Reactions of Group 2 Elements | 第2族元素的反应

📚 Reactions of Group 2 Elements | 第2族元素的反应

The Group 2 elements, known as the alkaline earth metals, consist of beryllium, magnesium, calcium, strontium, barium and radium. These s‑block metals have two electrons in their outermost s‑orbital (ns²) and readily lose them to form M²⁺ ions, making them strong reducing agents. Their reactivity increases down the group as atomic radius increases and ionisation energies decrease. This article covers the characteristic reactions of Group 2 elements and the trends observed, which are central to Cambridge A‑Level Chemistry.

第2族元素，又称碱土金属，包括铍、镁、钙、锶、钡和镭。这些s区金属最外层s轨道有两个电子（ns²），容易失去这两个电子形成M²⁺离子，因此是强还原剂。随着族往下，原子半径增大、电离能降低，反应性增强。本文涵盖第2族元素的特征反应及其变化趋势，这些内容是剑桥A‑Level化学的核心考点。

1. Reactivity Trends in Group 2 | 第2族反应性趋势

Down Group 2, atomic radius increases and shielding effect becomes greater, leading to a decrease in first and second ionisation energies. Consequently, the ease of losing the two 2s² electrons increases from Be to Ba. This means the reducing power of the metals strengthens down the group, making barium the strongest reducing agent among the stable members.

沿第2族向下，原子半径增大，屏蔽效应增强，导致第一和第二电离能降低。因此，从Be到Ba失去两个ns²电子的容易程度增加。这意味着金属的还原能力自上而下增强，钡是稳定元素中最强的还原剂。

The standard electrode potentials become more negative down the group: Mg²⁺/Mg = −2.37 V, Ca²⁺/Ca = −2.87 V, Sr²⁺/Sr = −2.89 V, Ba²⁺/Ba = −2.90 V. This trend directly influences the vigour of reactions with oxygen, water and acids.

标准电极电势沿族向下变得更负：Mg²⁺/Mg = −2.37 V，Ca²⁺/Ca = −2.87 V，Sr²⁺/Sr = −2.89 V，Ba²⁺/Ba = −2.90 V。这一趋势直接影响它们与氧气、水和酸反应的剧烈程度。

2. Reaction with Oxygen | 与氧气的反应

When heated in air, all Group 2 metals react with oxygen to form white, ionic oxides with the general formula MO. Magnesium burns with a brilliant white flame, while calcium produces a brick‑red flame and strontium a crimson flame.

在空气中加热时，所有第2族金属均与氧气反应，生成通式为MO的白色离子型氧化物。镁燃烧时发出耀眼的白光，钙产生砖红色火焰，锶产生深红色火焰。

2Mg(s) + O₂(g) → 2MgO(s)

2Ca(s) + O₂(g) → 2CaO(s)

Beryllium is an exception; it forms a hard, protective oxide layer at room temperature and reacts only slowly upon strong heating to give BeO, which is amphoteric.

铍是个例外；它在室温下形成坚硬、保护性的氧化层，只在强热下缓慢反应生成两性氧化物BeO。

Barium forms barium peroxide (BaO₂) in excess oxygen at high pressure, but under normal heating only BaO is produced. The oxides are basic, reacting with water to form alkaline solutions (except BeO).

钡在过量氧气和高压下可生成过氧化钡（BaO₂），但正常加热只产生BaO。这些氧化物呈碱性，能与水反应生成碱性溶液（BeO除外）。

3. Reaction with Water | 与水的反应

Group 2 metals react with water to produce the corresponding metal hydroxide and hydrogen gas. The reactivity increases down the group because the metals release electrons more readily.

第2族金属与水反应生成相应的金属氢氧化物和氢气。反应活性沿族向下增加，因为越向下金属越容易失去电子。

Magnesium reacts very slowly with cold water but vigorously with steam, forming magnesium oxide and hydrogen. Calcium, strontium and barium react readily with cold water, with barium reacting the most violently.

