Universiti Tunku Abdul Rahman ( Kampar Campus )
Bachelor of Science (Hons) Biotechnology
Year 1 Semester 1
Group 1
Laboratory 1A ( UESB 1112 )
(II) Atomic Structures and Periodicity
Lecturer: Ms. Chong Foon Yee
Group Member:
1. Ang Pei Wen ( 08AEB02216 )
2. Chan Pei Sin ( 08AEB03544 )
3. Cheah Hong Leong ( 08AEB03788 )
Experiment No. 4
Title: Preparation and Reaction of Boric Acid, H3BO3
Date: 25 June 2008
Title: Preparation and Reaction of Boric Acid, H3BO3
Objectives:
- Synthesis boric acid B(OH)3 from sodium tetraborate as starting material.
- Used crystallization technique to obtain the final product.
- Understand the physical properties of the crystal structure of boric acid.
Theory and Background:
Boron is a Period 2, Group 3 element. Therefore, it is the first element of the p-block
elements in periodic table. Boron has the atomic number of 5, meaning that each boron
atoms consist of 5 protons in the nucleus that is surrounded by 5 electrons attracted to
the nucleus.
The electronic configuration of Boron is 1s²2s²2p¹. Boron consists of two isotopes: B10 (19.6%) and B-11 (80.4%), making its average atomic mass in the periodic table
becomes 10.81. Having only 3 valence electron, boron have a lower electronegativity
compared to carbon, its neighbour in the same period. The electron deficiency in boron
atom also cause it to has a high tendency to form polyhedral molecules with other atoms
of other elements, for example: oxygen.
Boron is the first element in its group and it has properties different from other
elements in that group. Instead it has properties similar to the second element in the
next group (silicon) --------- Diagonal relationship. Similar to silicon, as pure element,
boron is a metalloid. Besides, boron also behaves semiconductivity and form a network
of giant covalent metalloid with strong covalent bonds between atoms. Similar to silicon,
boron mostly found in nature as oxygenated mineral, which is commonly known as
boron ore. Different from other elements in its group, boron also have a much higher
ionization energy than other elements in its group and much less reactive at room
temperature. Besides, boron also has very high melting point (2573K) and boiling point
(4203K). due to its high tendency to gain electron-rich atoms, especially oxygen, pure
boron is not exist in the nature ( just like silicon ).
Boron has electronic configuration of 1s² 2s²2p¹ and thus will undergoes sp²
hybridization when bonded to oxygen atoms to form a trigonal planar molecule or ion,
boron oxide, BO3. However, the boron atom in the trigonal planar BO3 is still not
achieve the octet electron configuration which is stable. Therefore, the boron atom can
still accept a pair of electrons from a oxygen atom, forming a dative bond with it. The
result of the dative bond is tetrahedral BO4 molecule.
ˉO
Oˉ
Oˉ
B
B
Oˉ
Trigonal planar BO3
ˉO
Oˉ
Oˉ
Tetrahedral BO4
Although there several kinds of boron ore in nature, for example: ulexite, colemanite,
and kernite, the most common boron ore is borax. Borax which is always found in the
dessert, is actually sodium tetraborate, Na2B4O7. 10H2O. Borax is often used as washing
agent, borax has been used as a house hold cleaning agent for decades. Borax consists of
mainly borate ion, B4O5(OH)4²-. Reaction of borax with acids, for examples:
hydrochloric acid and sulphuric acid, will results in boric acid.
Experiment Procedures:
Apparatus and Materials Sodium tetraborate
 Concentrated hydrochloric acid
 Concentrated sulphuric acid
 Methyl red indicator
 Mannitol
 Beaker
 Methanol
 Distilled water
 Electric heater
 Ice
 Test tube
 Test tube rack
 Glass rod
 Suction funnel
 Suction pump
 Filter flask
 Filter paper
 Spatula
 Watch glass
 Splint
ProceduresPart 1: Preparation of boric acid in crystalline form.
10g of Sodium tetraborate (borax) was dissolved in 40cm3 of water.
Solution produced was heated.
After the heating process was completed, 5cm3 of concentrated hydrochloric acid
HCl was added to the solution.
