📚 R3.1 Proton Transfer Reactions | R3.1 质子转移反应
Welcome to this comprehensive revision guide on R3.1 Proton Transfer Reactions from IB Chemistry. This topic unpacks the Brønsted–Lowry theory, where acids and bases are defined by their ability to donate or accept protons (H⁺). From the ionization of water to buffer systems and the colourful world of indicators, proton transfer lies at the heart of aqueous equilibrium chemistry. A solid grasp of these concepts is essential for understanding pH control in biological systems, industrial processes, and laboratory titrations.
欢迎阅读 IB 化学 R3.1 质子转移反应的全面复习指南。本专题深入剖析布朗斯特-劳里理论，该理论根据物种提供或接受质子 (H⁺) 的能力来定义酸和碱。从水的电离到缓冲体系以及指示剂的缤纷变色世界，质子转移是水溶液平衡化学的核心。扎实掌握这些概念对于理解生物系统、工业过程和实验室滴定中的 pH 控制至关重要。
1. Introduction to Brønsted–Lowry Theory | 布朗斯特-劳里理论导论
The Brønsted–Lowry theory defines an acid as a proton donor and a base as a proton acceptor. Unlike the earlier Arrhenius theory, this definition does not require the presence of water or the formation of OH⁻ ions. A classic example is the gas-phase reaction between hydrogen chloride and ammonia, where HCl transfers a proton to NH₃, forming the ammonium ion and chloride ion.
布朗斯特-劳里理论将酸定义为质子给体，将碱定义为质子受体。与早期的阿伦尼乌斯理论不同，这一定义不要求存在水或生成 OH⁻ 离子。一个经典例子是氯化氢与氨的气相反应，HCl 将质子转移给 NH₃，生成铵离子和氯离子。
HCl(g) + NH₃(g) → NH₄⁺(g) + Cl⁻(g)
In aqueous solution, an acid reacts with water to produce hydronium ions, H₃O⁺. For instance, hydrogen chloride dissolves in water to give hydrochloric acid, with the proton being donated to a water molecule. The Brønsted–Lowry perspective thus reframes any proton-exchange process as an acid-base reaction, even if water is not the solvent.
在水溶液中，酸与水反应产生水合氢离子 H₃O⁺。例如，氯化氢溶于水得到盐酸，质子被提供给水分子。因此，布朗斯特-劳里的视角将任何质子交换过程都视为酸碱反应，即使溶剂不是水。
2. Conjugate Acid-Base Pairs | 共轭酸碱对
Every Brønsted–Lowry acid has a corresponding conjugate base, formed by the loss of a proton. Equally, every base has a conjugate acid, formed by gaining a proton. A conjugate pair differs by exactly one H⁺. In the equilibrium below, HA donates a proton to B, leaving A⁻ as its conjugate base, while BH⁺ is the conjugate acid of B.
每一个布朗斯特-劳里酸都有其对应的共轭碱，由失去一个质子形成。同样，每一个碱都有其共轭酸，由获得一个质子形成。共轭对恰好相差一个 H⁺。在下面的平衡中，HA 将质子给予 B，A⁻ 成为其共轭碱，而 BH⁺ 是 B 的共轭酸。
HA + B ⇌ A⁻ + BH⁺
Consider the dissolution of HCl in water. HCl donates a proton to H₂O, so HCl is the acid and Cl⁻ is its conjugate base. Water accepts the proton, making H₂O the base and H₃O⁺ its conjugate acid. Recognizing conjugate pairs helps predict the direction of proton transfer: the equilibrium favours the weaker acid and weaker base.
考虑 HCl 在水中的溶解。HCl 把质子给予 H₂O，因此 HCl 是酸，Cl⁻ 是其共轭碱。水接受质子，使得 H₂O 成为碱，H₃O⁺ 成为其共轭酸。识别共轭对有助于预测质子转移的方向：平衡倾向于较弱的酸和较弱的碱。
3. Amphiprotic Species | 两性物种
A species that can both donate and accept a proton is described as amphiprotic. Water is the most important example. It can act as an acid, donating a proton to form a hydroxide ion, and as a base, accepting a proton to form a hydronium ion. This dual behaviour is the reason for water's ability to autoionize.
