H⁺ 的浓度等于酸的初始浓度。
Weak acids, such as ethanoic acid CH₃COOH, only partially dissociate. The equilibrium position lies far to the left, and the solution contains predominantly undissociated molecules.
弱酸（如乙酸CH₃COOH）仅部分电离。平衡位置远远偏左，溶液中主要存在未电离的分子。
Do not confuse strength with concentration. It is possible to have a concentrated weak acid and a dilute strong acid. ‘Strength’ refers to the degree of dissociation, not the amount of substance per volume.
切勿混淆强度与浓度。浓的弱酸和稀的强酸都是可能的。“强度”指电离程度，而非单位体积的物质的量。
| Strong Acids | Weak Acids |
|---|---|
| HCl, HBr, HI | CH₃COOH |
| HNO₃ | HCOOH |
| H₂SO₄ (first H⁺) | HF (weak but corrosive) |
4. The Ionic Product of Water, Kw | 水的离子积常数 Kw
Water undergoes slight self-ionisation: 2H₂O ⇌ H₃O⁺ + OH⁻. The equilibrium constant for this process is called the ionic product of water, Kw.
水会发生微弱的自耦电离：2H₂O ⇌ H₃O⁺ + OH⁻。该过程的平衡常数称为水的离子积 Kw。
Kw = [H⁺][OH⁻] = 1.0 × 10⁻¹⁴ mol² dm⁻⁶ at 298 K
The value of Kw increases with temperature because self-ionisation is endothermic. For any aqueous solution at a given temperature, the product [H⁺][OH⁻] is constant, which allows calculation of one concentration from the other.
Kw 的值随温度升高而增大，因为自耦电离是吸热的。在给定温度下，任何水溶液中的 [H⁺][OH⁻] 乘积为常数，因此可由一个浓度算出另一个浓度。
In pure water, [H⁺] = [OH⁻] = 1.0 × 10⁻⁷ mol dm⁻³ at 298 K, giving a neutral pH of 7.0.
纯水中，298 K 时 [H⁺] = [OH⁻] = 1.0 × 10⁻⁷ mol dm⁻³，此时中性 pH 为 7.0。
5. pH and pOH Scales | pH 与 pOH 标度
pH is defined as the negative logarithm to base 10 of the hydrogen ion concentration.
pH = -log₁₀[H⁺]
Similarly, pOH = -log₁₀[OH⁻]. At 298 K, the relationship pH + pOH = 14 holds. This means you can find [OH⁻] from pH directly, which is useful for both strong and weak base calculations.
类似地，pOH = -log₁₀[OH⁻]。在 298 K 时，pH + pOH = 14 成立。这意味着你可以直接从 pH 求出 [OH⁻]，这对强碱和弱碱的计算都很有用。
For a strong monoprotic acid of concentration C, [H⁺] = C, so pH = -log₁₀ C. For strong diprotic acids such as H₂SO₄, the second dissociation must be considered carefully; at A-Level you are often given the first as strong and the second as weak.
对于浓度为 C 的一元强酸，[H⁺] = C，因此 pH = -log₁₀ C。对于二元强酸（如H₂SO₄），需仔细考虑第二步解离；在A-Level中，通常将第一步视为强酸，第二步视为弱酸。
6. Weak Acid Equilibria: Kₐ and pKₐ | 弱酸平衡：Kₐ 与 pKₐ
For a weak monoprotic acid HA dissociating according to HA + H₂O ⇌ H₃O⁺ + A⁻, the acid dissociation constant is:
对于弱一元酸 HA，其解离反应为 HA + H₂O ⇌ H₃O⁺ + A⁻，酸解离常数为：
Kₐ = [H₃O⁺][A⁻] / [HA]
The smaller the Kₐ, the weaker the acid. pKₐ = -log₁₀ Kₐ; a low pKₐ indicates a stronger acid. You should be able to convert between Kₐ and pKₐ and use them to find the pH of weak acid solutions.
Kₐ越小，酸越弱。pKₐ = -log₁₀ Kₐ；pKₐ值越小表示酸性越强。你需要掌握 Kₐ 和 pKₐ 的相互换算，并利用它们计算弱酸溶液的 pH。
When the degree of dissociation is very small (<5%), the approximation [HA] ≅ initial concentration holds, allowing the simplified expression [H⁺] = √(Kₐ × C). Always check the approximation is valid.
当解离度非常小（<5%）时，近似 [HA] ≅ 初始浓度成立，从而得到简化式 [H⁺] = √(Kₐ × C)。务必验证该近似是否合理。
| Acid | Kₐ (mol dm⁻³) | pKₐ |
|---|---|---|
| CH₃COOH | 1.8 × 10⁻⁵ | 4.76 |
| HF | 5.6 × 10⁻⁴ | 3.25 |
7. Weak Base Equilibria: Kb and pKb | 弱碱平衡：Kb 与 pKb
The base dissociation constant Kb is used for weak bases such as ammonia. For the equilibrium B + H₂O ⇌ BH⁺ + OH⁻:
Kb = [BH⁺][OH⁻] / [B]
pKb = -log₁₀ Kb. A lower pKb means a stronger base. For a conjugate acid-base pair, the relationship Kₐ × Kb = Kw holds, and consequently pKₐ + pKb = 14 at 298 K.
pKb = -log₁₀ Kb。pKb 越小，碱性越强。对于共轭酸碱对，存在关系 Kₐ × Kb = Kw，因此在298 K时 pKₐ + pKb = 14。
When calculating the pH of a weak base solution, first find [OH⁻] using [OH⁻] = √(Kb × C), then convert to pOH and finally to pH. Alternatively, use Kw to find [H⁺].
