Reactions of Ions in Aqueous Solution | 水溶液中的离子反应

📚 Reactions of Ions in Aqueous Solution | 水溶液中的离子反应

Transition metal ions and certain post-transition metal ions, such as Al³⁺, do not exist as bare ions in water. Instead, each ion is surrounded by six water molecules forming a complex ion known as a hexaaqua ion. These ions undergo a range of characteristic reactions, including hydrolysis, precipitation, and ligand substitution, which produce striking colour changes and form the basis of many qualitative tests.

过渡金属离子以及某些后过渡金属离子（如 Al³⁺）在水中并不以裸露的离子形式存在。相反，每个离子被六个水分子包围，形成一种称为六水合离子的配离子。这些离子会发生一系列特征反应，包括水解、沉淀和配体取代，这些反应产生明显的颜色变化，是许多定性检验的基础。

1. Hexaaqua Complexes | 六水合配离子

In aqueous solution, the metal ion Mⁿ⁺ is surrounded by six water molecules with the lone pairs on oxygen donating into the metal’s empty orbitals. The general formula is [M(H₂O)₆]ⁿ⁺, where the water molecules act as ligands and the geometry is octahedral.

在水溶液中，金属离子 Mⁿ⁺ 被六个水分子包围，氧上的孤对电子进入金属的空轨道。通式为 [M(H₂O)₆]ⁿ⁺，其中水分子作为配体，几何构型为八面体。

Common examples include [Fe(H₂O)₆]²⁺ (pale green), [Fe(H₂O)₆]³⁺ (pale yellow/brown), [Cr(H₂O)₆]³⁺ (violet), [Cu(H₂O)₆]²⁺ (blue), and [Co(H₂O)₆]²⁺ (pink). These colours arise from d-d electronic transitions, which absorb visible light and leave complementary colours to be transmitted.

[Fe(H₂O)₆]²⁺ is pale green; [Fe(H₂O)₆]³⁺ is pale yellow/brown due to partial hydrolysis.

[Fe(H₂O)₆]²⁺ 为浅绿色；[Fe(H₂O)₆]³⁺ 因部分水解呈浅黄/棕色。
[Cr(H₂O)₆]³⁺ is violet, though aqueous Cr³⁺ often appears dark green because of inner-sphere ligand substitution.

[Cr(H₂O)₆]³⁺ 为紫色，但水溶液中的 Cr³⁺ 常因配体在内界取代而呈暗绿色。
[Cu(H₂O)₆]²⁺ is bright blue, and [Co(H₂O)₆]²⁺ is characteristic pink.

[Cu(H₂O)₆]²⁺ 为亮蓝色，[Co(H₂O)₆]²⁺ 呈特征性粉红色。

2. Acidity of Hexaaqua Ions | 六水合离子的酸性

Hexaaqua ions behave as Bronsted-Lowry acids because the coordinated water molecules lose protons to the surrounding solvent. The equilibrium for an ion of charge n+ is shown below, where the complex loses a proton to form an aquahydroxo species.

六水合离子是布朗斯特-洛瑞酸，因为配位的水分子会向周围溶剂释放质子。对于电荷为 n+ 的离子，平衡如下，配合物失去一个质子形成水合羟基物种。

[M(H₂O)₆]ⁿ⁺ ⇌ [M(H₂O)₅(OH)]⁽ⁿ⁻¹⁾⁺ + H⁺

The acidity increases with the charge density of the central metal ion. A higher charge and smaller radius pull electron density away from the O–H bonds, weakening them and making proton loss easier.

酸性随中心金属离子的电荷密度增大而增强。更高的电荷和更小的半径将电子密度从 O–H 键上拉走，削弱这些键，使质子更易失去。

[Fe(H₂O)₆]³⁺ is a stronger acid than [Fe(H₂O)₆]²⁺ because Fe³⁺ has a higher charge density.

[Fe(H₂O)₆]³⁺ 的酸性强于 [Fe(H₂O)₆]²⁺，因为 Fe³⁺ 的电荷密度更高。
[Al(H₂O)₆]³⁺ is noticeably acidic; it reacts with small amounts of OH⁻ to give [Al(H₂O)₅(OH)]²⁺.