4. The solution in the beaker was left until the temperature of the solution was falling
down to room temperature.
5. The beaker was immersed in cold water (with ice) to cool down the solution so that
boric acid can be crystallized out.
6. The crystal was filtered off by using suction pump, suction funnel, and filter papers
(2 pieces which the weight was already measured)
7. The beaker and glass rod used in filtration were washed with ice cold distilled water
into the suction funnel.
8. The product yield from the filtration was taken out from suction funnel together
with the filter papers.
9. The product together with the filter papers was placed on a watch glass (which the
weight was already measured).
10. The watch glass with the products on it was weight with analytical balance.
11. The weight value was recorded.
1.
2.
3.
Part 2: Chemical Test on The Prepared Boric Acid
1.
2.
3.
4.
5.
6.
7.
8.
Little Boric acid, H3BO3 was dissolved in test-tube 1.
A few drops of methyl red were added into the solution.
The colour changes were recorded.
Little mannitol was dissolved in test-tube 2.
A few drops of methyl red were added into the solution.
The colour changes were recorded.
The solution in test-tube 1 and test-tube 2 were added together in test-tube 3
Any changes of colour in test-tube 3 were recorded.
Part 3: Flame test o the prepared Boric acid
1.
2.
3.
4.
5.
Little Boric acid (crystal) was transferred into the test-tube
Little methanol and some concentrated sulphuric acid H2SO4 were added to the
boric acid in the test-tube.
The solution was heated
The vapour produced was ignited using a flame splint.
The colour of the flame produced was recorded.
Experiment Data:
Table 1: Weight of borax, Na2B4O7. 10H2O
Material and apparatus
Weight (g)
44.9572
55.1078
10.1506
Beaker
Beaker + Borax
Borax
Table 2: Weight of the product(crystal) prepared
Material and apparatus
Filter papers(2 pieces) + watch glass + product
Filter papers(2 pieces)
Watch glass
Product
Weight(g)
35.5356
0.6625
32.6659
2.2072
Table 3: Test on the reaction of boric acid
Samples
Boric acid + Distilled water + Methyl red---(a)
Mannitol + Distilled water + Methyl red---(b)
Mixture of (a) + (b)
Boric acid + Methanol + concentrated
sulphuric acid (flame test)
Observations
Turns from colourless to light orange
Turns from colourless to light yellow
Pink colour of solution is obtained
Flame appear to be in green colour, the
liquid remaining in the test tube turns
from colourless to dark brown colour
Analysis and Calculation:
Weight of sodium tetraborate,
Na2B4O7.10H2O(s) (g)
= 10.1506g
Weight of boric acid, H3BO3(s)
produced (g)
= 2.2075g
Relative molecular mass of sodium tetraborate = 2(23) + 4(10.81) + 7(16) + 10(2+16)
= 381.24
Relative molecular mass of boric acid = 3(1) + 10.81 + 3(16)
= 61.81
Number of moles of =
Weight of Na2B4O7.10H2O
.
Sodium tetraborate, Relative molecular mass of Na2B4O7.10H2O
Na2B4O7.10H2O
= 10.1506g
381.24
= 2.6628 x 10-2 mol
Na2B4O7. 10H2O(s) + 2HCl(aq)
4 H3BO3(aq) + 2NaCl(aq) + 5 H2O(l)
1 mol of tetraborate (Na2B4O7.10H2O) react with 2 moles of hydrochloric acid (HCl)
produce 4 mol of boric acid (H3BO3).
Number of moles of H3BO3 = Number of mol for Na2B4O7.10H2O x 4
= 2.6628 x 10-2 mol x 4
= 1.065 x 10-1 mol
Theoretical mass of H3BO3 produced = Number of mol for Boric acid x Relative
molecular mass of H3BO3
= ( 1.0672 x 10-1 mol ) x 61.81
= 6.5953g
Experimental yield of H3BO3 = 2.2075g
Percentage of yield of boric acid =
Weight of H3BO3
x 100%
Theoritical mass of H3BO3 produced
= 2.2072g x 100%
6.5953g
= 33.47 %
Discussion and Precaution:
DiscussionMethyl red is a pH indicator that change colour when the pH change. The pH range of
methyl red is from around 4 to 6. At pH 4, methyl red shows red colour and at pH 6 it
shows yellow colour
H+(aq) + In-(aq)
HIn(aq)
When the methyl red is added to the boric acid solution, the solution turns from
colourless to orange colour. This show that boric acid is a weak acid which has pH
probably around 5.