既能够提供又能够接受质子的物种被称为两性物种。水是最重要的例子。它既可以作为酸，失去一个质子形成氢氧根离子，又可以作为碱，获得一个质子形成水合氢离子。这种双重行为是水能够自电离的原因。
protic ions play key roles in biological buffer systems, such as the bicarbonate buffer in blood.
其他两性物种包括碳酸氢根离子 HCO₃⁻ 和硫酸氢根离子 HSO₄⁻。在强酸存在下，HCO₃⁻ 充当碱：HCO₃⁻ + H⁺ → H₂CO₃。在强碱存在下，它充当酸：HCO₃⁻ + OH⁻ → CO₃²⁻ + H₂O。两性离子在生物缓冲体系（如血液中的碳酸氢盐缓冲系统）中起关键作用。
4. The Ionization of Water and Kw | 水的电离与 Kw
Pure water exhibits a very small but measurable electrical conductivity because of autoionization: one water molecule donates a proton to another. This can be written as an equilibrium between water, hydronium ions, and hydroxide ions, or more simply as the dissociation of water into H⁺ and OH⁻.
纯水表现出极微小但可测量的电导率，这是由自电离引起的：一个水分子将质子给予另一个。这可以写成水、水合氢离子和氢氧根离子之间的平衡，或者更简单地写成水解离为 H⁺ 和 OH⁻。
H₂O(l) ⇌ H⁺(aq) + OH⁻(aq)
dm⁻³. Since autoionization is endothermic, an increase in temperature shifts the equilibrium to the right, increasing Kw. Consequently, the pH of neutral water decreases as temperature rises – neutral is no longer exactly 7 at higher temperatures.
此过程的平衡常数是水的离子积常数，Kw = [H⁺][OH⁻]。在 298 K 时，Kw 的值为 1.0 × 10⁻¹⁴。在此温度下纯水中，[H⁺] = [OH⁻] = 1.0 × 10⁻⁷ mol dm⁻³。由于自电离是吸热的，温度升高会使平衡向右移动，增大 Kw。因此，中性水的 pH 随温度升高而降低——在较高温度下，中性不再恰好为 7。
5. The pH Scale and Calculations | pH 标度与计算
pH is a logarithmic measure of the hydrogen ion concentration: pH = –log[H⁺]. Similarly, pOH = – such as HCl, complete dissociation means that [H⁺] equals the initial acid concentration.
pH 是氢离子浓度的对数度量：pH = –log[H⁺]。类似地，pOH = –log[OH⁻]。对 Kw 表达式取负对数，得到 298 K 下的有用关系：pH + pOH = 14.00。对于像 HCl 这样的一元强酸，完全解离意味着 [H⁺] 等于酸的初始浓度。
Example: Calculate the pH of 0.020 mol dm⁻³ HCl. [H⁺] = 0.020 mol dm⁻³, so pH = –0) = 1.70. For a strong base like 0.0050 mol dm⁻³ NaOH, [OH⁻] = 5.0 × 10⁻³ mol dm⁻³; pOH = –log(5.0 × 10⁻³) ≈ 2.30, and pH = 14.00 – 2.30 = 11.70.
示例：计算 0.020 mol dm⁻³ HCl 的 pH。[H⁺] = 0.020 mol dm⁻³，因此 pH = –log(0.020) = 1.70。对于强碱，如 0.0050 mol dm⁻³ NaOH，[OH⁻] = 5.0 × 10⁻³ mol dm⁻³；pOH = –log(5.0 × 10⁻³) ≈ 2.30，pH = 14.00 – 2.30 = 11.70。
6. Strong vs Weak Acids and Bases | 强酸强碱与弱酸弱碱
The strength of an acid or base refers to its degree of dissociation in water. Strong acids, including HCl, HNO₃ and H₂SO₄, dissociate essentially 100%, releasing all their available protons. Strong bases such as NaOH and KOH dissociate fully to give OH⁻ ions. Weak acids and bases only partially dissociate, establishing a dynamic equilibrium between the molecular form and the ions.
酸或碱的强度是指其在水中解离的程度。强酸，包括 HCl、HNO₃ 和 H₂SO₄，基本上 100% 解离，释放出所有可用的质子。强碱如 NaOH
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