📚 Reactions 1.2.1 – Acids, Alkalis and pH | 反应1.2.1 – 酸、碱与pH值 核心原理
Understanding acids, alkalis and the pH scale is essential for mastering chemical reactions, from neutralisation to industrial processes. This article explores the core principles behind acidic and alkaline behaviour, the measurement of pH, and the theories that define these substances.
理解酸、碱和pH标度对于掌握从中和反应到工业流程的化学反应至关重要。本文探索酸和碱行为的核心原理、pH的测量方法以及定义这些物质的理论。
1. Defining Acids and Alkalis | 酸与碱的定义
According to the Arrhenius theory, an acid is a substance that dissociates in water to produce hydrogen ions (H⁺), while an alkali is a soluble base that dissociates in water to release hydroxide ions (OH⁻). For example, hydrogen chloride gas dissolves in water to form hydrochloric acid, producing H⁺ and Cl⁻ ions.
根据阿伦尼乌斯理论，酸是在水中解离产生氢离子(H⁺)的物质，而碱(可溶碱)是在水中解离释放氢氧根离子(OH⁻)的物质。例如，氯化氢气体溶于水形成盐酸，产生H⁺和Cl⁻离子。
The Brønsted-Lowry theory expands the definition: an acid is a proton (H⁺) donor, and a base is a proton acceptor. This model explains reactions in non-aqueous solvents and includes substances like ammonia (NH₃), which accepts a proton to become NH₄⁺, making it a base even without a hydroxide group.
布朗斯特-劳里理论扩展了定义：酸是质子(H⁺)的供体，碱是质子的受体。该模型解释了非水溶剂中的反应，并包括了像氨(NH₃)这样的物质，它接受一个质子变成NH₄⁺，即使没有氢氧根也属于碱。
Common laboratory acids are hydrochloric acid (HCl), sulfuric acid (H₂SO₄), and nitric acid (HNO₃). Typical alkalis include sodium hydroxide (NaOH), potassium hydroxide (KOH), and calcium hydroxide (Ca(OH)₂).
常见的实验室酸有盐酸(HCl)、硫酸(H₂SO₄)和硝酸(HNO₃)。典型的碱包括氢氧化钠(NaOH)、氢氧化钾(KOH)和氢氧化钙(Ca(OH)₂)。
2. The pH Scale | pH值标度
The pH scale is a logarithmic scale ranging typically from 0 to 14 that measures the acidity or alkalinity of an aqueous solution. A pH less than 7 indicates an acidic solution, pH equal to 7 is neutral (pure water), and pH greater than 7 indicates an alkaline solution.
pH标度是一个通常范围在0到14之间的对数标度，用于测量水溶液的酸度或碱度。pH小于7表示酸性溶液，pH等于7为中性(纯水)，pH大于7表示碱性溶液。
A change of one pH unit corresponds to a tenfold change in hydrogen ion concentration. For instance, a solution with pH 3 is ten times more acidic than one with pH 4 and one hundred times more acidic than pH 5.
一个pH单位的变化对应氢离子浓度十倍的变化。例如，pH为3的溶液比pH为4的溶液酸性强十倍，比pH为5的溶液酸性强一百倍。
The term ‘pH’ stands for the negative logarithm (base 10) of the hydrogen ion activity, simplified as [H⁺] in mol dm⁻³. The relationship is central to quantitative acid-base chemistry.
“pH”一词代表氢离子活度(简化表示为[H⁺]，单位mol dm⁻³)的以10为底的负对数。这一关系是定量酸碱化学的核心。
3. Hydrogen Ion Concentration and pH | 氢离子浓度与pH值
The mathematical definition of pH is given by the equation:
pH的数学定义由以下方程给出：
pH = –log₁₀ [H⁺]
where [H⁺] is the concentration of hydrogen ions in mol dm⁻³. For a neutral solution at 25 °C, [H⁺] = 1.0 × 10⁻⁷ mol dm⁻³, so pH = 7. If [H⁺] = 0.01 mol dm⁻³, the pH is 2.