When the methyl red is added to the mannitol (C6H8(OH)6, the solution turns from
colourless to yellow colour. This show that mannitol is an even weaker acid which has
pH probably around 6.
When the boric acid is added to the mannitol, the solution turns to pink colour. the
boric acid forms a stronger complex acid with mannitol, which has pH probably around
4.
Boric acid react with acidified methanol to form B(OCH3)3, which gives green colour
flame when its vapour is ignited.
B(OH)3(aq) + 3CH3OH(l)
B(OCH3)3(aq) + 3H2O(l)
PrecautionMaterials such as hydrochloric acid, HCl and sulphuric acid, H2SO4 should be handle
with care to prevent any unwanted accident to happen since these materials are very
corrosive.
The flame test was carried out in vacuum incubator as the vapour release from the test
tube might be toxic.
Two layers of filter paper were used as the suction pump is too powerful and might
cause the filter paper to puncture if only one layer was used.
Watch glass was used while weighing the boric acid on the analytical balance to prevent
boric acid to spill on the pan.
Conclusion:
1.
2.
3.
4.
Sodium tetraborate reacts with hydrochloric acid produces boric acid.
Boric acid is a weak Lewis acid.
Boric acid can form a stronger acid complex with mannitol.
Boric acid can react with an acidified alcohol to liberate B(OCH3)3.
Reference:
Martin S. Silberberg, Chemistry the Molecular Nature of Matter and Change, Fifth
Edition, McGraw Hill, 2005.
F. Albert Cotton, Geoffrey Wilkinson, Carlos A. Murillo, and Manfred Bochmann,
Advanced Inorganic Chemistry, Sixth edition, John Wiley& Son.1999.
F. Albert Cotton, Geoffrey Wilkinson, Paul L. Gaus, Basic Inorganic Chemistry, Third
Edition, John Wiley& Son.1995.
John C. Kotz, Paul M. Treichel, and Gabriela C. Weaver, Chemistry and Chemical
Reactivity, Sixth Edition, Thomson 2006.
Questions:
1. Draw the structure of the borate anion.
H
O
B
H
O
O
B
O
O
O
B
O
B
O
H
O
H
2. Find out the properties and structure of boric acid.
Boric acid, H3BO3 or B(OH) 3 is the acid produced when borax is reacted with
acids. In solid form, boric acid exists as crystalline form where the B(OH)3 units
are linked together by hydrogen bonds, forming a infinite layers of hexagonal
symmetry. The boron atom in boric acid is still not achieving the octet electronic
configuration and can still accept an electron pair. In water, boric acid do not
dissociate directly in water to form proton, but accept an electron pair form the
oxygen atom in water molecule, thus making the whole reaction release hydrogen
ions according to the following equation:
B(OH)4-(aq) + H+(aq)
B(OH)3(s) + H2O(l)
Since the boric acid is an electron pair acceptor, hence it is a Lewis acid. Boric
acid is a weak acid in water. Boric acid can react with an acidified alcohol to
produces B(OR)3 , which gives bright green colour of flame when its vapor is
ignited. Structure of boric acid is:
H
O
H
B
O
O
H
3. How do you convert boric acid to boric oxide?
Boric oxide is formed by strong heating of boric acid. Boric acid is dehydrated
on heating. The conversion of boric acid to boron oxide is not direct. The heating
of boric acid will generates a metaboric acid (HBO2) first. Further heating of the
metaboric acid will then dehydrate it to boron oxide. The conversion of boric
acid to boron oxide is proceed stage by stage.
B(OH)3(s)
HBO2(l)
B2O3(l)
The overall equation of the conversion is shown below:
2B(OH)3(S)
B2O3(l) + 3H2O(g)