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.