Cambridge International AS & A Level Chemistry (9701)
Group 2
The big idea: Magnesium to barium all lose two outer electrons to form M²⁺ ions, so they react in the same way. Going down the group the atoms get bigger and lose those electrons more easily, so reactivity rises, the hydroxides get more soluble, the sulfates less soluble, and the carbonates and nitrates harder to decompose.
Summary — What This Chapter Covers
- Group 2 elements react with oxygen to give MO, with water to give M(OH)₂ (or MO with steam for Mg) and H₂, and with dilute acids to give salts and H₂.
- Reactivity increases down the group as ionisation energies decrease.
- Oxides and hydroxides are basic; carbonates react with acids to give CO₂.
- Carbonates decompose to MO + CO₂; nitrates to MO + NO₂ + O₂. Thermal stability increases down the group.
- Solubility of hydroxides increases down the group; solubility of sulfates decreases.
1. Properties of the Group 2 Metals and Their Compounds
Physical trends
Mg to Ba
| Property | Trend down the group | Reason |
|---|
| Atomic radius | Increases | Extra occupied shell each time |
| First and second ionisation energies | Decrease | Outer electrons further from the nucleus and more shielded |
| Reactivity | Increases | The two outer electrons are lost more easily |
| Melting point | Generally decreases | Larger ions, weaker attraction to the delocalised electrons |
Reactions of the elements
With oxygen, water and dilute acids
With oxygen
All burn to form the white ionic oxide: 2M(s) + O₂(g) → 2MO(s). Magnesium burns with a brilliant white flame.
With water
- Mg reacts very slowly with cold water: Mg + 2H₂O → Mg(OH)₂ + H₂. With steam it reacts readily: Mg(s) + H₂O(g) → MgO(s) + H₂(g).
- Ca, Sr, Ba react with cold water, increasingly vigorously: M(s) + 2H₂O(l) → M(OH)₂(aq) + H₂(g). Ca gives a cloudy white suspension because Ca(OH)₂ is only slightly soluble.
With dilute hydrochloric acid
M(s) + 2HCl(aq) → MCl₂(aq) + H₂(g): fizzing, faster down the group; all the chlorides are soluble.
With dilute sulfuric acid
M(s) + H₂SO₄(aq) → MSO₄ + H₂(g). With Mg the sulfate is soluble and the reaction continues. With Ca, and especially Sr and Ba, the sulfate is insoluble and forms a coating on the metal, so the reaction slows and stops.
Oxides, hydroxides and carbonates
With water and dilute acids
| Compound | With water | With dilute HCl | With dilute H₂SO₄ |
|---|
| Oxide MO | MO + H₂O → M(OH)₂; MgO gives pH about 9, BaO about 13–14 | MO + 2HCl → MCl₂ + H₂O | MO + H₂SO₄ → MSO₄ + H₂O |
| Hydroxide M(OH)₂ | Dissolves slightly (more down the group): alkaline solution | M(OH)₂ + 2HCl → MCl₂ + 2H₂O | M(OH)₂ + H₂SO₄ → MSO₄ + 2H₂O |
| Carbonate MCO₃ | Insoluble | MCO₃ + 2HCl → MCl₂ + H₂O + CO₂ (fizzing) | MCO₃ + H₂SO₄ → MSO₄ + H₂O + CO₂ (stops early for Ca, Sr, Ba: insoluble sulfate layer) |
The oxides and hydroxides are basic; the solution formed with water becomes more alkaline down the group because the hydroxides are more soluble.
Thermal decomposition
Carbonates and nitrates
Decomposition
MCO₃(s) → MO(s) + CO₂(g) 2M(NO₃)₂(s) → 2MO(s) + 4NO₂(g) + O₂(g)
Nitrates give off brown NO₂ gas and oxygen, which relights a glowing splint. Thermal stability increases down the group: MgCO₃ decomposes at a much lower temperature than BaCO₃, which needs very strong heating.