experiment p.1.2.2

KWAME NKRUMAH UNIVERSITY OF SCIENCE AND TECHNOLOGY

COLLEGE OF ENGINEERING DEPARTMENT OF CHEMICAL ENGINEERING

TITLE: ENTHALPY AND ENTROPY OF AN AQUEOUS BORAX SOLUTION

NAME: KWAKYI KOFI BOSOMPEMCOURSE:BSC. CHEMICAL ENGINEERING YEAR: SECOND YEAREXPERIMENT NO. :P.1.2.2.I.D. NO: 9664813DEMONSTRATOR: J. Y. GYESIDATE: 4TH FEBRUARY, 2015Aims and Objectives1. To measure the enthalpy (H) and entropy (S) of an aqueous solution of borax (sodium tetraborate decahydrate) from the temperature dependence of the equilibrium constant for the dissolution reaction of borax in water.2. To study a system of an aqueous solution of borax.

SAFETY PRECAUTIONS3. Safety glasses and aprons must be worn at all times in the lab.4. Dilute solutions of acid, other chemicals and experimental apparatus should be handled with care since spillage and breakages can cause injuries.5. When using volumetric pipettes, always use a pipette bulb, never pipette by mouth.

INTRODUCTION/THEORYThe changes in free energy (G), enthalpy (H), and entropy (S), are related to the equilibrium constant (K) for a chemical reaction at temperature (T) by the equation below:(1) G= -RT lnK = H - TSWhere R is the universal Gas Constant and equals 8.314 J/molK. Equation (1) can be rearranged to eliminate G and solve for the natural log of K in terms of H, S and T:(2) lnK = (-H/R) (1/T) + (S/R)Equation (2) can be compared to the point slope of a line (y = mx + b) so that lnK versus 1/T should result in a straight line with slope being equal to (-H/R) and the y-axis intercept (S/R). The solubility of salt is dependent on the temperature of the solution. When equilibrium is established in a saturated solution at a specific temperature, the rate of formation of ions in a solution is equal to the rate of deposition of the solid. The equilibrium constant for the dissociation of the solid in a solvent is called the solubility product constant (Ksp). It is equal to the product of the concentration of ions in solution. Since the concentration of ions can change with temperature, the equilibrium constant for the reaction is temperature dependent. Sodium tetraborate decahydrate (borax) dissociates in water to form sodium and borate ions and water.(3) Na2B4O7.10H2O(s) 2Na(aq) + B4O5(OH)42-(aq) + 8H2O(l)The equilibrium constant for the expression is: (4) Ksp = [Na+]2 [B4O5(OH)42-]Note that there are two sodium ions produced for each borate ion in the reaction. Therefore, the concentration of sodium ion is twice the concentration of borate ion. (5) [Na+] = 2[B4O5(OH)42-]And the equilibrium can now be re-written as total borate ion concentration by substituting the equations from (5) into (4):(6) Ksp = 4[B4O5(OH)42-]3+Thus the equilibrium constant for the reaction can be determined if the total borate ion concentration is known. Since borate ions are bases, their concentration can be determined by a simple acid-base titration requiring two moles of H+ ions for each mole of orate ion:(7) B4O5(OH)42- + 2H+ + 3H2O 4B(OH)3If a set saturated borax solutions are formed at various temperatures and titrated to determine borate ion concentration, the data can be used to compute Ksp values at each temperature and a plot of ln(Ksp) versus 1/T can then be used to provide sufficient information which will allow calculation of H and S.