其中[H⁺]是氢离子的浓度，单位为mol dm⁻³。对于25 °C下的中性溶液，[H⁺] = 1.0 × 10⁻⁷ mol dm⁻³，因此pH = 7。如果[H⁺] = 0.01 mol dm⁻³，则pH为2。
Similarly, the pOH scale is used to express hydroxide ion concentration: pOH = –log₁₀ [OH⁻]. At 25 °C, pH + pOH = 14. This relationship arises from the ionic product of water, which will be discussed later.
类似地，pOH标度用于表示氢氧根离子浓度：pOH = –log₁₀ [OH⁻]。在25 °C时，pH + pOH = 14。这一关系来源于后面将讨论的水的离子积。
4. Strong and Weak Acids | 强酸与弱酸
A strong acid is one that completely dissociates (ionises) in water to release H⁺ ions. Hydrochloric acid, sulfuric acid (first dissociation) and nitric acid are typical strong acids. In a solution of 0.1 mol dm⁻³ HCl, the [H⁺] is 0.1 mol dm⁻³, giving a pH of 1.
强酸是在水中完全解离(电离)释放出H⁺离子的酸。盐酸、硫酸(第一级解离)和硝酸是典型的强酸。在0.1 mol dm⁻³ HCl溶液中，[H⁺]为0.1 mol dm⁻³，pH为1。
A weak acid only partially dissociates in water, establishing an equilibrium between the undissociated acid and the ions. Ethanoic acid (CH₃COOH) is a common example. At the same concentration, a weak acid has a higher pH (less acidic) than a strong acid because [H⁺] is lower.
弱酸在水中仅部分解离，在未解离的酸和离子之间建立平衡。乙二酸(醋酸，CH₃COOH)是一个常见例子。在相同浓度下，弱酸的pH值比强酸高(酸性更弱)，因为[H⁺]较低。
The strength of an acid should not be confused with its concentration. Strength refers to the degree of dissociation, while concentration is the amount of acid dissolved per unit volume. A concentrated weak acid can still be corrosive but has a lower proportion of dissociated ions.
酸的强度不应与其浓度混淆。强度指的是解离程度，而浓度是单位体积中溶解的酸的量。浓的弱酸仍然可能具有腐蚀性，但其解离的离子比例较低。
5. Strong and Weak Alkalis | 强碱与弱碱
A strong alkali dissociates completely in water to give a high concentration of OH⁻ ions. Sodium hydroxide and potassium hydroxide are classic examples. A 0.1 mol dm⁻³ NaOH solution yields [OH⁻] = 0.1 mol dm⁻³, corresponding to a pOH of 1 and a pH of 13.
强碱在水中完全解离，产生高浓度的OH⁻离子。氢氧化钠和氢氧化钾是典型的例子。0.1 mol dm⁻³ NaOH溶液产生[OH⁻] = 0.1 mol dm⁻³，对应的pOH为1，pH为13。
Weak alkalis, such as aqueous ammonia (NH₃), only partially react with water to produce OH⁻ ions. Ammonia accepts a proton from water, forming NH₄⁺ and OH⁻ in an equilibrium that lies mostly on the side of the reactants. Thus, a solution of ammonia with the same nominal concentration has a lower pH than that of NaOH.
弱碱，如氨水(NH₃)，只部分与水反应生成OH⁻离子。氨从水中接受一个质子，生成NH₄⁺和OH⁻，该平衡强烈偏向反应物一侧。因此，相同名义浓度的氨溶液的pH值低于NaOH溶液。
The base strength of oxides and carbonates is also relevant. Metal oxides like calcium oxide (CaO) react with water to form the corresponding hydroxide, while carbonates such as sodium carbonate (Na₂CO₃) hydrolyse in water to give alkaline solutions, even though they are not hydroxides themselves.
