Acids and Bases Exam Guide for Edexcel A-Level | Edexcel A-Level 酸与碱考点精讲

📚 Acids and Bases Exam Guide for Edexcel A-Level | Edexcel A-Level 酸与碱考点精讲

This comprehensive guide covers all essential concepts for the Edexcel A-Level Chemistry topic of acids and bases. From Brønsted–Lowry theory to buffer calculations, you will find clear explanations, key equations, and exam-focused tips to help you master this fundamental topic.

本篇综合指南涵盖了 Edexcel A-Level 化学中酸与碱的所有核心概念。从布朗斯特-劳里理论到缓冲溶液计算，你将获得清晰的解释、关键方程式以及针对考试的实用技巧，帮助你熟练掌握这一基础专题。

1. Brønsted–Lowry Theory | 布朗斯特-劳里酸碱理论

According to the Brønsted–Lowry theory, an acid is a proton (H⁺) donor, and a base is a proton acceptor. This definition broadens the scope beyond aqueous solutions and focuses on the transfer of a hydrogen ion.

根据布朗斯特-劳里理论，酸是质子（H⁺）的给予体，碱是质子的接受体。这一定义将酸碱概念拓展至非水溶液体系，其核心在于氢离子的转移过程。

When HCl gas dissolves in water, HCl donates a proton to H₂O, forming H₃O⁺ and Cl⁻. Here, HCl acts as the acid and H₂O acts as the base.

当氯化氢气体溶于水时，HCl 将质子给予 H₂O，生成 H₃O⁺ 和 Cl⁻。在此过程中，HCl 是酸，而 H₂O 是碱。

A key feature is that the theory requires a base to accept the proton; acids do not simply ‘release’ protons — there must be a proton transfer to another species.

该理论的一个重要特征是必须有碱来接受质子；酸并非简单地‘释放’质子，而是必定存在质子向另一物种的转移。

2. Conjugate Acid-Base Pairs | 共轭酸碱对

A conjugate acid-base pair consists of two species that differ by one proton (H⁺). The acid has one more proton than its conjugate base. For example, CH₃COOH (acid) and CH₃COO⁻ (conjugate base) form a pair.

共轭酸碱对由相差一个质子（H⁺）的两个物种组成。酸比其共轭碱多一个质子。例如，CH₃COOH（酸）与 CH₃COO⁻（共轭碱）构成一对。

In the reaction HCl + H₂O → H₃O⁺ + Cl⁻, we can identify two conjugate pairs: HCl/Cl⁻ and H₃O⁺/H₂O. The stronger the acid, the weaker its conjugate base, and vice versa.

在反应 HCl + H₂O → H₃O⁺ + Cl⁻ 中，可以找到两组共轭对：HCl/Cl⁻ 和 H₃O⁺/H₂O。酸越强，其共轭碱就越弱，反之亦然。

Water is an amphoteric substance — it can act as either an acid or a base depending on the reaction partner. This is crucial for understanding autoionisation of water.

水是一种两性物质——它既可作为酸，也可作为碱，取决于反应搭档。这对理解水的自解离至关重要。

3. Strong and Weak Acids and Bases | 强酸强碱与弱酸弱碱

A strong acid or base is one that dissociates completely in aqueous solution. HCl, HNO₃, and H₂SO₄ are typical strong acids, while NaOH and KOH are strong bases. Their dissociation goes to completion, so [H⁺] or [OH⁻] equals the initial concentration of the acid or base (taking stoichiometry into account).

强酸或强碱在水溶液中完全解离。典型的强酸有 HCl、HNO₃ 和 H₂SO₄，强碱则有 NaOH 和 KOH。它们的解离进行到底，因此 [H⁺] 或 [OH⁻] 等于酸或碱的初始浓度（需考虑化学计量比）。

Weak acids and bases only partially dissociate, establishing an equilibrium. For a weak acid HA, the equilibrium is HA(aq) ⇌ H⁺(aq) + A⁻(aq). The extent of dissociation is given by the acid dissociation constant Ka.

弱酸和弱碱仅部分解离，建立起平衡。对于弱酸 HA，平衡反应为 HA(aq) ⇌ H⁺(aq) + A⁻(aq)。解离程度由酸解离常数 Ka 给出。

You must be able to distinguish between the terms ‘strong’ and ‘concentrated’. A concentrated weak acid may still have a high total amount of acid but low [H⁺] due to limited dissociation.

