Enthalpy Change of Atomisation and Electron Affinity | 原子化焓变与电子亲和能

📚 Enthalpy Change of Atomisation and Electron Affinity | 原子化焓变与电子亲和能

This article provides a comprehensive revision guide for two key thermodynamic terms in the Cambridge A-Level Chemistry syllabus: standard enthalpy change of atomisation and electron affinity. We will explore their definitions, sign conventions, periodic trends, and their essential roles in Born–Haber cycles.

本文是剑桥 A-Level 化学考纲中两个重要热力学术语的复习指南：标准原子化焓变与电子亲和能。我们将深入探讨它们的定义、符号约定、周期趋势及其在 Born–Haber 循环中的关键作用。

1. Standard Enthalpy Change of Atomisation | 标准原子化焓变

The standard enthalpy change of atomisation, Δ_atH°, is the enthalpy change when one mole of gaseous atoms is formed from the element in its standard state under standard conditions (298 K and 100 kPa).

标准原子化焓变（Δ_atH°）是指在标准条件（298 K 和 100 kPa）下，由处于标准状态的元素生成 1 摩尔气态原子时的焓变。

For example, the atomisation of graphite involves breaking the covalent bonds in the giant covalent structure to produce isolated carbon atoms:

例如，石墨的原子化涉及破坏巨型共价结构中的共价键，以产生孤立的碳原子：

C(s, graphite) → C(g) Δ_atH° = +716.7 kJ mol⁻¹

Notice that atomisation is always endothermic because energy is required to break bonds and separate atoms against attractive forces.

请注意，原子化过程总是吸热的，因为需要输入能量来破坏键并克服引力将原子分离。

2. Key Features of Atomisation Enthalpy | 原子化焓变的关键特征

Several important points must be remembered when applying Δ_atH° in calculations:

在计算中应用 Δ_atH° 时，必须记住几个要点：

It always refers to the formation of gaseous atoms, not ions or molecules. The final state is a monatomic gas.

它总是指形成气态原子，而不是离子或分子。最终状态是单原子气体。
For elements that exist as diatomic molecules (e.g., Cl₂), atomisation includes bond dissociation of the molecule: ½Cl₂(g) → Cl(g). For standard atomisation, the element in its standard state is used, so for chlorine this is ½Cl₂(g), not Cl₂(s).

对于以双原子分子存在的元素（如 Cl₂），原子化包括分子的键解离：½Cl₂(g) → Cl(g)。对于标准原子化，使用标准状态的元素，因此对于氯来说是 ½Cl₂(g)，而不是 Cl₂(s)。
The value is always positive for stable elements. Endothermic bond breaking dominates.

对于稳定元素，该值始终为正。吸热的键断裂过程占主导。
Metallic elements have positive atomisation enthalpies because the metallic lattice must be broken into gaseous atoms.

金属元素的原子化焓为正值，因为必须将金属晶格破碎成气态原子。

3. First Electron Affinity | 第一电子亲和能

The first electron affinity (EA₁) is the enthalpy change when one mole of gaseous atoms gains one mole of electrons to form one mole of gaseous singly charged anions under standard conditions.

第一电子亲和能（EA₁）是指在标准条件下，1 摩尔气态原子获得 1 摩尔电子形成 1 摩尔气态单电荷阴离子时的焓变。

For chlorine:

对于氯：

Cl(g) + e⁻ → Cl⁻(g) EA₁ = −349 kJ mol⁻¹

The first electron affinity is usually exothermic because the added electron experiences an attractive force from the nucleus, releasing energy.

第一电子亲和能通常是放热的，因为加入的电子受到原子核的吸引力，释放出能量。

4. Second Electron Affinity | 第二电子亲和能

The second electron affinity (EA₂) is the enthalpy change when one mole of gaseous singly charged anions gains one mole of electrons to form one mole of gaseous doubly charged anions.

第二电子亲和能（EA₂）是指 1 摩尔气态单电荷阴离子获得 1 摩尔电子形成 1 摩尔气态双电荷阴离子时的焓变。

For oxygen:

对于氧：

O⁻(g) + e⁻ → O²⁻(g) EA₂ = +844 kJ mol⁻¹

The second electron affinity is endothermic because the electron must be forced onto an already negative ion, and the electrostatic repulsion must be overcome. This is why oxide ions O²⁻ do not form spontaneously from O⁻ in the gas phase.

第二电子亲和能是吸热的，因为电子必须被强行加到已经带负电的离子上，需要克服静电排斥。这就是为什么 O²⁻ 不能在气相中由 O⁻ 自发形成。

5. Sign Conventions and Terminology | 符号约定与术语

In Cambridge A-Level, electron affinities are reported as enthalpy changes for the process of electron addition. A negative sign means exothermic, a positive sign means endothermic.

在剑桥 A-Level 中，电子亲和能报告为电子加成过程的焓变。负号表示放热，正号表示吸热。

Be careful: some textbooks use the opposite sign for electron affinity (defining it as the energy released). However, in Born–Haber cycles and thermodynamic calculations, the ΔH value with the sign is used directly. Always check the convention in the question — Cambridge uses the actual enthalpy change.