[Al(H₂O)₆]³⁺ 明显呈酸性；它与少量 OH⁻ 反应生成 [Al(H₂O)₅(OH)]²⁺。

This stepwise deprotonation is the basis of precipitation reactions when excess hydroxide is added.

这种逐步去质子化是加入过量氢氧化物时产生沉淀反应的基础。

3. Precipitation with Hydroxide Ions | 氢氧化物沉淀反应

When aqueous sodium hydroxide is added to a solution containing a hexaaqua ion, successive deprotonation occurs until a neutral hydroxo complex precipitates. The overall reactions for key ions are written as follows:

当氢氧化钠水溶液加入含有六水合离子的溶液时，会发生连续去质子化，最终生成中性羟基配合物而沉淀。关键离子的总反应如下：

[Fe(H₂O)₆]³⁺ + 3OH⁻ → Fe(OH)₃(s) + 3H₂O

[Cu(H₂O)₆]²⁺ + 2OH⁻ → Cu(OH)₂(s) + 2H₂O

[Fe(H₂O)₆]²⁺ + 2OH⁻ → Fe(OH)₂(s) + 2H₂O

It is important to recognise that these ionic equations are simplified; the true precipitates are hydrated solids, but writing Fe(OH)₃(s) is accepted in AQA examination practice.

需要注意的是，这些离子方程式是简写形式；实际沉淀是含水固体，但在 AQA 考试中写成 Fe(OH)₃(s) 是接受的。

Ion \| 离子	Precipitate colour \| 沉淀颜色	Precipitate formula \| 沉淀化学式
Fe²⁺	Green \| 绿色	Fe(OH)₂
Fe³⁺	Orange-brown \| 橙棕色	Fe(OH)₃
Cu²⁺	Pale blue \| 浅蓝色	Cu(OH)₂
Al³⁺	White \| 白色	Al(OH)₃

4. Amphoteric Hydroxides | 两性氢氧化物

Some hydroxides are amphoteric, meaning they react with both acids and bases. Aluminium hydroxide is the classic example required at A-Level.

某些氢氧化物是两性的，意味着它们既能与酸反应也能与碱反应。氢氧化铝是 A-Level 阶段要求的典型例子。

With excess sodium hydroxide, freshly precipitated Al(OH)₃ dissolves to form a colourless solution containing the tetrahydroxoaluminate(III) ion:

在过量氢氧化钠中，新生成的 Al(OH)₃ 沉淀溶解，形成含有四羟基合铝(III)酸根离子的无色溶液：

Al(OH)₃(s) + OH⁻ → [Al(OH)₄]⁻

With excess aqueous ammonia, however, aluminium hydroxide does not dissolve because ammonia is a weaker base; this distinguishes Al³⁺ from Zn²⁺ and Cu²⁺ in qualitative analysis.

然而，在过量氨水中，氢氧化铝不溶解，因为氨是较弱的碱；这可以在定性分析中区分 Al³⁺ 与 Zn²⁺ 和 Cu²⁺。

Zn(OH)₂ is also amphoteric, dissolving in excess OH⁻ to form [Zn(OH)₄]²⁻.

Zn(OH)₂ 也是两性的，溶于过量 OH⁻ 生成 [Zn(OH)₄]²⁻。
Cr(OH)₃ is amphoteric as well, giving [Cr(OH)₆]³⁻ or [Cr(OH)₄]⁻ depending on conditions; AQA commonly tests Cr(OH)₃ with excess NaOH forming a dark green solution.

Cr(OH)₃ 同样是两性的，依条件生成 [Cr(OH)₆]³⁻ 或 [Cr(OH)₄]⁻；AQA 常见考点是 Cr(OH)₃ 溶于过量 NaOH 形成深绿色溶液。

5. Reactions with Ammonia | 与氨的反应

Adding aqueous ammonia to a solution of a hexaaqua ion initially precipitates the hydroxide in the same way as sodium hydroxide. For example, Cu(OH)₂ is formed as a pale blue precipitate.

向六水合离子溶液中加入氨水时，最初与氢氧化钠一样生成氢氧化物沉淀。例如，Cu(OH)₂ 以浅蓝色沉淀形式生成。

In excess ammonia, however, the hydroxide precipitate dissolves where a more stable ammine complex can form. Copper(II) hydroxide dissolves in excess NH₃ to give a deep blue solution:

然而，在过量氨水中，当能形成更稳定的氨配合物时，氢氧化物沉淀会溶解。氢氧化铜(II)溶于过量 NH₃ 得到深蓝色溶液：

Cu(OH)₂(s) + 4NH₃ ⇌ [Cu(NH₃)₄(H₂O)₂]²⁺ + 2H₂O

This is a valuable test for Cu²⁺: a pale blue precipitate that dissolves in excess NH₃ to a deep blue solution.

