Vapor Pressure Measurement Discussion Examples | How to Interpret a Clausius–Clapeyron Plot

What Is Vapor Pressure?

Vapor pressure is the pressure exerted by a vapor when a liquid and its vapor are in equilibrium.
When a liquid is placed in a sealed container, molecules evaporate from the liquid surface and move into the gas phase.
At the same time, some molecules in the gas phase return to the liquid surface.
When the rate of evaporation becomes equal to the rate of condensation, the pressure in the gas phase becomes constant, and this pressure is the vapor pressure.

Vapor pressure becomes larger when liquid molecules can move into the gas phase more easily.
Liquids with weak intermolecular interactions or high volatility tend to have high vapor pressures.
In contrast, liquids with strong intermolecular interactions tend to have lower vapor pressures because the molecules are more strongly retained in the liquid.

Main Items to Include in the Results

In the results of vapor pressure measurement, organize the vapor pressure at each temperature, conversion of temperature to absolute temperature, 1/T, lnP, the Clausius-Clapeyron plot, the equation of the fitted line, the slope, and the enthalpy of vaporization.
Because calculations will be incorrect if temperature is handled in °C, it must always be converted to K before analysis.

Main Items to Include in the Results

• Name of the liquid measured
• Measured temperature
• Absolute temperature T
• Vapor pressure P
• 1/T
• lnP
• Clausius-Clapeyron plot
• Equation of the fitted line
• Coefficient of determination R²
• Slope of the fitted line
• Enthalpy of vaporization
• Comparison with literature values
• Error rate
• Temperature- and pressure-measurement errors

Reference Experimental Values for Vapor Pressure Measurement and an Example of Clausius-Clapeyron Plot Analysis

Here, reference experimental values are organized for measuring the vapor pressure of a liquid at different temperatures and determining the enthalpy of vaporization from a Clausius-Clapeyron plot.
Temperature, vapor pressure, lnP, 1/T, plot slope, boiling-point estimation, and measurement errors are presented in a form that is easy to discuss in a report.

The vapor pressure of a liquid increases as the temperature rises.
This is because an increase in temperature increases the kinetic energy of liquid molecules and increases the number of molecules that move into the gas phase.
The relationship between vapor pressure and temperature can be analyzed in relation to the enthalpy of vaporization using the Clausius-Clapeyron equation.

Reference Experimental Conditions

Form of the Clausius-Clapeyron Equation

If the enthalpy of vaporization can be regarded as approximately constant over the measured temperature range, the relationship between vapor pressure P and absolute temperature T can be expressed as follows.

lnP = −ΔH_vap / R × 1/T + C

Here, ΔH_vap is the enthalpy of vaporization, R is the gas constant, T is the absolute temperature, and C is a constant.
Therefore, when lnP is plotted on the vertical axis and 1/T on the horizontal axis, the slope is −ΔH_vap/R.

ΔH_vap = −Slope × R

Using the gas constant R = 8.314 J/(mol·K), the enthalpy of vaporization can be determined from the slope of the plot.

Example Measurements of Temperature and Vapor Pressure

Reference data for measuring vapor pressure at different temperatures are shown below using ethanol as an example.

Vapor pressure increases rapidly as the temperature rises.
Because the relationship between vapor pressure and temperature is not linear but increases exponentially, it is analyzed by converting the data to lnP and 1/T.

Converted Data for the Clausius-Clapeyron Plot

The following is an example of converting the measured temperature and vapor pressure to 1/T and lnP.
Here, P is used in kPa.

As 1/T becomes smaller, in other words as temperature becomes higher, lnP becomes larger.
When lnP is plotted on the vertical axis and 1/T on the horizontal axis, a relationship close to a downward-sloping straight line is obtained.

