A-Level Chemistry: Thermal Decomposition of Group 2 Carbonates and Nitrates | 第二主族碳酸盐与硝酸盐的热分解

📚 A-Level Chemistry: Thermal Decomposition of Group 2 Carbonates and Nitrates | 第二主族碳酸盐与硝酸盐的热分解

Group 2 metal carbonates and nitrates are common compounds tested in CIE A-Level Chemistry. When heated strongly, they decompose to form metal oxides and release gases. The key challenge is explaining why the thermal stability of these compounds increases down the group, and linking this to ionic polarisation and charge density.

第二主族金属的碳酸盐和硝酸盐是 CIE A-Level 化学中常考的化合物。强热时，它们分解生成金属氧化物并释放气体。核心难点在于解释为什么这些化合物的热稳定性随原子序数增大而增强，并将此与离子极化、电荷密度联系起来。

1. What Does “Thermal Decomposition” Mean? | 什么是热分解？

Thermal decomposition is the breaking down of a single compound into simpler substances using heat. It is an endothermic process because energy must be supplied to break chemical bonds.

热分解是指在加热条件下，一种化合物分解为更简单物质的过程。由于需要提供能量来断裂化学键，该过程为吸热反应。

For Group 2 compounds, the general pattern is:

第二主族化合物的通式如下：

MCO₃(s) → MO(s) + CO₂(g)

M(NO₃)₂(s) → MO(s) + 2NO₂(g) + ½O₂(g)

where M is a Group 2 metal such as Mg, Ca, Sr or Ba.

其中 M 代表第二主族金属，如 Mg、Ca、Sr 或 Ba。

2. The Thermal Stability Trend Down the Group | 同族热稳定性的变化趋势

As you move down Group 2 from magnesium to barium, both carbonates and nitrates become more thermally stable. This means that a higher temperature is required to decompose them.

从镁到钡，沿第二主族向下移动，碳酸盐和硝酸盐的热稳定性逐渐增强。这意味着需要更高的温度才能使它们分解。

For example:

例如：

MgCO₃ decomposes at around 350 °C.

MgCO₃ 约在 350 °C 时分解。
CaCO₃ decomposes at around 840 °C.

CaCO₃ 约在 840 °C 时分解。
BaCO₃ requires a temperature above 1300 °C.

BaCO₃ 需要超过 1300 °C 的温度才能分解。

This shows that thermal stability increases with increasing cation size down the group.

这说明热稳定性随阳离子半径在同族中增大而增强。

3. Explaining the Trend: Ion Polarisation | 解释趋势：离子极化

Thermal stability depends on how easily the anion (CO₃²⁻ or NO₃⁻) is distorted by the metal cation. A large, highly charged cation has a high charge density and strongly polarises the nearby negatively charged anion.

热稳定性取决于阴离子（CO₃²⁻ 或 NO₃⁻）被金属阳离子扭曲的难易程度。电荷高、体积小的阳离子具有较大的电荷密度，能够强烈极化邻近的带负电荷的阴离子。

The carbonate ion is a large anion with a high electron density. When a small, highly charged cation approaches, it attracts the oxygen electron cloud, weakening the C–O bonds.

碳酸根离子是体积大、电子密度高的阴离子。当一个体积小、电荷高的阳离子靠近时，会吸引氧原子的电子云，从而削弱 C–O 键。

This polarisation makes it easier for the carbonate to break apart and release CO₂. Therefore, the greater the polarising power of the cation, the less stable the carbonate.

这种极化作用使碳酸根更容易破裂并释放 CO₂。因此，阳离子的极化能力越强，碳酸盐的稳定性越低。

Cation polarising power ∝ charge density = charge ÷ radius²

阳离子极化能力 ∝ 电荷密度 = 电荷 ÷ 半径²

Down Group 2, the ionic radius increases significantly while the charge (+2) stays the same. Hence charge density decreases, polarising power decreases, and the anion remains more stable.

