10.1 Trends in Group 2 metals and compounds
- Syllabus
- 9701–2028–2029
- Topic
- 10.1
- Level
- AS
Learning objectives
10.1.1Reactions of the elements with oxygen• Describe/write equations: the reactions of the elements with oxygen, water and dil. hydrochloric and sulfuric acids10.1.2Reactions of the oxides, hydroxides• Describe/write equations: the reactions of the oxides, hydroxides and carbonates with water and dil. hydrochloric and sulfuric acids10.1.3Thermal decomposition trends• Describe/write equations: the thermal decomposition of the nitrates and carbonates, to include the trend in thermal stabilities10.1.4Describe, and make predictions• Describe, and make predictions from, the trends in physical/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 10.1.510.1.5Variation in the solubilities• State the variation in the solubilities of the hydroxides and sulfates
Group 2 metals show linked reactions with oxygen, water and dilute acids
| Reagent | General equation and Group 2 pattern |
|---|---|
| oxygen | 2M + O₂ → 2MO; reactivity increases Mg → Ba. Sr and Ba can also form peroxides, MO₂, in oxygen |
| cold water | M + 2H₂O → M(OH)₂ + H₂; Mg reacts very slowly, then Ca, Sr and Ba react increasingly rapidly |
| steam with Mg | Mg + H₂O(g) → MgO + H₂; this is much faster than Mg with cold water |
| dilute HCl | M + 2HCl → MCl₂ + H₂; colourless chloride solution and hydrogen form |
| dilute H₂SO₄ | M + H₂SO₄ → MSO₄ + H₂; Mg continues, but Ca slows and Sr/Ba stop as sparingly soluble or insoluble sulfate coats the metal |
Magnesium burns with a brilliant white flame. Calcium, strontium and barium react increasingly vigorously with cold water, producing hydrogen and hydroxide; their hydroxides become more soluble down the group, so the mixtures become more strongly alkaline.
Every metal loses two outer electrons to form M²⁺. Down the group, an extra occupied shell increases radius and shielding, so the attraction to those electrons weakens and the first two ionisation energies decrease. Electron loss—and therefore reactivity—becomes easier.
A sulfate coating can make the observed sulfuric-acid reaction slow or stop even though intrinsic metal reactivity increases down the group. Do not use bubbling rate in H₂SO₄ alone as the reactivity trend.
Oxides, hydroxides and carbonates have distinct water and acid reactions
| Compound | Behaviour with water |
|---|---|
| oxide, MO | MO + H₂O → M(OH)₂; MgO reacts slowly, while the reaction and resulting alkalinity generally increase down the group |
| hydroxide, M(OH)₂ | dissolves to an extent: M(OH)₂(s) ⇌ M²⁺(aq) + 2OH⁻(aq); solubility increases down the group |
| carbonate, MCO₃ | essentially insoluble and no reaction with water |
| Compound + dilute HCl | Balanced general equation |
|---|---|
| oxide | MO + 2HCl → MCl₂ + H₂O |
| hydroxide | M(OH)₂ + 2HCl → MCl₂ + 2H₂O |
| carbonate | MCO₃ + 2HCl → MCl₂ + CO₂ + H₂O |
| Compound + dilute H₂SO₄ | Balanced general equation |
|---|---|
| oxide | MO + H₂SO₄ → MSO₄ + H₂O |
| hydroxide | M(OH)₂ + H₂SO₄ → MSO₄ + 2H₂O |
| carbonate | MCO₃ + H₂SO₄ → MSO₄ + CO₂ + H₂O |
With sulfuric acid, CaSO₄ is sparingly soluble and SrSO₄/BaSO₄ are insoluble. A sulfate layer can coat an oxide, hydroxide or carbonate surface and inhibit further reaction; powdering and stirring expose more surface.
Acid + carbonate produces CO₂ as well as salt and water; acid + oxide or hydroxide does not. Dissolution of a hydroxide in water is not the same chemical job as neutralisation by an acid.
Group 2 nitrates and carbonates become more thermally stable down the group
Thermal stability is resistance to decomposition by heat. Both the carbonates and nitrates require progressively stronger heating from Mg to Ba.
| Salt heated | Balanced general equation | Observable gas evidence |
|---|---|---|
| carbonate | MCO₃(s) → MO(s) + CO₂(g) | CO₂ turns limewater milky |
| nitrate | 2M(NO₃)₂(s) → 2MO(s) + 4NO₂(g) + O₂(g) | brown toxic NO₂ fumes; O₂ relights a glowing splint |
All cations have charge +2, but M²⁺ radius increases down the group, so charge density and polarising power decrease. The larger cation distorts the electron cloud of CO₃²⁻ or NO₃⁻ less, so bonds within the anion are less weakened and more heat is needed for decomposition.
MgCO₃ therefore decomposes at a lower temperature than BaCO₃ under comparable conditions. The trend concerns the temperature or heating needed, not a change in the products: the same product pattern applies throughout Mg to Ba.
One set of particle trends predicts Group 2 properties
| Property from Mg → Ba | Trend | Particle-level reason or linked consequence |
|---|---|---|
| atomic and M²⁺ ionic radius | increase | each step adds an occupied shell |
| first and second ionisation energies | decrease | greater distance and shielding outweigh increased nuclear charge |
| melting point | general decrease, with irregularities | larger ions weaken attraction to delocalised electrons overall; packing differences prevent a perfectly smooth trend |
| density | general increase | atomic mass increases overall faster than atomic volume |
| metal reactivity with O₂, H₂O and dilute acid | increases | losing two outer electrons becomes easier |
| oxide-water reaction and hydroxide-solution alkalinity | generally increase | hydroxides become more soluble, releasing a higher concentration of OH⁻ |
| nitrate/carbonate thermal stability | increases | larger M²⁺ has lower polarising power |
| hydroxide solubility | increases | Mg(OH)₂ sparingly soluble → Ba(OH)₂ much more soluble |
| sulfate solubility | decreases | MgSO₄ soluble → BaSO₄ insoluble |
To predict an unfamiliar Group 2 case, first place it relative to Mg, Ca, Sr and Ba. Select only the row relevant to the question, state the direction, and give the matching causal explanation or reaction equation.
Not every property follows one smooth arrow: melting points are irregular, and observed sulfuric-acid rates can be suppressed by a sulfate coating. Keep intrinsic metal reactivity separate from passivation and keep thermal stability separate from solubility.
Hydroxide solubility rises while sulfate solubility falls
| Series from Mg to Ba | Required variation | Useful endpoints |
|---|---|---|
| M(OH)₂ | solubility increases down Group 2 | Mg(OH)₂ is sparingly soluble; Ba(OH)₂ is much more soluble |
| MSO₄ | solubility decreases down Group 2 | MgSO₄ is soluble; CaSO₄ is sparingly soluble; SrSO₄ and BaSO₄ are effectively insoluble |
More soluble hydroxide gives a higher OH⁻ concentration and a more alkaline saturated solution. Falling sulfate solubility explains white sulfate precipitates and why Ca, Sr or Ba compounds can become coated during reactions with sulfuric acid.
The two trends run in opposite directions. This AS outcome requires the variation to be stated and applied; a full lattice-enthalpy/hydration-enthalpy calculation belongs to later A Level energetics.