注意：一些教科书对电子亲和能使用相反的符号（将其定义为释放的能量）。然而，在 Born–Haber 循环和热力学计算中，直接使用带符号的 ΔH 值。请务必检查题目中使用的约定——剑桥使用实际焓变。

For atomisation enthalpy, the sign is always positive for stable elements. No ambiguity exists for Δ_atH°.

对于原子化焓，稳定元素的符号始终为正。Δ_atH° 没有歧义。

6. Trends in Atomisation Enthalpy | 原子化焓变的周期趋势

Atomisation enthalpy varies across periods and down groups due to changes in atomic radius, metallic bonding strength, and covalent bond strength.

由于原子半径、金属键强度和共价键强度的变化，原子化焓在周期内和族间呈现规律性变化。

Element	Type	Δ_atH° / kJ mol⁻¹
Na	Metal	+107
Mg	Metal	+148
Al	Metal	+326
Si	Covalent	+456
Cl	Diatomic	+121

Across Period 3, atomisation enthalpy rises from Na to Si due to increasing nuclear charge and stronger bonding. It falls sharply for P, S, Cl, and Ar because these form weak covalent bonds (or none for Ar) and have smaller atoms with lower melting points in the solid state.

在第三周期中，从 Na 到 Si，原子化焓升高，原因是核电荷增加、键合增强。但对于 P、S、Cl 和 Ar，由于形成较弱的共价键（或对于 Ar 没有键），且固态时原子较小、熔点较低，原子化焓急剧下降。

Down a group, atomisation enthalpy generally decreases for metals because larger atoms have weaker metallic bonds. For non-metals, bond energy trends are less regular, but Δ_atH° still tends to decrease down a group for elements with similar structures.

对于金属，在同一族中向下，原子化焓通常减小，因为原子越大，金属键越弱。对于非金属，键能趋势不太规则，但对于结构相似的元素，Δ_atH° 仍然倾向于向下减小。

7. Trends in First Electron Affinity | 第一电子亲和能的周期趋势

First electron affinity generally becomes more negative across a period because nuclear charge increases while atomic radius decreases, making the electron attracted more strongly.

在同一周期中，第一电子亲和能通常变得更负，因为核电荷增加而原子半径减小，使电子受到更强的吸引力。

Down a group, first electron affinity becomes less negative because the atomic radius increases and the added electron enters a shell further from the nucleus, experiencing more shielding and less attraction.

在同一族中向下，第一电子亲和能变得不那么负，因为原子半径增大，加入的电子进入离核更远的壳层，受到更强的屏蔽效应和较弱的吸引力。

However, there are anomalies. For example, nitrogen has a very negative electron affinity because the 2p subshell is half-filled (stable), but adding an electron requires pairing, which is less favourable than expected. Conversely, chlorine has the most negative electron affinity among the halogens because its small size and effective nuclear charge create a strong attraction for an additional electron.

然而，存在异常情况。例如，氮的电子亲和能并不是预期的那么负，因为 2p 亚层处于半充满状态（稳定），加入一个电子需要配对，不太有利。相反，氯在卤素中具有最负的电子亲和能，因为它的小尺寸和有效核电荷对额外电子产生强烈吸引。

8. Electron Affinity and Born–Haber Cycles | 电子亲和能与 Born–Haber 循环

Electron affinity is essential in Born–Haber cycles for ionic compounds. The cycle uses Hess’s law to relate lattice enthalpy to other enthalpy changes, including atomisation enthalpy, ionisation energy, and electron affinity.

电子亲和能在离子化合物的 Born–Haber 循环中至关重要。该循环利用赫斯定律将晶格焓与其他焓变（包括原子化焓、电离能和电子亲和能）联系起来。

For a metal M and non-metal X forming MX:

对于金属 M 和非金属 X 形成 MX：

Δ_fH°(MX) = Δ_atH°(M) + Δ_atH°(X) + IE₁(M) + EA₁(X) + Lattice Enthalpy

Notice that lattice enthalpy is always negative (exothermic) for formation of the lattice from gaseous ions. The electron affinity term may be positive or negative depending on the step.

请注意，对于由气态离子形成晶格，晶格焓始终为负（放热）。电子亲和能项可能是正也可能是负，取决于具体步骤。

For compounds containing O²⁻, the sum of EA₁ and EA₂ for oxygen is positive overall, which means that the formation of O²⁻ from O(g) is endothermic. This endothermic input is overcome by the very large lattice enthalpy of the metal oxide.

对于含有 O²⁻ 的化合物，氧的 EA₁ 和 EA₂ 之和总体为正，这意味着从 O(g) 形成 O²⁻ 是吸热的。该吸热输入由金属氧化物非常大的晶格焓来克服。

9. Worked Example: Lattice Enthalpy of NaCl | 实例：NaCl 的晶格焓

Given the following data, calculate the lattice enthalpy of NaCl.

