Precision Glass Isoteniscope Cell
Features a sample reservoir bulb, vapor reflux trap, and integral differential U-tube manometer containing pure liquid ethanol to indicate pressure equality without external contamination.
Chemistry / Clausius-Clapeyron Vapor Pressure
FREE TO EXPLORELABORATORY 15 · NATURAL SCIENCES
Determination of the Enthalpy of Vaporization from Liquid Vapor Pressure via the Clausius-Clapeyron Equation
Determine the standard molar enthalpy of vaporization (ΔH_vap°) of pure absolute ethanol by measuring its equilibrium vapor pressure as a function of temperature using a static isoteniscope apparatus, linearizing the liquid-vapor phase boundary with the Clausius-Clapeyron equation, and extracting ΔH_vap° and the normal boiling point via ordinary least-squares regression. Explore interactive analytical gravimetry, Class A volumetric glassware and density determination.
MEASURE · CALIBRATE · ANALYSE
Your virtual chemistry laboratory.
EQUILIBRIUM VAPOR PRESSURE P—
MEASUREMENT STATUS
Ready to dispense 50.0 °C of liquid.
Adjust bath temperature V, then weigh the dispensed aliquot.
Mass observations are determined by the high-precision thermodynamic equation of state for pure liquid water. Readings reflect thermal equilibrium and analytical balance force restoration.
Physical chemistry assumptions and metrological limits
The linear Clausius-Clapeyron model assumes constant enthalpy of vaporization over the 30–78 °C range, ideal gas behavior of ethanol vapor, and negligible liquid molar volume compared to gas volume. Over this modest 48 °C interval, ethanol vapor compressibility Z remains between 0.98 and 1.00, and ΔH_vap varies by less than 1.5%, which is effectively smoothed by the linear regression fit. The physical simulation model calculates equilibrium vapor pressure across temperature using the integrated Clausius-Clapeyron relation ln(P / P_b) = −(ΔH_vap / R) · (1/T − 1/T_b), where P_b = 101.325 kPa is standard atmospheric pressure at the normal boiling point T_b = 351.52 K (78.37 °C). Physicochemical parameters for pure absolute ethanol: molar mass M = 46.069 g/mol; universal gas constant R = 8.31446 J/(mol·K) = 0.00831446 kJ/(mol·K); accepted standard molar enthalpy of vaporization: ΔH_vap° = 38.560 kJ/mol (38560 J/mol). Scaled regression coordinates: independent variable x = 1000.0 / T (in K⁻¹), dependent variable y = ln(P in kPa). Theoretical regression slope: m_slope = −ΔH_vap / (1000 · R) = −38560.0 / 8314.46 = −4.63854 K. Theoretical intercept: C = ln(P_b) + (ΔH_vap / R) · (1/T_b) = ln(101.325) + 4.63854 · (1000 / 351.52) = 4.6183 + 13.1957 = 17.8140. Across water bath temperatures from 30.0 °C to 78.0 °C, vapor pressures span P = 12.34 kPa to 99.93 kPa. Linear regression across the six temperature settings isolates ΔH_vap = −m_slope · (1000 · R) with R² = 1.00000; the null-manometer isoteniscope design completely prevents non-condensable air bubble contamination and capillary depression artifacts.
Analytical chemistry apparatus & tools.
Constant-Temperature Circulating Water Bath (±0.02 °C)
Maintains uniform, highly regulated thermal equilibration around the submerged isoteniscope cell using a motorized impeller and PID electronic heater.
Digital Absolute Pressure Transducer (±0.01 kPa)
Measures the absolute gas pressure inside the external ballast manifold with high precision (0.01 kPa = 0.08 mmHg) when the isoteniscope U-tube is nulled.
5-Liter Vacuum Ballast Surge Reservoir and Needle Valves
Buffers pressure fluctuations and allows ultra-fine throttling of air bleed and vacuum evacuation to precisely balance the isoteniscope manometer liquid levels.
Two-Stage Rotary Vane Vacuum Pump and Cold Trap
Evacuates the manifold down to 0.1 kPa during initial boiling and degassing of the sample to purge trapped air from the reservoir bulb.
NIST-Calibrated Precision Digital Thermometer (±0.01 °C)
Measures the water bath temperature directly adjacent to the isoteniscope sample bulb ensuring exact thermodynamic temperature registration.
FOLLOW THE METHOD
A reason behind every step.
Determine the standard molar enthalpy of vaporization (ΔH_vap°) of pure absolute ethanol by measuring its equilibrium vapor pressure as a function of temperature using a static isoteniscope apparatus, linearizing the liquid-vapor phase boundary with the Clausius-Clapeyron equation, and extracting ΔH_vap° and the normal boiling point via ordinary least-squares regression. Suggested independent volumetric settings: 30.0, 40.0, 50.0, 60.0, 70.0, 78.0 °C.
01
Isoteniscope Loading and System Assembly
Fill the isoteniscope reservoir bulb two-thirds full with pure anhydrous ethanol (20–25 mL). Tilt the apparatus to introduce enough liquid into the short U-tube to form a 3–4 cm liquid manometer seal. Mount the isoteniscope inside the water bath and connect to the vacuum ballast manifold.
02
Degassing and Trapped Air Purging
Start the vacuum pump and carefully open the evacuation valve until the ethanol in the bulb boils vigorously under reduced pressure. Vapors sweep out all residual non-condensable air from the bulb and inner U-tube leg. Boil for 1–2 minutes, then slowly admit air through the fine needle valve until the U-tube levels match.
03
Thermal Bath Equilibration
Set the circulating water bath controller to the initial assigned temperature (30.0 °C). Allow at least 10 minutes of continuous motorized stirring for the ethanol sample in the submerged bulb to achieve true thermodynamic equilibrium with the bath.
04
Manometer Nulling and Vapor Pressure Acquisition
Carefully manipulate the vacuum valve and air bleed needle valve to adjust the pressure in the ballast manifold until the liquid ethanol menisci in both arms of the isoteniscope U-tube are at the exact same horizontal height. Record the digital pressure transducer reading P (kPa) and bath temperature t (°C).
05
Temperature Series Progression
Increase the water bath temperature systematically to 40.0, 50.0, 60.0, 70.0, and 78.0 °C. At each plateau, re-null the U-tube levels by admitting air into the manifold as vapor pressure increases, allowing 8 minutes for thermal equilibration before logging pressure.
06
Clausius-Clapeyron Linear Regression and Analysis
Convert all temperatures to Kelvin (T = t + 273.15 K) and calculate inverse temperatures 1000/T (K⁻¹). Calculate natural logarithms of vapor pressure ln(P/kPa). Perform ordinary least-squares regression of ln(P) vs 1000/T, extract the slope m_slope, compute ΔH_vap° = −m_slope · 8.31446 kJ/mol, and extrapolate the normal boiling point at 101.325 kPa.
YOUR NOTEBOOK · 0 OBSERVATIONS
Turn measurements into quantitative chemical insight.
Points are your recorded gravimetric observations. The regression line validates the linear mass-volume proportionality and extracts density.
ORDINARY LEAST-SQUARES FIT · m = ρ·V + m₀
ln(P/kPa) = −[ΔH_vap / (1000·R)] · (1000/T) + C ==> ln(P) = −4.639 · (1000/T) + 17.814
| # | Water bath temperature t (°C) (°C) | Equilibrium vapor pressure P (kPa) | Inverse absolute temperature 1000/T (K⁻¹) | Natural logarithm of vapor pressure ln(P/kPa) | Remove |
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