Chapter 7 Group 2

Question 1

Introduction of Group 2

Answer 1

• The Group 2 nitrates and carbonates become more thermally stable going down the group
• The charge density of the cation (Group 2 metal ion) and the polarisationoftheanion (the nitrate and carbonate ion) attribute towards this increased stability

Question 2

Trends in thermal stability going down the group

Answer 2

• All Group 2 metals form 2+ ions as they lose two electrons from their valence shells
• The metal cations at the top of the group are smaller in size than those at the bottom
• The metal cations at the top of Group 2, therefore, have the greatest charge density as the same charge (2+) is packed into a smaller volume
• As a result, smaller Group 2 ions have a greater polarising effect on neighbouring negative ions
• When a carbonate or nitrate ion approaches the cation, it becomes polarised
  
  • This is because the metal cation draws the electrons in the carbonate or nitrate ion towards itself
• The more polarised the anion is, the less heat is required to thermally decompose them

Question 3

Therefore, the thermal stability increases down the group

Answer 3

• As down the group, the cation becomes larger
• Thus has a smaller charge density
• And a smaller polarising effect on the carbonate or nitrate anion
• So the anion is less polarised
• Therefore, more heat is required to thermally decompose them

Question 4

Trends in Solubility & Enthalpy Change of Solution of Group 2 Hydroxides & Sulfates

Answer 4

• The solubility of Group 2 hydroxides increases down the group
• In contrast, the Group 2 sulfates show a decrease in solubility going down the group
• Compounds that have very low solubility are said to be sparingly soluble
  
  • For example, calcium sulfate (CaSO₄) has low solubility as only 0.21 g of CaSO₄ dissolves in 100 g of water
• Most of the sulfates are soluble in warm water with the exception of barium sulfate which is insoluble

Question 5

Solubility of Group 2 elements table

Answer 5

Question 6

The Standard enthalpy of solution

Answer 6

• (ΔH_sol^ꝋ) is the energy absorbed or released when 1 mole of ionic solid dissolves in enough water to form a dilute solution (under standard conditions)
  
  • The ΔH_sol^ꝋ can be either exothermic or endothermic
• For example, the ΔH_sol^ꝋ of sodium chloride (NaCl) is +3.9 kJ mol^-1

NaCl (s) + aq → NaCl (aq)

OR

NaCl (s) + aq → Na⁺ (aq) + Cl^– (aq)

• This means, that 3.9 kJ mol^-1 of energy is absorbed when one mole of NaCl is dissolved in enough water to form a dilute solution
• The ΔH_sol^ꝋ is the sum of the lattice energy (ΔH_latt^ꝋ) and the standard enthalpy change of hydration (ΔH_hyd^ꝋ)

ΔH_sol^ꝋ = ΔH_latt^ꝋ + ΔH_hyd^ꝋ

Question 7

The lattice (formation) energy

Answer 7

• is the energy released when gaseous ions combine to form one mole of an ionic compound under (standard conditions)
  
  • Since energy is released when an ionic compound is formed, the ΔH_latt^ꝋ is always exothermic
  • For example, the ΔH_latt^ꝋ of NaCl is -787 kJ mol^-1

Na⁺ (g) + Cl^– (g) → NaCl (s)

• This means, that 787 kJ mol^-1 of energy is released when NaCl is formed from its gaseous ions

Question 8

The standard enthalpy of hydration

Answer 8

• is the energy released when gaseous ions dissolve in enough water to form a dilute solution (under standard conditions)
  
  • Since energy is released when gaseous ions become hydrated, the ΔH_hyd^ꝋ is always exothermic
  • For example, the ΔH_hyd^ꝋ of the sodium (Na⁺) ion is -406 kJ mol^-1

Na⁺ (g) → Na⁺ (aq)

• This means, that 406 kJ mol^-1 of energy is released when Na⁺ ions become hydrated

Question 9

Trends of enthalpy change of solution

Answer 9

• Going down the group, the ΔH_latt^ꝋ of the ionic compounds decreases
• Going down the group, the ΔH_hyd^ꝋ also decreases
• The more exothermic the ΔH_sol^ꝋ the more soluble the compound

Question 10

Trends of enthalpy change of solution: Going down the group, the ΔH_latt^ꝋ of the ionic compounds decreases

Answer 10

• Going down the group, the positively charged cations become larger
• There is more space between the negatively and positively charged ions in the ionic compound so there are weaker attractive forces between the ions
• As there are weaker electrostatic forces between the ions, there is less energy released upon formation of the ionic compound from its gaseous ions
• Therefore, the ΔH_latt^ꝋ becomes less exothermic

Question 11

Trends of enthalpy change of solution: Going down the group, the ΔH_hyd^ꝋ also decreases

Answer 11

• Again, the positively charged ions become larger going down the group
• As a result, the ion-dipole bonds between the cations and water molecules get weaker
• This means that less energy is released when the gaseous Group 2 ions become hydrated
• The ΔH_hyd^ꝋ , therefore, becomes less exothermic

Question 12

Trends of enthalpy change of solution: For Group 2 hydroxides

Answer 12

• Hydroxide ions are relatively small ions
• The ΔH_latt^ꝋ falls faster than the ΔH_hyd^ꝋ
• The enthalpy change of solution is, therefore, more exothermic going down the group

Question 13

Trends of enthalpy change of solution: For Group 2 sulfates

Answer 13

• Sulfate ions are relatively large ions
• The ΔH_latt^ꝋ falls slower than the ΔH_hyd^ꝋ enthalpy
• The ΔH_sol^ꝋ will become more endothermic going down the group

Question 14

Trends of enthalpy change of solution: The more exothermic the ΔH_sol^ꝋ the more soluble the compound

Answer 14

• This is why the sulfates become less soluble going down the group and the hydroxides more soluble