Trends in chemical properties of group 2
Reactivity
Group 2 are less reactive than group 1.
Na vigorously reacts with water.
But Ca reacts very slowly.
The difference is due to the amount of energy required to form the metal ion.
Energy needed
Li 519 kj/mol
Na 494
K 418
Rb 402
Cs 376
Energy needed
Be 2660 kj/mol
Mg 2189
Ca 1735
Sr 1614
Ba 1468
• The combined ionisation energies decrease down the group.
• This means that reactivity increases as it becomes progressively easier to form the M2+ ion.
Energy needed
Be 2660 kj/mol
Mg 2189
Ca 1735
Sr 1614
Ba 1468
Ionisation energies of Group II
0
500
1000
1500
2000
2500
3000
3500
4000
4500
5000
Mg Ca Sr Ba
element
ion
isat
ion
en
erg
y (K
j/mo
l)
Combined ionisationenergies
2nd ionisation energy
1st ionisation energy
Reactions of magnesium with water.
• Magnesium reacts slowly with cold water;
• Mg + 2H2O → Mg(OH)2 + H2
• Magnesium reacts more readily with steam to form its oxide;
• Mg + H2O(g) → MgO + H2
Reactions of magnesium hydroxide
Mg(OH)2 neutralises acids.
Mg(OH)2 + 2HCl→ MgCl2 + 2H2O
Treating indigestion.
HCl is secreted in the stomach to aid
digestion.
Excess acid causes
indigestion.
“Milk of Magnesia” contains
magnesium hydroxide so neutralises
excess acid.
Reactions of Ca, Sr and Ba with water
• Calcium reacts rapidly with cold water to give hydrogen and a white percipitate of calcium hydroxide.
• Ca + 2H2O → Ca(OH)2 + H2
• Strontium and barium react in the same way, but more vigorously.
Reactions of calcium hydroxide
Ca(OH)2 neutralises acids.
Ca(OH)2 + 2HCl→ CaCl2 + 2H2O
It is commonly known as “slaked lime” and is used by farmers to lower soil acidity. It is also used in some anti acids.
Relative solubilities of
the hydroxides
• The solubilities of group 2 hydroxides increase from Mg to Ba.
• Magnesium hydroxide is almost insoluble whereas barium hydroxide readily dissolves.
Compound Solubility
mol/100g at 298K
Mg(OH)2 2 x 10-5
Ca(OH)2 1.5 x 10-3
Sr(OH)2 3.4 x 10-3
Ba(OH)2 1.5 x 10-2
Relative solubilities of the sulphates
Compound Solubility
mol/100g at 298K
MgSO4 1.8 x 10-1
CaSO4 4.7 x 10-3
SrSO4 7.1 x 10-5
BaSO4 9.4 x 10-7
The solubilities of group 2 sulphates decrease from Mg to Ba.
Testing for the presence of sulphate ions.
• Barium chloride is used as a reagent to test for the presence of sulphate ions.
• First dilute hydrochloric acid is added to remove any carbonate ions present.
• CO32- + 2H+ → CO2 + H2O
• Otherwise white barium carbonate might be precipitated.
• BaCl2 + SO42- → BaSO4 (s) + 2Cl-
• Barium sulphate forms a white ppt.
Barium meals• Soft tissue normally
lets through X rays.• But barium is opaque,
its heavy atom absorbs X rays.
• A “barium meal”, barium sulphate, can be administered to examine the gut.
• Although barium salts are highly toxic the sulphate is safe to use as it is insoluble.