这是检验 Cu²⁺ 的重要方法：浅蓝色沉淀溶于过量 NH₃ 得到深蓝色溶液。

For aluminium, the white Al(OH)₃ precipitate is insoluble in excess NH₃, which is why the qualitative analysis scheme uses both NaOH and NH₃ to distinguish Al³⁺ from other ions.

对于铝，白色 Al(OH)₃ 沉淀不溶于过量 NH₃，因此定性分析方案使用 NaOH 和 NH₃ 来区分 Al³⁺ 和其他离子。

6. Ligand Substitution with Chloride | 氯离子的配体取代

Concentrated hydrochloric acid provides a high concentration of Cl⁻ ions, which can replace water molecules in certain complexes. The change in ligand field strength alters the colour dramatically.

浓盐酸提供高浓度的 Cl⁻ 离子，这些离子可以取代某些配合物中的水分子。配体场强度的变化会显著改变颜色。

[Co(H₂O)₆]²⁺ + 4Cl⁻ ⇌ [CoCl₄]²⁻ + 6H₂O

The pale pink [Co(H₂O)₆]²⁺ is converted to the intense blue [CoCl₄]²⁻, with the solution turning deep blue. This reaction is reversible; adding water reverses it, and it is also affected by temperature change (used in Le Chatelier demonstrations).

浅粉色的 [Co(H₂O)₆]²⁺ 转化为深蓝色的 [CoCl₄]²⁻，溶液变为深蓝色。该反应可逆；加水可使反应逆转，温度变化也会影响平衡（常用于勒夏特列原理的演示）。

Similarly, copper(II) reacts with concentrated HCl to give the yellow-green [CuCl₄]²⁻ ion:

类似地，铜(II)与浓盐酸反应生成黄绿色 [CuCl₄]²⁻ 离子：

[Cu(H₂O)₆]²⁺ + 4Cl⁻ ⇌ [CuCl₄]²⁻ + 6H₂O

Note that chloride ions are weaker ligands than water for many transition metals, but the large excess of Cl⁻ drives the equilibrium to the right.

注意，对许多过渡金属，氯离子是比水更弱的配体，但大量过量的 Cl⁻ 驱动平衡向右移动。

7. Colours and Observations Summary | 颜色与现象总结

Mastering these observations is essential for examination success. The table below collects the key transition metal reactions in aqueous solution.

熟练记忆这些现象对考试成功至关重要。下表汇集了水溶液中关键过渡金属反应。

Reaction \| 反应	Initial colour \| 初始颜色	Final observation \| 最终现象
Fe²⁺ + NaOH	Pale green \| 浅绿色	Green gelatinous precipitate \| 绿色胶状沉淀
Fe³⁺ + NaOH	Yellow/brown \| 黄色/棕色	Orange-brown precipitate \| 橙棕色沉淀
Cu²⁺ + excess NH₃	Blue \| 蓝色	Deep blue solution \| 深蓝色溶液
Al³⁺ + excess NaOH	Colourless \| 无色	White precipitate dissolves \| 白色沉淀溶解
Co²⁺ + conc HCl	Pink \| 粉红色	Blue solution \| 蓝色溶液

A useful mental model: for each metal ion, memorise the colour of the aqueous ion, the colour of the hydroxide precipitate, and what happens in excess NaOH and excess NH₃.

一个有用的记忆思路：对每种金属离子，记住水合离子颜色、氢氧化物沉淀颜色，以及在过量 NaOH 和过量 NH₃ 中发生的变化。

8. Equilibria and the Effect of Concentration | 平衡与浓度影响

The ligand substitution reactions above are equilibria. For example, the cobalt chloride equilibrium is affected by changes in chloride concentration and temperature.

