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Group 2

Inorganic Chemistry Weightage: AS · Papers 1, 2 and 3 (assumed in Papers 4 and 5) Topic 10
“Welcome! Group 2 is a gift topic: four trends, all explained by the same idea. The metals get more reactive down the group, so the oxides and hydroxides get more basic and more soluble, while the sulfates get less soluble and the carbonates and nitrates get harder to decompose. Learn the four arrows, then the equations that go with them.”
— SCORECHEM ACADEMIC TEAM

0. What Examiners Want

How marks are earned in Group 2
  • Equations: the correct formula of each product (MO, M(OH)2, MCl2, MSO4), balanced, with state symbols when asked.
  • Trends: say the direction and the reason. For reactivity the reason is the ionisation energy; for the hydroxide pH it is the concentration of OH−.
  • Observations: colours, gases and precipitates (the test for each gas matters).
  • Predictions: for radium or strontium, extend the trend and use the words “more” or “less” than barium or calcium.
  • Calculations: use of the gas volume (24.0 dm3 at room conditions) and solubility data in mol per 100 g.

1. The Elements and Their Reactivity

Group 2 (the alkaline earth metals) is Be, Mg, Ca, Sr and Ba. Each atom has two electrons in an outer s sub-shell, and forms a 2+ ion with a noble-gas configuration. The metals are reducing agents: the atoms are oxidised (oxidation number 0 to +2) in every reaction.

Mg → Mg2+ + 2e−   (oxidation number 0 to +2)

Physical trends

Reactivity increases down the group. The Data Booklet values show why:

Be Mg Ca Sr Ba
1st IE / kJ mol−1 900 736 590 548 502
2nd IE / kJ mol−1 1760 1450 1150 1060 966
1st + 2nd IE / kJ mol−1 2660 2186 1740 1608 1468
1st + 2nd ionisation energy / kJ mol−1​ 2660 Be 2186 Mg 1740 Ca 1608 Sr 1468 Ba radius / nm 0.122 0.160 0.197 0.215 0.217 easier to remove 2e−​ reactivity increases more shells, more shielding, bigger radius
The energy needed to remove the two outer electrons falls down Group 2. The extra shells increase the shielding and the distance of the outer electrons from the nucleus, which outweighs the greater nuclear charge. So the metals become more reactive (better reducing agents).

The sum of the first two ionisation energies decreases down the group, so it is easier to form the 2+ ion. The nuclear charge increases, but the extra inner shells increase the shielding and the outer electrons are further from the nucleus; these two effects outweigh the greater nuclear charge.

⚠️ Examiner Trap: Reactivity is about losing electrons. Say “the outer electrons are lost more easily” and quote shielding and distance. Do not write that the atoms “want” to lose electrons, and do not use only “the atom is bigger”. Use both ionisation energies (1st and 2nd), because two electrons are lost.
Mg Ca Sr Ba atomic radius increases down the group reactivity increases down the group hydroxide solubility increases down the group (solution more alkaline) sulfate solubility decreases down the group carbonate / nitrate thermal stability increases down the group
Group 2 trends from Mg to Ba. Two are opposite and easy to mix up: hydroxides become more soluble, sulfates become less soluble.

2. Reactions with Oxygen, Water and Acids

With oxygen

The metals burn to give white ionic oxides: 2M(s) + O2(g) → 2MO(s). Magnesium burns with a bright white flame: 2Mg(s) + O2(g) → 2MgO(s). The reactions are more vigorous down the group; barium is stored under oil to keep it from air.

With water

Metal Reaction Equation
Mg (cold) extremely slow; weakly alkaline solution (pH about 10) Mg(s) + 2H2O(l) → Mg(OH)2(aq) + H2(g)
Mg (steam) vigorous; white oxide forms Mg(s) + H2O(g) → MgO(s) + H2(g)
Ca steady stream of H2; cloudy white suspension (Ca(OH)2 is slightly soluble) Ca(s) + 2H2O(l) → Ca(OH)2(aq) + H2(g)
Sr, Ba increasingly vigorous M(s) + 2H2O(l) → M(OH)2(aq) + H2(g)

The gas is hydrogen: it burns with a squeaky pop with a lighted splint.

With dilute acids

M MCO3​ M(NO3​)2​ MO M(OH)2​ MCl2​ / MSO4​ + O2​ (burn) heat, + CO2​ heat, + NO2​ + O2​ + H2​O + acid (salt + water) + H2​O (Mg: slow; steam gives MO) + H2​ + acid M also reacts with dilute acid: M + 2HCl → MCl2​ + H2​ (Ca, Sr, Ba with H2​SO4​ stop: insoluble sulfate layer)
Reaction map for a Group 2 metal M. The oxide is the hub: it is made by burning the metal or by heating the carbonate or nitrate, and it gives the hydroxide with water or a salt with acid.