Example of Determining the Enthalpy of Vaporization From the Plot Slope

Suppose linear fitting of the Clausius-Clapeyron plot gives the following equation.

lnP = −5100 × 1/T + 19.17

In this case, the slope is −5100 K.
According to the Clausius-Clapeyron equation, slope = −ΔH_vap/R, so

ΔH_vap = −Slope × R

ΔH_vap = −(−5100) × 8.314 = 42400 J/mol = 42.4 kJ/mol

In this reference example, the enthalpy of vaporization is determined to be 42.4 kJ/mol.

Simple Example of Determining the Slope From Two Points

The slope can also be roughly estimated using the two points at 20°C and 60°C.

Slope = (3.850 − 1.775) ÷ (0.003002 − 0.003411)

Slope = 2.075 ÷ (−0.000409) = −5073 K

ΔH_vap = 5073 × 8.314 = 42170 J/mol = 42.2 kJ/mol

The value determined from two points is also close to the value determined from linear fitting.
However, to reduce the effect of experimental error, linear fitting using multiple points is preferable.

Example of Boiling-Point Estimation

The boiling point is the temperature at which the vapor pressure of a liquid becomes equal to the external pressure.
If atmospheric pressure is 101.3 kPa, lnP is as follows.

ln(101.3) = 4.618

Substitute this into the linear equation lnP = −5100 × 1/T + 19.17.

4.618 = −5100 × 1/T + 19.17

−5100 × 1/T = 4.618 − 19.17 = −14.552

1/T = 14.552 ÷ 5100 = 0.002853

T = 350.5 K = 77.4°C

From this calculation, the boiling point is estimated to be approximately 77°C.
This value is close to the boiling point of ethanol, suggesting that the measurement and analysis were generally reasonable.

Differences in Vapor Pressure Among Liquids

Even at the same temperature, vapor pressure differs depending on the type of liquid.
Liquids with weaker intermolecular forces evaporate more readily and have higher vapor pressures.

Water has stronger hydrogen bonding, so at the same temperature its vapor pressure is lower than that of ethanol or acetone.
In contrast, acetone has relatively weak intermolecular forces and is highly volatile because of its high vapor pressure.

Effect of Temperature-Measurement Error

Because 1/T is used in the Clausius-Clapeyron plot, errors in temperature measurement affect the analysis results.

Temperature is converted to K rather than used in °C for calculations.
In addition, it is important not only that the thermometer display be constant but also that the liquid and gas phase have sufficiently reached thermal equilibrium.

Change in Vapor Pressure With Equilibrium Time

Immediately after changing the temperature, the liquid and gas phase may not yet have reached equilibrium, and the vapor pressure may not be stable.

Using values measured before equilibrium may cause vapor pressure to be underestimated.
It is important to wait until the measured value becomes constant before recording it.

Deviation in Pressure Measurement Caused by Air Contamination

When vapor pressure is measured with a pressure gauge, if air remains in the container, the measured pressure includes not only the vapor pressure of the liquid but also the partial pressure of air.

If total pressure is treated directly as vapor pressure, air contamination may cause vapor pressure to be overestimated.
The effect of air must be checked according to the structure of the experimental apparatus and the measurement principle.

Sources of Error in Vapor Pressure Measurement

In theory, the enthalpy of vaporization determined from the slope does not change even if the pressure unit is changed from kPa to Pa.
However, if units are mixed within the same data set, lnP shifts unnaturally and the data cannot be analyzed correctly.

Example When an Outlier Is Present

If vapor pressure at one temperature deviates greatly from the linear trend, insufficient equilibrium, reading errors, or air contamination may be possible causes.

If only the value at 40°C is too high, the cause should not simply be attributed to the properties of the liquid; the measurement operation and condition of the apparatus should be checked.
If an outlier is excluded, the reason for exclusion must be clearly stated.

Precautions When Changing Pressure Units

When calculating lnP, the value of lnP changes depending on whether P is expressed in kPa or Pa.
However, if the same unit is used for all data points, the slope of the plot does not change and the enthalpy of vaporization is also unchanged.

