12th Class Chemistry Chapter 3 Group III-A and Group IV-A Elements Short Question Answers

The elements of group III-A have three electrons in the outermost principal quantum number.One of the electrons is promoted from s-orbital to one of the p-orbitals and three unpaired unpaired electrons make the valency three.

It is due to the reason that the atoms of elements with greater atomic number are less tightly packed due to their bigger sizes.

The size of B⁺³ ion is very small.It has high charged density and immediately forms the covalent bonding due to greater polarizing power.AI⁺³ ion gives the ionic bonding and its compounds are dissociated in water to give AI⁺³ ions.

B₂O₃ is acidic in nature.AI₂O₃  and Ga₂O₃ are amphoteric.In₂O₃ and TI₂O₃ are basic in character.

Boron gives polymeric hydrides having bridge structures.B₂H₆ and B₄H₁₀ have bridge structures and they are electron deficient molecules.The hydrides of other family members are not bridged molecules.

Boron gives four importand acids.These acids are  as follows:
i) Orthoboric Acid,H₃Bo₃ or B₂O₃,3H₂O also called boric acid.
ii)Metaboric Acid,HBO₂ or B₂O₃,H₂O.
iii)Tetraboric Acid,H₂B₄O₇ or 2B₂O₃,H₂O.
iv) Pyroboric Acid, H₆B₄O₉ or B₂O₃,3H₂O.
Orthoboric acid is the most important  and is also called boric acid.It is a stable compound while the other acids are stale in solid state and change to orthoboric acid in solution.

When orthoboric  acid H₃BO₃ is neutralized by caustic soda we get borax.
4H₃BO₃ + 2NaOH………………. Na₂B₄O₇+7H₂O
When borax is treated with concentrated H₂SO₄ then boric acid is produced.On cooling the crystals of boric  acid are separated out.
Na₂B₄O₇+H₂SO₄+5H₂O…………….. Na₂SO₄+4H₃BO₃.

Borax is sparingly soluble in cold water.3 grams is dissolved in 100 grams of water at 10ᵒC.At 100ᵒC,99.3 grams of H3BO3 becomes soluble in 100 grams of water.

Below 62ᵒC decahydrated crystals are formed.Above 62ᵒC octahedral crystals of pentahydrated borax are obtained.

Na₂B₄O₇ is hydrolyzed by water to give H₃BO₃ and NaOH.Since the base is strong so the solution turns red litmus blue.

When borax is treated with HCI or H₂SO₄  in the presence of water then orthoboric acid is produced.
Na₂B₄O₇+2HCI+5H₂O………………… 2NaCI+4H₃BO₃
Na₂B₄O₇+H₂SO₄+5H₂O………………….Na₂SO₄+4H₃BO₃

Borax is used: i) To prepare borate glass which is heat resistant. ii) In the softening of water. iii) In borax bead test for the detection of metallic cations. iv) In metallurgical operations. v) As a flux in welding. vi) In making washing powders. vii) In leather industry for tanning and dyeing. Viii) In cosmetics soaps textiles paints medicines match industry and as a preservative.

When borax is heated it swell up.When it is heated further it melts into a clear transparent glass.
2Na₂B₄O₇.10H₂O………………. Na₂B₄O₇.2NaBO₂+B₂O₃.

There are seven basic radicals in salt analysis scheme which  give colored beads when  borax bead test is performed.These radicals are CU⁺²,Fe⁺²,Fe⁺³,Mn⁺²,Co⁺²,Ni⁺²,Cr⁺³.They have different colors  in the cold and hot states in the oxidizing and reducing flames.

Those metals which give the borax bead test are converted into their  metaborates.These metaborates appear in the form of beads.They show different colors in the cold and hot state  and in oxidizing  and reducing flame.Fe⁺², Fe⁺³,Cr⁺³,Cu⁺², Co⁺²,Ni⁺² and Mn⁺² give borax lead tests.

Orthoboric acid H₃BO₃
Metaboric acid HBO₂.
Tetraboric acid H₂B₄O₇.
Pyroboric acid H₆B₄O₉.

Colemanite is suspended in boling water and SO₂ gas is passed through it.H₃Bo₃ crystallizes out from the solution.
Ca₂B₆O₁₁ + 2SO₂+9H₂O…………………. 2CaSO₃ + 6H₃BO₃

Borax is treated with calculated quantity of conc.H₂SO₄.When the solution is cooled H₃BO₃ settles down in the form of precipitate.
Na₂B₄O₇+H₂SO₄+5H₂O…………………. NaSO₄ + 4H₃BO₃

H₃BO₃ reacts with water  and releases H⁺ ions from water.It accepts OH^– ion and acts  as a monobasic acid.

When H₃BO₃ is heated it releases water molecules in three stages and B₂O₃ is produced in the long run.
H₃BO₃……….100ᵒC……..HBO₂+H₂O        4HBO₂…….140ᵒC……H₂B₄O₇+H₂O
H₂B₄O₇……Red/heat…….2B₂O₃+H₂O.