你必须要区分‘强’与‘浓’这两个术语。一个浓的弱酸可能含有大量的酸分子，但由于解离程度有限，[H⁺] 可能仍然不高。

4. pH and the Ionic Product of Water | pH 与水的离子积

pH is defined as:

pH = -log[H⁺]

pH 定义为：

pH = -log[H⁺]

Here [H⁺] is the hydrogen ion concentration in mol dm⁻³. Similarly, pOH = -log[OH⁻]. At 298 K, the ionic product of water Kw = [H⁺][OH⁻] = 1.0 × 10⁻¹⁴ mol² dm⁻⁶, and therefore pH + pOH = 14.

此处 [H⁺] 表示氢离子浓度，单位为 mol dm⁻³。类似地，pOH = -log[OH⁻]。在 298 K 时，水的离子积 Kw = [H⁺][OH⁻] = 1.0 × 10⁻¹⁴ mol² dm⁻⁶，因此 pH + pOH = 14。

For a neutral solution, [H⁺] = [OH⁻] = 1.0 × 10⁻⁷ mol dm⁻³, giving pH 7 at 25°C. The value of Kw increases with temperature because the autoionisation of water is endothermic.

对于中性溶液，[H⁺] = [OH⁻] = 1.0 × 10⁻⁷ mol dm⁻³，在 25°C 时 pH 为 7。由于水的自解离是吸热过程，Kw 值随温度升高而增大。

In exam calculations, always check if the temperature is 298 K before assuming Kw = 1.0 × 10⁻¹⁴.

在考试计算中，在假定 Kw = 1.0 × 10⁻¹⁴ 之前，务必先确认温度是否 298 K。

5. Acid Dissociation Constant Ka and pKa | 酸解离常数 Ka 与 pKa

For a weak acid HA ⇌ H⁺ + A⁻, the acid dissociation constant is:

Ka = [H⁺][A⁻] / [HA]

对于弱酸 HA ⇌ H⁺ + A⁻，酸解离常数表达式为：

Ka = [H⁺][A⁻] / [HA]

The units of Ka are mol dm⁻³. A larger Ka indicates a stronger weak acid. The pKa value is defined as pKa = -log(Ka). The smaller the pKa, the stronger the acid.

Ka 的单位是 mol dm⁻³。Ka 值越大，弱酸相对越强。pKa 定义为 pKa = -log(Ka)。pKa 越小，酸性越强。

pKa values allow easy comparison: if pKa of ethanoic acid is 4.76 and that of chloroethanoic acid is 2.86, the latter is a much stronger acid because its pKa is lower.

通过 pKa 值可方便地比较酸性强弱：若乙酸的 pKa 为 4.76，而一氯乙酸的 pKa 为 2.86，则后者酸性强得多，因为其 pKa 更小。

6. Calculating pH of Weak Acids | 弱酸 pH 计算

To find the pH of a weak acid solution, use the approximation that the equilibrium [HA] is approximately equal to the initial concentration c, and [H⁺] = [A⁻] at equilibrium. Then Ka = [H⁺]² / c, giving:

[H⁺] = √(Ka × c)

计算弱酸溶液的 pH 时，可使用近似处理：平衡时 [HA] 约等于初始浓度 c，且 [H⁺] = [A⁻]。于是 Ka = [H⁺]² / c，得到：

[H⁺] = √(Ka × c)

This approximation holds if the acid is weak (typically pKa > 3) and the solution is not extremely dilute. You can then calculate pH = -log[H⁺]. For example, for 0.10 mol dm⁻³ CH₃COOH with Ka = 1.8 × 10⁻⁵, [H⁺] = √(1.8×10⁻⁵ × 0.1) = 1.34 × 10⁻³, pH ≈ 2.87.

若酸足够弱（通常 pKa > 3）且溶液不是极稀，该近似成立。随后可计算 pH = -log[H⁺]。例如，0.10 mol dm⁻³ CH₃COOH，Ka = 1.8 × 10⁻⁵，则 [H⁺] = √(1.8×10⁻⁵ × 0.1) = 1.34 × 10⁻³，pH ≈ 2.87。

If the pH is measured experimentally, you can rearrange to find Ka. Be prepared to use the quadratic formula if the degree of dissociation is not negligible; however, the Edexcel specification typically favours the approximation method.