Converting from kPa to Pa increases lnP by a constant amount.
The intercept changes, but because the slope does not change, ΔH_vap is unaffected.
However, if units are mixed partway through the data, the correct linear relationship cannot be obtained.

Example of How to Write the Results

When the vapor pressure of ethanol was measured over the range of 20–60°C, the vapor pressure increased from 5.9 kPa to 47.0 kPa as the temperature rose.
This is considered to have occurred because the kinetic energy of the molecules increased with increasing temperature, increasing the number of molecules moving from the liquid phase to the gas phase.
The increase in vapor pressure was not linear but showed a tendency to increase strongly as the temperature rose.

Based on the Clausius-Clapeyron equation, 1/T was plotted on the horizontal axis and lnP on the vertical axis.
As a result, an approximately linear relationship was obtained, and the fitted equation was lnP = −5100 × 1/T + 19.17.
Because this slope corresponds to −ΔH_vap/R, the enthalpy of vaporization was calculated as −(−5100) × 8.314 = 42400 J/mol, or 42.4 kJ/mol.

Furthermore, when the temperature at which the vapor pressure became 101.3 kPa was determined from the fitted equation, T = 350.5 K, or approximately 77.4°C, was obtained.
This is close to the boiling point of ethanol, suggesting that the measured values and analysis using the Clausius-Clapeyron plot were generally reasonable.

Points for Connecting the Results to the Discussion

In a discussion of vapor pressure measurement, it is important to explain not only the relationship between temperature and vapor pressure but also the linearity of lnP and 1/T, the enthalpy of vaporization determined from the slope, boiling-point estimation, and sources of error in relation to one another.

• Can you explain why vapor pressure increases with increasing temperature in relation to molecular motion?
• Have you converted °C to K and calculated 1/T correctly?
• Have you calculated lnP from vapor pressure P and prepared a Clausius-Clapeyron plot?
• Can you explain that the slope of the plot corresponds to −ΔH_vap/R?
• Can you calculate the enthalpy of vaporization from the slope?
• Can you estimate the boiling point as the temperature at which vapor pressure equals the external pressure?
• Can you discuss differences in vapor pressure among liquids in relation to intermolecular forces?
• Can you explain measurement before equilibrium, air contamination, temperature-measurement error, and pressure-gauge deviation as sources of error?
• Do you understand the need to use consistent pressure units?
• If an outlier is present, can you discuss its cause from the measurement operation or the condition of the apparatus?

Example Discussion

In this experiment, the vapor pressure of a liquid was measured at different temperatures, and the enthalpy of vaporization was determined from a Clausius-Clapeyron plot.
The measured vapor pressure was 5.9 kPa at 20°C and 47.0 kPa at 60°C, showing a large increase in vapor pressure as the temperature rose.
This is because as the temperature increases, the kinetic energy of liquid molecules increases and more molecules can overcome intermolecular forces and move into the gas phase.

To analyze the relationship between vapor pressure and temperature, the temperature was converted to absolute temperature K, and 1/T and lnP were calculated.
When lnP was plotted on the vertical axis and 1/T on the horizontal axis, an approximately linear relationship was obtained.
According to the Clausius-Clapeyron equation, the slope of this line corresponds to −ΔH_vap/R.
Because the slope in this experiment was −5100 K, the enthalpy of vaporization was determined to be 42.4 kJ/mol.

The enthalpy of vaporization is an indicator of the energy required for liquid molecules to become gas.
The stronger the intermolecular forces in a liquid, the more energy is required for molecules to move from the liquid to the gas phase, and the larger the enthalpy of vaporization becomes.
In addition, when compared at the same temperature, liquids with stronger intermolecular forces tend to have lower vapor pressures.
Water has a lower vapor pressure than ethanol or acetone because hydrogen bonding acts strongly between water molecules.