3C₂H₅OH + H₃BO₃…..H₂SO₄….( C₂H₅)₃BO₃+3H₂O
4H₃BO₃.+Na₂CO₃……………… Na₂B₄O_&+6H₂O+CO

Orthoboric acid is used: i) In medicines as antiseptic as dusting powder in boric ointment and as eye-wash. ii) In pottery as a glaze.The reason is that borate glazes are more fusible than silicate glazes.They have high co-efficient of expansion. iii)In candle industry for stiffening of wicks.

Some important minerals of aluminium are as follows:
Feldspar                             KALSi₃O₈
Mica(muscovite)              KH₂AI₃(SiO₄)₃
Kaolin(china clay)             H₂AI₂(SiO₄)₂.H₂O
Bauxite                               AI₂O₃.2H₂O
Cryolite                               Na₃AIF₆

The layer of AI₂O₃ on the surface of aluminium is very useful.Its thickness is increased purposely in the industry by electrolytic process  which is called anodizing.

Aluminium reacts with oxygen vigorously and this reaction is highly exothemic.This heat can be used for the melting of those metals whose oxides are reduced by aluminium powder.This strong affinity of AI for O2 does not allow AI to be free in nature.

Aluminium reacts with dilute and conc. H₂SO₄ as follows:
2AI+3 H₂SO₄………..dil…………AI₂(SO₄)₃+3H₂
2AI+6 H₂SO₄………..conc…………. AI₂(SO₄)₃+3SO₂+6H₂O

Conc.HNO₃ has no action on aluminium.A protective layer of AI₂O₃ is developed which does not allow  aluminium to react with HNO₃ further.

Aluminium dissolves in alkalies with the evolution of hydrogen gas  and soluble  aluminates are produced.The reaction is as follows:
2AI + 2NaOH………..2Na₂AIO₂+3H₂

Sn and Pb can hardly promote their electrons from s-orbitals to vacant p-orbtials.So they show the valency of two rather than four.Well C,Si Ge do not show the inert pair effect.

i) It is the hardest substance in its family. ii) It shows allotropy. iii) It shows catenation. iv) It is fundamental element in organic chemistry.

The property of allotropy is associated with all the elements of group IV-A except Pb.Carbon shows two.SI two Ge two and Sn has three allotropic forms.Carbon shows as diamond and graphite.This is due to parallel layers in graphite ad loosely held electrons that it is a good conductor of electricity.

Important ores of silicon are as follows:
Analcite (a zeolite)                    NaAI(Sio₃)₂.H₂O
Asbestos                                     Ca Mg₃(SiO₃)₄
Kaolin(china clay)                      H₂AI₂(SiO₄)₂.H₂O
Zircon                                          ZrSiO₄
Talc(soap stone)                       H₂Mg₃ (SiO₃)₄

i) Rock crystal ii) Amethyst quartz iii) Smoky quartz iv) Rose quartz V) Milky quartz.

Quartz is one of the common crystalline form of SiO2.It is hard brittle refactor and a colorless soild.

Graphite has layered structure.Free electrons are present in them which are responsible for conduction of current.In diamond there are no free electrons.All the bonds in diamond C-Co bonds.

Co₂ is a gas at room temperature while Sio₂ is a solid.The reason is that CO₂exists in the form of discrete molecules which exist independently.In case  of SiO₂ there is a network structure.It is three dimensional structure giving a giant molecule.

It is also called fused quartz.It is obtained by heating SiO₂.It has  random structure.When it is cooled it is not  crystallized readily.

When Na₂CO₃ is reacted with pure sand and heated in furnace Na₂SiO₃ is obtained.It is called water  glass or soluble glass.

It is water soluble and its aqueous solution is strongly alkaline.It is used in the preparation of chemical gardens and is used in soap manufacture.

It is used as a filler in the manufacture of soap as a fire proof in textile in fumiture polish in calicoprinting and in preservation of eggs.

When the crystals of various soluble colored salts like NiCI₂ FeSO₄ and CuSO₄ etc. Are placed in a solution of water glass there happens a beautiful growth of crystals.This is called silica garden or chemical garden.

It is used to make porcelin and china clay.Impure clay is used to make make bricks tiles and stone wares.

Clay is soaked in water and it is hydrated with the passage of time.It becomes more and more plastic in nature.On heating water of hydration is lost and hard rock like mass is obtained.

Mg₃H₂ (SiO₃) is the formula of soap stone.It is greasy to touch and is used in making cosmetics and household articles.

Asbestos is mixed silicate of Ca and Mg with formula CaMg₃(SiO₃)₄.
It is used to  make incombustible fabrics and hard board.

There happens a very small change in viscosity with the change in temperature for silicones.When the temperature drops from 100ᵒC to ᵒC then the viscosities of petrolem oils which are used as lubricants increase their viscosity 100 times.Anyhow viscosities of silicon oils increase less than four times for this change of temperature.Hence silicones are preferred over ordinary organic lubricants.

Silicon germanium and selenium are good semiconductors.The important semiconductors among the compounds are PbS,SiC,CdS,etc.

When a semiconductor is heated its resistance decreases and conductivity increases.Similarly when a semiconductor absorbs light the electrons become more mobilized and conductivity increases.

A semiconductor is a substance which has different resistances for the passage of electrical current under different circumstances.

Litharge varies in colors from pale yellow to reddish yellow.Actually it has two forms: i) Rhombic (yellow) ii) Tetragonal(red).