Experiment 4

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. Oˉ

Oˉ

B ˉO

B Oˉ

Trigonal planar BO3

ˉ

O

Oˉ

Tetrahedral BO4

Oˉ

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. 2. Solution produced was heated. 3. 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.

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)

Analysis and Calculation:

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

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 HIn(aq)

H+(aq) + In-(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 (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(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. Sodium tetraborate reacts with hydrochloric acid produces boric acid. 2. Boric acid is a weak Lewis acid. 3. Boric acid can form a stronger acid complex with mannitol. 4. 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)