How To Calculate Heat of Vaporization

Clausius-Clapeyron Equation:

\[ \Delta H_{vap} = - R \frac{\ln (P_2 / P_1)}{1/T_2 - 1/T_1} \]

Vapor Pressure at T₁ (P₁):

Vapor Pressure at T₂ (P₂):

Temperature 1 (T₁):

Temperature 2 (T₂):

Heat of Vaporization (ΔH_vap):

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1. What is Heat of Vaporization?

Heat of vaporization (ΔH_vap) is the amount of energy required to convert a given amount of a substance from liquid to gas at constant temperature and pressure. It's an important thermodynamic property that varies between different substances.

2. How Does the Calculator Work?

The calculator uses the Clausius-Clapeyron equation:

\[ \Delta H_{vap} = - R \frac{\ln (P_2 / P_1)}{1/T_2 - 1/T_1} \]

Where:

ΔH_vap — Heat of vaporization (J/mol)
R — Gas constant (8.314 J/mol·K)
P₁ — Vapor pressure at temperature T₁ (Pa)
P₂ — Vapor pressure at temperature T₂ (Pa)
T₁ — Temperature 1 (K)
T₂ — Temperature 2 (K)

Explanation: This equation relates the vapor pressures of a substance at two different temperatures to its heat of vaporization, assuming ideal gas behavior and that the molar volume of liquid is negligible compared to that of vapor.

3. Importance of Heat of Vaporization

Details: Heat of vaporization is crucial in various applications including distillation processes, refrigeration systems, weather forecasting, and understanding phase transitions in chemical engineering and physical chemistry.

4. Using the Calculator

Tips: Enter vapor pressures in Pascals (Pa) and temperatures in Kelvin (K). Ensure T₁ and T₂ are different values. All values must be positive and non-zero.

5. Frequently Asked Questions (FAQ)

Q1: Why must temperatures be in Kelvin?
A: The Clausius-Clapeyron equation requires absolute temperature for thermodynamic calculations, and Kelvin is the SI unit for absolute temperature.

Q2: What are typical values for heat of vaporization?
A: Water has a high heat of vaporization of about 40.7 kJ/mol at 100°C. Most organic liquids range from 20-50 kJ/mol.

Q3: When is this equation most accurate?
A: The equation works best when the temperature range is small and the substance behaves ideally. For large temperature ranges, integrated forms may be more appropriate.

Q4: Can I use different pressure units?
A: Yes, but both pressures must be in the same units. The calculator currently expects Pascals, but you can convert from other units before entering values.

Q5: What if my temperatures are too close together?
A: The calculation becomes less accurate with very small temperature differences. A temperature difference of at least 5-10K is recommended for reliable results.