3.2.2 Group 2, the alkaline earth metal

Group 2 atomic radius trend

• Atomic radius increases down the group

• The atoms have more shells of electrons

Melting points trend

• Melting point decreases down the group

• The metallic bonding weakens as the atomic size increases

• The distance between the positive ions and the sea of delocalised electrons increases

• So the electrostatic force of attraction between the positive ions and the delocalised electrons weaken.

1st Ionisation energy trends?

• As you go down the group ionisation energy decreases

• As shielding increases, the outermost electrons gets further away from the nucleus

• So experience less attraction from the nucleus

Group 2 reactions with oxygen?

• Group 2 metals will burn in oxygen

• Mg burns with a bright white flame

• 2Mg + O₂ —> 2MgO

• MgO is a white solid with high melting point due to ionic bonding

Group 2 reaction with water trend and equation and observations

• Reactivity with water increases down the group

• Due to ionisation energy decreasing down the group

General equation:

• Mg (s) + 2H₂O (l) —> Mg(OH)₂ (s) + H₂ (g)

• Ca (s) + 2H₂O (l0 —> Ca(OH)₂ (aq) + H₂ (g)

• Group 2 hydroxide state symbol depends in the group 2 metal below Ca is definitely (aq)

Observations:

• Fizzing (more vigorous down group)

• The metallic bonding weakens dissolving

• With calcium a white precipitate forms. - less/no precipitate forms down the group

Solubility of hydroxides trend

+Use of magnesium hydroxide and calcium hydroxide?

• Solubility increases down the group

• Mg(OH)₂ is the least soluble and barium hydroxide is the most soluble

• Magnesium hydroxide is used in medicine as antacid as it is alkaline and can neutralise acids. - low solubility so wouldn’t make stomach too alkaline

• Weakly alkaline. Preferable to CaCO₃ produces CO₂ gas

• Also calcium hydroxide is used in agriculture to neutralise acidic soils

Magnesium equation with steam

• Mg (s) + H₂O (g) —> MgO (s) + H₂ (g)

• Mg would burn with a bright white light/ flame

• White solids/ ppt

Magnesium equation with warm water?

• Mg + 2H₂O —> Mg(OH)₂ + H₂

• This is a much slower reaction than the reaction with steam + no flame

Solubility of Sulfates?

• Group 2 sulfates becomes less soluble down the group

• MgSO₄ is the most soluble, whereas BaSO₄ is the least soluble

Equation for the formation of the precipitate and simplest ionic equation

Full equation:

• SrCl₂ (aq) + Na₂SO₄ (aq) —> 2Nacl (aq) + SrSO₄ (s)

Ionic equation:

• Sr²⁺ (aq) + SO₄^2- (aq) —> SrSO₄ (s)

Use of BaSO₄?

• Used in medicine as a ‘Barium meal’

• Given to patients who need x - rays of their intestines.

• These are a form of medical tracers that allow internal tissues and organs to be imaged.

• Barium sulfate is toxic if it enters the bloodstream, however because it is insoluble it cannot be absorbed into the blood

How can we test for sulfate ion?

• Use BaCl₂ solution acidified with HCl - to react with carbonate impurities that are found in salts which would form a white barium carbonate precipitate - false result

• If barium chloride is added to a solution that contains sulfate ions a white precipitate of barium sulfate forms.

Generic steps when making insoluble salts?

• Remove the salts by filtration

• Then wash the salts with distilled water to remove soluble impurities

• Fry with filter paper

Titanium extraction steps?

• TiO₂ (s) is converted to TiCl₄ (l) at 900 c

• The TiCl₄ is purified by fractional distillation in an argon atmosphere

• The Ti is extracted by Mg in an argon atmosphere at 500 C

Titanium extraction equations

• TiO₂ + 2Cl₂ + 2C —> TiCl₄ + 2CO

• TiCl₄ + 2Mg —> Ti + 2MgCl₂

Why is titanium expensive?

• Expensive cost of Mg

• Batch process which makes it expensive because the process is slower

• High temperature is required

Flue gas removal?

• Used to prevent SO₂

• CaO + 2H₂O + SO₂ —> CaSO₃ + 2H₂O

Or

• CaCO₃ (s) + O₂ + SO₂ (g) —> CaSO₄ (S) + CO₂

• However it produces carbon dioxide a green house gas