计算弱碱溶液的 pH 时，首先利用 [OH⁻] = √(Kb × C) 求出 [OH⁻]，然后换算为 pOH，最后得到 pH。也可利用 Kw 求出 [H⁺]。
8. Buffer Solutions | 缓冲溶液
A buffer solution minimises changes in pH when small amounts of acid or base are added. It consists of a weak acid and its conjugate base (e.g. CH₃COOH/CH₃COO⁻) or a weak base and its conjugate acid (e.g. NH₃/NH₄⁺).
缓冲溶液能抵抗因加入少量酸或碱而引起的 pH 变化。它由弱酸及其共轭碱（如 CH₃COOH/CH₃COO⁻）或弱碱及其共轭酸（如 NH₃/NH₄⁺）组成。
The pH of an acidic buffer can be calculated using the Henderson-Hasselbalch equation:
pH = pKₐ + log₁₀([A⁻] / [HA])
When [A⁻] = [HA], pH = pKₐ, which is the point of maximum buffering capacity. Dilution does not change the pH of a buffer as long as the ratio remains constant, although capacity decreases.
当 [A⁻] = [HA] 时，pH = pKₐ，此时缓冲能力最强。只要比值保持不变，稀释不会改变缓冲液的 pH，但缓冲容量会下降。
Buffer action relies on the equilibrium shifting: added acid is neutralised by the conjugate base, and added base is neutralised by the weak acid. The concentrations of both components must be high enough relative to the added species.
缓冲作用依赖于平衡移动：加入的酸被共轭碱中和，加入的碱被弱酸中和。两种组分的浓度必须相对于添加的物种足够高。
9. Acid-Base Titration Curves | 酸碱滴定曲线
Titration curves plot pH against the volume of titrant added and reveal the equivalence point and neutralisation characteristics. The shape depends on the strengths of the acid and base involved.
滴定曲线描绘了 pH 随滴定剂加入体积的变化，揭示了等当点和中和特性。曲线形状取决于参与反应的酸和碱的强度。
-
Strong acid – strong base: equivalence point at pH 7; a steep vertical region from about pH 3.5 to 10.5.
强酸-强碱：等当点 pH=7；从约 pH 3.5 到 10.5 出现陡峭的垂直区域。
-
Weak acid – strong base: equivalence point > 7 (e.g. ~8.7 for ethanoic acid with NaOH); buffer region before equivalence with pH changing slowly.
弱酸-强碱：等当点 pH > 7（如乙酸与氢氧化钠滴定约为 8.7）；等当点前存在缓冲区域，pH 变化缓慢。
-
Strong acid – weak base: equivalence point < 7 (e.g. ~5.3 for ammonia with HCl).
强酸-弱碱：等当点 pH < 7（如氨水与盐酸滴定约为 5.3）。
-
Weak acid – weak base: no sharp vertical region; difficult to detect the endpoint and rarely used for quantitative analysis.
弱酸-弱碱：无明显垂直区域；终点难以检测，很少用于定量分析。
The half-equivalence point is particularly important: for a weak acid titration, pH = pKₐ at this exact point, allowing experimental determination of pKₐ.
半等当点尤为重要：在弱酸滴定中，此点 pH = pKₐ，从而实现 pKₐ 的实验测定。
10. Choosing Indicators for Titrations | 指示剂的选择
Acid-base indicators are weak acids or bases whose acid form (HIn) and conjugate base (In⁻) have different colours. The colour change occurs over a pH range of approximately pKin ± 1.
酸碱指示剂是弱酸或弱碱，其酸式体（HIn）和共轭碱式体（In⁻）具有不同颜色。颜色变化发生在约 pKin ± 1 的 pH 范围内。
For an indicator to be suitable, its colour change interval must fall entirely within the steep vertical section of the titration curve. This ensures a sharp colour change at the equivalence point.
适用的指示剂必须使其变色区间完全落在滴定曲线陡峭的垂直段内，这样才能在等当点发生鲜明的颜色变化。
| Indicator | pH range | Suitable titration |
|---|---|---|
| Methyl orange | 3.1 – 4.4 | Strong acid – strong base / strong acid – weak base |
| Phenolphthalein | 8.2 – 10.0 | Strong acid – strong base / weak acid – strong base |
Exam questions often ask you to justify the choice of indicator by comparing the indicator’s range with the pH range of the vertical section of the titration curve.
考题经常会要求你通过比较指示剂的变色范围与滴定曲线垂直段的 pH 范围，来论证指示剂的选择。
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