The Boiling Point of Glacial Acetic Acid: A Comprehensive Guide

The boiling point of glacial acetic acid, a pure form of acetic acid, is a crucial parameter in various scientific and industrial applications. At standard atmospheric pressure (760 mmHg), the boiling point of glacial acetic acid is 118.1°C, a value that is consistent with the boiling points of acetic acid-water mixtures at even percentages in the liquid phase.

Understanding the Boiling Point of Glacial Acetic Acid

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. The boiling point of glacial acetic acid is influenced by several factors, including:

1. Molecular Structure: Acetic acid (CH3COOH) is a carboxylic acid with a polar, hydrogen-bonding functional group. This structure contributes to the relatively high boiling point compared to other organic compounds of similar molar mass.

2. Intermolecular Forces: The strong hydrogen bonding between acetic acid molecules results in higher intermolecular forces, which require more energy to overcome and cause the liquid to boil.

3. Pressure: The boiling point of glacial acetic acid, like any liquid, is directly proportional to the surrounding pressure. As the pressure increases, the boiling point also increases, and vice versa.

4. Boiling Point-Pressure Relationship: The relationship between the boiling point and pressure of a liquid can be described by the Clausius-Clapeyron equation:

   ln(P2/P1) = (ΔHvap/R) * (1/T1 - 1/T2)

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

5. Example Calculation: If the boiling point of glacial acetic acid is 118.1°C at 760 mmHg, the boiling point at 500 mmHg can be calculated using the Clausius-Clapeyron equation:

   ln(P2/760) = (ΔHvap/R) * (1/391.25 - 1/500)
P2 = 500 mmHg
T2 = 111.6°C

   This shows that the boiling point of glacial acetic acid decreases as the pressure is reduced.

6. Dissolved Substances: The presence of dissolved salts or other solutes in the acetic acid can affect its boiling point, either increasing or decreasing it depending on the nature of the solute.

7. Boiling Point Elevation: The addition of a non-volatile solute, such as a salt, to a solvent (in this case, acetic acid) increases the boiling point of the solution. This is known as boiling point elevation and can be described by the following equation:

   ΔTb = Kb * m

   Where:
   – ΔTb is the boiling point elevation
   – Kb is the boiling point elevation constant (for acetic acid, Kb = 3.07°C/m)
   – m is the molality of the solution

8. Boiling Point Depression: Conversely, the addition of a volatile solute, such as water, to acetic acid decreases the boiling point of the solution. This is known as boiling point depression and can be described by the following equation:

   ΔTb = Kb * m

   Where:
   – ΔTb is the boiling point depression
   – Kb is the boiling point depression constant (for acetic acid, Kb = 3.07°C/m)
   – m is the molality of the solution

Applications of the Boiling Point of Glacial Acetic Acid

The boiling point of glacial acetic acid has several important applications in various fields, including:

1. Petroleum Distillation: The boiling point of glacial acetic acid is used as a reference point in determining the boiling ranges of various fractions in petroleum distillation. This information is crucial for the efficient separation and purification of different hydrocarbon components in crude oil.

2. Example: Table 1 in the provided source shows the boiling point ranges of various petroleum fractions, with the boiling point of glacial acetic acid (118.1°C) serving as a reference point.

3. Chemical Synthesis: The boiling point of glacial acetic acid is an important parameter in chemical synthesis reactions, as it helps determine the appropriate temperature conditions for the reaction to occur efficiently.

4. Analytical Chemistry: The boiling point of glacial acetic acid is used as a reference point in various analytical techniques, such as gas chromatography, where it helps in the identification and quantification of organic compounds.

5. Thermodynamic Studies: The boiling point of glacial acetic acid is a crucial parameter in thermodynamic studies, as it is used to calculate other thermodynamic properties, such as the enthalpy of vaporization and the vapor pressure of the liquid.

6. Industrial Applications: The boiling point of glacial acetic acid is relevant in various industrial processes, such as the production of acetic anhydride, which is used in the manufacture of cellulose acetate and other acetylated compounds.

Factors Affecting the Boiling Point of Glacial Acetic Acid

The boiling point of glacial acetic acid can be influenced by several factors, including:

1. Purity: The presence of impurities, such as water or other organic compounds, can affect the boiling point of glacial acetic acid. Higher purity levels generally result in a more accurate and consistent boiling point.

2. Pressure: As mentioned earlier, the boiling point of glacial acetic acid is directly proportional to the surrounding pressure. Changes in pressure can significantly alter the boiling point.

3. Dissolved Substances: The addition of dissolved salts or other solutes can either increase or decrease the boiling point of glacial acetic acid, depending on the nature of the solute.

4. Experimental Conditions: Factors such as the accuracy of temperature measurement, the rate of heating, and the presence of nucleation sites can all influence the observed boiling point of glacial acetic acid.

Numerical Examples and Calculations

1. Boiling Point Elevation: Calculate the boiling point of a 0.5 molal solution of sodium acetate in glacial acetic acid.

Given:
– Boiling point elevation constant (Kb) for acetic acid = 3.07°C/m
– Molality of sodium acetate solution = 0.5 m

Using the boiling point elevation equation:
ΔTb = Kb * m
ΔTb = 3.07°C/m * 0.5 m = 1.535°C

Therefore, the boiling point of the 0.5 molal sodium acetate solution in glacial acetic acid is:
Boiling point = 118.1°C + 1.535°C = 119.635°C

1. Boiling Point Depression: Calculate the boiling point of a 10% (by mass) acetic acid-water solution.

Given:
– Boiling point depression constant (Kb) for acetic acid = 3.07°C/m
– Mass fraction of acetic acid in the solution = 0.10 (10% by mass)

To calculate the molality of the solution, we need to convert the mass fraction to mole fraction:
Mole fraction of acetic acid = (0.10 * 60.05 g/mol) / [(0.10 * 60.05 g/mol) + (0.90 * 18.02 g/mol)] = 0.1667

Molality of the solution = 0.1667 / (0.90 * 18.02 g/mol) = 0.5556 m

Using the boiling point depression equation:
ΔTb = Kb * m
ΔTb = 3.07°C/m * 0.5556 m = 1.706°C

Therefore, the boiling point of the 10% acetic acid-water solution is:
Boiling point = 100°C – 1.706°C = 98.294°C

These examples demonstrate how the boiling point of glacial acetic acid can be affected by the presence of dissolved substances, and how the Clausius-Clapeyron equation and boiling point elevation/depression equations can be used to calculate the new boiling point under different conditions.

Conclusion

The boiling point of glacial acetic acid is a crucial parameter in various scientific and industrial applications. Understanding the factors that influence the boiling point, such as molecular structure, intermolecular forces, pressure, and the presence of dissolved substances, is essential for accurately predicting and manipulating the boiling behavior of this important chemical compound. The numerical examples provided in this guide illustrate the practical applications of the boiling point of glacial acetic acid and the relevant equations used to calculate its value under different conditions.

References:

1. Boiling Point of Acetic Acid-Water Mixtures
2. Boiling Point Ranges of Petroleum Fractions
3. Effect of Dissolved Salts on Boiling Point of Acetic Acid
4. Clausius-Clapeyron Equation and Boiling Point-Pressure Relationship