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1.
Diboron trioxide and boric acid
The crystalline B 2 O 3 is colorless and one of the most difficult to crystallize substances, and it is white in powder form
.
The element B is burned in the air to obtain B 2 O 3 , and the boric acid can be dehydrated under heating to obtain B 2 O 3 , but the products are all glassy
.
Only when boric acid is dehydrated very slowly at a lower temperature can B 2 O 3 crystals be obtained
B 2 O 3 is the anhydride of boric acid, which is easily soluble in water to generate boric acid
.
B 2 O 3 +3H 2 O=2H 3 BO 3
Therefore, powdered B 2 O 3 can be used as a water absorbing agent
.
B 2 O 3 is an acidic oxide.
It reacts with metal oxides at high temperatures to obtain metaborate with characteristic colors, which is called the boron bead experiment
.
E.
In metaborate, Cu(BO 2 ) 2 is blue, Fe(BO 2 ) 3 is yellow, Cr(BO 2 ) 3 is green, Mn(BO 2 ) 2 is purple, and Ni(BO 2 ) 2 is Green
.
Na 2 B 4 O 7 +H 2 SO 4 +5H 2 O=4H 3 BO 3 +Na 2 SO 4
The direct reaction of natural boronite and sulfuric acid can also produce boric acid
.
Mg 2 B 2 O 5 ·H 2 O+2H 2 SO 4 =2H 3 BO 3 +2MgSO 4
H 3 BO 3 crystals are colorless and have a lamellar structure
.
The central B atom adopts sp 2 hybridization, and each B(OH) 3 molecule forms a lamellar structure through intermolecular hydrogen bonds, as shown in Figure 13-12
H 3 BO 3 is a weak monobasic acid
.
Its acidic mechanism is due to the lack of center B
Electronic
.
The empty p orbital where the central B atom does not participate in the hybridization can accept the H 2 O solution
The electron pair of oxygen in the separated OH - forms [B(OH) 4 ] - , thereby destroying the water’s
The dissociation equilibrium makes the concentration of H + in the solution greater than the concentration of OH -and becomes acidic
.
Figure 13-12 The lamellar structure of boric acid
B(OH) 3 +H 2 O=[B(OH) 4 ] - +H+ Ka Θ =5.
4×10 -10
[B (OH) .
4 ] - in the form of a regular tetrahedron structure, as shown in Figure 13-13 of (A)
H 3 BO 3 is dehydrated at 100°C to generate metaborate HBO 2
.
Adding polyols (such as glycerin) to boric acid can increase the acidity of boric acid
.
This is mainly because boric acid combines with polyols such as glycerin to form a more stable boric acid ester [Figure 13-13(b)].
Boric acid is easier to dissociate from H + , thus increasing the acidity of boric acid
.
FIG.
13-13 of [B (OH) .
4 ] - and borate of formula
After the boric acid solution is mixed with ethanol, sulfuric acid is added to produce volatile ethyl borate (C 2 H 5 O) 3 B.
After being ignited, a green flame can be observed.
This property can be used to identify boric acid
.
B(OH) 3 +3C 2 H 5 OH=B(C 2 H 5 O) 3 +3H 2 O
2.
Borax
Borax is white, glassy crystals
.
The chemical composition is Na 2 [B 4 O 5 (OH) 4 ]·8H 2 O, and its anionic structure is shown in Figure 13-14(a), consisting of 2 BO 3 triangles and 2 BO 4 tetrahedrons alternately connected.
Into
.
Borax is easy to weather and loses crystal water.
When heated, it will eventually lose 8 crystal water and hydroxyl group to remove 2 water.
Therefore, the chemical composition of borax can also be written as Na 2 B 4 O 7 ·10H 2 O, and its anion structure is shown in Figure 13- Shown in 14(b)
.
[B .
4 O .
7 ] by the two 2- [BO 2 ] - and a B 2 O .
3 composition, borax and thus can be used in place of laboratory B 2 O .
3 boron bead experiments
.
Na 2 B 4 O 7 ·10H 2 O+CuO=Cu(BO 2 ) 2 +2NaBO 2 +10H 2 O
Borax is a binary base, which can be used as a benchmark for acid-base titration
.
The hydrolysis of borax produces a weak acid H 3 BO 3 and its salt [B(OH) 4 ] - , forming a buffer system with pH=9.
24.
Therefore, borax solution can be used as a buffer solution
.
Figure 13-14 [B 4 O 5 (OH) 4 ] 2- and [B 4 O 7 ] 2- structure
Industrially, natural boron magnesia ore is used as raw material to produce borax
.
NaBO 2 is called sodium metaborate, which can react with H 2 O 2 to produce sodium perborate.
The essence of this reaction is the transfer reaction of the peroxy chain, that is, one O atom of sodium metaborate is replaced by the peroxy chain -OO-
.
Borax is an important chemical raw material, widely used in the preparation of enamel, hard glass ( borosilicate ), foam insulation materials, washing products, cosmetics and flame retardants
.
Related Links: Boron Hydride