沿第二主族向下，离子半径显著增大，而电荷保持 +2 不变。因此电荷密度减小，极化能力减弱，阴离子更加稳定。

4. Thermal Decomposition of Group 2 Carbonates | 第二主族碳酸盐的热分解

All Group 2 carbonates decompose according to the same general equation:

所有第二主族碳酸盐都遵循相同的通式分解：

MCO₃(s) → MO(s) + CO₂(g)

For magnesium carbonate:

例如碳酸镁：

MgCO₃(s) → MgO(s) + CO₂(g)

Observations: the solid remains white (though some carbonates may change slightly), and a colourless, odourless gas is evolved. This gas turns limewater milky, confirming the presence of CO₂.

现象：固体仍为白色（部分碳酸盐可能有轻微变化）；逸出一种无色无味气体，能使石灰水变浑浊，由此可验证 CO₂ 的生成。

In the laboratory, the decomposition of magnesium carbonate is used as a quick demonstration. Calcium carbonate requires the higher temperature of a Bunsen burner flame.

在实验室中，常用碳酸镁的分解作快速演示。碳酸钙则需要本生灯火焰的更高温度才能分解。

5. Thermal Decomposition of Group 2 Nitrates | 第二主族硝酸盐的热分解

Group 2 nitrates are less thermally stable than the corresponding carbonates. They decompose into the metal oxide, nitrogen dioxide and oxygen.

第二主族硝酸盐的热稳定性低于相应的碳酸盐。它们受热分解生成金属氧化物、二氧化氮和氧气。

2M(NO₃)₂(s) → 2MO(s) + 4NO₂(g) + O₂(g)

For magnesium nitrate:

例如硝酸镁：

2Mg(NO₃)₂(s) → 2MgO(s) + 4NO₂(g) + O₂(g)

The key observation is the evolution of a brown, pungent gas, NO₂, which confirms nitrate decomposition. Oxygen gas can also be tested using a glowing splint, which relights.

关键现象是逸出一种棕色、有刺激性气味的气体 NO₂，它可用来确认硝酸盐的分解。氧气则可用带火星的木条复燃来检验。

6. Comparing Group 1 and Group 2 Carbonates | 第一、二主族碳酸盐的比较

Group 1 carbonates are generally more thermally stable than Group 2 carbonates. For example, sodium carbonate does not decompose even when heated strongly in a Bunsen flame.

第一主族碳酸盐通常比第二主族碳酸盐更稳定。例如，碳酸钠即使在本生灯高温加热下也不会分解。

This is explained by the lower charge on the Group 1 cation (+1 instead of +2) and its larger ionic radius. Group 1 cations therefore have a much smaller polarising power, so they distort the carbonate ion far less.

这是因为第一主族阳离子的电荷较低（+1 而不是 +2），且离子半径较大。因此，第一主族阳离子的极化能力要小得多，对碳酸根离子的扭曲程度也远低于第二主族。

Group 1 nitrates decompose to the nitrite and oxygen, not to the oxide.

第一主族硝酸盐受热分解生成亚硝酸盐和氧气，而非氧化物。
Group 2 nitrates decompose straight to the oxide because of the stronger polarisation.

第二主族硝酸盐因极化作用更强，直接分解为氧化物。

7. Practical Tests for Decomposition Products | 分解产物的检验方法

In the CIE practical exam or written questions, you need to know how to identify the gases released.

在 CIE 实验考试或笔试题目中，你需要知道如何鉴别释放出的气体。

Gas \| 气体	Test \| 检验方法	Result \| 现象
CO₂	Bubble through limewater \| 通入石灰水	Limewater turns milky/cloudy \| 石灰水变浑浊
NO₂	Observe colour and smell \| 观察颜色与气味	Brown pungent gas \| 棕色、刺激性气体
O₂	Glowing splint \| 带火星的木条	Splint relights \| 木条复燃

8. Explaining the Decomposition Temperature Using Lattice Enthalpy | 用晶格焓解释分解温度

Some textbooks explain thermal stability using lattice enthalpy. The decomposition of a carbonate involves the formation of a metal oxide. Since the oxide ion O²⁻ has a higher charge density than the carbonate ion, the lattice enthalpy of the oxide is much more exothermic than that of the carbonate.

