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 Chan Pei Pei Sin ( 08A 08AEB0 EB0354 3544 4) 3. Cheah Cheah Hong Hong Leong Leong ( 08AEB03 08AEB03788 788 ) Experiment No. 4 Title: Preparation Preparation and Reaction of Boric Acid, H 3BO3 Date: 25 June 2008
Title: Preparation Preparation and Reaction of Boric Acid, H 3BO3 Objectives: Synthesis boric acid B(OH)3 from sodium tetraborate as starting material. Used crystallization technique to obtain the final product. 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 Boron has elect electro ronic nic config configur urati ation on of 1s² 2s²2p¹ 2s²2p¹ and and thus thus will will un unde derg rgoe oess sp² 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 BO 3 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 BO 4 molecule. O
O
B O
B O
Trigonal planar BO3
O
O
Tetrahedral BO4
O
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, Na 2B4O7. 10H2O. Borax is often used as washing agent, borax has been used as a house hold cleaning agent for decades. Borax consists of main mainly ly bora borate te ion, ion, B4O5(OH)4² . Reac Reacti tion on of bora borax x with with acid acids, s, for for exa example mples: s: hydrochloric hydrochloric acid and sulphuric acid, will results in boric acid.
Experiment Procedures: Apparatus and Materials•
Sodium tetraborate tetraborate
•
Concentrated hydrochloric acid
•
Concentrated 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 40cm 3 of water. 2. Solu Soluti tion on pro produ duce ced d was was heat heated ed.. 3. After the heating process was completed, 5cm 3 of concentrated hydrochloric acid HCl was added to the solution. 4. The solutio solution n in the beaker beaker was was left until until the the temperatu temperature re of the the solution solution was was falling falling down to room temperature. 5. The beaker beaker was immerse immersed d in cold cold water water (with (with ice) ice) to cool cool down the soluti solution on so that that boric acid can be crystallized out. 6. The crysta crystall was filter filtered ed off by by using using suction suction pump, pump, suction suction funne funnel, l, and filter filter paper paperss (2 pieces which the weight was already measured) 7. The beaker beaker and glass glass rod rod used in filtra filtration tion were were washed washed with with ice cold cold distilled distilled water 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 products on it was weight weight with analytical analytical balance. balance. 11. The weight weight value value was rec recorde orded. d. 1.
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 drops of of methyl methyl red were were added added into the solution. solution. The colour colour chang changes es were were reco recorde rded. d. Little Little man mannit nitol ol was was disso dissolve lved d in testtest-tub tubee 2. A few drops drops of of methyl methyl red were were added added into the solution. solution. The colour colour chang changes es were were reco recorde rded. d. The solutio solution n in test-tub test-tubee 1 and test-t test-tube ube 2 were were added added together together in test-tub test-tubee 3 Any chan changes ges of of colour colour in in testtest-tub tubee 3 were were recor recorded ded..
Part 3: Flame test o the prepared Boric acid 1. 2.
3. 4. 5.
Little Little Boric Boric acid acid (cryst (crystal) al) was was transf transferr erred ed into into the the test-tu test-tube be Little methanol and some concentrated sulphuric acid H 2SO4 were added to the boric acid in the test-tube. test-tube. Thee sol Th solut utio ion n was was heat heated ed The vapo vapour ur produ produced ced was was ignite ignited d using using a flame flame splint. splint. The colo colour ur of of the flam flamee produ produce ced d was rec record orded. ed.
Experiment Data: Table 1: Weight of borax, Na 2B4O7. 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 acid + Distilled Distilled water + Methy Methyll red---(a) red---(a) Mannitol Mannitol + Distilled Distilled wate waterr + Meth Methyl yl red--red---(b) (b) Mixture of (a) + (b) Bor Boric acid acid + Metha ethano noll + conce oncen ntrat trateed sulphuric acid (flame test)
Analysis and Calculation:
Observations Turns from from colourles colourlesss to light orange orange Turns from from colourles colourlesss to light yellow 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
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, tetraborate, Relative molecular mass of Na 2B4O7.10H2O 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 (Na 2B4O7.10H2O) 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 Theor The oret etica icall mass mass of H 3BO3 produ produced ced = Number Number of mol for Boric Boric acid acid x Relat Relative ive molecular mass of H3BO3 = ( 1.0672 x 10 -1 mol ) x 61.81 = 6.5953g Experimental Experimental yield of H3BO3 = 2.2075g Percentage of yield of boric acid =
Weight of H 3BO3 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 HIn(aq)
H+(aq) + In-(aq)
When the methyl When methyl red red is added added to the boric acid solut solution ion,, the the solut solution ion tur turns ns from 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 (C 6H8(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(OCH 3)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, H 2SO4 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. Sodium Sodium tetraborat tetraboratee reacts reacts with hydrochl hydrochloric oric acid produc produces es boric acid. acid. 2. Boric Boric acid acid is a weak weak Lewis Lewis acid. acid. 3. Boric acid can can form form a stron stronger ger acid comple complex x with mannitol. mannitol. 4. Boric acid can react with an acidified alcohol to liberate B(OCH 3)3. Reference:
Martin Martin S. Silberber Silberberg, g, Chem Chemistr istryy the Molecul Molecular ar Nature Nature of Matter Matter and Change Change,, Fifth Edition, McGraw Hill, 2005. F. Albert Albert Cotton, Cotton, Geoffre Geoffrey y Wilkinso Wilkinson, n, Carlos Carlos A. Murillo, Murillo, and Manfred Manfred Bochmann, Bochmann, Advanced Inorganic Chemistry, Chemistry, Sixth edition, John Wiley& Son.1999. F. Albert Cotton, Geoffrey Wilkinson, Paul L. Gaus, Basic Inorganic Chemistry, Chemistry , Third Third Edition, John Wiley& Son.1995. John C. Kotz, Paul M. Treichel, and Gabriela C. Weaver, Chemistry and Chemical Reactivity, Reactivity, Sixth Edition, Thomson 2006.
Questions: 1. Draw the structur structuree of of the the borat boratee anion. anion.
H O B H
O O
B
O O
O
B O
B
O H
O H
2. Find out out the the propert properties ies and and structu structure re of boric 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 directl y 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)3(s) + H2O(l)
B(OH)4
(aq)
+ H+(aq)
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 Structure of boric acid is:
H O
H
B O
O H
3. How do do you you convert convert boric acid to boric boric oxide? 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)