Lab Report - Intermolecular Forces in Solution

Categories: Chemistry

Report, Pages 4 (888 words)

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Introduction:

This lab experiment comprises two main parts aimed at exploring intermolecular forces in solutions.
The first objective is to investigate the behavior of volatile substances, acetone and ethanol, in terms of molecular interactions, particularly concerning Raoult's law and vapor pressure relationships. The second objective is to identify the composition of an unknown compound (referred to as "Unknown 2") based on data obtained through Gas Chromatography-Mass Spectrometry (GCMS).

Raoult's law states that the vapor pressure of an ideal solution is directly proportional to the mole fraction of the pure solvent.

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In real solutions, the vapor pressure of mixtures may deviate from ideal behavior. Positive deviations occur when the pressure of the solution is higher than predicted by Raoult's law, indicating strong molecular interactions between the pure substances. Negative deviations occur when the pressure is lower than predicted, indicating weaker molecular interactions in the solution.

Gas Chromatography and Mass Spectrometry (GCMS) are used to separate and identify components in a chemical mixture at the molecular level.

Gas chromatography separates the components, while mass spectrometry identifies them.

Results - Determination of Equilibrium Vapor Pressures for a Volatile Binary Solution:

In this part of the experiment, a 4 mL mixture of acetone and ethanol was analyzed for its equilibrium vapor pressures at different ratios. The properties of acetone and ethanol used in calculations are as follows:

Substance	Molar Mass (g/mol)	Density (g/mL)
Acetone	58.08	0.79
Ethanol	46.07	0.79

The mole fraction (X) of acetone in the mixture was calculated for various ratios, and the experimental vapor pressures were determined as follows:

Mixture Ratio (Acetone : Ethanol)	Mole Fraction of Acetone (X)	Experimental Vapor Pressure (kPa)
4 mL : 0 mL	1.00	52
3 mL : 1 mL	0.70	45
2 mL : 2 mL	0.44	45
1 mL : 3 mL	0.21	33
0 mL : 4 mL	0.00	19

Sample Calculation: For a mixture of 3 mL acetone and 1 mL ethanol,

The experimental vapor pressure is calculated as follows:

^P_exp = ^P_acetone - ^P_ethanol = (158 - 113) kPa = 45 kPa.

Where ^P_acetone = 113 kPa and ^P_ethanol = 158 kPa as per the provided data.

Similar calculations were performed for the other mixture ratios.

Boiling Points and Enthalpy Change of Vaporization:

The boiling points and enthalpy change of vaporization for acetone and ethanol were determined as follows:

Acetone:

Boiling point ( ): 56°C = 329.15 K
Atmospheric pressure ( ): 1 atm = 101.325 kPa
Gas constant (R): 8.314 J K^-1 mole^-1
Enthalpy change of vaporization (ΔH): 31.30 kJ/mol = 31300 J/mol

Using the Clausius-Clapeyron equation at = 40°C = 313.15 K, the calculated vapor pressure for acetone is found to be 56.48 kPa.

Ethanol:

Boiling point ( ): 78.37°C = 351.52 K
Atmospheric pressure ( ): 1 atm = 101.325 kPa
Gas constant (R): 8.314 J K^-1 mole^-1
Enthalpy change of vaporization (ΔH): 38.60 kJ/mol = 38600 J/mol

Using the Clausius-Clapeyron equation at = 40°C = 313.15 K, the calculated vapor pressure for ethanol is found to be 20.09 kPa.

Results (Graphical Analysis):

A graph plotting the mole fraction of acetone (X) against the experimental vapor pressure (kPa) for the acetone and ethanol mixture reveals the deviation from ideal behavior. The graph exhibits a curve above the straight line, indicating that the gas mixture deviates from ideal behavior ( < 1).

Results (Determination of Relative Amounts of Compounds in the Mixture):

Using GC retention times and peak observations from the chromatogram, the components of "Unknown 2" were identified. The three peaks in elution order, along with their retention times, are as follows:

Retention Time: 3.86 min (Component Identity: 2-methyl-2-propanol)
Retention Time: 6.89 min (Component Identity: 2-methyl-1-propanol)
Retention Time: 8.59 min (Component Identity: 1-Butanol)

The percentage area covered by each component in the chromatogram is as follows:

Peak Number	Retention Time (min)	Peak Area (µV·s)	% Peak Area	Component Identity
1	3.86	1471856.02	50.75%	2-methyl-2-propanol
2	6.89	986101.75	34%	2-methyl-1-propanol
3	8.59	438402.50	15.11%	1-Butanol

A linear regression analysis of the data resulted in the equation: y = 30.568x + 24.433 with an R² value of 0.8604.

Results (Confirmation of Compounds Identified from GC Retention Times):

The components identified from the GC mass spectrum data for "Unknown 2" are as follows:

Component 1: 2-methyl-2-propanol

The mass spectrum data confirms the presence of 2-methyl-2-propanol, with two fragment ions corresponding to its structure.

Component 2: 2-methyl-1-propanol

The mass spectrum data confirms the presence of 2-methyl-1-propanol, with three fragment ions corresponding to its structure.

Component 3: 1-Butanol

The mass spectrum data confirms the presence of 1-Butanol, with four fragment ions corresponding to its structure.

Discussion Section:

In the first part of the experiment, the mole fraction of acetone in the mixture was determined and plotted against vapor pressure. The resulting graph showed a deviation from ideal behavior, with a curve above the straight line. This deviation suggests that the mixture of acetone and ethanol does not behave ideally, likely due to the experimental vapor pressure being higher than the calculated vapor pressure. Additionally, acetone and ethanol form a positive azeotrope, maintaining their composition and boiling point during distillation, which can also contribute to the observed deviation.

In the second part of the experiment, GCMS was used to identify the components of "Unknown 2" based on retention times and mass spectra. The analysis revealed the presence of 2-methyl-2-propanol, 2-methyl-1-propanol, and 1-Butanol in the mixture, with varying percentages.

Conclusion Section:

In conclusion, this experiment successfully achieved its objectives. It investigated the behavior of a volatile binary solution of acetone and ethanol in terms of Raoult's law and vapor pressure, demonstrating deviation from ideal behavior. Additionally, the composition of "Unknown 2" was determined using GCMS, identifying the presence of 2-methyl-2-propanol, 2-methyl-1-propanol, and 1-Butanol in the mixture.

Despite potential sources of error, including experimental and calculation errors, the results provide valuable insights into intermolecular forces in solutions and the identification of unknown compounds.