- 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
Group 2
A-Level Chemistry · Topic 10
10.1
The Group 2 metals
Syllabus
Source: Cambridge International syllabus
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.
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:
$$2\text{Mg} + \text{O}_2 \rightarrow 2\text{MgO}$$
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.
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) |
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 |
10.1
Exam tips
- Reactivity increases down Group 2: ionisation energy falls as atoms get larger, so electrons are lost more easily.
- Reactions with water become more vigorous down the group; Mg reacts slowly with cold water but fast with steam (→ MgO).
- Hydroxides get more soluble down the group; sulfates get less soluble ($\text{BaSO}_4$ insoluble — the basis of the sulfate test).
- Write equations with the correct $+2$ ion and state the observations (fizzing, dissolving).