Group 2: magnesium to barium — key trends

· Group 2 metals studied here: Mg, Ca, Sr, Ba.
· All form M²⁺ ions and mainly ionic compounds.
· Reactivity increases down the group: Mg < Ca < Sr < Ba.
· Reason for metal reactivity trend: outer electrons are further from the nucleus, have more shielding, and are lost more easily.
· Common exam phrase: first ionisation energy decreases down Group 2.

Reactions of Group 2 metals with oxygen

· General equation: 2M(s) + O₂(g) → 2MO(s).
· Product: Group 2 oxide, usually a white solid.
· Example: 2Mg(s) + O₂(g) → 2MgO(s).
· These oxides are basic oxides.
· Burning becomes generally easier/more vigorous down the group, linked to increasing reactivity.

Group 2 metals react with oxygen to form basic metal oxides, MO. The equation pattern is the same for Mg, Ca, Sr and Ba, so students should be able to substitute the metal symbol into the general equation. Source

Reactions of Group 2 metals with water

· General equation for Ca, Sr, Ba: M(s) + 2H₂O(l) → M(OH)₂(aq/s) + H₂(g).
· Mg reacts very slowly with cold water.
· Mg reacts with steam: Mg(s) + H₂O(g) → MgO(s) + H₂(g).
· Ca reacts steadily, Sr reacts faster, Ba reacts vigorously with cold water.
· Trend: reactions become more vigorous down the group.
· Products: metal hydroxide + hydrogen gas.
· Test for hydrogen: squeaky pop with a lit splint.

The diagram shows that Group 2 metals form hydroxides and hydrogen with water. The key exam trend is that reactivity increases from Mg to Ba. Source

Reactions of Group 2 metals with dilute acids

· With dilute hydrochloric acid: M(s) + 2HCl(aq) → MCl₂(aq) + H₂(g).
· Example: Ca(s) + 2HCl(aq) → CaCl₂(aq) + H₂(g).
· With dilute sulfuric acid: M(s) + H₂SO₄(aq) → MSO₄(s/aq) + H₂(g).
· Reactions with acids usually produce salt + hydrogen.
· With H₂SO₄, reactions may slow down for Ca, Sr and Ba because insoluble sulfate coatings form on the metal surface.
· BaSO₄ is very insoluble, so barium’s reaction with sulfuric acid may stop quickly due to a protective coating.

Reactions of Group 2 oxides with water and acids

· Oxides are basic oxides.
· With water: MO(s) + H₂O(l) → M(OH)₂(aq/s).
· Example: CaO(s) + H₂O(l) → Ca(OH)₂(aq/s).
· With hydrochloric acid: MO(s) + 2HCl(aq) → MCl₂(aq) + H₂O(l).
· With sulfuric acid: MO(s) + H₂SO₄(aq) → MSO₄(s/aq) + H₂O(l).
· Acid reactions are neutralisation reactions: basic oxide + acid → salt + water.

Reactions of Group 2 hydroxides with water and acids

· Group 2 hydroxides are alkaline when dissolved in water.
· Dissociation in water: M(OH)₂(s/aq) → M²⁺(aq) + 2OH⁻(aq).
· With hydrochloric acid: M(OH)₂(s/aq) + 2HCl(aq) → MCl₂(aq) + 2H₂O(l).
· With sulfuric acid: M(OH)₂(s/aq) + H₂SO₄(aq) → MSO₄(s/aq) + 2H₂O(l).
· More soluble hydroxides produce a higher concentration of OH⁻ ions, so solutions become more alkaline.

Solubility trends: hydroxides and sulfates

· Hydroxide solubility increases down Group 2.
· Trend: Mg(OH)₂ least soluble → Ba(OH)₂ most soluble.
· Consequence: alkalinity of hydroxide solutions increases down the group.
· Sulfate solubility decreases down Group 2.
· Trend: MgSO₄ most soluble → BaSO₄ least soluble.
· BaSO₄ is insoluble, which explains why it forms a coating in reactions involving sulfuric acid.
· Exam shortcut: hydroxides get more soluble; sulfates get less soluble.

Reactions of Group 2 carbonates with water and acids

· Group 2 carbonates are insoluble or sparingly soluble in water.
· They do not react significantly with water under normal conditions.
· With hydrochloric acid: MCO₃(s) + 2HCl(aq) → MCl₂(aq) + H₂O(l) + CO₂(g).
· With sulfuric acid: MCO₃(s) + H₂SO₄(aq) → MSO₄(s/aq) + H₂O(l) + CO₂(g).
· Carbonates with acids produce salt + water + carbon dioxide.
· Test for CO₂: turns limewater milky.

Thermal decomposition of Group 2 carbonates

· General equation: MCO₃(s) → MO(s) + CO₂(g).
· Example: MgCO₃(s) → MgO(s) + CO₂(g).
· Thermal stability increases down the group.
· Order: MgCO₃ least thermally stable → BaCO₃ most thermally stable.
· This means more heat is needed to decompose carbonates lower down the group.
· Products are a metal oxide and carbon dioxide.

Thermal decomposition of Group 2 nitrates

· General equation: 2M(NO₃)₂(s) → 2MO(s) + 4NO₂(g) + O₂(g).
· Example: 2Mg(NO₃)₂(s) → 2MgO(s) + 4NO₂(g) + O₂(g).
· Products: metal oxide + nitrogen dioxide + oxygen.
· NO₂ is a brown gas.
· Thermal stability increases down the group.
· Order: Mg(NO₃)₂ least thermally stable → Ba(NO₃)₂ most thermally stable.
· Exam comparison: both nitrates and carbonates become more thermally stable down Group 2.

Required equations to memorise

· Metal + oxygen: 2M + O₂ → 2MO.
· Metal + water: M + 2H₂O → M(OH)₂ + H₂.
· Mg + steam: Mg + H₂O → MgO + H₂.
· Metal + HCl: M + 2HCl → MCl₂ + H₂.
· Metal + H₂SO₄: M + H₂SO₄ → MSO₄ + H₂.
· Oxide + water: MO + H₂O → M(OH)₂.
· Oxide + HCl: MO + 2HCl → MCl₂ + H₂O.
· Hydroxide + HCl: M(OH)₂ + 2HCl → MCl₂ + 2H₂O.
· Carbonate + HCl: MCO₃ + 2HCl → MCl₂ + H₂O + CO₂.
· Carbonate decomposition: MCO₃ → MO + CO₂.
· Nitrate decomposition: 2M(NO₃)₂ → 2MO + 4NO₂ + O₂.

Practical and observation points

· Hydrogen gas: gives a squeaky pop with a lit splint.
· Carbon dioxide: turns limewater milky.
· Nitrogen dioxide: brown gas formed when nitrates decompose.
· Group 2 oxides are usually white solids.
· Group 2 carbonates are usually white solids.
· Flame colours may help identify ions: Ca²⁺ brick-red/orange-red, Sr²⁺ crimson red, Ba²⁺ apple green.

Flame tests show characteristic colours caused by metal ions. For Group 2, calcium, strontium and barium are especially useful to recognise in practical chemistry. Source

Common exam traps

· Do not say Group 2 metals form M⁺ ions; they form M²⁺ ions.
· Do not forget to balance nitrate decomposition: 2M(NO₃)₂ → 2MO + 4NO₂ + O₂.
· Do not confuse solubility trends: hydroxides increase, sulfates decrease.
· Do not write Mg + cold water as vigorous; Mg is very slow with cold water but reacts with steam.
· Do not forget BaSO₄ is insoluble, so sulfuric acid reactions can be slowed or stopped by a sulfate coating.