Reactions and Stoichiometry of Alkaline Earth and Alkali Metal Compounds
This cluster is a working set of specific, high-value alkaline-earth and alkali-metal reactions: hydrolysis of a nitride and a carbide, the limestone-to-bicarbonate cycle behind hard water and stalactites, bleaching powder manufacture, and two stoichiometry-based calculation problems.
Identifying the Carbide That Gives Propyne on Hydrolysis is the central idea in this question pattern. Identify the condition in the stem before applying the method below.
Nitride and carbide hydrolysis: predictable products from the anion
Ionic nitrides hydrolyse in water to give the metal hydroxide plus ammonia: Mg3N2 + 6H2O → 3Mg(OH)2 + 2NH3, so ONE mole of magnesium nitride (which contains 2 nitrogen atoms per formula unit, as N3-) releases TWO moles of ammonia on complete hydrolysis with excess water, not one. Different metal carbides hydrolyse to different hydrocarbons depending on the specific carbon-containing anion present: methanides (C4-, as in Be2C or Al4C3) give methane; acetylides/dicarbides (C2²-, as in CaC2) give ethyne (acetylene); and the less common sesquicarbide/allylide ion (C3⁴-, as in Mg2C3) gives propyne (methylacetylene) specifically — recognise Mg2C3 as the one classic exam example that breaks from the more familiar methane/acetylene pattern.
Bleaching powder and the limestone-lime-bicarbonate cycle
Bleaching powder (calcium oxychloride, roughly CaOCl2) is made by passing chlorine gas over slaked lime, Ca(OH)2 (not over quicklime, CaO, directly)
Separately, a classic cycle links three calcium compounds: heating limestone (CaCO3, compound X) releases CO2 gas and leaves quicklime (CaO) as a residue; mixing that residue with water gives slaked lime, Ca(OH)2 (compound Y); passing EXCESS CO2 through a suspension/solution of Y converts it to the water-soluble calcium bicarbonate, Ca(HCO3)2 (compound Z, giving a clear solution, since Ca(HCO3)2 is soluble while Ca(OH)2 and CaCO3 are not); and boiling that clear solution of Z drives off CO2 and reforms solid CaCO3 (compound X), completing the cycle. This exact cycle (limestone to lime to bicarbonate solution to limestone again on boiling) is the chemistry behind temporary hard water and behind stalactite/stalagmite formation in caves.
Titration stoichiometry for a two-metal mixture
When a mixture of two Group 1 metals reacts completely with water, each mole of metal (regardless of which one) releases exactly one mole of hydroxide ion, since each is a +1 cation. Neutralising the resulting mixed hydroxide solution with a known volume and molarity of a diprotic acid like H2SO4 (which neutralises 2 moles of OH- per mole of acid) lets you calculate the TOTAL moles of metal present from the acid used; combining that total-moles equation with the given total MASS of the mixture (via each metal atomic mass) gives two simultaneous equations in two unknowns, solvable for the mass of each individual metal — the calculation works the same way regardless of which two alkali metals are mixed, since basicity (charge) is identical across the group.
Milk of magnesia is a suspension, not a solution
Magnesium hydroxide, Mg(OH)2, is only very sparingly soluble in water. Milk of magnesia (the familiar antacid) is prepared as a SUSPENSION of finely divided, undissolved Mg(OH)2 particles dispersed in water, not a true solution (which would require the compound to actually dissolve at the molecular/ionic level) and not a colloid either (a suspension has larger, visible/settling particles, distinct from the much smaller, stably dispersed particles of a true colloid) — the deliberately low aqueous solubility of Mg(OH)2 is exactly what makes it useful as a slow-acting, sustained-release antacid rather than a fast-dissolving one.
OlympIQ exam lens
- ✓Mg3N2 + 6H2O gives 3Mg(OH)2 + 2NH3 — one mole of nitride releases TWO moles of ammonia, not one
- ✓Mg2C3 (the sesquicarbide/allylide, C3⁴-) hydrolyses to propyne, distinct from methanides (to methane) and acetylides like CaC2 (to ethyne)
- ✓Bleaching powder is made from Cl2 + Ca(OH)2 (slaked lime), not Cl2 + CaO directly
- ✓The limestone cycle: CaCO3 to CaO (heat) to Ca(OH)2 (add water) to Ca(HCO3)2 (excess CO2, gives a clear soluble solution) to CaCO3 again (boiling drives off CO2)
- ✓For a two-metal titration problem, each mole of Group 1 metal gives one mole of OH-; combine total-moles-from-acid with total-mass to solve for each metal individual mass
- ✓Milk of magnesia is a SUSPENSION of Mg(OH)2, not a true solution or a colloid — its low solubility is what makes it a sustained-release antacid
Do not assume one mole of a binary nitride gives one mole of ammonia on hydrolysis. Count the nitrogen atoms in the formula first — Mg3N2 contains two nitrogens per formula unit, so complete hydrolysis of one mole of Mg3N2 gives two moles of NH3, not one.
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