有些教材用晶格焓解释热稳定性。碳酸盐分解涉及金属氧化物的生成。由于 O²⁻ 的电荷密度高于碳酸根离子，氧化物的晶格焓比碳酸盐的晶格焓更加放热。

As the metal cation becomes larger down the group, both lattice enthalpies decrease. However, the decrease is much larger for the carbonate than for the oxide. This makes decomposition progressively less favourable for heavier Group 2 metals.

随着金属阳离子在同族中体积增大，两种晶格焓都减小。但碳酸盐晶格焓的下降幅度远大于氧化物晶格焓的下降幅度。这使得重质第二主族金属的分解变得越来越困难。

In short, the increasing thermal stability down Group 2 can be explained either by decreasing polarisation or by the changing lattice enthalpy competition.

简言之，第二主族热稳定性增强可从极化减弱的角度解释，也可以从晶格焓竞争变化的角度解释。

9. Common Exam Mistakes | 常见考试错误

Students often lose marks by giving vague or incorrect explanations. Here are some common traps to avoid.

学生常因表述含糊或错误而失分。以下是一些需要避免的常见陷阱。

Saying “thermal stability increases down the group because the metal becomes larger” without linking it to polarisation charge density.

只说“同族向下金属变大，所以热稳定性增加”，却没有联系极化作用或电荷密度。
Writing the wrong equation for nitrate decomposition: forgetting the 2 in front of the nitrate or writing NO₂ instead of the oxide products.

写错硝酸盐分解方程式：忘记硝酸盐前的系数 2，或将产物写错为 NO₂ 而非氧化物。
Confusing the decomposition products of Group 1 nitrates (nitrite + O₂) with Group 2 nitrates (oxide + NO₂ + O₂).

混淆第一主族硝酸盐的分解产物（亚硝酸盐 + O₂）与第二主族硝酸盐的产物（氧化物 + NO₂ + O₂）。
Forgetting to mention that the carbonate ion is easily polarised because it is large.

忘记说明碳酸根离子因体积大而容易被极化。

10. Summary and Key Equations | 总结与关键方程式

For CIE A-Level Chemistry, you should be able to write balanced equations, describe qualitative observations, and explain the stability trend using polarisation.

对于 CIE A-Level 化学，你需要能够写出配平的方程式、描述实验现象，并用极化理论解释稳定性趋势。

Key ideas to remember:

需要记住的关键点：

Group 2 carbonates: MCO₃ → MO + CO₂.

第二主族碳酸盐：MCO₃ → MO + CO₂。
Group 2 nitrates: 2M(NO₃)₂ → 2MO + 4NO₂ + O₂.

第二主族硝酸盐：2M(NO₃)₂ → 2MO + 4NO₂ + O₂。
Thermal stability increases down the group because cation radius increases and charge density decreases.

热稳定性随同族向下增强，是因为阳离子半径增大、电荷密度减小。
Polarisation weakens the anion; the least stable compound is at the top of the group (Mg).

极化作用削弱阴离子；同族中最不稳定的是顶部的 Mg 化合物。

Mg < Ca < Sr < Ba (increasing thermal stability)

Mg < Ca < Sr < Ba（热稳定性依次增强）

Practise writing these equations and explanations until they become automatic. This is a favourite topic for structured questions because it combines bonding, periodicity and practical chemistry.

反复练习书写这些方程式和解释，直到能够熟练运用。这是结构化问答题的常考主题，因为它综合了化学键、元素周期律和实验化学。

Published by TutorHao | Chemistry Revision Series | aleveler.com

更多咨询请联系16621398022（同微信）

Gas \| 气体	Test \| 检验方法	Result \| 现象
CO₂	Bubble through limewater \| 通入石灰水	Limewater turns milky/cloudy \| 石灰水变浑浊
NO₂	Observe colour and smell \| 观察颜色与气味	Brown pungent gas \| 棕色、刺激性气体
O₂	Glowing splint \| 带火星的木条	Splint relights \| 木条复燃

A-Level Chemistry: Thermal Decomposition of Group 2 Carbonates and Nitrates | 第二主族碳酸盐与硝酸盐的热分解