Acids and Bases | 酸碱

📚 Acids and Bases | 酸碱

This AQA A-Level Chemistry topic covers Brønsted-Lowry definitions, pH, K_w, weak acid equilibria, buffers, titration curves and indicators. It links equilibrium constants to acid strength and provides quantitative tools for predicting pH in aqueous solutions.

本主题涵盖 AQA A-Level 化学中的布朗斯特-劳里酸碱定义、pH、水的离子积、弱酸平衡、缓冲溶液、滴定曲线和指示剂。它将平衡常数与酸性强度联系起来，为预测水溶液的 pH 提供定量工具。

1. Brønsted-Lowry Acids and Bases | 布朗斯特-劳里酸碱理论

A Brønsted-Lowry acid is a proton donor, and a Brønsted-Lowry base is a proton acceptor. For example, when HCl dissolves in water, HCl donates a proton to H₂O, forming H₃O⁺ and Cl⁻.

布朗斯特-劳里酸是质子给体，布朗斯特-劳里碱是质子受体。例如，HCl 溶于水时，HCl 把质子给 H₂O，生成 H₃O⁺ 和 Cl⁻。

HCl + H₂O → H₃O⁺ + Cl⁻

This definition is broader than the Arrhenius definition because it includes non-aqueous systems and ions such as NH₄⁺ or CO₃²⁻.

该定义比阿伦尼乌斯定义更广，因为它包括非水体系以及 NH₄⁺、CO₃²⁻ 等离子。

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2. Conjugate Acid-Base Pairs | 共轭酸碱对

A conjugate acid-base pair differs by one proton. In the forward reaction, HCl is the acid and Cl⁻ is its conjugate base; H₂O is the base and H₃O⁺ is its conjugate acid.

共轭酸碱对相差一个质子。在正反应中，HCl 是酸，Cl⁻ 是其共轭碱；H₂O 是碱，H₃O⁺ 是其共轭酸。

NH₃ + H₂O ⇌ NH₄⁺ + OH⁻

Every acid-base reaction has two conjugate pairs. The stronger the acid, the weaker its conjugate base.

每个酸碱反应都有两个共轭对。酸越强，其共轭碱越弱。

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3. The Ionic Product of Water, K_w | 水的离子积 K_w

Water undergoes slight self-ionisation: 2H₂O ⇌ H₃O⁺ + OH⁻. The equilibrium constant expression multiplied by [H₂O]² gives the ionic product of water.

水发生轻微的自电离：2H₂O ⇌ H₃O⁺ + OH⁻。平衡常数表达式乘以 [H₂O]² 得到水的离子积。

K_w = [H⁺][OH⁻] = 1.0 × 10⁻¹⁴ mol² dm⁻⁶ at 298 K

Because the reaction is endothermic, K_w increases with temperature; therefore the pH of pure water falls as temperature rises, even though it remains neutral.

由于该反应吸热，K_w 随温度升高而增大；因此纯水的 pH 随温度升高而下降，但水仍保持中性。

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4. pH and Strong Acids | pH 与强酸

ong acid dissociates completely in water, so [H⁺] equals the concentration of the acid multiplied by the number of protons released per formula unit.

pH 定义为 pH = −log₁₀[H⁺]。强酸在水中完全电离，因此 [H⁺] 等于酸浓度乘以每个化学式释放的质子数。

pH = −log₁₀[H⁺]

. For example, 0.050 mol dm⁻³ HCl has [H⁺] = 0.050 mol dm⁻³, so pH = −log₁₀(0.050) = 1.30.

对于 HCl 或 HNO₃ 等一元强酸，[H⁺] = [酸]。例如，0.050 mol dm⁻³ HCl 中 [H⁺] = 0.050 mol dm⁻³，因此 pH = −log₁₀(0.050) = 1.30。

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5. Weak Acids and Kₐ | 弱酸与 Kₐ

A weak acid only partially dissociates in water. Its acid dissociation constant Kₐ is given by the equilibrium expression:

弱酸在水中仅部分电离。其酸解离常数 Kₐ 由以下平衡表达式给出：

Kₐ = [H⁺][A⁻] / [HA]

en assume [H⁺] = [A⁻] and [HA] ≈ c. This gives the approximation Kₐ = [H⁺]² / c, so [H⁺] = √Kₐ c.

对于浓度为 c 的弱酸 HA，通常假设 [H⁺] = [A⁻] 且 [HA] ≈ c。由此得出近似式 Kₐ = [H⁺]² / c，因此 [H⁺] = √Kₐ c。

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6. pKₐ and Acid Strength | pKₐ 与酸性强度

onds to a larger Kₐ and therefore a stronger acid. Each unit decrease in pKₐ increases acid strength by a factor of ten.

pKₐ = −log₁₀Kₐ。pKₐ 越小，Kₐ 越大，因此酸越强。pKₐ 每减小 1 个单位，酸性强度增大十倍。

ic acid has a lower pKₐ because the electron-withdrawing chlorine stabilises the conjugate base.

pKₐ 值可以快速比较弱酸的强度。例如，乙酸的 pKₐ 约为 4.76，而氯乙酸的 pKₐ 更低，因为吸电子的氯原子稳定了共轭碱。

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7. Buffer Solutions | 缓冲溶液

An acidic buffer is a mixture of a weak acid and one of its salts, such as CH₃COOH and CH₃COONa. It resists changes in pH when small amounts of acid or base are added.

酸性缓冲溶液由弱酸及其一种盐混合而成，例如 CH₃COOH 和 CH₃COONa。当加入少量酸或碱时，它能抵抗 pH 的变化。

The weak acid component neutralises added OH⁻, while the conjugate base component neutralises added H⁺. Blood plasma uses H₂CO₃/HCO₃⁻ as an important biological buffer.

弱酸组分中和加入的 OH⁻，共轭碱组分中和加入的 H⁺。血浆使用 H₂CO₃/HCO₃⁻ 作为重要的生物缓冲体系。

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8. Buffer Calculations | 缓冲溶液计算

For a buffer made from a weak acid HA and its conjugate base A⁻, the pH can be calculated using the Henderson-Hasselbalch equation:

对于由弱酸 HA 及其共轭碱

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