Measure the saturated vapor pressure of pure ethyl alcohol (ethanol) across temperatures from 20.0 °C to 80.0 °C using an immersed vapor pressure tube and precision mercury manometer, verify the Clausius-Clapeyron equation ln(P) = −(ΔH_vap / R)(1/T) + C, and determine the molar enthalpy of vaporization ΔH_vap. Explore a virtual apparatus, collect readings and analyse a graph.
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Adjust the apparatus, then take a reading.
Readings are generated by an ideal model. The diagram is schematic and is not to scale.
The teaching model assumes the enthalpy of vaporization ΔH_vap remains constant over the 20 °C to 80 °C temperature range and that the ethanol vapor behaves as an ideal gas. In rigorous thermodynamics, ΔH_vap decreases slightly with temperature due to non-zero heat capacity differences (ΔC_p = C_p,v − C_p,l), and ethanol vapor exhibits slight non-ideality near its normal boiling point. The Clausius-Clapeyron equation provides the thermodynamic cornerstone connecting macroscopic phase equilibria with microscopic intermolecular binding forces. The measured molar enthalpy of vaporization ΔH_vap = 41.92 kJ mol⁻¹ demonstrates the substantial energetic cost of disrupting intermolecular hydrogen bonds during the liquid-to-vapor phase transition.
A specialized glass U-tube containing an evacuated upper bulb partially filled with degassed liquid ethanol, with the liquid-vapor meniscus positioned inside the submerged bulb and communicating with a mercury column.
Measures the difference in mercury column heights between the vapor-filled closed limb and open atmospheric limb to register the differential gauge vapor pressure Δh to ±0.2 mmHg.
Surrounds the vapor bulb in a deep water bath, heated by an adjustable electrical resistance coil to maintain steady operating temperatures between 20.0 °C and 80.0 °C.
Provides continuous, vigorous fluid circulation throughout the heating bath, eliminating vertical thermal stratification and ensuring identical temperature across the entire vapor bulb.
Calibrated mercury-in-glass thermometer with 0.1 °C subdivisions, suspended directly alongside the vapor bulb to record the liquid equilibrium temperature to ±0.05 °C.
Determines the ambient atmospheric pressure B in mmHg and kPa, required to convert open-limb manometer differential readings into total absolute vapor pressure P = B + Δh.
Measure the saturated vapor pressure of pure ethyl alcohol (ethanol) across temperatures from 20.0 °C to 80.0 °C using an immersed vapor pressure tube and precision mercury manometer, verify the Clausius-Clapeyron equation ln(P) = −(ΔH_vap / R)(1/T) + C, and determine the molar enthalpy of vaporization ΔH_vap. Suggested independent settings: 20.0, 35.0, 50.0, 65.0, 80.0 °C.
Read the laboratory Fortin mercurial barometer to 0.1 mmHg, applying temperature and brass scale corrections to determine the ambient atmospheric pressure B (baseline B = 760.0 mmHg = 101.325 kPa). Inspect the vapor pressure tube to verify that the upper bulb contains pure liquid ethanol free of trapped atmospheric air bubbles and that the mercury surfaces are clean and reflective.
Because the open limb of the manometer is exposed to the atmosphere, the absolute vapor pressure is calculated from P = B + Δh. An accurate barometric reading is indispensable for evaluating absolute pressure.
Fill the water bath with tap water adjusted to 20.0 °C. Position the vapor bulb completely submerged beneath the liquid surface and run the motorized stirrer for at least five minutes. Confirm that the bath thermometer reading remains steady at 20.0 °C and the mercury menisci in the manometer limbs have ceased drifting. Read the heights of the mercury columns in both limbs.
Sufficient equilibration time guarantees that the liquid ethanol and its vapor achieve true dynamic phase equilibrium at the measured bath temperature before taking data.
Energize the electric heating coil to warm the bath progressively in 15.0 °C intervals (35.0 °C, 50.0 °C, 65.0 °C, 80.0 °C). Keep the propeller stirrer operating continuously at moderate speed to circulate the water evenly around the glass vapor chamber and thermometer stem.
Active stirring prevents convective thermal gradients from forming in the bath, ensuring that the enclosed ethanol sample attains the identical temperature indicated by the thermometer.
Upon reaching each target temperature, adjust the heater power to maintain a stable plateau and wait for a dwell interval of at least three to four minutes. Confirm that the temperature remains constant to within ±0.05 °C and the mercury column elevation stabilizes.
Because glass has finite thermal conductivity and liquid evaporation absorbs latent heat, a dwell period is required to re-establish thermal and phase equilibrium between the bath, liquid ethanol, and saturated vapor.
Read the mercury column levels in both the closed vapor limb (h_closed) and open atmospheric limb (h_open) using the mirror-backed millimeter scale. Align the optical line of sight so that the mercury meniscus directly occludes its reflected image in the mirror, eliminating parallax error. Compute the gauge pressure Δh = h_open − h_closed and determine the absolute pressure P = B + Δh in kilopascals (kPa).
Mirror-backed sighting removes visual parallax, guaranteeing reliable millimeter height measurements across all thermal stations.
Turn off the electric heater and record repeat readings at the same temperature stations as the bath cools naturally to verify the absence of thermal hysteresis or air ingress. Convert all temperatures to absolute kelvins (T = t + 273.15 K) and compute the reciprocal temperature 1/T. Calculate the natural logarithm of vapor pressure ln(P / Pa), plot ln(P) versus 1/T, and determine the slope m to evaluate the molar enthalpy of vaporization ΔH_vap = −m R.
Verifying identical pressures during cooling proves that the sample is free from non-condensable air contamination and that the system remained in reversible thermodynamic equilibrium throughout the cycle.
Points are your observations. The fit uses SI x and y values before display scaling.
| # | Bath temperature t (°C) | Saturated vapor pressure P (kPa) | Reciprocal temperature 1/T (10⁻³ K⁻¹) | Log vapor pressure ln(P / Pa) | Remove |
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