Calculating Vapor Pressure From Boiling Point And Enthalpy Of

Clausius-Clapeyron Equation:

\[ P = \exp\left[ -\frac{\Delta H_{vap}}{R} \left( \frac{1}{T} - \frac{1}{T_b} \right) + \ln P_0 \right] \]

Enthalpy of Vaporization (ΔH_vap):

J/mol

Temperature (T):

Boiling Point (T_b):

Reference Pressure (P_0):

Vapor Pressure (P):

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1. What is the Clausius-Clapeyron Equation?

The Clausius-Clapeyron equation describes the relationship between vapor pressure and temperature for a substance. It's derived from thermodynamic principles and is particularly useful for estimating vapor pressure at different temperatures when the boiling point and enthalpy of vaporization are known.

2. How Does the Calculator Work?

The calculator uses the Clausius-Clapeyron equation:

\[ P = \exp\left[ -\frac{\Delta H_{vap}}{R} \left( \frac{1}{T} - \frac{1}{T_b} \right) + \ln P_0 \right] \]

Where:

\( P \) — Vapor pressure at temperature T (Pa)
\( \Delta H_{vap} \) — Enthalpy of vaporization (J/mol)
\( R \) — Gas constant (8.314 J/mol·K)
\( T \) — Temperature (K)
\( T_b \) — Boiling point (K)
\( P_0 \) — Reference pressure at boiling point (typically 101325 Pa)

Explanation: The equation calculates how vapor pressure changes with temperature based on the energy required for vaporization and the reference point at the boiling temperature.

3. Importance of Vapor Pressure Calculation

Details: Vapor pressure calculations are essential in chemical engineering, atmospheric science, and material science for predicting evaporation rates, designing distillation processes, and understanding phase behavior of substances.

4. Using the Calculator

Tips: Enter all values in the specified units. Temperature and boiling point must be in Kelvin. The reference pressure is typically 101325 Pa (standard atmospheric pressure at sea level).

5. Frequently Asked Questions (FAQ)

Q1: Why use Kelvin instead of Celsius?
A: The Clausius-Clapeyron equation requires absolute temperature values, making Kelvin the appropriate unit as it starts from absolute zero.

Q2: What is the typical range for enthalpy of vaporization?
A: Enthalpy of vaporization varies by substance but typically ranges from 20-50 kJ/mol for common liquids at their boiling points.

Q3: Can this equation be used for all substances?
A: The equation works best for substances where the vapor behaves ideally and the enthalpy of vaporization is constant over the temperature range.

Q4: How accurate is this calculation?
A: The accuracy depends on the constancy of ΔH_vap over the temperature range. For large temperature differences, the integrated form with temperature-dependent ΔH_vap may be more accurate.

Q5: What if I don't know the reference pressure?
A: For most applications, use 101325 Pa (standard atmospheric pressure) as this is the pressure at which boiling points are typically measured.