CHEM C10: Group 2 Compounds

Today we’re covering:

  • Properties of Group 2 compounds
    • Reactions
      • Oxides with water
      • Carbonates with acid
    • Thermal decomposition
      • Carbonates
      • Nitrates
    • Solubility
      • Hydroxides
      • Sulfates

Let’s go!


Here we will be talking about:

  • Oxides
  • Hydroxides
  • Carbonates
  • Nitrates
  • Sulfates

Group 2 Oxides
Characteristics:

  • White ionic solids
  • All are basic oxides
    EXCEPT BeO

    • BeO: amphoteric
    • The small Be2+ ion polarizes the O2- ion
    • BeO has some covalent character

Reactions of Oxides with water

In general MO (s) + H2O (l) → M(OH)2 (aq)
  • Vigorous reaction
  • Forms alkaline solution (metal hydroxide)
BeO Insoluble in water Due to high lattice energy
MgO MgO (s) + H2O (l) → Mg(OH)2 (aq)
  • Slightly soluble
  • Reacts slowly
  • Forms suspension with pH 9-11
CaO CaO (s) + H2O (l) → Ca(OH)2 (aq)
  • Dissolves in water
  • Forms suspension with pH 10-13
SrO SrO (s) + H2O (l) → Sr(OH)2 (aq)
  • Dissolves in water
  • Forms solution with pH 10-13
BaO BaO (s) + H2O (l) → Ba(OH)2 (aq)
  • Dissolves in water
  • Forms solution with pH 10-13

As you can see, solubility of the hydroxides increase going down the group (see below).


Group 2 Carbonates
Characteristics:

  • Insoluble in water
  • React with acids
  • Undergo thermal decomposition to oxide + CO2

 Reactions of Carbonates with Acid

MgCO3 + H2SO4 MgCO3 + H2SO4
MgSO4 (aq) + H2O (l) + CO2
  • Effervescence
  • Forms soluble salt, water & CO2 gas
  • Complete reaction
Other carbonates + H2SO4 MCO3 + H2SO4
MSO4 (aq) + H2O (l) + CO2
Initially:

  • Effervescence
  • Forms insoluble salt, water & CO2 gas

 

However:

  • Insoluble sulfate layer forms on surface of carbonate
  • Reaction cannot be completed
All carbonates + HNO3 MCO3 + HNO3
M(NO3)2 (aq) + H2O (l) + CO2
  • Effervescence
  • Forms soluble salt, water & CO2 gas
  • Complete reaction
All carbonates + HCl MCO3 + HCl →
MCl2 (aq) + H2O (l) + CO2
  • Effervescence
  • Forms soluble salt, water & CO2 gas
  • Complete reaction

Thermal Decomposition
We will look at both Carbonates & Nitrates, & explain the trends going down the group.

Compound Carbonates Nitrates
General Equation MCO3 (s) → MO (s) + CO2 (g) M(NO3)2 (s) → MO (s) + 2NO2 (g) + ½O2 (g)
Observation
  • White solid is formed:
    metal oxide
  • Colourless gas is evolved: CO2
  • White solid is formed:
    metal oxide
  • Brown toxic gas is evolved: NO2
Trend going down group Higher temperature is needed Higher temperature is needed

 


Why is a higher temperature needed going down the group?
This is due to ION POLARISATION:

  1. Carbonate & nitrate anions have large ionic radii: they are easily polarised
  2. Group 2 cations increase in ionic radius going down the group
  3. Polarising power decreases going down the group
  4. The greater the polarisation, the easier it is to weaken bonds within the anion (carbon-oxygen / nitrogen-oxygen)
  5. Thus, as polarisation decreases going down the group, the more difficult it is to weaken bonds within the anion
  6. Thus, more energy is needed to thermally decompose the compound

 


Solubility of Group 2 Compounds
We will look at hydroxides & sulfates.

Compound Hydroxides Sulfates
Trend going down group Solubility increases Solubility decreases

What affects solubility?
Enthalpy change of solution = ΔHsol.
The solubility INCREASES when the ΔHsol becomes more EXOTHERMIC (more negative).

What affects the value of ΔHsol?

Screenshot 2019-04-28 at 10.12.40 PM.png

Screenshot 2019-04-28 at 10.13.52 PM.png

Using Hess’ Law, we can express ΔHsol as:
Enthalpy change of hydration of Cation + Enthalpy change of hydration of Anion – Lattice energy.

  • ΔHsol = ΔHhyd(M2+) + 2ΔHhyd(OH) – ΔHlatt(M(OH)2)
  • ΔHsol = ΔHhyd(M2+) + ΔHhyd(SO42-) – ΔHlatt(MSO4)

Thus, ΔHsol becomes more EXOTHERMIC as:

  • ΔHhyd of cation & anion decreases
  • Lattice energy increase

These cause solubility to INCREASE.

Now let’s compare the values of these enthalpies:

ΔHhyd of Metal Lattice Energies of Hydroxides Lattice Energies of Sulfates
Going down group:
more ENDOTHERMIC
Going down group:
decreases with a large difference
Going down group:
decreases with a small difference

The net effect is:

Compound Hydroxides Sulfates
Trend going down group ΔHsol becomes EXOTHERMIC ΔHsol becomes ENDOTHERMIC

 

 

Screenshot 2019-04-28 at 10.12.40 PM.png

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