The Boiling Point of Ethylene: A Comprehensive Guide

The boiling point of ethylene, a colorless, flammable gas, is a crucial parameter in understanding its physical properties and behavior. At standard atmospheric pressure (760 mmHg), the normal boiling point of ethylene is -154.7°F (-103.7°C). This value represents the temperature at which the vapor pressure of the liquid equals the surrounding atmospheric pressure, causing the liquid to transition into a gaseous state.

Understanding the Boiling Point of Ethylene

The boiling point of a substance is influenced by various factors, including temperature, pressure, and the presence of other substances. In the case of ethylene, the boiling point can be affected by changes in these conditions.

Pressure-Dependent Boiling Point

The boiling point of ethylene is directly related to the surrounding pressure. As the pressure increases, the boiling point also increases, and vice versa. This relationship can be described by the Clausius-Clapeyron equation:

ln(P2/P1) = (ΔHvap/R) * (1/T1 - 1/T2)

Where:
– P1 and P2 are the vapor pressures at temperatures T1 and T2, respectively
– ΔHvap is the enthalpy of vaporization
– R is the universal gas constant

By rearranging this equation, we can calculate the boiling point of ethylene at different pressures:

T2 = (ΔHvap/R) / (ln(P2/P1) + 1/T1)

For example, at a pressure of 1 atm (760 mmHg), the boiling point of ethylene is -103.7°C. However, at a higher pressure of 2 atm (1520 mmHg), the boiling point increases to -95.8°C.

Colligative Effects on Boiling Point

The boiling point of ethylene can also be affected by the presence of other substances, such as in the case of aqueous solutions. This phenomenon is known as boiling point elevation, a colligative property.

When a nonvolatile solute, such as ethylene glycol (a common antifreeze ingredient), is added to a solvent (e.g., water), the boiling point of the solution increases compared to the pure solvent. The magnitude of the boiling point elevation depends on the molality of the solute (moles of solute per kilogram of solvent) and the molal boiling point elevation constant of the solvent.

The boiling point elevation can be calculated using the following equation:

ΔTb = Kb * m

Where:
– ΔTb is the boiling point elevation (the increase in boiling point)
– Kb is the molal boiling point elevation constant (0.51°C/m for water)
– m is the molality of the solute (moles of solute per kilogram of solvent)

For example, in a 1.00 m aqueous solution of a nonvolatile molecular solute, the boiling point elevation would be 0.51°C. Similarly, a 1.00 m aqueous NaCl solution would have a boiling point elevation approximately twice as large as that of a glucose or sucrose solution due to the production of 2 mol of dissolved ions per mol of NaCl.

Ethylene Phase Diagram

The phase behavior of ethylene can be visualized using a phase diagram, which shows the relationship between temperature, pressure, and the different phases (solid, liquid, and gas) of the substance.

The phase diagram of ethylene reveals the following key information:

• The normal boiling point of ethylene is -103.7°C at 760 mmHg (1 atm).
• As the pressure increases, the boiling point of ethylene also increases, as shown by the liquid-vapor equilibrium line.
• The triple point, where all three phases (solid, liquid, and gas) coexist, occurs at a temperature of -169.2°C and a pressure of 0.11 MPa (0.83 atm).
• The critical point, where the distinction between the liquid and gas phases disappears, is located at a temperature of -9.3°C and a pressure of 5.04 MPa (49.7 atm).

Understanding the phase diagram of ethylene is crucial for applications where precise control of temperature and pressure is required, such as in cryogenic engineering, refrigeration systems, and chemical processes involving ethylene.

Numerical Examples and Calculations

To further illustrate the concepts related to the boiling point of ethylene, let’s consider some numerical examples and calculations.

1. Calculating the boiling point of ethylene at a different pressure:
   Given:
2. Normal boiling point of ethylene: -103.7°C at 760 mmHg
3. Pressure: 1500 mmHg

Using the Clausius-Clapeyron equation:
ln(P2/P1) = (ΔHvap/R) * (1/T1 - 1/T2)
ln(1500/760) = (ΔHvap/R) * (1/(-103.7+273.15) - 1/T2)
T2 = -97.8°C
Therefore, the boiling point of ethylene at 1500 mmHg is -97.8°C.

1. Calculating the boiling point elevation of a 1.00 m aqueous ethylene glycol solution:
   Given:
2. Molal boiling point elevation constant for water (Kb): 0.51°C/m
3. Molality of ethylene glycol: 1.00 m

Using the boiling point elevation equation:
ΔTb = Kb * m
ΔTb = 0.51°C/m * 1.00 m = 0.51°C
Therefore, the boiling point of the 1.00 m aqueous ethylene glycol solution is 0.51°C higher than the boiling point of pure water.

These examples demonstrate how to apply the relevant equations and principles to determine the boiling point of ethylene under different conditions, as well as the impact of colligative effects on the boiling point.

Additional Considerations

• Ethylene is a highly flammable gas, and its boiling point is well below room temperature. Proper safety precautions must be taken when handling and storing ethylene.
• The boiling point of ethylene is an important parameter in various industrial applications, such as in the production of polyethylene, the manufacture of ethylene oxide, and the use of ethylene as a refrigerant.
• The phase behavior of ethylene, including its boiling point, is crucial in cryogenic engineering, where the liquefaction and storage of ethylene are essential for various processes.
• Accurate knowledge of the boiling point of ethylene is also important in the design and operation of chemical reactors, distillation columns, and other equipment involved in the processing and handling of ethylene.

Conclusion

The boiling point of ethylene is a fundamental property that plays a crucial role in understanding the behavior and applications of this important chemical compound. By exploring the factors that influence the boiling point, such as pressure and the presence of other substances, as well as the visualization of the ethylene phase diagram, we can gain a comprehensive understanding of this parameter and its implications in various scientific and industrial contexts.

References:

1. PubChem. (n.d.). Ethylene. Retrieved from https://pubchem.ncbi.nlm.nih.gov/compound/Ethylene
2. LibreTexts. (n.d.). 13.5: Colligative Properties. Retrieved from https://chem.libretexts.org/Bookshelves/General_Chemistry/Map%3A_Chemistry_-The_Central_Science(Brown_et_al.)/13%3A_Properties_of_Solutions/13.05%3A_Colligative_Properties
3. Wikipedia. (n.d.). Ethylene phase diagram. Retrieved from https://en.wikipedia.org/wiki/File:Ethylene_phase_diagram.svg