10.1 similarities and trend in the properties of the group 2 metals (Mg-Ba) & their compounds

Explain how Mg reacts with oxygen

• burns easily with bright white flame

• 2Mg (s) + O₂ (g) → 2MgO (s)

Explain how Ca reacts with oxygen

• Difficult to ignite Red flame

• 2Ca (s) + O₂ (g) → 2CaO (s)

Explain how Sr reacts with oxygen

• difficult to ignite: red flame

• 2Sr (s) + O₂ (g) → 2SrO (s) Sr (s) + O₂ (g) → SrO₂ (s)

Explain how Ba reacts with oxygen

2Ba (s) + O₂ (g) → 2BaO (s) Ba (s) + O₂ (g) → BaO₂ (s)

Difficult to ignite

Green flame

Explain how Mg reacts with water

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

• *Mg reacts with steam: Mg (s) + 2H₂O (g) → MgO (s) + H₂ (g)

• Vigorous reaction with steam

• Very slow reaction

Explain how Ca reacts with water

• Ca (s) + 2H₂O (l) → Ca(OH)₂ (s) + H₂ (g)

• Reacts moderately

• Hydroxide formed

Explain how Sr reacts with water

• Sr (s) + 2H₂O (l) → Sr(OH)₂ (aq) + H₂ (g)

• reacts rapidly

• Hydroxide formed

Explain how Ba reacts with water

• reacts vigorously

• Hydroxide formed

• Ba (s) + 2H₂O (l) → Ba(OH)₂ (aq) + H₂(g)

Explain how Mg reacts with dilute HCl

• reacts vigorously

• Mg (s) + 2HCl (aq) → MgCl₂ (aq) + H₂ (g)

Explain how Ca reacts with dilute HCl

Reacts vigorously

Ca (s) + 2HCl (aq) → CaCl₂ (aq) + H₂ (g)

Explain how Sr reacts with dilute HCl

Reacts vigorously

Sr (s) + 2HCl (aq) → SrCl₂ (aq) + H₂ (g)

Explain how Ba reacts with dilute HCl

Reacts vigorously

Ba (s) + 2HCl (aq) → BaCl₂ (aq) + H₂ (g)

Explain how Mg reacts with dilute H2SO4

Reacts vigorously

Mg (s) + H₂SO₄ (aq) → MgSO₄ (aq) + H₂ (g)

Explain how Ca reacts with dilute H2SO4

• Reaction is slowed by the formation of a sparingly soluble sulfate layer which coats the metal surface stopping hydrogen bubbles from rising

Ca (s) + H₂SO₄ (aq) → CaSO₄ (s) + H₂ (g)

Explain how Sr reacts with dilute H2SO4

Reaction is quickly stopped by the formation of an insoluble sulfate layer on the metal surface

Sr (s) + H₂SO₄ (aq) → SrSO₄ (s) + H₂ (g)

Explain how Ba reacts with dilute H2SO4

Reaction is quickly stopped by the formation of an insoluble sulfate layer on the metal surface

Ba (s) + H₂SO₄ (aq) → BaSO₄ (s) + H₂ (g)

Explain how MgO reacts with water

MgO (s) + H₂O (l) → Mg(OH)₂(s) 

MgO is only slightly soluble in water, therefore a weakly alkaline solution (pH 10.0) is formed

Explain how CaO reacts with water

CaO (s) + H₂O (l) → Ca(OH)₂(s) 

A vigorous reaction which releases a lot of energy, causing some of the water to boil off as the solid lump seems to expand and open (pH 11.0)

Explain how SrO reacts with water

SrO (s) + H₂O (l) → Sr(OH)₂(aq) 

Explain how BaO reacts with water

BaO (s) + H₂O (l) → Ba(OH)₂(aq)

All group 2 carbonates except for… are insoluble in water

BeCO3

All group 2 carbonates will form … when reacted with dilute HCl

Soluble chloride salts

Water

CO2

When the carbonate of Ca, Sr and Ba are reacted with sulfuric acid

• an insoluble sulfate layer forms

• This stops any further reaction after initial effervescence of CO2

Explain the thermal decomposition of carbonates

• The Group 2 carbonates decompose when they are heated to form the metal oxide and give off carbon dioxide gas

• Going down the group, more heat is needed to break down the carbonates

Explain the thermal decomposition of nitrates

• Group 2 nitrates also undergo thermal decomposition 

• Group 2 nitrates decompose to form the metal oxide, nitrogen dioxide gas and oxygen gas

• Nitrogen dioxide gas is observed as brown fumes and is toxic

Explain the trend in thermal stabilities

• Going down Group 2, more heat is needed to break down the carbonate and nitrate ions

• The thermal stability of the Group 2 carbonates and nitrates therefore increases down the group 

  • The smaller positive ions at the top of the groups will polarise the anions more than the larger ions at the bottom of the group

    • The small positive ion attracts the delocalised electrons in the carbonate ion towards itself

    • The higher the charge and the smaller the ion the higher the polarising power

  • The more polarised they are, the more likely they are to thermally decompose as the bonds in the carbonate and nitrate ions become weaker

Explain the trend in solubility of group 2 hydroxides

• Down the group, lattice enthalpy of group 2 hydroxides decreases. (i.e. it becomes less negative/less exothermic)

• enthalpy of hydration of the metal ion becomes less negative down the group (both change due to the decrease in charge density down the group).

• This causes enthalpy of sol to become more exothermic down the group, as lattice enthalpy decreases more quickly than enthalpy of hydration of the metal ion.

• So the more exothermic the enthalpy of sol, the more soluble the metal hydroxide

 

Explain the trend in solubility of group 2 sulfates

• Down the group, lattice enthalpy of group 2 sulfates decreases. ( becomes less negative/less exothermic)

• enthalpy of hydration of the metal ion becomes less negative down the group (both change due to the decrease in charge density down the group).

• This causes enthalpy of solution to become more endothermic/less exothermic down the group, as enthalpy of hydration of the metal ion decreases more quickly than lattice enthalpy.

• So the more endothermic the enthalpy of solution, the less soluble the metal sulfate

Magnesium sulfate - barely soluble (low pH, around 10)

Barium hydroxide - very soluble (high pH, around 12-13)