若通过实验测得 pH，可通过公式变形求出 Ka。如果解离度不可忽略，则需要用到二次方程；不过 Edexcel 考纲通常倾向于使用近似法。

7. Base Dissociation Constant Kb and pKb | 碱解离常数 Kb 与 pKb

Weak bases such as ammonia react with water: NH₃(aq) + H₂O(l) ⇌ NH₄⁺(aq) + OH⁻(aq). The base dissociation constant Kb = [NH₄⁺][OH⁻] / [NH₃], with units mol dm⁻³. pKb = -log(Kb).

弱碱如氨与水发生反应：NH₃(aq) + H₂O(l) ⇌ NH₄⁺(aq) + OH⁻(aq)。碱解离常数 Kb = [NH₄⁺][OH⁻] / [NH₃]，单位为 mol dm⁻³。pKb = -log(Kb)。

For a conjugate acid-base pair, the relationship pKa + pKb = 14 (at 298 K) holds. Thus, if you know the Ka of a weak acid, you can find the Kb of its conjugate base.

对于共轭酸碱对，存在关系 pKa + pKb = 14（298 K 时）。因此，若已知弱酸的 Ka，就可求出其共轭碱的 Kb。

Calculating pH of a weak base involves finding [OH⁻] via a similar square root approximation: [OH⁻] = √(Kb × c), then determining pOH and finally pH = 14 – pOH.

计算弱碱的 pH 涉及通过类似的平方根近似求 [OH⁻]：[OH⁻] = √(Kb × c)，接着求出 pOH，最后利用 pH = 14 – pOH。

8. Buffer Solutions | 缓冲溶液

A buffer solution resists changes in pH upon addition of small amounts of acid or base. It consists of a weak acid and its conjugate base (such as CH₃COOH/CH₃COO⁻) or a weak base and its conjugate acid (such as NH₃/NH₄⁺).

缓冲溶液能抵抗因加入少量酸或碱而引起的 pH 变化。它通常由弱酸及其共轭碱（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 + log([A⁻] / [HA])

酸性缓冲液的 pH 可通过 Henderson–Hasselbalch 方程计算：

pH = pKa + log([A⁻] / [HA])

When [A⁻] = [HA], pH = pKa. On adding H⁺, the conjugate base A⁻ neutralises it; on adding OH⁻, the weak acid HA releases H⁺ to restore equilibrium. This dual action gives the buffer its capacity.

当 [A⁻] = [HA] 时，pH = pKa。加入 H⁺ 时，共轭碱 A⁻ 中和之；加入 OH⁻ 时，弱酸 HA 释放 H⁺ 以恢复平衡。这种双重作用赋予了缓冲能力。

Common exam question: calculate the mass of sodium ethanoate needed to prepare a buffer of a specific pH. You must be comfortable with converting moles, concentration, and volume.

常见考题：计算配制特定 pH 缓冲液所需醋酸钠的质量。你必须熟练掌握物质的量、浓度和体积之间的转换。

9. Titration Curves and Equivalence Points | 滴定曲线与等当点

Titration curves plot pH against the volume of titrant added. Key features are vertical regions where pH changes rapidly, and the equivalence point where the amount of base equals the amount of acid (stoichiometrically).

滴定曲线绘出了 pH 随滴定剂加入体积的变化。关键特征包括 pH 急剧变化的突跃区域，以及碱的量与酸的量恰好按化学计量比完全反应的等当点。

The equivalence point for a strong acid–strong base titration occurs at pH 7. The curve has a sharp vertical jump.

强酸-强碱滴定的等当点出现在 pH 7，曲线具有一个陡峭的垂直突跃。
For a weak acid–strong base titration, the equivalence point is > pH 7 because the conjugate base of the weak acid hydrolyses to produce OH⁻. The curve starts higher and has a smaller initial slope.

弱酸-强碱滴定时，等当点 pH > 7，因为弱酸的共轭碱水解产生 OH⁻。曲线起点较高，且初始斜率较小。
Strong acid–weak base titrations have an equivalence point < pH 7. Weak acid–weak base titrations lack a sharp jump and are generally not suitable for indicator detection.

强酸-弱碱滴定的等当点 pH < 7。弱酸-弱碱滴定缺乏明显突跃，通常不适于用指示剂检测终点。

The half-equivalence point is the stage where exactly half the acid has been neutralised, and pH = pKa. This is a crucial concept for buffer zones and indicator selection.

