Group 2, the alkaline earth metals (3.2.2)

What are the trends in atomic radius and ionisation energies down the group?

The nuclear charge increases down the group but so does the shielding this causes the atomic radius to increase and the attraction between the nucleus and outer electron to decrease, so less energy is needed to remove it thus decreases the ionisation energy

What is the trend in melting point down the group?

General trend - the melting point decreases down the group as the ionic radius (size of ion) and shielding increases, reducing the strength of attraction between the cation nucleus and the delocalised electrons so less energy is required to break the metallic bonds

Mg has a low melting point as the strength of the metallic bond is influenced by how the cations are packed together and they are packed together differently in Mg

How does the group react with water (and how does Mg react differently + observations)?

They react to form a metal hydroxide and hydrogen:

M_(s) + 2H₂O_(l) → M(OH)_2(s/aq) + H₂

Mg reacts differently with steam vs liquid water, forming MgO with steam (burns bright white and forms a white powder) and forms Mg(OH)₂ with liquid water

What is the trend in the solubility of the hydroxides down the group?

The solubility of the hydroxides increases down the group so Mg(OH)₂ (partially soluble) is the least soluble and Ba(OH)_2(aq) the most

What is the trend in the solubility of the sulfates down the group?

The solubility of the sulfates decreases down the group so MgSO₄ is very soluble, CaSO₄ is sparingly soluble and SrSO₄ and BaSO₄ are insoluble

How is Mg used in the extraction of Ti?

1) First TiO₂ is extracted and turned into TiCl₄:

TiO₂ + 2Cl₂ + 2C → TiCl₄ + 2CO

2) It is then reduced using Mg_(l) - heated to 1200 degrees celsius with an inert atmosphere:

TiCl_4(l) + 2Mg_(l) → Ti_(l) + 2MgCl_2(l)

Ti: +4 → 0 - reduced

Mg: 0 → +2 - oxidised

The MgCl₂ can be extracted via electrolysis and recycled to recover the Mg

How can the group 2 hydroxides be used?

Mg(OH)₂:

• Used as an antacid as its alkaline so it neutralises acids - it is sparingly soluble so only slightly alkaline

• Used in dental as it can neutralise acidic toxins produced by bacteria

• Used for treating digestive upsets as it acts as a laxative

Ca(OH)₂:

• Called slaked lime which is used to neutralise soil acidity

What is the use of BaSO₄?

A ‘barium meal’ is given to a patient who needs a x-ray on their intestines. Barium absorbs x-rays so the gut shows up white on the image, helping to identify abnormalities.

It is insoluble so although barium is toxic it can’t be absorbed by the blood

How can calcium be used to remove SO₂ from flue gas?

Both CaO and CaCO₃ can be used and they both produce CaSO₃ which is oxidised to form CaSO₄

CaCO₃:

• SO_2(g) + CaCO_3(s) → CaSO_3(aq) + CO_2(g)

• 2CaSO_3(s) + O_2(g) → 2CaSO_4(s)

CaO

• SO_2(g) + CaO_(s) → CaSO_3(aq)

• 2CaSO_3(s) + O_2(g) → 2CaSO_4(s)

How can sulfate ions be tested for?

To test for SO₄^2- add a solution of BaCl₂, forming a white precipitate, if the ion is present, of BaSO₄

Ba²⁺_(aq) + SO₄^2-_(aq) → BaSO_4(s)

It is acidified with dilute HCl / HNO₃ to react with any carbonate ions that might be present, which also form a white precipitate of BaCO₃, leading to a misleading result