给定以下数据，计算 NaCl 的晶格焓。

Δ_fH°(NaCl) = −411 kJ mol⁻¹
Δ_atH°(Na) = +107 kJ mol⁻¹
½Δ_bondH°(Cl₂) = +121 kJ mol⁻¹ (i.e. Δ_atH°(Cl))
IE₁(Na) = +496 kJ mol⁻¹
EA₁(Cl) = −349 kJ mol⁻¹

Using the cycle:

使用循环：

Lattice Enthalpy = Δ_fH° − [Δ_atH°(Na) + Δ_atH°(Cl) + IE₁(Na) + EA₁(Cl)]

= −411 − [107 + 121 + 496 + (−349)]

= −411 − 375 = −786 kJ mol⁻¹

The negative value indicates that forming the crystal lattice from gaseous ions is highly exothermic, which stabilises the ionic compound.

负值表明由气态离子形成晶格是高度放热的，从而使离子化合物稳定。

10. Common Mistakes and Exam Tips | 常见错误与考试技巧

Students often lose marks on these topics due to small but crucial errors:

学生常因细小但关键的失误在这些主题上丢分：

Forgetting to use the correct stoichiometric coefficients for atomisation of diatomic elements. Always write ½Cl₂(g) → Cl(g), not Cl₂(g) → 2Cl(g) when defining Δ_atH° per mole of Cl atoms.

忘记使用双原子元素原子化的正确化学计量系数。定义 Δ_atH°（每摩尔 Cl 原子）时，应写 ½Cl₂(g) → Cl(g)，而不是 Cl₂(g) → 2Cl(g)。
Confusing electron affinity with ionisation energy. Ionisation energy removes an electron from an atom (endothermic); electron affinity adds an electron to an atom (usually exothermic).

混淆电子亲和能与电离能。电离能是从原子中移除一个电子（吸热）；电子亲和能是向原子加入一个电子（通常放热）。
Using the incorrect sign for second electron affinity. Remember EA₂ for most elements is positive because of repulsion.

使用错误的第二电子亲和能符号。请记住，对于大多数元素，EA₂ 为正，因为存在排斥。
In Born–Haber cycles, check that the arrows show the correct direction. The cycle is a closed loop, and the sum of enthalpy changes around the cycle is zero.

在 Born–Haber 循环中，检查箭头方向是否正确。循环是一个闭合回路，循环中所有焓变之和为零。
Always state units (kJ mol⁻¹) and sign for each enthalpy change in your answer.

在答案中始终注明单位（kJ mol⁻¹）和每个焓变的符号。

11. Practice Questions | 练习问题

Test your understanding with these short questions:

通过以下简短问题测试你的理解：

Write the equation that defines Δ_atH° for bromine. State the sign of this enthalpy change.

写出定义溴的 Δ_atH° 的方程式，并说明该焓变的符号。
Explain why the second electron affinity of oxygen is positive.

解释为什么氧的第二电子亲和能为正。
Using the Born–Haber cycle for MgO, explain why the endothermic atomisation of Mg and O and the positive second electron affinity of O are compensated by the very exothermic lattice enthalpy.

利用 MgO 的 Born–Haber 循环，解释为什么 Mg 和 O 的吸热原子化以及 O 的正第二电子亲和能被非常放热的晶格焓所补偿。

Answers: 1. ½Br₂(l) → Br(g), positive. 2. Adding an electron to O⁻ requires overcoming electrostatic repulsion between the negative ion and the incoming electron. 3. The large exothermic lattice enthalpy of MgO (from small Mg²⁺ and O²⁻ ions) provides enough energy to overcome the unfavourable positive enthalpy terms.

答案：1. ½Br₂(l) → Br(g)，正。2. 向 O⁻ 加入电子需要克服负离子与入射电子之间的静电排斥。3. MgO 的巨大放热晶格焓（来自小的 Mg²⁺ 和 O²⁻ 离子）提供了足够的能量来克服不利的正焓项。

12. Summary | 总结

Atomisation enthalpy measures the energy needed to form gaseous atoms from an element in its standard state, always positive for stable elements. Electron affinity measures the energy change when gaseous atoms gain electrons; first electron affinity is generally negative (exothermic), while second electron affinity is positive (endothermic) due to electron–electron repulsion.

原子化焓衡量从标准状态的元素形成气态原子所需的能量，对于稳定元素始终为正。电子亲和能衡量气态原子获得电子时的能量变化；第一电子亲和能通常为负（放热），而由于电子-电子排斥，第二电子亲和能为正（吸热）。

Both quantities are indispensable in constructing Born–Haber cycles and understanding why ionic compounds form. Mastering their definitions, signs, and trends is essential for success in Cambridge A-Level Chemistry Paper 4 and Paper 5.

这两个量在构建 Born–Haber 循环和理解离子化合物形成原因中不可或缺。掌握它们的定义、符号和趋势，对于在剑桥 A-Level 化学 Paper 4 和 Paper 5 中取得成功至关重要。

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Enthalpy Change of Atomisation and Electron Affinity | 原子化焓变与电子亲和能