Alkaline Earth metals Flashcards
Why do the size of the atoms increase down group II?
On descending a group, the number of shells increases. Therefore the outer shells are more shielded from the nucleus, are less closely held and move further away.
Why do the first ionisation energies of the atoms decrease down group II?
On descending a group, the number of shells increases. Therefore the outer shells are more shielded from the nucleus, are less closely held and are easier to remove.
Why do electronegativies of the atoms decrease down group II?
On descending a group, the number of shells increases. Therefore the outer shells are more shielded from the nucleus and bonded pairs of electrons are less strongly held.
Why do the melting points of the elements decrease down group II?
On descending a group, the size of the cations increases and the charge on the cations remains constant. The charge density thus decreases and the attraction between the cations and the delocalized electrons also decreases. The melting points and hardness therefore decrease.
The vigour of reactions of group II elements with water
depends on:
- How easily two electrons can be removed from an atom
2. How soluble the hydroxides of the metals are
Why does the reaction with water becomes increasingly vigorous on descending group II?
Ionisation energies decrease down a group, the electrons are more easily removed from the atoms
Why are the reactions of magnesium and beryllium with water are very slow?
Since Be(OH)2 and Mg(OH)2 are insoluble in water, the hydroxide formed remains on the surface of the metal, preventing further reaction.
Be with water or steam
Doesn’t react
Mg with water or steam
Mg does not react with water but reacts with steam. Since Mg(OH)2 actually decomposes at high temperatures into the oxide, MgO is the main product when magnesium reacts with steam:
Mg(s) + H2O(g) -> MgO(s) + H2(g)
Ca with water
Calcium reacts steadily with cold water. The solution goes cloudy as the hydroxide is not very soluble:
Ca(s) + 2H2O(l) -> Ca(OH)2(s) + H2(g)
Sr + Ba with water
Strontium and barium react vigorously with cold water, barium even more vigorously than strontium:
Sr(s) + 2H2O(l) -> Sr(OH)2(aq) + H2(g)
Ba(s) + 2H2O(l) -> Ba(OH)2(aq) + H2(g)
Trend in solubility of sulphates
MgSO4 is soluble, CaSO4 is sparingly soluble and SrSO4 and BaSO4 are insoluble.
If sulphuric acid or sodium sulphate is added to aqueous solutions of calcium, strontium or barium ions
a white precipitate will be formed
Test for sulphate ions
Add 1 cm3 of aqueous barium chloride, and also1 cm3 of dilute hydrochloric acid, to 1 cm3 of the unknown solution. If a thick white precipitate is formed, and that precipitate does not dissolve in dilute HCl, then sulphate or hydrogensulphate ions are present.
Trend in solubility of hydroxides
Mg(OH)2 is insoluble, Ca(OH)2 is sparingly soluble and Sr(OH)2 and Ba(OH)2 are soluble.
