Vernier Software and Technology
Vernier Software & Technology

Understanding Intermolecular Forces Using a Gas Chromatograph: Enthalpy of Vaporization

Introduction

One well-known application of gas chromatography is its analytical capability used to obtain purely physiochemical data such as activity coefficients of solutes in various solvents, heats of solution, and enthalpies of vaporization of volatile compounds. It can also be used to demonstrate colligative properties. Here, we introduce the determination of the enthalpy of vaporization using retention times measured with a gas chromatograph (GC).

Gas chromatography is based on a solute in a mixture partitioning itself between the mobile phase and the stationary phase. With the Vernier Mini GC, the mobile phase is air and the stationary phase is a nonpolar phase capillary column. The amount of time a given chemical spends in the stationary phase relative to the amount of time it spends in the mobile phase is a very important quantity in elution chromatography; it is called the capacity factor, k′, and is given by:

k^{\prime} = \frac{t_{R} - t_{M}}{t_{M}}

where tR is the retention time of the compound; that is, the amount of time the chemical spends in the column from the point of injection to the point of detection. The time it takes for the mobile phase to pass through the column is referred to as tM; it is typically the retention time of a non-retained species. In this experiment, the non-retained compound you will be using is acetone and it functions as a very important standard to help normalize the amount of time it takes a species to run through the column, enabling calculation of k′. As part of this calculation, we are assuming that the retention time of the non-retained species (acetone) is independent of temperature.

To relate the capacity factor to the enthalpy of vaporization, the following equation is used:

\text{ln}\left(\frac{k^{\prime}}{T}\right) = \frac{\Delta\text{H}_{VAP}}{\text{R}}\left(\frac{1}{T}\right)+\text{C}

where ΔHvap is the standard enthalpy (heat) of vaporization of the compound. This value is assumed to be independent of temperature. T is the temperature in Kelvin, R is the gas constant in appropriate units, and C is a constant. The equation is written in the slope-intercept form where the value of ΔHvap is determined by plotting ln(k′/T) vs. 1/T.

Objectives

In this experiment, you will

  • Collect and analyze GC data from various samples.
  • Calculate the enthalpy of vaporization of various compounds from GC temperature dependent data.
  • Identify an unknown sample based on its enthalpy of vaporization using values calculated from known standards.

Sensors and Equipment

This experiment features the following Vernier sensors and equipment.

Additional Requirements

You may also need an interface and software for data collection. What do I need for data collection?

Organic Chemistry with Vernier

See other experiments from the lab book.

1Determining Melting Temperature
2Recrystallization
3Determination of a Boiling Point
4Identifying an Unknown Analgesic by Three Methods
5Separation of Organic Compounds by Acid-Base Extraction Techniques
6Understanding Polarimetry
7Identification of Organic Unknowns Using Polarimetry
8Investigating Gas Chromatography
9Fractional Distillation of Esters
10Understanding Intermolecular Forces Using a Gas Chromatograph: Enthalpy of Vaporization
11Investigating Thermodynamic Relationships of Substituted Hydrocarbons
12Extraction of Spinach Pigments and Analysis by Electronic Absorption Spectroscopy
13SN1: Synthesis of t-butyl chloride
14SN2: Synthesis of 1-bromobutane
15Observing the Reaction Kinetics of Sucrose with Polarimetry
16The Synthesis and Analysis of Aspirin
17Isolation of R-(+)-Limonene from Oranges using Steam Distillation
18Synthesizing Ethyl Acetate by Fisher Esterification
19Synthesis of Dibenzalacetone by Aldol Condensation
20The Diels-Alder Reaction of Anthracene with Maleic Anhydride
21Friedel-Crafts Acylation of Ferrocene
22Grignard Formation of Crystal Violet
23Synthesis of Fluorescein
24Synthesis of Methyl Orange and Its Application to Textiles
25Analysis of Natural Products
26Using a Gas Chromatograph: Identifying an Unknown Compound

Experiment 10 from Organic Chemistry with Vernier Lab Book

<i>Organic Chemistry with Vernier</i> book cover

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