CHEM 254 EXPERIMENT 5. Solubility and Enthalpy of Fusion of Ammonium Oxalate in Water

Transcription

1 CHEM 254 EXPERIMENT 5 Solubility and Enthalpy of Fusion of Ammonium Oxalate in Water In general solubility (g/100 ml) is defined as amount of substance that dissolved in a given solvent at a given temperature. The process may be exothermic or endothermic depending on the intermolecular forces that exist in pure solute, pure solvent and solution. In a saturated solution the undissolved solute is in equilibrium with the dissolved solute. Therefore, in a saturated solution the chemical potential of pure solid solute B, μ B (s) and the chemical potential of solute B, μ B (l) in solution are equal (* is used for pure substances). μ B (l) = μ B (s) and μ B (l) = μ B (l) + RT ln x B thus; μ B (s) = μ B (l) + RT ln x B RT ln x B = μ B (s) μ B (l) and fus G = (μ B (s) μ B (l)) thus; RT ln x B = fus G Application of the Gibbs-Helmholtz equation: ( ( fus G T ) t ) = fush T 2 then ( d ln x B ) = fush dt RT 2 Assuming that fus H is independent of temperature in the temperature range interested. ln x B = fush R ( 1 T f 1 T ) The slope of the line obtained by plotting variation of ln x B with 1 T in a saturated solution is equal to the - fush R of the solute (Figure 1). Figure 1. Variation of ln x B with 1 T

2 Purpose: The aim of this experiment is to determine solubility and enthalpy of fusion of ammonium oxalate in water. Apparatus and Chemicals Apparatus: Landolt pipettes, stirrer, thermometer, pipettes, flasks. Chemicals: Ice, dilute H 2 SO 4, ammonium oxalate, potassium permanganate. Procedure I. Solubility at the freezing point of water 1. Put 4 g (NH 4 ) 2 C 2 O 4 and 20 ml of H 2 O into a clean tube. ice-water bath Solution + solute Figure 2. Experimental set-up for determining solubility at the freezing point of water 2. Immerse the tube into the ice-bath. 3. Stir the solution for 20 min. 4. Wait for 10 min for settlement of the undissolved particles. Record the temperature T 1 in report sheet- Table Take 10 ml from the upper part of the solution by using a pipette and pour it into a flask. (Caution: be careful not to take any precipitate). 6. Add 30 ml dilute H 2 SO 4 to the flask. 7. Titrate 10 ml of this solution with KMnO 4 at 80 C (Caution: heating should be done in water bath) until the appearance of a permanent pink color (indicating presence of excess KMnO 4 ). The redox reaction taking place is given below. 5C 2 O 4 2- (aq) + 2MnO 4 -(aq) + 16H + (aq) 10CO 2(g) + 8H 2 0 (l) + 2Mn 2+ (aq)

3 II. Solubility at room temperature 1. Put 6 g (NH 4 ) 2 C 2 O 4 and 20 ml of H 2 O into a clean test tube. 2. Fix the tube with a clamp. 3. Repeat the procedure given in Part I.3-7. (Caution: the recorded temperature is T 2 now) III. Solubility at the boiling temperature of water 1. Put 15 g (NH 4 ) 2 C 2 O 4 and 20 ml of H 2 O into a clean test tube. puar stirrer water bath Landolt pipette Solution + solute Figure 3. Experimental set-up for determining solubility at the boiling point of water 2. Immerse the tube into a water bath heated up to boiling point. 3. Repeat the procedure given in Part I.3-5. (Caution: the recorded temperature is T 3 now) 4. Take out the Landolt pipette, cool down to room temperature and weight with its stopper. 5. Pour the clear solution to half of the volume of the Landolt pipette. 6. Put on the stopper of the pipette and weigh the pipette after cooling to room temperature. 7. Place a graduated cylinder below the Landolt pipette. 8. Pour H 2 SO 4 solution into the Landolt pipette and when all the precipitate is dissolved then pour all the solution into the graduated cylinder. 9. Record the volume of the solution in the graduated cylinder as V sol. 10. Titrate 5 ml of the solution in the graduated cylinder with KMnO 4 at 80 C until the appearance of a permanent pink color.

4 Treatment of Data I. Calculation of the solubility of (NH 4 ) 2 C 2 O 4 at T 1, T 2 and T Calculate the number of moles of (NH 4 ) 2 C 2 O 4 ) from the titration, n A1. 2. Calculate the number of moles of (NH 4 ) 2 C 2 O 4 n A2, the amount of present in the (NH 4 ) 2 C 2 O 4 in the saturated solution 3. Calculate the mass of (NH 4 ) 2 C 2 O 4 in the saturated solution. 4. Calculate the mass of the solvent present in the saturated solution. (Assume that density of solution is equal to 1.0 g/ml, do not forget to consider the amount of solute in solution) 5. Calculate the solubility of (NH 4 ) 2 C 2 O 4 in units of g/100 g of solvent. solubility = m ((NH 4 ) 2 C 2 O 4 ) / m solvent x 100 II. Calculation of fus H of (NH 4 ) 2 C 2 O 4 ). 1. Calculate the mole fraction of (NH 4 ) 2 C 2 O 4 ) in the saturated solutions at each temperature studied. 2. Plot ln x B versus 1 T graph. 3. Calculate slope. 4. Calculate fus H of (NH 4 ) 2 C 2 O 4 ). Questions 1. What is saturated solution? 2. Why is dilute H 2 SO 4 added? 3. Why is the Landolt pipette used at high temperature? 4. Discuss the possible sources of errors for this experiment and prepare an error analysis.

5 5. DATA SHEET Experiment 5-Solubility and Enthalpy of Fusion of Ammonium Oxalate in Water Group Number: Date: Assistant name and signature: I. A.1. Solubility of (NH 4 ) 2 C 2 O 4 at T 1 V (KMnO 4 ) = n A1 = n A2 = m ((NH 4 ) 2 C 2 O 4 ) = m solvent = solubility = 2. Solubility of (NH 4 ) 2 C 2 O 4 at T 2 V (KMnO 4 ) = n A1 = n A2 = m ((NH 4 ) 2 C 2 O 4 ) = m solvent = solubility = B. Solubility of (NH 4 ) 2 C 2 O 4 at T 3 V (H 2 O + (NH 4 ) 2 C 2 O 4 + H 2 SO 4 ) = V (KMnO 4 ) = n A1 = n A2 = m ((NH 4 ) 2 C 2 O 4 ) = m solvent = solubility =

6 II. Mole fraction of (NH 4 ) 2 C 2 O 4 at each temperature At T 1 x B [(NH 4 ) 2 C 2 O 4 ] = At T 2 x B [(NH 4 ) 2 C 2 O 4 ] = At T 3 x B [(NH 4 ) 2 C 2 O 4 ] = 2. Fill the following table and plot ln x B versus 1 T graph. T(K) 1 T (K 1 ) x B ((NH 4 ) 2 C 2 O 4 ) ln x B fus H of (NH 4 ) 2 C 2 O 4 ) =