上述配体取代反应都是平衡。例如，氯化钴平衡受氯离子浓度和温度变化的影响。

[Co(H₂O)₆]²⁺(aq) + 4Cl⁻(aq) ⇌ [CoCl₄]²⁻(aq) + 6H₂O(l)

Adding concentrated HCl increases Cl⁻ and decreases water concentration, favouring the blue [CoCl₄]²⁻. Adding water dilutes Cl⁻ and increases water, favouring the pink [Co(H₂O)₆]²⁺.

加入浓盐酸会增加 Cl⁻ 并减少水浓度，有利于蓝色 [CoCl₄]²⁻ 生成。加水会稀释 Cl⁻ 并增加水，有利于粉红色 [Co(H₂O)₆]²⁺ 生成。

In the exam, you may be asked to predict the colour change when a reagent is added. Always consider whether the change shifts the position of equilibrium and which species is thereby favoured.

考试中可能要求你预测加入试剂后的颜色变化。始终考虑该变化如何移动平衡位置以及因此有利于哪种物质。

9. Hydrolysis and Partial Deprotonation | 水解与部分去质子化

Even without added base, hexaaqua ions partly hydrolyse in water. The extent depends on the acidity of the ion. For example, Fe³⁺ solutions are noticeably acidic and appear yellow-brown because of the formation of species such as [Fe(H₂O)₅(OH)]²⁺ and [Fe(H₂O)₄(OH)₂]⁺.

即使不加入碱，六水合离子在水中也会部分水解。水解程度取决于离子的酸性。例如，Fe³⁺ 溶液明显呈酸性并显示黄棕色，是因为形成了 [Fe(H₂O)₅(OH)]²⁺ 和 [Fe(H₂O)₄(OH)₂]⁺ 等物种。

This is why Cr³⁺ aqueous solutions often look green rather than violet; equilibria involving coordinated hydroxo ligands are present, and chloride ions from the preparation can also coordinate.

这也是为什么 Cr³⁺ 水溶液常呈绿色而非紫色的原因；其中存在涉及配位羟基配体的平衡，且制备过程中的氯离子也能配位。

[Cr(H₂O)₆]³⁺ + H₂O ⇌ [Cr(H₂O)₅(OH)]²⁺ + H₃O⁺

This partial deprotonation is also the reason why adding small amounts of acid or base can affect the colour of some transition metal solutions before any precipitate forms.

这种部分去质子化也是为什么在沉淀生成前，加入少量酸或碱会影响某些过渡金属溶液颜色的原因。

10. Exam-Focused Practice Questions | 考点针对练习

To consolidate understanding, consider these typical AQA-style questions and check the reasoning carefully.

为巩固理解，请思考以下典型 AQA 风格问题，并仔细核对推理过程。

Question 1: A solution of copper(II) sulfate is treated with a small amount of sodium hydroxide, then with excess ammonia. State what is observed at each stage and write an ionic equation for the final reaction.

问题 1： 将少量氢氧化钠加入硫酸铜溶液，然后加入过量氨水。说明每个阶段的现象，并写出最终反应的离子方程式。

Answer 1: A pale blue precipitate forms initially (Cu(OH)₂). In excess NH₃, the precipitate dissolves to give a deep blue solution of [Cu(NH₃)₄(H₂O)₂]²⁺. Equation: Cu(OH)₂(s) + 4NH₃ → [Cu(NH₃)₄(H₂O)₂]²⁺ + 2H₂O.

答 1： 初始形成浅蓝色沉淀 Cu(OH)₂。在过量 NH₃ 中，沉淀溶解，得到深蓝色 [Cu(NH₃)₄(H₂O)₂]²⁺ 溶液。方程式：Cu(OH)₂(s) + 4NH₃ → [Cu(NH₃)₄(H₂O)₂]²⁺ + 2H₂O。

Question 2: Explain why [Al(H₂O)₆]³⁺ is a stronger acid than [Mg(H₂O)₆]²⁺.

问题 2： 解释为什么 [Al(H₂O)₆]³⁺ 的酸性比 [Mg(H₂O)₆]²⁺ 强。

Answer 2: Al³⁺ has a higher charge and a smaller ionic radius than Mg²⁺, so its charge density is greater. The stronger polarising power draws electron density away from the O–H bonds of coordinated water molecules, weakening those bonds and making loss of H⁺ easier.

