2.2 Group 2 the alkaline earth metals

The elements in Group 2 are called the alkaline earth metals.

AS_Inorganic___Physical_II_pdf

Atomic radii and 1st ionisation energy Group 2

Melting point and boiling point Group 2

 

Atomic radius increases down the group Mg–Ba
Explanation: the number of shells of electrons increases in each element as the group is descended.

First ionisation energy decreases down the group Mg–Ba
Explanation
: the distance between the nucleus and the outermost valence electrons is increased (due to an increase in the number of shells and the increased effect of *electron shielding) as the group is descended. Hence, the valence electron is easier to remove despite the increasing nuclear charge.

Melting point of the elements Mg–Ba
With the exception of Mg, there is a progressive decrease in melting point as the group is descended. There is no obvious pattern in the group’s boiling points.
Explanation: as the group is descended, the metal positive ions increase in size (by having more electron shells), hence delocalised electrons are further away from the positive ions. Therefore, the element has weaker attraction between its positive ions and the delocalised electrons and thus weaker metallic bonding.
Explanation for Mg: it has been suggested that the lower than expected melting point of Mg is a consequence of its different crystalline structure (arrangement of metal ions).

*electron shielding: the nuclear attractive force on the outer valence electrons is ‘shielded’ by the fully occupied inner electron shells. 


Reactivity of with water (and solubility of metal hydroxides) increases down the group

Explanation: First ionisation energy decreases as the group is descended making it easier for successive elements to lose electrons and form metal ions and therefore react with water.

Trend of reactivity with water
Be  doesn’t react
Mg  very slowly with cold water, but fast with steam the reaction is rapid: Mg + H2O → MgO + H2
Ca   steadily
Sr    quickly
Ba   rapid and vigorously

In general, group 2 metals react with water to give a metal hydroxide [(aq) or (s)] and hydrogen gas:
M + 2H2O → M(OH)2+ H2
e.g. Ca + 2H2O → Ca(OH)2+ H2           

Magnesium reacts differently with cold water compared to its reaction with steam
Cold water: Mg + 2H2O → Mg(OH)2+ H2
Steam: Mg + H2O → MgO + H2

Burning magnesium reacts extremely exothermically with water or steam. Hence, water should not be used to put out a fire in which Mg metal is burning because hydrogen gas is rapidly produced and a highly flammable and explosive mixture is thus formed.


Solubility trends depend on the compound anion

Group 2 element Hydroxide ion Sulfate ion or
Carbonate ion
Mg least soluble most soluble
Ca
Sr
Ba most soluble least soluble

Generally, Group 2 elements that form compounds with single charged negative ions (e.g. OH) increase in solubility as the group descends. So, Mg(OH)2 is less soluble than Ba(OH)2 .

Mg2+(aq) reacts with NaOH to form a white precipitate because Mg(OH)2 is insoluble (only sparingly soluble)

Ca2+(aq), Sr2+(aq) and Ba2+(aq) ions all react with NaOH to produce their respective soluble metal hydroxide solutions: as the hydroxide products are all colourless and soluble these reactions are often recorded as “no (observed) reaction.”

Compounds that contain doubly-charged negative ions (e.g.  SO42− or CO32−) decrease in solubility as the group descends. So, MgSO4 is more soluble than BaSO4 .


Special properties of Beryllium compounds

BeCl2 and NaOH forms a white precipitate because Be(OH)2 is insoluble.

BeCl2  + 2NaOH →Be(OH)2 + 2NaCl (white precipitate)

However, adding excess NaOH causes the precipitate to dissolve as Be(OH)42, a colourless complex solution, is formed. This means Be(OH)2 is amphoteric (reacts with both acids and bases).

Be(OH)2  + 2OH → Be(OH)42−

With the exception of beryllium chloride, Group II chlorides are classed as ionic. However, Be2+ ion has a relatively high charge density (charge/size ratio) and electronegativity value (1.5 for Be, compared to 1.2 for Mg). Hence, there is less of a difference in electronegativities between Be and Cl (electronegativity 3.0) causing a greater degree of covalency of BeCl2.


Test for sulfate ions

A white precipitate, BaSO4 , is formed when acidified BaCl2 solution is added to a solution containing SO42− .

Ba2+(aq) +SO42−(aq) → BaSO4(s)

Acidification with HCl is necessary as this reacts with any sulfites or carbonates present in the test solution that may otherwise give an invalid (false positive) test result with BaCl2 solution .


Uses

BaSO4 is used clinically as a radio-contrast agent for X-ray imaging . It is most often used in gastrointestinal tract imaging. The investigation is known as a ‘barium meal’. 

Mg(OH)2 is a common component of antacids and laxatives.

Ca(OH)2 is used in agriculture to neutralise soil acidity. High levels of soil acidity can reduce root growth and reduce nutrient availability.

Mg is used in the extraction of titanium from TiCl4 .
The Kroll process for Ti extraction is slow and has at least two steps: 
Step 1- titanium oxide ore is reacted with Cl2 to make titanium chloride: C acts as a reducing agent, Cl2 acts as an oxidising agent

TiO2 + 2Cl2 + C→ TiCl4 + CO2      OR
TiO2 + 2Cl2 + 2C→ TiCl4 + 2CO

Step 2- titanium chloride is reduced by heating with magnesium at 850°C in the presence of Argon gas (prevents oxidation of Mg and Ti by air)
2Mg + TiCl4 → 2MgCl2+ Ti

CaO or CaCOare used in Flue-gas desulfurization (FGD).
FGD is a set of technologies used to remove SO2 from exhaust flue gases of fossil-fuel power plants.
CaCO3 +SO2 →CaSO3 + CO2                    CaSOis calcium sulfite
Ca(OH)2 +SO2 →CaSO3 + H2O
Ca(OH)2 +SO2 +½O2 → CaSO4 +H2O
CaSO3.½H2O +½O2 + 1½H2O →CaSO4.2H2O


 

 

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