半等当点是指酸恰好一半被中和的时刻，此时 pH = pKa。这是理解缓冲区域和指示剂选择的关键概念。

10. Indicators and Their Selection | 指示剂及其选择

Acid–base indicators are weak acids that change colour when the ratio of their conjugate base to acid changes. The colour change occurs over a pH range approximately pKin ± 1, where pKin is the indicator’s own pKa.

酸碱指示剂是弱酸，当其共轭碱与酸的比例发生变化时显示颜色变化。颜色转变的 pH 范围大约在 pKin ± 1，其中 pKin 是指示剂本身的 pKa。

Indicator	pKin	pH range	Colour change (acid → base)
Methyl orange	3.7	3.1–4.4	Red → Yellow
Phenolphthalein	9.3	8.2–10.0	Colourless → Pink

选择指示剂时，必须确保其变色范围完全落在滴定曲线的突跃范围内。对于强酸-强碱滴定，两者均可；若滴定弱酸，则应选用酚酞，因为等当点在碱性区域。

Methyl orange is suitable for strong acid–strong base and strong acid–weak base titrations, but not for weak acid–strong base because its range does not cover the high equivalence pH.

甲基橙适用于强酸-强碱以及强酸-弱碱滴定，但不适用于弱酸-强碱滴定，因为其变色范围无法覆盖较高的等当点 pH。

11. Dilution and Mixing Calculations | 稀释与混合计算

When two solutions are mixed, the total volume changes, and concentrations must be recalculated using n = cV. For an acid–base mixture, first identify the limiting reagent and calculate the moles of excess H⁺ or OH⁻.

两溶液混合时，总体积发生变化，必须用 n = cV 重新计算浓度。对于酸碱混合，首先确定限量试剂，并计算过量的 H⁺ 或 OH⁻ 的物质的量。

If equal moles of strong acid and strong base are mixed, the resulting solution is neutral (pH 7 at 298 K). If the acid is in excess, [H⁺] = (moles of excess H⁺) / total volume.

若强酸与强碱等物质的量混合，所得溶液为中性（298 K 时 pH 7）。若酸过量，则 [H⁺] =（过量 H⁺ 的物质的量）/ 总体积。

For a mixture containing a weak acid and its salt (a buffer), you can directly apply the Henderson–Hasselbalch equation after accounting for dilution, as the ratio [A⁻]/[HA] remains unchanged by dilution if both are present in the same solution.

对于含有弱酸及其盐的缓冲混合体系，考虑稀释后可直接应用 Henderson–Hasselbalch 方程，因为 [A⁻]/[HA] 比值在同溶液中不受稀释影响。

12. Common Exam Pitfalls and Tips | 常见考试雷区与技巧

Always state assumptions: e.g., neglect [H⁺] from water when calculating weak acid pH, and check if approximation is valid (typically [H⁺] < 5% of c).

务必说明假设条件：例如计算弱酸 pH 时忽略水的自解离 H⁺，并检查近似是否有效（通常要求 [H⁺] < 初始浓度的 5%）。
Do not confuse endpoint (indicator colour change) with equivalence point. The indicator must change exactly where the equivalence point falls.

勿混淆终点（指示剂变色）与等当点。指示剂的变色必须恰好落在等当点的位置上。
For buffer calculations, don’t forget that adding small amounts of acid/base changes the ratio [A⁻]/[HA], and you must recalculate the moles of each component after the neutralisation reaction.

进行缓冲计算时，不要忘记加入少量酸或碱会改变 [A⁻]/[HA] 的比值，须在中和反应后重新计算各组分的物质的量。
Write correct units for Kw, Ka, and Kb. Kw has units mol² dm⁻⁶; Ka and Kb are mol dm⁻³. Leaving out units can lose marks.

写出 Kw、Ka 和 Kb 的正确单位。Kw 单位是 mol² dm⁻⁶；Ka 和 Kb 是 mol dm⁻³。漏写单位会丢失分数。
When explaining buffer action, use chemical equations and Le Chatelier’s principle — don’t just say ‘it neutralises acid/base’.

解释缓冲作用时，务必使用化学方程式和勒夏特列原理——不要简单地说‘它能中和酸/碱’。

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Acids and Bases Exam Guide for Edexcel A-Level | Edexcel A-Level 酸与碱考点精讲