镁与冷水反应非常缓慢，但可与水蒸气剧烈反应生成氧化镁和氢气。钙、锶和钡与冷水即可反应，其中钡反应最为剧烈。

Ca(s) + 2H₂O(l) → Ca(OH)₂(aq) + H₂(g)

Beryllium does not react with water or steam at any appreciable rate because its oxide layer is impenetrable and its ionisation energy is relatively high.

铍不与水或蒸汽发生明显反应，因为其氧化层无法穿透，且电离能较高。

Key observation: the resulting solutions are alkaline. Calcium hydroxide (Ca(OH)₂) is sparingly soluble, so it appears as a white suspension (limewater); strontium and barium hydroxides are much more soluble and give clear, strongly alkaline solutions.

关键现象：所得溶液呈碱性。氢氧化钙（Ca(OH)₂）微溶，呈现白色悬浊液（石灰水）；锶和钡的氢氧化物溶解性高得多，形成澄清的强碱性溶液。

4. Reaction with Dilute Acids | 与稀酸的反应

All Group 2 metals react readily with dilute hydrochloric and sulfuric acids to produce a salt and hydrogen gas. The reaction rate increases down the group, mirroring the trend in reducing strength.

所有第2族金属都容易与稀盐酸和稀硫酸反应，生成对应的盐和氢气。反应速率沿族向下增加，与还原性强弱的趋势一致。

Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g)

Ba(s) + H₂SO₄(aq) → BaSO₄(s) + H₂(g)

An interesting observation occurs with barium and dilute sulfuric acid: the reaction is initially vigorous but quickly slows down because an insoluble layer of barium sulfate (BaSO₄) coats the metal, preventing further attack. Calcium shows similar behaviour because CaSO₄ is sparingly soluble.

钡与稀硫酸反应有一个有趣的现象：反应起初剧烈，但很快减慢，因为不溶的硫酸钡（BaSO₄）层覆盖在金属表面，阻止了进一步反应。钙也呈现类似行为，因为CaSO₄微溶。

Magnesium reacts violently with dilute acids, whereas beryllium resists attack by cold nitric acid due to passivation, but dissolves in warm hydrochloric acid.

镁与稀酸反应剧烈，而铍由于钝化作用耐冷硝酸侵蚀，但可溶于温热盐酸。

5. Reaction with Chlorine | 与氯气的反应

When heated in chlorine gas, Group 2 metals burn to form white, ionic chlorides with the general formula MCl₂. The reactions become more vigorous down the group.

在氯气中加热时，第2族金属燃烧生成通式为MCl₂的白色离子型氯化物。反应沿族向下加剧。

Mg(s) + Cl₂(g) → MgCl₂(s)

Ca(s) + Cl₂(g) → CaCl₂(s)

Beryllium chloride (BeCl₂) is an exception: it is covalent and exists as a polymeric chain in the solid state, reflecting the high charge density of the Be²⁺ ion which polarises the chloride ions.

氯化铍（BeCl₂）是个例外：它呈共价性，固态时以聚合链形式存在，反映出Be²⁺离子的高电荷密度能极化氯离子。

All other Group 2 chlorides are ionic solids with high melting points, soluble in water giving neutral solutions (except for slight hydrolysis by the smaller ions).

其他第2族氯化物都是离子型固体，熔点高，溶于水形成中性溶液（较小的离子会稍微水解）。

6. Solubility Trends of Hydroxides and Sulfates | 氢氧化物和硫酸盐的溶解性趋势

The solubility of Group 2 hydroxides increases down the group, while the solubility of the sulfates decreases. This dual trend is a defining feature of the Group 2 chemistry and is frequently examined.

