Group 2

A-Level Chemistry · Topic 10

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Handout Vocabulary

10.1

The Group 2 metals

Syllabus

describe, and write equations for, the reactions of the elements with oxygen, water and dilute hydrochloric and sulfuric acids
describe, and write equations for, the reactions of the oxides, hydroxides and carbonates with water and dilute hydrochloric and sulfuric acids
describe, and write equations for, the thermal decomposition of the nitrates and carbonates, to include the trend in thermal stabilities
describe, and make predictions from, the trends in physical and chemical properties of the elements involved in the reactions in 10.1.1 and the compounds involved in 10.1.2, 10.1.3 and 10.1.5
state the variation in the solubilities of the hydroxides and sulfates

Source: Cambridge International syllabus

Coastal limestone cliffs of calcium carbonate — Limestone is calcium carbonate — a compound of the Group 2 metal calcium.

Group 族 2 holds the metals magnesium, calcium, strontium and barium. They all have two outer electrons, which they lose to form $2+$ ions. Going down the group, the atoms get larger and the outer electrons are easier to lose, so the metals get more reactive — their reactivity 反应活性 increases down the group.

Group 2 atoms growing larger from magnesium down to barium, beside a downward arrow labelled reactivity increases — Reactivity increases down Group 2: larger atoms hold their two outer electrons less tightly, so they are lost more easily

Reactions of the elements

With oxygen, they burn to form an oxide:

$$2\text{Mg} + \text{O}_2 \rightarrow 2\text{MgO}$$

A strip of magnesium held in pliers burns with a dazzling white light, giving off white smoke of magnesium oxide — Magnesium burns in air with a brilliant white flame, forming white magnesium oxide

With water, they form a hydroxide and hydrogen. The reaction gets faster down the group:

$$\text{Ca} + 2\text{H}_2\text{O} \rightarrow \text{Ca(OH)}_2 + \text{H}_2$$

Magnesium is slow with cold water but reacts fast with steam, giving $\text{MgO}$ and $\text{H}_2$.

With dilute hydrochloric or sulfuric acid, they form a salt and hydrogen:

$$\text{Mg} + 2\text{HCl} \rightarrow \text{MgCl}_2 + \text{H}_2 \qquad \text{Mg} + \text{H}_2\text{SO}_4 \rightarrow \text{MgSO}_4 + \text{H}_2$$

With sulfuric acid the reaction slows down the group, because the sulfates 硫酸盐 formed (such as $\text{BaSO}_4$) are insoluble and coat the metal.

Reactions of the compounds

The oxides 氧化物 and hydroxides 氢氧化物 are basic. They react with water and with dilute acids:

$$\text{MgO} + \text{H}_2\text{O} \rightarrow \text{Mg(OH)}_2 \qquad \text{Mg(OH)}_2 + 2\text{HCl} \rightarrow \text{MgCl}_2 + 2\text{H}_2\text{O}$$

The carbonates 碳酸盐 react with dilute acids to give a salt, water and carbon dioxide:

$$\text{CaCO}_3 + 2\text{HCl} \rightarrow \text{CaCl}_2 + \text{H}_2\text{O} + \text{CO}_2$$

Thermal decomposition

Thermal decomposition 热分解 means breaking a compound apart by heating it.

the carbonates break into the oxide and carbon dioxide:

$$\text{CaCO}_3 \rightarrow \text{CaO} + \text{CO}_2$$

the nitrates 硝酸盐 break into the oxide, brown nitrogen dioxide gas and oxygen:

$$2\text{Ca(NO}_3)_2 \rightarrow 2\text{CaO} + 4\text{NO}_2 + \text{O}_2$$

The thermal stability 热稳定性 of both the carbonates and the nitrates increases down the group. A larger metal ion pulls less on the carbonate or nitrate ion, so the compound is harder to break apart — it needs a higher temperature. So magnesium carbonate decomposes most easily, and barium carbonate is the hardest.

A small magnesium ion strongly distorting a carbonate ion next to a large barium ion that barely distorts it — Thermal stability rises down the group: a small cation polarises (distorts) the carbonate ion more, weakening it so it decomposes more easily

Trends in solubility

Compound	Trend in solubility 溶解度 down the group
hydroxides	increase (Mg(OH)$_2$ almost insoluble; Ba(OH)$_2$ soluble)
sulfates	decrease (MgSO$_4$ soluble; BaSO$_4$ insoluble)

Two log-scale lines crossing: hydroxide solubility rising from Mg to Ba and sulfate solubility falling — Down Group 2 the hydroxides become more soluble while the sulfates become less soluble (note the log scale)

From these trends you can predict the properties of the next element down, radium, and of its compounds.

Worked example. $\text{MgCO}_3$ decomposes at about $540\ °\text{C}$ and $\text{BaCO}_3$ at about $1360\ °\text{C}$. Explain the trend and predict where $\text{CaCO}_3$ sits. Going down Group 2 the cation gets larger, so its charge is spread over a bigger surface and its polarising power falls. A less polarising cation distorts the carbonate ion less, so the $\text{C-O}$ bond is weakened less and more heat is needed to break it. Thermal stability therefore increases down the group, and $\text{CaCO}_3$, lying between Mg and Ba, decomposes at an intermediate temperature of about $900\ °\text{C}$. Argue the whole chain - cation size, then polarising power, then distortion of the anion; "it is more reactive" explains nothing here.

Explore

Group 2 reactivity ladder

Move down Group 2 and see why reactions become more vigorous.

Explore

Group 2 trend lab

Compare thermal stability and solubility trends down Group 2.

Vocabulary Train

English	Chinese	Pinyin
group	族	zú
reactivity	反应活性	fǎn yìng huó xìng
sulfate	硫酸盐	liú suān yán
oxide	氧化物	yǎng huà wù
hydroxide	氢氧化物	qīng yǎng huà wù
carbonate	碳酸盐	tàn suān yán
thermal decomposition	热分解	rè fēn jiě
nitrate	硝酸盐	xiāo suān yán
thermal stability	热稳定性	rè wěn dìng xìng
solubility	溶解度	róng jiě dù

Exercise sheet