Silicon Oxide : Silica (SiO2):

Silicon is unable to form pp - pp bond with oxygen atom due to its relatively large size. Thus it satisfies its all four valency with four oxygen atoms and constitutes three - dimensional network. In this structure each oxygen atom is shared by two silicon atoms. Three crystalline modification of SiO₂ are quartz, cristobalite and tridymite of which quartz and cristobalite are important.

Quartz (rock crystal) is the purest form of silica. It is used in preparation of costly glasses and lenses. It is also used as piezoelectric material (crystal oscillators and transducers).

Several amorphous forms of silica such as silica gel and fumed silica are known. Silica gel in made by acidification of sodium silicate and when dehydrates, is extensively used as a drying agent in chromatographic and catalyst support.

STRUCTURE OF SILICA:

PREPARATION OF SILICA:

Artificially silica can be obtained by following methods

PROPERTIES OF SILICA:

Pure silica is colourless but sand is brownish or yellowish due to presence of impurities of iron oxide.

Question: Why SiO₂ is solid while CO₂ is a gas?

Answer: This is because SiO₂ exists in silicate form where Si forms tetrahedron SiO₄^2- ions which are regularly attached with each other and form a giant network structure hence remains solid.

What is Steric Inhibition in Resonance of SIR effect ?

SIR stand for Steric Inhibition in Resonance, means as per name steric means size, inhibition means some kind of hindrance, little hurdle, and resonance means delocalisation of conjugate points.

We know that planarity is the main condition for resonance that means resonance can occur only when all the atom involved in resonance lie in the same plane or nearly in the same plane. Any change in structure which destroys planarity of molecule will restrict or inhibit resonance, this phenomenon is known as steric inhibition of resonance.

SIR effect finds immense use in explaining Stability of carbocation, acidity and basicity of organic compounds:

For example(1): Ortho substituted Benzoic acid is more acidic then meta or para substituted benzoic acid irrespective of nature of group (electron donating group or electron withdrawing group) 

When a group present in the ortho position with respect to carboxylic group creates steric strain resulting in rotation of the carboxylic group and shifting it out of plane of the benzene ring as a result the carboxylic group can no longer participate in ring resonance and thereby the acidity increases as delocalization of negative charge equally on the both oxygen atom of  conjugate base ( carboxylate ion) of the benzoic acid , hence carboxylate ion more stablised. This is  also called Ortho effect.

Note:  However for groups like -NH₂ or -OH does not experience SIR effect due to small size and –CN is linear group so SIR effect not applicable.

For example (2):  N,N,2,6-Tetramethyl aniline is more basic than  N,N-Dimethyl aniline. In cas of N,N-Dimethyl aniline its dialkyl derivatives the NMe₂ group is coplanar with benzene ring.  So p-electron (lone pair) on N atom and pi orbitals are remain in the same plane. For this the p-electron on N-atom can delocalized via pi orbital in the benzene ring and its result the electron availability in para position is high and less available at nitrogen atom and hence less basic.

On the other hand , in case of N,N,2,6 tetramethyl aniline having two bulky methyl group in ortho position of the benzene ring , the NMe₂ group can not remain in the same plane. That is why the p-electron on N-atom cannot delocalized through pi orbital in para position. (due to steric inhibition of resonance)  It is localized at nitrogen atom Thus 2,6-Dimethyl aniline  derivative is more basic.

Titration of Borax: Tincal : Suhaga :

When Borax dissolved in water, it is subject to completely dissociate, and then, hydrolyzes to orthoboric acid (B(OH)₃  and [B(OH)₄]^- anions,  and resulting PH  of  solution about 9.13.

According to the equation:

Na₂[B₄O₅(OH)₄]⋅8H₂O(s)⟶2Na⁺(aq) + B₄O₅(OH)₄²⁻ (aq) + 8H₂O(l)

B₄O₅(OH)₄²⁻(aq) + 5H₂O(l) ⟶ 4 B(OH)₃(aq) + 2OH⁻(aq)

The liberated B(OH)₃(aq)  acts as a Lewis acid on OH⁻ ions from auto ionization of water, and hence, in equilibrium with water:

B(OH)₃(aq)+2H₂O(l) ⟶ B(OH)₄⁻(aq) + H₃O⁺(aq)

Therefore, one mole of borax reacts with two moles of strong acid solutions. This is  because when Borax dissolved in water Both B(OH)₃(aq)  and B(OH)₄⁻(aq) are formed but only  B(OH)₄⁻(aq) react with acid.  

For example:

                Na₂[B₄O₅(OH)₄].8H₂O(aq) + 2HCl(aq) ⟶ 4B(OH)₃(aq)+ 2NaCl (aq) + 5 H₂O

ILLUSTRATIVE EXAMPLE: Aqueous solution of borax reacts with two moles of acids.

This is because of:

(A) Formation of 2mol of B(OH)₃ only

(B) Formation of 2mol of Na[B(OH)₄]¯ only

(C) Formation of 1mol each of B(OH)₃ and Na[B(OH)₄]¯

(D) Formation of 2mol each of Na[B(OH)4]¯and B(OH)₃ of which only Na[B(OH)₄]¯ reacts with acid.

Answer: (D)

Solution: 

 Na₂B₄O₇ + 7H₂O ⟶ 2B(OH)₃ + 2Na[B(OH)₄]

B(OH)₃ or H₃BO₃ is an acid and does not react with acid. Hence Na[B(OH)₄] reacts with acid.

What is structure of Borax: Tincal or Suhaga ?

The name borax is derived from the Persian word borak, meaning white. IUPAC name of borax is sodium tetraborate decahydrate (Na₂B₄O₇⋅10 H₂O ). Also note that, after analyzing its crystal structure, borax is more properly formulated as Na₂[B₄O₅(OH)₄]⋅8H₂O

Structure:

(1) It is also named as sodiumtetra borate decahydrate

(2) Common Indian name is Suhaga.

(3) Na₂B₄O₇. 5H₂O is known as Jeweller's Boran

(4) Na₂B₄O₇ is known as Boron glass

(5) It is also represent as Na₂O.2B₂O₃.10H₂O.

IIT UPDATE: (2016):

Titration of Boric Acid:

Boric acid is a weak monoprotic acid (Ka=6.4×10⁻¹⁰). Therefore, it cannot be directly titrated with standard strong alkali (NaOH). But by the addition of an organic poly hydroxy compound such as 1,2- or 1,3-diol, (Like Sugar, Mannitol, Catechol)  it is converted to a much stronger acid, which can be titrated using phenolphthalein. This is due to the complex formation between hydrated borate ion and either 1,2- or 1,3-diol  for example .

Ploy hydroxy (Cis-diol) increases acidity of boric acid due to formation of complex with metaborate and reaction goes to farward to produce more H⁺.

IIT UPDATE: (2006):