第2族氢氧化物的溶解度沿族向下增加，而硫酸盐的溶解度则降低。这种双重变化趋势是第2族化学的一个标志性特征，经常出现在考试中。

Hydroxide solubility:

Mg(OH)₂	Insoluble
Ca(OH)₂	Sparingly soluble
Sr(OH)₂	More soluble
Ba(OH)₂	Soluble; strong alkali

氢氧化物溶解度：Mg(OH)₂不溶，Ca(OH)₂微溶，Sr(OH)₂较溶，Ba(OH)₂易溶形成强碱。溶液碱性随溶解度增大而增强。

Sulfate solubility:

MgSO₄	Soluble
CaSO₄	Sparingly soluble
SrSO₄	Insoluble
BaSO₄	Very insoluble

硫酸盐溶解度：MgSO₄可溶，CaSO₄微溶，SrSO₄不溶，BaSO₄极难溶。硫酸钡常用作胃肠道X射线造影剂，因其不溶于水和酸。

The opposite trends are explained by lattice energy and hydration enthalpy: for hydroxides, the drop in lattice energy dominates; for sulfates, the sharp fall in hydration enthalpy of the cation outweighs the lattice energy change.

这两种相反趋势可由晶格能和 hydration 焓变解释：对于氢氧化物，晶格能下降占主导；对于硫酸盐，阳离子水合焓急剧降低超过了晶格能变化。

7. Thermal Decomposition of Group 2 Carbonates | 第2族碳酸盐的热分解

Heating Group 2 carbonates causes them to decompose into the metal oxide and carbon dioxide. The thermal stability of the carbonates increases down the group.

加热第2族碳酸盐会使其分解为金属氧化物和二氧化碳。碳酸盐的热稳定性沿族向下增强。

MgCO₃(s) → MgO(s) + CO₂(g) (decomposes readily)

CaCO₃(s) → CaO(s) + CO₂(g) (requires strong heating)

BaCO₃(s) → BaO(s) + CO₂(g) (needs very high temperature)

Beryllium carbonate is unstable and decomposes easily even at low temperatures, reflecting the high polarising power of the tiny Be²⁺ ion, which distorts the carbonate ion.

碳酸铍极不稳定，低温下也易分解，这反映了极小Be²⁺离子的强极化能力，能显著扭曲碳酸根离子。

The rising thermal stability is due to the decreasing polarising power of the larger M²⁺ cations down the group, which distorts the carbonate ion less, making it harder to break the C−O bonds.

热稳定性递增是由于沿族向下阳离子半径增大、极化能力减弱，对碳酸根离子的扭曲作用减小，使C−O键更难断裂。

A common laboratory test: limewater turns milky when CO₂ is passed through it, confirming the decomposition of a carbonate.

常见实验室检验：将产生的气体通入石灰水，石灰水变浑浊，证明碳酸盐分解产生了CO₂。

8. Thermal Decomposition of Group 2 Nitrates | 第2族硝酸盐的热分解

Group 2 nitrates decompose upon heating to give the metal oxide, nitrogen dioxide (brown gas) and oxygen. The thermal stability of nitrates also increases down the group.

第2族硝酸盐受热分解生成金属氧化物、二氧化氮（棕色气体）和氧气。硝酸盐的热稳定性同样沿族向下增强。

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

2Ca(NO₃)₂(s) → 2CaO(s) + 4NO₂(g) + O₂(g)

Lithium nitrate (Group 1) decomposes in a similar way, but the other Group 1 nitrates give nitrites and oxygen only — a useful comparison question.

硝酸锂（第1族）分解方式类似，但其他第1族硝酸盐仅生成亚硝酸盐和氧气——这是一个有用的对比考点。

Observations: brown fumes of NO₂ are seen, and the solid residue is the alkaline earth oxide. A glowing splint relights in the evolved oxygen.

现象：可见棕色NO₂烟雾，固体残留物为碱土金属氧化物。带火星的木条在放出的氧气中复燃。

As with carbonates, the stability trend originates from the decreasing charge density and polarising power of the cation. Barium nitrate requires the highest temperature to decompose, while magnesium nitrate decomposes at moderate heat.

与碳酸盐一样，稳定性趋势源于阳离子电荷密度和极化能力的递减。硝酸钡需最高温度才能分解，而硝酸镁在中等加热下即可分解。

9. Anomalous Behaviour of Beryllium | 铍的反常行为

Beryllium, the first member of Group 2, shows several anomalies due to its extremely small ionic radius and high charge density. Be²⁺ has a radius of only 31 pm, compared to Mg²⁺ at 65 pm.