答 2： Al³⁺ 的电荷比 Mg²⁺ 高，离子半径更小，因此电荷密度更大。更强的极化力将配位水分子 O–H 键的电子密度拉走，削弱这些键，使失去 H⁺ 更容易。

Question 3: A green precipitate formed with NaOH stays when excess NaOH is added, but the same metal gives a blue solution with excess NH₃. Identify the metal ion.

问题 3： 某种金属离子与 NaOH 生成绿色沉淀，且过量 NaOH 不溶解；但过量 NH₃ 得到蓝色溶液。请鉴别该金属离子。

Answer 3: Ni²⁺ or Cu²⁺? Actually a green precipitate with excess NaOH insoluble and blue ammine complex points to Ni²⁺, since Ni(OH)₂ is green and [Ni(NH₃)₆]²⁺ is blue. But Cu(OH)₂ is pale blue, not green. Hence the ion is Ni²⁺.

答 3： Ni²⁺ 还是 Cu²⁺？绿色沉淀不溶于过量 NaOH 且氨合物为蓝色，指向 Ni²⁺，因为 Ni(OH)₂ 为绿色，[Ni(NH₃)₆]²⁺ 为蓝色。而 Cu(OH)₂ 是浅蓝色，不是绿色。因此该离子是 Ni²⁺。

11. Common Mistakes | 常见错误

Students frequently lose marks by using incorrect notation or omitting the state symbols. Practice these corrections:

学生常因使用错误符号或省略状态符号而失分。请练习这些修正：

Writing Fe³⁺(aq) + 3OH⁻(aq) → Fe(OH)₃(aq) instead of Fe(OH)₃(s). The precipitate is solid, not aqueous.

把 Fe³⁺(aq) + 3OH⁻(aq) → Fe(OH)₃(aq) 写成水溶液而不是 Fe(OH)₃(s)。沉淀是固体，不是水溶液。
Forgetting that excess NaOH distinguishes amphoteric hydroxides such as Al(OH)₃ from non-amphoteric ones like Fe(OH)₃.

忘记过量 NaOH 可以区分两性氢氧化物如 Al(OH)₃ 和部两性氢氧化物如 Fe(OH)₃。
Confusing the colour of Fe(OH)₂ (green) and Fe(OH)₃ (orange-brown). Both precipitate from their respective aqueous ions.

混淆 Fe(OH)₂（绿色）和 Fe(OH)₃（橙棕色）的颜色。两者分别从其水合离子沉淀生成。
Writing [Cu(NH₃)₆]²⁺ instead of [Cu(NH₃)₄(H₂O)₂]²⁺. The copper ammine complex has four NH₃ ligands and two water molecules.

错误写成 [Cu(NH₃)₆]²⁺，实际铜氨配合物含四个 NH₃ 配体和两个水分子，即 [Cu(NH₃)₄(H₂O)₂]²⁺。

12. Conclusion | 总结

Reactions of ions in aqueous solution can be mastered by remembering three core themes: hydrolysis (acidity), precipitation, and ligand substitution. Linking each metal ion to its colour changes across different reagents forms the foundation of many practical and exam questions.

水溶液中离子反应可以通过三个核心主题掌握：水解（酸性）、沉淀和配体取代。将每种金属离子在不同试剂下的颜色变化联系起来，是解决许多实验题与考试题的基础。

Always write balanced equations with state symbols, memorise key colours, and apply equilibrium reasoning where relevant. With this structured approach, aqueous ion reactions become predictable and rewarding.

始终书写带状态符号的配平方程式，熟记关键颜色，并在相关情境中运用平衡推理。通过这种结构化方法，水溶液中的离子反应将变得可预测且令人收获满满。

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Reaction \| 反应	Initial colour \| 初始颜色	Final observation \| 最终现象
Fe²⁺ + NaOH	Pale green \| 浅绿色	Green gelatinous precipitate \| 绿色胶状沉淀
Fe³⁺ + NaOH	Yellow/brown \| 黄色/棕色	Orange-brown precipitate \| 橙棕色沉淀
Cu²⁺ + excess NH₃	Blue \| 蓝色	Deep blue solution \| 深蓝色溶液
Al³⁺ + excess NaOH	Colourless \| 无色	White precipitate dissolves \| 白色沉淀溶解
Co²⁺ + conc HCl	Pink \| 粉红色	Blue solution \| 蓝色溶液

Reactions of Ions in Aqueous Solution | 水溶液中的离子反应