EXPERIMENTAL APPARATUS AND CHEMICAL Erlenmeyer flask Test tubes Burette Pipette Laboratory thermometer Burner Beaker Digital analytical balance

PROCEDURE 5 clean test tubes were labelled and 5.00ml of water was transferred into each test tube with a volumetric pipette. The exact level of the liquid in the test tube was carefully marked after which the liquid was discarded and the test tube dried. Approximately 30g of borax was dissolved in 50mL of distilled water in a beaker. The mixture was then heated gently to 65C and a little more borax was added to the solution until excess solid was present and the solution was completely saturated. The solution was allowed to cool to approximately 60C and was poured into the test tube #1 until the level of the solution reached the exact 5.00mL mark on the test tube. The exact temperature of the solution was noted. The solution was allowed to cool to approximately 50C and exactly 5.00mL of it was poured into test tube #2. The exact temperature of the solution temperature was noted. The steps were repeated and the solution was cooled to approximately 40C, 30C and 20C in the tubes #3, #4 and #5 respectively. The contents of test tube #1 were transferred into a clean 125mL Erlenmeyer flask and the test tube was rinsed with several portions of distilled water, adding the rinse to the flask. Water was added to the crystallized solid remains in the test tube, and was heated gently until the borax dissolved and then transferred to the flask. 2 to 3 drops of bromocresol green indicator was added to the flask. A clean 50mL burette was filled with 0.5M HCl solution. A few mL of the acid was drained into an empty beaker and 1 drop of the bromocresol green was added to the acid. The content of the beaker was titrated o the end point with the HCl solution. The steps were repeated and the contents of each remaining test tube were titrated.

OBSERVATIONSIt was observed that not all the borax dissolved wen approximately 30g was dissolved in 50ml of distilled water at room temperature.Also the color of bromocresol green when added to the flask containing the borax solution turned blue.A pale yellow color was observed at the end point with the HCl solution.

TABLE OF RESULTS

Temperature /oCVolume B4O5(OH)45-/mlVolume HCl/ml

605.015.40

505.014.80

405.08.60

305.05.60

205.04.30

CALCULATIONS

Temp (oC)T (Kelvin)1/T (Kelvin-1)[Oxalate]KspIn(Ksp)

603330.0030030.771.8260.60213

503230.0030960.741.6210.48304

403130.0031950.430.318-1.1457

303030.00330.280.088-2.4304

202930.0034130.2150.0398-3.22389

1) At T = 60oC = 273 + 60 = 333K T-1 = 333-1 = 3.003 10-3 K-1From reaction equation: B4O5(OH)42(aq) + 2HCl(aq) + 3H2O(l) 4B(OH)3(aq) + 2Cl (aq)

n(borate) =1n(H+) 2

n(borate) = 0.5 n(H+)

n(H+) = [H+] volume of H+ = 0.5 15.40 = 7.7mol

Hencen(borate) = 7.7 0.5= 3.85molC= n/vConcentration of borax, C ((B4O5(OH)45) = 3.85/5 = 0.77M

Ksp = 4 [borate] 3 = 4 (0.77)3 = 1.826

InKsp= 0.6022

2) At T = 50oC = 273 + 50 = 323K T-1 = 333-1 = 3.096 10-3 K-1




Ksp = 4 [borate] 3 = 4 (0.74)3 = 1.671

InKsp= 0.4830

3) At T = 40oC = 273 + 40 = 313K T-1 = 333-1 = 3.195 10-3 K-1




Ksp = 4 [borate] 3 = 4 (0.43)3 = 0.318

InKsp= -1.1457

4) At T = 30oC = 273 + 40 = 303K T-1 = 303-1 = 3.300 10-3 K-1




Ksp = 4 [borate] 3 = 4 (0.28)3 = 0.088

InKsp= -2.4304

5) At T = 20oC = 273 + 20 = 293K T-1 = 293-1 = 3.413 10-3 K-1




Ksp = 4 [borate] 3 = 4 (0.215)3 = 0.0398

InKsp= -3.2239

Graph of lnKsp versus 1/T

Slope of the line From graph,

Gradient= Y2 Y1 X2 X1

= -0.5- (-3) . 0.003104 0.003308 = -12254.90

But from equation (2), slope = -Ho = 12254.90 R

-H = 12254.90 R R = 8.314J/molK

Ho = 8.314 -12254.90 = 101887.25 KJ/molK

3. The two points are: (0.003104, -0.5) and (0.003308, -3)

Substituting into equation (2)-0.5 =-Ho (0.003104) + S(1) R R

-3 = -Ho (0.003308) + S(2) R R

But R = 8.314 J/mol and multiplying both sides by the value of R 8.314(-0.5) = -Ho (0.003104) + S(1)