铍作为第2族第一个元素，因其极小的离子半径和高电荷密度而表现出多种反常性质。Be²⁺半径仅31 pm，而Mg²⁺为65 pm。

Beryllium oxide and hydroxide are amphoteric, dissolving in both acids and alkalis, whereas other Group 2 oxides and hydroxides are basic.

氧化铍和氢氧化铍为两性，既能溶于酸也能溶于碱，而其他第2族氧化物和氢氧化物为碱性。
Beryllium does not react with cold water and reacts only slowly with steam.

铍不与冷水反应，与水蒸气反应也很缓慢。
Beryllium chloride (BeCl₂) is covalent and forms a chain polymer, unlike the ionic chlorides of the rest of the group.

氯化铍（BeCl₂）为共价化合物并形成链状聚合物，不同于本族其他离子型氯化物。
Beryllium forms complexes, e.g. [Be(OH)₄]²⁻ in excess alkali, and its bonding shows a strong tendency towards covalency.

铍能形成配合物，如在过量碱中生成[Be(OH)₄]²⁻，其化学键倾向共价性。

These differences explain why beryllium is often compared to aluminium in a diagonal relationship rather than to its fellow Group 2 metals.

这些差异说明了为何铍常与铝呈对角相似关系，而不是与同族其他金属相似。

10. Uses of Group 2 Elements and Compounds | 第2族化合物的重要用途

The characteristic reactivity and insolubility of certain compounds lead to important applications:

第2族元素特有的反应性和某些化合物的不溶性带来了重要应用：

Magnesium ribbon is used in flares, fireworks and incendiary devices because it burns with an intense white light. It is also used to extract titanium from TiCl₄ in the Kroll process.

镁带用于信号弹、烟花和燃烧装置，因其燃烧时发出夺目白光。它也用于Kroll法中从TiCl₄提炼钛：TiCl₄ + 2Mg → Ti + 2MgCl₂。
Calcium oxide (quicklime, CaO) and calcium hydroxide (slaked lime, Ca(OH)₂) are used to neutralise acidic soil in agriculture and to remove acidic gases in industry.

氧化钙（生石灰）和氢氧化钙（熟石灰）用于农业调节酸性土壤，以及工业上脱除酸性气体。
Barium sulfate is used as a ‘barium meal’ in medical X‑ray imaging because it is opaque to X‑rays and insoluble in body fluids, hence non‑toxic.

硫酸钡用作医学X射线检查的“钡餐”，因为它对X射线不透明且在体液中不溶，因此无毒。
Strontium compounds produce the crimson colour in fireworks; calcium gives brick‑red, barium gives green.

锶的化合物在烟花中产生深红色；钙产生砖红色，钡产生绿色。

11. Summary and Exam Tips | 总结与应试技巧

Group 2 reactions are systematic and predictable. Remember the key word ‘increase’: reactivity with water, oxygen and acids increases down the group; hydroxide solubility increases; basic strength of oxides and hydroxides increases; thermal stability of carbonates and nitrates increases. The only property that decreases down the group is sulfate solubility.

第2族反应系统而有规律。记住关键词“增加”：与水、氧气和酸的反应性向下增加；氢氧化物溶解度增加；氧化物和氢氧化物的碱性增强；碳酸盐和硝酸盐的热稳定性增强。唯一向下递减的性质是硫酸盐溶解度。

Always link explanations to ionisation energy, atomic radius and polarising power of the M²⁺ ion.

解释时务必联系电离能、原子半径和M²⁺离子的极化能力。
Practise writing balanced equations with state symbols, and be ready to predict products for unfamiliar Group 2 reactions using the patterns described.

练习书写带物态符号的配平方程式，并能运用上述规律预测陌生第2族反应的产物。
Be prepared to compare Group 2 with Group 1, especially for thermal decomposition of nitrates and carbonates.

准备将第2族与第1族进行比较，尤其是硝酸盐和碳酸盐的热分解。

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Reactions of Group 2 Elements | 第2族元素的反应