Vapor Pressure & Phase Equilibrium
Vapor pressure is the pressure exerted by vapor in equilibrium with its liquid at a given temperature. The Antoine equation log P = A - B/(C+T) and Clausius-Clapeyron relate vapor pressure to temperature. Boiling occurs when vapor pressure equals atmospheric pressure.
Why This Chemistry Calculation Matters
Why: Vapor pressure governs distillation, evaporation, humidity, and phase equilibria. Essential for chemical engineering, meteorology, and process design.
How: Use Clausius-Clapeyron with two known (T,P) points and ΔHvap for small temperature ranges. Use Antoine equation with substance-specific constants for higher accuracy over wider ranges.
- ●Antoine equation is empirical; constants A, B, C are substance-specific (NIST, CRC).
- ●ΔHvap varies with temperature; Clausius-Clapeyron assumes it constant.
- ●At 100°C, water vapor pressure = 101.325 kPa = 1 atm (normal boiling).
Compact Examples
Inputs
For educational and informational purposes only. Verify with a qualified professional.
🔬 Chemistry Facts
Water vapor pressure doubles roughly every 10°C near room temperature.
— NIST
Antoine equation constants A, B, C are substance-specific.
— CRC
ΔHvap varies with T; Clausius-Clapeyron assumes it constant.
— Thermodynamics
Ethanol has higher vapor pressure than water at same T (more volatile).
— NIST
📋 Key Takeaways
- • Vapor pressure | Pressure of vapor in equilibrium with liquid; increases with temperature
- • Clausius-Clapeyron | ln(P₂/P₁) = -ΔHvap/R × (1/T₂ - 1/T₁)
- • Antoine equation | log₁₀(P) = A - B/(C + T); more accurate over wide ranges
- • Boiling | Occurs when vapor pressure equals atmospheric pressure
Did You Know?
Water vapor pressure doubles roughly every 10°C near room temperature.
Source: NIST
Antoine equation is empirical; constants A, B, C are substance-specific.
Source: Perry's Handbook
ΔHvap varies with temperature; Clausius-Clapeyron assumes it constant.
Source: Thermodynamics
At 100°C, water vapor pressure = 101.325 kPa = 1 atm (normal boiling).
Source: IUPAC
High altitude: lower P_atm → water boils below 100°C.
Source: Meteorology
Ethanol has higher vapor pressure than water at same T (more volatile).
Source: Chemistry
How Vapor Pressure Calculation Works
Clausius-Clapeyron relates P at two temperatures using ΔHvap. Antoine equation uses empirical constants for higher accuracy over wider ranges.
Clausius-Clapeyron
ln(P₂/P₁) = -ΔHvap/R × (1/T₂ - 1/T₁)
Antoine
log₁₀(P) = A - B/(C + T)
Expert Tips
Use Kelvin
Always convert T to K for Clausius-Clapeyron.
Antoine Range
Use Antoine within its validated T range.
Small ΔT
Clausius-Clapeyron best for <50°C range.
NIST Data
Consult NIST WebBook for precise values.
Common Substances
| Substance | Formula | ΔHvap (kJ/mol) | Boiling Point (K) |
|---|---|---|---|
| Water | H_{2}O | 40.66 | 373.15 |
| Ethanol | C_{2}H₅ ext{OH} | 38.56 | 351.44 |
| Methanol | CH_{3} ext{OH} | 35.21 | 337.65 |
| Acetone | C_{3}H₆O | 29.1 | 329.2 |
| Benzene | C₆H₆ | 30.72 | 353.25 |
| Toluene | C₇H₈ | 33.18 | 383.75 |
| Chloroform | CHCl_{3} | 29.24 | 334.33 |
| Diethyl Ether | C₄H_{1}_{0}O | 26.52 | 307.65 |
| Hexane | C₆H_{1}₄ | 28.85 | 341.88 |
| Octane | C₈H_{1}₈ | 34.41 | 398.82 |
Key Concepts
Temperature Dependence
Vapor pressure increases exponentially with temperature. Higher temperatures give molecules more kinetic energy to escape.
Enthalpy of Vaporization
The energy required to convert liquid to vapor. Higher ΔHvap means stronger intermolecular forces and lower vapor pressure.
Boiling Point
The temperature at which vapor pressure equals atmospheric pressure. Normal boiling point is at 1 atm.
FAQ
What is vapor pressure?
Pressure exerted by vapor in equilibrium with its liquid at a given T. Higher T → higher P.
Clausius-Clapeyron vs Antoine?
CC: thermodynamic, good for small ΔT. Antoine: empirical, more accurate over wide ranges.
Why use Kelvin?
Clausius-Clapeyron requires absolute temperature; 1/T terms must use K.
When does boiling occur?
When vapor pressure equals atmospheric pressure.
Limitations?
Near critical point, high P, mixtures need Raoult's law. ΔHvap assumed constant.
Where to get Antoine constants?
NIST WebBook, Perry's Handbook, DIPPR database.
Key Numbers
📚 Sources
⚠️ Disclaimer: This calculator uses NIST vapor pressure data and the Antoine equation. For precise work, consult NIST Chemistry WebBook, IUPAC Gold Book (vapor pressure definitions), and Perry's Chemical Engineers' Handbook for Antoine constants. Actual results may vary with temperature range, substance purity, and pressure conditions.
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