8.314(-3) = -Ho (0.003308) + S(2)

(1) (2)20.785 = 0.000204Ho Ho = 101887.25 J/mol= 101887.25 KJ/molSubstituting Ho into (1)-4.157 = -(0.003104 101887.25) + SS = -4.157 + 316.26S = 312.10 J/mol

4. The value of the enthalpy of the dissociation of borax solution from the experiment conducted was 101.887KJ/mol. Comparing this value to the literature value (110 KJ/mol), it can be noticed that the value has deviated to some extent. This deviation can be attributed to certain factors:a) Temperature reading.b) Titer value obtained as a result of poor volume reading as a result of parallax.c) The nature of the borax solution (it solidifies quickly).d) Some of the solution clanged to the stirrer which was used as a result not all the 5.00ml borax solution was used in the titration.e) Recording the amount of acid used for titrating at very low temperatures was impossible due to the room temperature which was higher.

5. From the experiment conducted and the values obtained, it is realized that as the temperature increased, there was an increase in solubility as indicated by the Ksp values and their corresponding temperatures.On the other hand, when the temperature was decreasing, the borax solution was observed to be solidifying indicating also that a decrease in temperature reduces the solubility

6. The positive value of Ho obtained signifies an endothermic reaction. It was observed during the course of the experiment that beaker was cold to touch until heating and when left standing when the borax was dissolved in water, the system caked and became cold implying an endothermic process taking place.

7. From the calculated value, the entropy of the system is positive. This is so because the entropy of the borax (initial state) is lower than the borax solution (final state). In the reaction, the reactants and products are of the same phase but the number of moles of the products is greater than the number of moles of the reactants which leads to an increase in entropy (positive).

DISCUSSIONFor a series of borax containing samples taken at different temperatures, the concentration of borate ions will be determined. The borate ions react with a monoprotic acid HCL in a 1:2 ratio:

B4O5 (OH)42-(aq) +2 HCl(aq) + 3 H2O(l) 4 B (OH)3 (aq) + 2Cl-(aq)

The experiment requires that the solubility of borax be found at various temperatures.

The solubility of the borate ion depends on temperature such that it increases as temperature increases. When equilibrium is established in the saturated solution at a specific temperature, the rate of formation of borate ions is equal to the rate of deposition of the borax. The equilibrium constant for the dissolution of a solid in a solvent is called the solubility product constant (Ksp) which is equal to the product of the concentration of the ions in solution.Heat is applied to the borate ion in the conical flask so as to allow the full reaction of the borate ions and the HCl in solution.Recording the amount of acid used for titrating at very low temperatures was impossible due to the room temperature which was higher.

It was observed during the course of the experiment that beaker was cold to touch until heating and when left standing when the borax was dissolved in water, the system caked and became cold implying an exothermic process taking place.

EXPERIMENTAL PRECAUTION1. The volumetric pipette was never used with heated solutions.2. The exact level of the liquid on the test tube to ensure accuracy.3. It was ensured that the solid borax was not transferred from the beaker to the test tube.

CONCLUSION From the experiment conducted and the values obtained, it is realized that as the temperature increased, there was an increase in solubility as indicated by the Ksp values and their corresponding temperatures.On the other hand, when the temperature was decreasing, the borax solution was observed to be solidifying indicating also that a decrease in temperature reduces the solubility. In effect, there is a linear relationship between temperature and solubility.

The positive value of Ho obtained signifies an endothermic reaction.

Bromocresol green as an indicator turn yellow in an acidic region of 3.6-5.2. The color of the solution changed to light yellow indicating the end point of the reaction. This shows that the indicator was suitable for the experiment.

REFFERENCES1. Chemistry the central science 7th edition by Brown Lemay Bursten (Page 597-8)

experiment p.1.2.2

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