WJEC CH2 Chemistry Topic 6.2: Trends in properties of the elements of the s-block and Group 7

All alkali metals (group 1) react with water with increasing violence down the group.

e.g. 2Na(s) + 2H2O(l)  ----> 2NaOH(aq) + H2(g)

Group 2 metals all react with water.

Magnesium will burn in steam.

Mg(s) + H2O(g)  ----> MgO(s) + H2(g)

The other members of the group will form the hydroxide.

Ca(s) + 2H2O(l)  ----> Ca(OH)2(aq) + H2(g)        Calcium hydroxide may be seen as a white solid

All s-block elements burn in air to form oxides.

e.g. 2Ca(s) + O2(g)  ----> 2CaO(s)

Elements such as potassium can form K2O2 and KO2, potassium peroxide and potassium superoxide.

If magnesium is burnt in air, a little magnesium nitride is formed.

All s-block elements react with dilute acids to give hydrogen. 

Remember the ionic equation:

Mg(s) + 2H+(aq)  ----> Mg2+(aq) + H2(g)

All oxides/hydroxides of Group 2 are bases and react with acids to form salt and water.

e.g. CaO(s) + 2HCl(aq) ---> CaCl2(aq) + H2O(l)

The solubility of the Group 2 oxides increases down the group. Barium hydroxide is sufficiently soluble for barium hydroxide to be used in volumetric analysis.

All oxides/hydroxides of Group 1 dissolve in water to form the corresponding alkali.

e.g. K2O(s) + H2O(l) ---> 2KOH(aq)

Need to know all oxides and hydroxides of Group 1 and 2 elements.

Group 1 oxides all have general formula X2O and hydroxides XOH

Group 2 oxides all have general formula XO and hydroxides X(OH)2

When many of the s-block elements are introduced to a hot flame they emit a colour as an emission spectrum which can be used to identify them.

Lithium - red 

Sodium - golden yellow

Potassium - lilac

Calcium- brick red

Strontium - crimson

Barium - apple green

Magnesium - no colour

Calcium and phosphorus are extremely important in skeletons.

Calcium is the most abundant mineral in the body and is found mainly in teeth and bones, but is also important in cell function.

Phosphorus is also necessary in bone formation. 

Ca:P in bone is about 2:1.

Deficiencies in calcium intake in children may lead to rickets.

Minerals in bone include calcium carbonate and calcium hydroxyapaptite.

Sedimentary rocks (eg limestone) are often formed by accumulation of skeletal remains and shells. 

These deposits have industrial importance.

Magnesium is part of the chlorophyll molecule.

Fluorine is a pale yellow gas at room temperature with MP -220 and BP -188

Chlorine is a green-yellow gas at room temperature with MP -101 and BP -35

Bromine is a red-brown liquid at room temperature with MP -8 and BP 59

Iodine is a grey-black solid at room temperature with MP 114 and BP 184. Its vapour is purple.

Volatility decreases down the group due to increasing number of electrons increases van der Waals' strength. These elements are oxidising agents usually gaining electrons to form halide ions.

A more reactive halogen will oxidise the halide ion of a less reactive halogen eg when chlorine gas/chlorine water is added to potassium bromide(aq), red brown colouration of bromine is observed. All oxidises potassium iodide(aq), brown colouration made. Often called displacement reactions.

This is a redox reaction. Halogen has gained electrons to become a halide ion. Aqueous halide ion has lost an electron.

Ion/electron half equations of chlorine with potassium bromide(aq) are:

Cl2(g) + 2e- ---> 2Cl-(aq)

2Br-(aq) ---> Br2(l) + 2e-

Overall:

Cl2(g) + 2Br-(aq) ---> Br2(l) + 2Cl-(aq)

For chlorine with potassium iodide(aq):

Cl2(g) + 2e- ---> 2Cl-(aq)

2I-(aq) ---> I2(s) + 2e-

Overall:

Cl2 + 2I-(aq) ---> 2Cl-(aq) + I2(s)

Aqueous chloride, bromide and iodide ions may be tested for and identified by the following procedures. 

Acidify solution with nitric acid.

Add aqueous silver nitrate.

Chloride ions produce a white curdy precipitate of silver chloride which darkens on standing. Silver chloride dissolves in dilute aqueous ammonia to form a colourless solution.

Bromide ions produce a cream precipitate of silver bromide which will dissolve in concentrated aqueous ammonia.

Iodide ions form a primrose yellow precipitate of silver iodide which is insoluble in aqueous ammonia.