The Boiling Point of Dichloromethane: A Comprehensive Guide for Science Students

Dichloromethane, also known as methylene chloride, is a widely used organic solvent with a boiling point of approximately 39.7°C (103.5°F) or 312.85 Kelvin. This property has been extensively studied and documented, making it a crucial piece of information for science students and professionals working with this versatile compound.

Understanding the Boiling Point of Dichloromethane

The boiling point of a substance is the temperature at which the vapor pressure of the liquid equals the pressure surrounding the liquid, and bubbles of vapor form inside the liquid. This point is a fundamental physical property that is influenced by various factors, including the strength of intermolecular forces, the molar mass of the substance, and the pressure of the system.

For dichloromethane, the boiling point is relatively low compared to other common solvents, such as water (100°C or 212°F) and ethanol (78.5°C or 173.3°F). This lower boiling point is a result of the weaker intermolecular forces present in dichloromethane, which is a non-polar molecule with a relatively low molar mass of 84.93 g/mol.

Factors Affecting the Boiling Point of Dichloromethane

The boiling point of dichloromethane can be influenced by several factors, including:

1. Pressure: The boiling point of a substance is inversely proportional to the pressure of the system. As the pressure decreases, the boiling point also decreases, 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, and R is the universal gas constant.

1. Temperature: The boiling point of dichloromethane is directly proportional to the temperature of the system. As the temperature increases, the vapor pressure of the liquid also increases, leading to a higher boiling point.

2. Presence of Impurities: The presence of impurities in dichloromethane can affect its boiling point. Impurities can interact with the dichloromethane molecules, altering the intermolecular forces and, consequently, the boiling point.

3. Solute Concentration: If dichloromethane is used as a solvent, the presence of solutes can also affect its boiling point. The boiling point elevation, described by Raoult’s law, is a colligative property that depends on the mole fraction of the solute in the solution.

Practical Applications of Dichloromethane’s Boiling Point

The low boiling point of dichloromethane makes it a useful solvent for various applications, including:

1. Chemical Reactions: Dichloromethane is commonly used as a solvent in organic synthesis reactions, where its low boiling point allows for easy separation and recovery of the product.

2. Extraction Processes: The low boiling point of dichloromethane makes it suitable for extraction processes, where it can be used to selectively remove target compounds from complex mixtures.

3. Cleaning and Degreasing: Dichloromethane’s low boiling point and ability to dissolve a wide range of organic compounds make it a useful cleaning and degreasing agent, particularly in industrial and laboratory settings.

4. Pharmaceutical and Cosmetic Industries: Dichloromethane is used as a solvent in the production of certain pharmaceuticals and cosmetic products, taking advantage of its low boiling point and ability to dissolve a variety of substances.

Numerical Examples and Calculations

To illustrate the practical application of dichloromethane’s boiling point, let’s consider a few numerical examples:

1. Boiling Point Elevation: Suppose a solution is prepared by dissolving 5 grams of sucrose (molar mass = 342.30 g/mol) in 100 grams of dichloromethane. Calculate the boiling point elevation of the solution.

Given:
– Mass of sucrose = 5 g
– Mass of dichloromethane = 100 g
– Molar mass of sucrose = 342.30 g/mol
– Boiling point of pure dichloromethane = 39.7°C

Using Raoult’s law:
ΔTb = Kb * m
where ΔTb is the boiling point elevation, Kb is the boiling point elevation constant for dichloromethane (5.39°C/m), and m is the molality of the solution.

Calculating the molality:
m = (5 g / 342.30 g/mol) / (100 g / 18.02 g/mol) = 0.0833 mol/kg

Calculating the boiling point elevation:
ΔTb = 5.39°C/m * 0.0833 mol/kg = 0.449°C

Therefore, the boiling point of the solution is 39.7°C + 0.449°C = 40.149°C.

1. Vapor Pressure and Boiling Point: Suppose the vapor pressure of dichloromethane at 25°C is 47.3 kPa. Calculate the boiling point of dichloromethane at this pressure.

Given:
– Vapor pressure of dichloromethane at 25°C = 47.3 kPa

Using 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 of dichloromethane (28.42 kJ/mol), and R is the universal gas constant (8.314 J/mol·K).

Rearranging the equation to solve for T2:
T2 = 1 / ((1/T1) - (R/ΔHvap) * ln(P2/P1))

Substituting the values:
T2 = 1 / ((1/298.15 K) - (8.314 J/mol·K / 28.42 kJ/mol) * ln(47.3 kPa / 101.325 kPa))
T2 = 312.15 K

Therefore, the boiling point of dichloromethane at a pressure of 47.3 kPa is approximately 312.15 K or 39.0°C.

These examples demonstrate how the boiling point of dichloromethane can be calculated and applied in various scenarios, highlighting the importance of understanding this fundamental property.

Conclusion

The boiling point of dichloromethane is a well-defined and quantifiable property that has been extensively studied and documented. With a boiling point of approximately 39.7°C (103.5°F) or 312.85 Kelvin, dichloromethane is a useful solvent for a variety of applications in chemistry, industry, and beyond. By understanding the factors that influence the boiling point of dichloromethane, science students and professionals can make informed decisions and effectively utilize this versatile compound in their work.

References

1. O’Neil, M. J. (2013). The Merck Index: An Encyclopedia of Chemicals, Drugs, and Biologicals. Royal Society of Chemistry.
2. U.S. Coast Guard. (1984). CHRIS – Hazardous Chemical Data. Washington, DC: U.S. Government Printing Office.
3. Raoult’s Law: https://chem.libretexts.org/Bookshelves/Physical_and_Theoretical_Chemistry_Textbook_Maps/Supplemental_Modules_(Physical_and_Theoretical_Chemistry)/Equilibria/Solutions_and_Mixtures/Raoult’s_Law
4. Clausius-Clapeyron Equation: https://chem.libretexts.org/Bookshelves/Physical_and_Theoretical_Chemistry_Textbook_Maps/Supplemental_Modules_(Physical_and_Theoretical_Chemistry)/Thermodynamics/Phase_Transitions/The_Clausius-Clapeyron_Equation
5. Dichloromethane Properties: https://www.epa.gov/sites/default/files/2016-09/documents/methylene-chloride.pdf