氧化物和碳酸盐的碱强度也相关。像氧化钙(CaO)这样的金属氧化物与水反应生成相应的氢氧化物，而碳酸钠(Na₂CO₃)等碳酸盐在水中水解产生碱性溶液，尽管它们本身不是氢氧化物。
6. Indicators and Their Colour Changes | 指示剂及其颜色变化
Acid-base indicators are weak organic dyes whose colour depends on the pH of the solution. The table below summarises three common indicators and their colour transitions.
酸碱指示剂是弱有机染料，其颜色取决于溶液的pH值。下表总结了三种常见指示剂及它们的颜色转变。
| Indicator | Colour in acid | Colour in alkali | pH range of colour change |
|---|---|---|---|
| Litmus | Red | Blue | 5.0 – 8.0 |
| Phenolphthalein | Colourless | Pink | 8.2 – 10.0 |
| Methyl orange | Red | Yellow | 3.1 – 4.4 |
常见的指示剂包括石蕊、酚酞和甲基橙。下表总结了它们的颜色变化和pH范围。
| 指示剂 | 酸性中的颜色 | 碱性中的颜色 | 颜色变化的pH范围 |
|---|---|---|---|
| 石蕊 | 红 | 蓝 | 5.0 – 8.0 |
| 酚酞 | 无色 | 粉红 | 8.2 – 10.0 |
| 甲基橙 | 红 | 黄 | 3.1 – 4.4 |
Universal indicator is a mixture of several indicators that produces a continuous spectrum of colours across the pH scale, typically from red (pH 1) through green (pH 7) to violet (pH 14).
通用指示剂是由多种指示剂混合而成，在pH标度上产生连续的色谱，通常从红色(pH 1)经过绿色(pH 7)到紫色(pH 14)。
7. Measuring pH: Indicators vs pH Meter | pH的测量：指示剂与pH计
pH can be estimated using indicator papers or solutions, and measured accurately with a pH meter. Indicator papers are inexpensive and easy to use but only provide an approximate pH value, limited by the colour resolution of the human eye.
pH可以使用指示试纸或指示剂溶液进行估算，并使用pH计进行精确测量。指示试纸价格便宜、使用方便，但只能提供近似的pH值，受限于人眼对颜色的分辨率。
A pH meter consists of a glass electrode sensitive to H⁺ ions and a reference electrode. The device measures the potential difference across the electrode system, which is then converted to a pH reading. It can give readings to 0.01 pH units, making it essential for precise titrations and quality control.
pH计由一个对H⁺离子敏感的玻璃电极和一个参比电极组成。该装置测量电极系统中的电位差，然后转换为pH读数。它可以给出精确到0.01个pH单位的读数，因此对于精密滴定和质量控制至关重要。
When choosing a method for a specific reaction, such as a strong acid–strong base titration, phenolphthalein is suitable because its colour change coincides with the steep part of the titration curve. Methyl orange is preferred for strong acid–weak base titrations.
在为特定反应(如强酸-强碱滴定)选择方法时，酚酞是合适的，因为它的颜色变化与滴定曲线的陡峭部分重合。甲基橙则更适用于强酸-弱碱滴定。
8. Neutralisation Reactions | 中和反应
Neutralisation is the reaction between an acid and a base to form a salt and water. The general equation is: Acid + Base → Salt + Water. For example, hydrochloric acid reacts with sodium hydroxide to produce sodium chloride and water:
中和反应是酸与碱反应生成盐和水的过程。通用方程式为：酸 + 碱 → 盐 + 水。例如，盐酸与氢氧化钠反应生成氯化钠和水：
HCl + NaOH → NaCl + H₂O
The ionic equation for the neutralisation between a strong acid and a strong alkali highlights the essential chemical change:
强酸与强碱中和的离子方程式突出了关键的化学变化：
H⁺ + OH⁻ → H₂O
Neutralisation reactions are exothermic. The heat released per mole of water
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