Unit 17: Group 2 Elements — Short Questions

The barium hydroxide solution has the higher pH. Ca(OH)₂ has a solubility of only 1.5 × 10⁻³ mol/100 g water, while Ba(OH)₂ is about ten times more soluble at 1.5 × 10⁻² mol/100 g water. Since 0.1 mol exceeds the solubility of both, each forms a saturated solution with excess undissolved solid, and the dissolved [OH⁻] in each solution equals twice its solubility limit. Because Ba(OH)₂ dissolves to a much greater extent, its saturated solution has a higher [OH⁻] concentration, giving it the higher pH.

Calcium dichromate would be more soluble than strontium dichromate. Like sulphate, dichromate is a large anion, so lattice enthalpy is dominated by the anion and changes little with cation size, while hydration enthalpy of the cation decreases (becomes less negative) down the group. Since the dichromate ion is even larger than sulphate, this effect is expected to be at least as strong, so solubility should decrease down the group — making the calcium salt (smaller cation, higher hydration enthalpy) more soluble than the strontium salt.

Ba(s) + 2H₂O(l) → Ba(OH)₂(aq) + H₂(g)

Be²⁺ is the smallest and most highly charge-dense Group 2 cation, so it forms an exceptionally strong, tightly bonded ionic lattice with the O²⁻ ion. This produces a thin but very strong, dense and adherent BeO layer that firmly coats the metal surface and prevents oxygen or water from reaching the metal underneath. The larger Group 2 cations (Mg²⁺, Ca²⁺, etc.) have lower charge density, so their bonding with O²⁻ is weaker, giving a less strongly bonded, more porous oxide coating that does not protect the metal as effectively, allowing further reaction.

Barium nitrate decomposes to give barium oxide, nitrogen dioxide gas and oxygen gas: 2Ba(NO₃)₂(s) → 2BaO(s) + 4NO₂(g) + O₂(g).

Going down the group, both the hydration enthalpy of M²⁺ and the lattice enthalpy of the hydroxide decrease in magnitude as the cation gets larger. However, because OH⁻ is a small anion comparable in size to the Group 2 cations, lattice enthalpy is strongly dependent on cation size and falls off faster than hydration enthalpy as the cation grows. This makes the overall enthalpy of solution more negative (more favourable) down the group, so solubility of the hydroxides increases down the group.

Mg²⁺(aq) + Ca(OH)₂(s) → Mg(OH)₂(s) + Ca²⁺(aq)

BaSO₄ is used in "barium meals" for X-ray imaging of the digestive system — it is opaque to X-rays and, being highly insoluble, passes through the body safely without being absorbed. Mg(OH)₂ ("Milk of Magnesia") is used to neutralize excess stomach acid and as a mild laxative; its low solubility makes it safe to use.

Group 1 metals have only one valence electron to lose and much lower ionization energies than Group 2 metals in the same period, so they lose their outer electron more readily to form stable +1 ions. Group 2 metals must remove two electrons (a higher total ionization energy) to form +2 ions, which requires more energy, making them generally less reactive than their Group 1 counterparts.

Calcium hydroxide, Ca(OH)₂, is a base. When added to acidic soils it neutralizes excess soil acidity, raising the soil pH to a level more suitable for crop growth, which improves nutrient availability and supports healthier plant growth.