PChem Lab 7 - Mandatory lab work, assignments and final review PDF

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Mandatory lab work, assignments and final review...

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INTRODUCTION A liquid-vapour phase diagram for a two-component system was created in order to test Raoult's law for non-ideal solutions. To create the liquid-vapour phase diagram, fractional distillation was used. Fractional distillation is a common technique used in organic synthesis and to separate compounds in a mixture. Raoult's law states that the vapour pressure of an ideal liquid-vapour solution is directly proportional to its mole fraction. P i = X i P i*

(1)

Where, Pi= vapour pressure of the liquid in the solution Xi= mole fraction of the liquid in the solution Pi*= vapour pressure of the pure liquid Raoult's law holds best when applied to an ideal liquid-vapour mixture. For real mixtures, Raoult's law holds for the solvent but begins to break down in dilute concentrations. In a nonideal solution Raoult's law may deviate from the observed vapour pressure positively or negatively. In a mixture containing two compounds, it is possible to have homogeneous and heterogeneous intermolecular forces. In the case where homogeneous interactions are stronger than heterogeneous reactions, Raoult's law deviates positively. However, if Raoult's law deviates negatively from the observed vapour pressure it due to heterogeneous intermolecular interaction being stronger than the homogeneous attractions. In order to determine the phase of a system at equilibrium the degrees of freedom must be known. The degree of freedom represents the number of variables that can be manipulated independently without distrusting the phases in equilibrium. Typically these variables are pressure, temperature and/or composition. The degree of freedom is related to the components and phases present in the system. A component is a chemically independent constituent of te system; a component can specify the compositions of the system. A phase is defined as the state of matter is uniform throughout its composition and physical state. The relationship between the degrees of freedom, components and phases can be calculated using Gibbs Phase Law: F=C-P+2

(2)

Where, F=degrees of freedom C=components P=phases

Raoult's law typically results in maxima and minima in the vapour pressure. The maxima and minima compositions of the liquid and vapour are identical, known as azerotropic composition. At azerotropic compositions the mixture will boil constantly without a change in the overall composition of the mixture. EXPERIMENTAL PROCEDURE Refer to "Laboratory Manual for Introductory Physical Chemistry CHMI-2516" p.97-103. RESULTS Table 7.1 Data from Experiment Collection Temp (°C) Initial Final 144.15 150.10 152.90 154.20 156.88 156.89 152.23 156.22 156.79 160.98

N/A N/A 153.13 154.22 156.99 156.90 153.39 156.59 159.68 161.12

Vapour Sample Vial # Refraction Index (nD) V16 1.4933 V15 1.4880 V18 1.4845 V7 1.4824 V3 1.4750 V12 1.4708 V17 1.4490 V15 1.4557 V13 1.4615 V9 1.4640

Liquid Sample Vial # Refraction Index (nD) L6 1.4924 L20 1.4834 L9 1.4791 L19 1.4772 L3 1.4730 L17 1.4692 L13 1.4490 L12 1.4593 L7 1.4632 L18 1.4665

Table 7.2 Refraction Index and Mole Fraction of Cyclohexanone Log

Weight % C6H10O

Moles of C6H10O

Moles of tetrachloroethane

0.1744 0.1730 0.1716 0.1701 0.1686 0.1672 0.1658 0.1647 0.1636 0.1626 0.1616

0.0000 0.1000 0.2000 0.3000 0.4000 0.5000 0.6000 0.7000 0.8000 0.9000 1.0000

0.0000 0.1019 0.2038 0.3057 0.4075 0.5094 0.6113 0.7132 0.8151 0.9170 1.0189

0.6030 0.5427 0.4824 0.4221 0.3618 0.3015 0.2412 0.1809 0.1206 0.0603 0.0000

SAMPLE CALCULATIONS Mole Fraction of cyclohexanone Xa = na/(na+nb) ncyclohexanone= mass of cyclohexanone/molar mass = 10g/98.148g/mol =0.102mol ntetrachloroethane= mass of tetrachloroethane/molar mass =90g/167.8325g/mol =0.536mol

Xcyclohexanone= ncyclohexanone / (ncyclohexanone + ntetrachloroethane) =0.102mol/ (0.102mol/0.536mol) =0.1581

Mole Fraction of C6H10O 0.0000 0.1581 0.2970 0.4200 0.5297 0.6282 0.7171 0.7977 0.8711 0.9383 1.0000

Refraction index 1.4942 1.4893 1.4844 1.4794 1.4745 1.4696 1.4649 1.4613 1.4575 1.4540 1.4507

Table 7.3 Interpolated Mole Fractions Using Figure 7.1 and the Refraction Index of Each Trail Trial

Refraction Index of Liquid

1 2 3 4 5 6 7 8 9 10

1.4924 1.4834 1.4791 1.4772 1.4730 1.4692 1.4490 1.4593 1.4632 1.4665

Interpolated Mole Fraction of Cyclohexanone 0.100 0.300 0.400 0.465 0.530 0.620 1.050 0.820 0.750 0.670

To interpolate the mole fraction of cyclohexaone, the refractive index was looked up on the xaxis and then a line was drawn to the trendline. When the trendline was intercepted another line was drawn down the y-axis. Where this line intercepted with the x-axis indicated the mole fraction of cyclohexanone and tetracyclohexane. DISCUSSION It can be seen in Figure 7.2 that the refraction index and mole fraction of cyclohexanone have a negative linear relationship. As the composition of the mixture becomes more pure in cyclohexanone the refractive index decreases. An explanation for this is due to the molar mass of cyclohexanone and tetrachloroethane. Cyclohexanone has a smaller molar mass of 98.16g/mol compared to tetrachloroethane molar mass of 167.85. The liquid-vapour phase diagrams constructed in this experiment varied tremendously to that of a liquid-vapour phase diagram of an idea binary. This is most likely due to errors while conducting the experiment. There are several sources of error in this experiment. The largest source of error is in the refraction index. If the refraction index was not measured immediately after being collected the sample could have decomposed. It can also be noted that the temperatures at which dew points and bubble points occurred have been reversed during the experiment. There is also a large error in the interpolation of the mole fraction of cyclohexanone. Furthermore, if the two compounds were not pure before they were mixed the results will be skewed. It can be concluded that the liquid-vapour phase diagram constructed in the experiment significantly deviates from Raoult's law because of the non-ideal solution.

REFERENCE ATKINS, P. Atkins' Physical Chemistry. 9th Ed., New York: W.H. Freeman and Comp., 2011. Print. KUGEL, R. Raoult's Law: Binary Liquid-Vapor Phase Diagrams: A Simple Physical Chemistry Experiment. 9th Ed., St. Mary's University of Minnesota, J. Chem. Educ., 1998. Print. SHEPHERD, J. Laboratory Manual CHMI 2516 Fall 2014.Sudbury, Ontario: Laurentian University, 2014. p.97-103.

Experiment 7 Binary Liquid-Vapour Phase Diagram CHMI-2526 Kyle Hartman 11/11/14...