3.2.2 ALKALINE EARTH METALS

Question 1

Explain the trend in first ionisation energy for this group

Answer 1

• Decreases
• Atom gets larger
• Outer negative electron becomes further
  away from positive nucleus
• Less electrostatic attraction so is more easily
  lost

Question 2

Explain the trend in atomic radius down the group

Answer 2

• Increases
• As there are more energy levels filled
• So distance between outermost orbital and
  nucleus increases

Question 3

Describe the trend in melting point down the group

Answer 3

• GENERAL decrease
  There does not appear to be a trend in boiling points going down the group. Again magnesium is an anomaly. If you exclude it, the boiling points decrease from beryllium to strontium, then increase to radium.

General decrease due to increased size of ions in metallic lattice, decreasing size of electrostatic attraction between them and delocalised electrons

Question 4

Describe the group 2 metals’ reactions with water

Answer 4

• Beryllium doesn’t react with water
• Mg is slightly reactive (needs heat)
• React more vigorously down the group
• Form metal hydroxides

X (s) + 2H2O —– X(OH)2 + H2

Mg also produces an oxide when reacting with STEAM

Mg(s) + H2O (g) —- MgO + H2

Question 5

Describe how the solubility of group 2 hydroxides changes down the group

Answer 5

• Increase in solubility
• Mg(OH) appears to be insoluble however a
  tiny amount dissolves (sparingly soluble)
• Ca(OH)2 is sparingly soluble (bit more)
• Greater solubility down the group

Question 6

Describe some uses of group 2 hydroxides

Answer 6

• Ca(OH)2 (limewater) is used in agriculture to
  raise soil pH
• Mg(OH)2 is used to aid indigestion, by raising
  the pH of the stomach

Question 7

Describe the use of CaO

Answer 7

• CaO made through thermal decomposition of
  CaCO3
• Used to remove flue gases from power
  stations

CaO(s) + SO2(g) —- CaSO3 (s)

• Produces Calcium SULFITE which isn’t a gas

Question 8

Describe the use of CaO

Answer 8

• CaO made through thermal decomposition of
  CaCO3
• Used to remove flue gases from power
  stations

CaO(s) + SO2(g) —- CaSO3 (s)

• Produces Calcium SULFITE which isn’t a gas

Question 9

Describe the use of CaCO3 (limestone)

Answer 9

• Used to remove SO2 from flue gases

CaCO3(s) + SO2(g) —- CaSO3(s) + CO2(g)

Question 10

Describe how the solubility of group 2 sulfates changes down the group

Answer 10

• Decreases down the group
• MgSO4 is soluble
• CaSO4 is sparingly soluble
• SrSO4 and BaSO4 are insoluble

Question 11

Is hydrogen soluble in water?

Answer 11

No it is practically insoluble,

Question 12

Write the net ionic equation for the reaction of magnesium with water

Answer 12

Mg + 2H2O —- Mg(OH)2 + H2

• It doesn’t change as Mg(OH)2 is insoluble

Question 13

Describe how to test for the presence of sulfates in a compound

Answer 13

• Acidify the solution with HCL or HNO3
• Add BaCl2
• White ppt forms in presence of sulfate
  [Ba2+ + SO42- —- BaSO4]

Question 14

Why is a solution acidified before testing for sulfates?

Answer 14

• Removes carbonates which would also -
  produce a white ppt
• Removes sulfites by dissolving them, which
  would also give a false positive

Question 15

Write the equations for the reactions of magnesium nitrate with sodium hydroxide and sodium sulfate

Answer 15

Mg(NO3)2 + 2NaOH —- Mg(OH)2 + 2NaNO3

Mg(NO3)2 + Na2SO4 —- MgSO4 + 2NaNO3

Question 16

Describe how magnesium is used in the extraction of titanium from titanium tetrachloride

Answer 16

Titanium removed from ore via carbon smelting

TiO2 + 2C + 2Cl2 —- TiCl4 + 2CO

TiCl4 + 2Mg —- Ti + 2MgCl2