3. Oxides and Hydroxides

Oxides. The oxides are ionic and basic. They react with dilute acid to give a salt and water:

Oxides with water. The oxide ion is a strong base: O2− + H2O → 2OH−.

Hydroxides are bases and are neutralised by dilute acids:

Carbonates are insoluble but react with dilute acid to give a salt, water and carbon dioxide:

⚠️ Examiner Trap: CaO and water. The product is Ca(OH)2, not “Ca(OH)” and not CaO(aq). The pH of about 11 belongs to the saturated solution, because Ca(OH)2 is only slightly soluble. Write “exothermic” if the question asks for an observation of the temperature.

4. Solubility of Hydroxides and Sulfates

Mg Ca Sr Ba
Hydroxide solubility / mol per 100 g water (298 K) 2.0 × 10−5 1.5 × 10−3 3.4 × 10−3 1.5 × 10−2
Sulfate soluble slightly soluble insoluble insoluble
hydroxides: solubility / mol per 100 g water 2.0×10−5​ Mg 1.5×10−3​ Ca 3.4×10−3​ Sr 1.5×10−2​ Ba (log scale) more soluble: more OH−​, higher pH sulfates: solubility falls MgSO4​ soluble CaSO4​ slightly soluble SrSO4​ insoluble BaSO4​ insoluble BaSO4​: white precipitate used to test for SO4​2−​
Hydroxides get more soluble down the group, so the saturated solution of Ba(OH)2 has a higher pH than Mg(OH)2 (about 10 for Mg(OH)2 and 11 for Ca(OH)2). Sulfates get less soluble.
⚠️ Examiner Trap: Solubility data in a calculation. The solubility is given in mol per 100 g of water, so scale to the mass of water, then convert moles to mass. Example: 3.4 × 10−3 mol of Sr(OH)2 per 100 g, so 250 g of water dissolves 3.4 × 10−3 × 2.5 = 8.5 × 10−3 mol, which is 8.5 × 10−3 × 121.6 = 1.03 g.

5. Carbonates and Nitrates: Thermal Decomposition

Carbonates decompose to the oxide and carbon dioxide: MCO3(s) → MO(s) + CO2(g)

Nitrates give the oxide, brown nitrogen dioxide and oxygen: 2M(NO3)2(s) → 2MO(s) + 4NO2(g) + O2(g)

Trend: both carbonates and nitrates need a higher temperature to decompose as you go down the group. Thermal stability increases from Mg to Ba.

Explanation (needed for Papers 4 and 5): a small cation with a high charge density (Mg2+) polarises (distorts) the electron cloud of the carbonate or nitrate ion more, which weakens the bonds in the anion so the compound breaks down more easily. Down the group the cation is larger, its charge density is lower, the polarisation is smaller, and the compound is more stable.

Mg2+​: small, charge density high Mg2+​ CO3​2−​ electron cloud pulled and distorted C–O bond weakened decomposes at a LOWER temperature Ba2+​: large, charge density low Ba2+​ CO3​2−​ anion hardly distorted C–O bonds stay strong needs a HIGHER temperature
Why thermal stability increases down Group 2 (this explanation is needed for Papers 4 and 5). A small cation with a high charge density polarises the carbonate (or nitrate) anion more, so the anion breaks down more easily. Going down the group the cation is larger, the charge density is lower, and the compound is more stable.
⚠️ Examiner Trap: Three products from a nitrate. Writing 2M(NO3)2 → 2MO + 2NO2 + O2 is a common slip: it is 4NO2. Check the oxygen: 12 oxygen atoms on the left, 2 + 8 + 2 = 12 on the right. Also state the trend as increasing stability, not “more reactive”.

6. Predicting Properties

Use the trend, then add a comparison word. For radium (below barium) predict:

For beryllium (above magnesium): the least reactive, the highest ionisation energies, and its compounds have the lowest thermal stability, but Be does not follow all of the trends (for example its melting point is the highest).

7. Quick Sheet and Checklist

Property (down the group) Trend Reason
Atomic radius increases extra shell
1st and 2nd ionisation energy decrease more shielding and distance outweigh nuclear charge
Reactivity with O2, H2O, acid increases easier to lose 2 electrons
Solubility of hydroxides increases higher [OH−], higher pH
Solubility of sulfates decreases BaSO4 is a white precipitate (test for SO42−)
Thermal stability of carbonates and nitrates increases lower charge density, less polarisation

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