Some important compounds of group 3A

Borax [Na2B4O710H2O]

 Purified borax is soft, colourless and crystalline. Borax is found in glass, in glazes used to decorate ceramics and in fertilizers. Borax occurs naturally as sodium tetraborate decahydrate but we can manufacture it using the following methods;

  1. Heating the mixture of colemanite and sodium carbonate. In this process, colemanite, Ca2B6O11 and sodium carbonate, Na2CO3 are heated as illustrated in the equation below:

Ca2B6O11(s) + 2Na2CO3(s)  2 CaCO3(aq) + Na2B4O7 + 2NaBO2(s)

Na2B4O7 is then cooled when the borax crystals have been precipitated. Carbon dioxide is an essential raw material in this process since it converts the Na2BO2 to form Na2B4O

4NaBO2(aq) + CO2(g) Na2B4O7+ Na2CO3

  1. Preparation of borax from orthoboric acid. We can obtain borax by reacting orthoboric acid with sodium carbonate

4H3BO3(aq) + Na2CO3(s)  Na2B4O7+ 10H2O(l)

 

 

Chemical properties of borax

Borax losses water of crystallization when heated. However, it forms a colourless borax bead when heated at temperatures above 750∘C.

 Na2B4O7 2NaBO2(s)+ B2O3

Borax reacts with dilute hydrochloric acid to form boric acid.

  5H2O(l)+2HCl(aq)+Na2B4O7(s) 4H3BO3(aq)+ 2NaCl(aq)

 

 

Chemical test for borax bead

The borax beads formed can chemically be tested by use of Ni2+, Co2+, Cr3+, Cu2+ and Mn2+ ions. Borax reacts with these ions to form coloured ions known as metaborates which can help chemists in identifying the positive ions present in the salts.

 

 

Uses of borax

  1. It is used in the  purification of gold

  2. Used as a flux in the welding of metals

  3. Used in the production of glasses in industries

 

 

Boron hydride

The diborane compounds are the binary compounds containing boron and hydrogen. They are divided into two distinct groups;

  1. B2H6 B5H9 B6H10 B10H14        (BnHn+4)

  2. B4H10 B5H11 B6H12 B9H15         (BnHn+6)

These diborane are important compounds in the synthesis of chemical reactions and also their confirmation helps us understand the basic concepts about the structure of electron-deficient substances.

 

 

Preparation of boron hydride

Boron hydride can be prepared by reacting boron halides such as boron chloride with excess hydrogen gas

2BCl2(aq) + 6H2(g)  B2H6(aq)+ 6HCl(aq)

Furthermore, diborane can be prepared by treating NaBH4 with concentrated acids such as sulphuric acid and phosphoric acid.

2NaBH4(aq) + H2SO4(l)  2H2(g)+ B2H6(aq)+Na2SO4(aq)

2NaBH4(aq) +2 H3PO4(l)  2H2(g)+ B2H6(aq)+2NaH2PO4(aq)

In the industry, diborane can be prepared by heating BF3 and NaH under temperatures of about 450K

BF3(aq) + 6NaH 6NaF + B2H6(aq)

 

 

Physical and Chemical properties of boron hydride

  1. Boron hydrides are colourless gases and boils at 183K

  2. Readily decomposed by water to form boric acid and hydrogen gas.

  3. Boron hydride mixes with oxygen in the air spontaneously releasing a lot of heat energy to the surrounding.

  4. Boron hydride undergoes additional reaction with hydrocarbons such as alkene and alkynes in organic solvents at RTP to produce organoboranes. The reaction is referred to as a hydroboration reaction.

  5. Boron hydrides undergoes the following cleavage reactions:

 B2H6(aq)+ 2Me3N 2Me3NBH3

B2H6(aq)+ 2Me3P 2Me3PBH3

Diborane reacts with carbon monoxide at a temperature of 473K and pressure of 20 atmospheres to form borane carbonyl as shown in the equation.

B2H6(g)+CO(g)2BH3CO

 

 

Aluminium

The major ore of aluminium is bauxite, Al2O3. 2H2O. It is manufactured by the process known as the Hall-Heroult process. Bauxite is first purified and then mixed with cryolite, Na3AlF6 and CaF2 and finally fused. The mixture undergoes electrolytic reduction where molten aluminium is produced at the positive terminal of the cell.

 

 

Physical properties of boron hydride

  1. Aluminium is a silvery metal which is a good conductor of both heat and electricity.

  2. It has a density of 2.7g/cm3 and a melting point of 933K.

  3. Aluminium metal is both malleable and ductile.

 

 

Chemical properties of boron hydride

  1. Aluminium reacts with moist air though it forms a protective layer of aluminium oxide, Al2O3.

Al(s) + O2(g)  Al2O3(s)

       At high temperature, aluminium reacts with both oxygen and nitrogen in the air to form aluminium nitride and aluminium oxide.

  1. Aluminium reacts with halogens to form anhydrous halides.

Al(s) + 3Cl2(g)   2AlCl3(s)

 

  1. Aluminium reacts with warm sodium hydroxide solution to form sodium aluminate and hydrogen gas

2Al(s)+ 2NaOH(aq) 2NaAlO2(aq)+ H2(g)

 

  1. Aluminium reacts with dilute acids such as sulphuric acid and hydrochloric acid to a salt and hydrogen gas. We should note that concentrated acids are not reacted with aluminium because it forms a protective layer on its surface when reacted with concentrated ac

  2Al(s)+ 6HCl(aq) 2AlCl3(aq)+ 3H2(g)

   2Al(s)+ 3H2SO4(aq) Al2(SO4)3(aq)+ 3H2(g)

 

  1. Aluminium reacts with warm water to form aluminium hydroxide and hydrogen gas is also liberated.

2Al(s)+ 2H2O(g) Al(OH)3(aq)+ 3H2(g)

            N/B. aluminium does not react with cold water.

 

  1. Aluminium is used as a reducing agent. It reduces oxides of metals that are below it in the reactivity series.

Fe2O3(s) + Al(s)  Al2O3(s) + Fe(s)

 

Uses of Aluminium

  1. Aluminium is used in making of cooking pans and other household utensils.

  2. Aluminium is used in the production of electric cables

  3. Because of its low density, aluminium is used to make aircraft parts, rail coaches and surgical equipment.

  4. Used to make plating tanks, pipes and other steel-related substances to prevent corrosion.

 

Some important compounds of Aluminium

  1. Aluminium Oxide

Aluminium oxide is also known as alumina and occurs naturally as bauxite and corundum. Aluminium oxide is mainly found in the of gems.

 

Preparation

Aluminium oxide can be prepared by igniting ammonium alum, aluminium sulphate and aluminium hydroxide.

Al2(SO4)3Al2O3(s) + 3SO3(g)

Al2(OH)3Al2O3(s) + 3HO2(g)

 

 

Properties of Aluminium oxide

  1. Aluminium oxide is amphoteric in nature since it can with both acids and alkalies. It is a white amorphous powder that is insoluble in water but soluble in acids.

 

Uses of aluminium oxide

  1. Aluminium oxide is used for the preparation of important compounds of aluminium.

  2. Can be used in the extraction of aluminium metal

  3. Aluminium oxide is used to speed up the rate of organic reactions. (used as a catalyst)

  4. Used for making artificial gems such as Emerald-green.

 



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