Possible sources of error include reading the vapor pressure before the temperature had sufficiently stabilized, residual air in the container, pressure-gauge reading errors, and calibration errors of the thermometer.
In particular, if air is present, the measured pressure includes not only the vapor pressure of the liquid but also the partial pressure of the air, so the vapor pressure may be overestimated.
In addition, if measurement is performed before equilibrium is reached, the vapor pressure may be underestimated.

In the analysis, temperature must be handled in K rather than °C, and the pressure unit must be consistent throughout all data.
If kPa and Pa are mixed, the lnP values shift unnaturally and the correct linear relationship cannot be obtained.
However, if all data are handled using the same pressure unit, the difference in pressure units mainly affects the intercept and has little effect on the enthalpy of vaporization determined from the slope.

Summary

Vapor pressure increases as temperature rises, and the relationship between lnP and 1/T can be analyzed using a Clausius-Clapeyron plot.
The enthalpy of vaporization can be determined from the slope of the plot, and the boiling point can also be estimated from the temperature at which the vapor pressure equals the external pressure.

This reference example covered vapor pressure at different temperatures, calculation of 1/T and lnP, the Clausius-Clapeyron plot, enthalpy of vaporization, boiling-point estimation, differences among liquids, temperature errors, equilibrium attainment, air contamination, pressure units, and outliers.
In a report, it is useful to organize the measured values in a table and discuss the relationship between the slope of the line, intermolecular forces, and the enthalpy of vaporization.

Why Vapor Pressure Increases as Temperature Rises

As the temperature rises, the thermal motion of liquid molecules becomes more active.
More molecules are able to overcome the intermolecular interactions in the liquid and move into the gas phase, making evaporation easier.
As a result, the number of molecules in the gas phase inside a sealed container increases, and the vapor pressure becomes larger.

The increase in vapor pressure with temperature is generally not linear and tends to become more rapid as the temperature rises.
Therefore, plotting vapor pressure P directly against temperature may produce a curve.
The Clausius-Clapeyron plot linearizes this relationship for analysis.

What Is the Clausius-Clapeyron Equation?

The Clausius-Clapeyron equation expresses the temperature dependence of vapor pressure.
If the enthalpy of vaporization can be regarded as approximately constant within the temperature range, it can be expressed in the following form.

lnP = −ΔH_vap / R × 1/T + C

Here, P is vapor pressure, T is absolute temperature, ΔH_vap is the enthalpy of vaporization, R is the gas constant, and C is a constant.
This equation corresponds to the straight-line equation y = ax + b.
When lnP is plotted on the vertical axis and 1/T on the horizontal axis, the slope is −ΔH_vap/R.

How to Create a Clausius-Clapeyron Plot

To create a Clausius-Clapeyron plot, first convert the measured temperature to absolute temperature T.
Next, calculate 1/T for each temperature and determine the natural logarithm lnP of the vapor pressure P.
Create a scatter plot with 1/T on the horizontal axis and lnP on the vertical axis, and draw a fitted straight line.

Basic Procedure

1. Measure the vapor pressure P at each temperature
2. Convert the temperature from °C to K
3. Calculate 1/T
4. Calculate lnP
5. Plot 1/T on the horizontal axis and lnP on the vertical axis
6. Draw a fitted straight line
7. Determine the enthalpy of vaporization from the slope

Meaning of the Plot Slope

In a Clausius-Clapeyron plot, 1/T is placed on the horizontal axis and lnP on the vertical axis.
The slope of the fitted line corresponds to −ΔH_vap/R.
Because the enthalpy of vaporization ΔH_vap is positive, the slope is negative.

Slope = −ΔH_vap / R

ΔH_vap = −Slope × R

The larger the absolute value of the slope, the more strongly vapor pressure changes with temperature.
This is related to the large amount of energy required to transfer molecules from the liquid to the gas phase.

Meaning of the Intercept

The intercept of the Clausius-Clapeyron plot corresponds to the constant C in the equation.
This constant is related to the entropy of vaporization, the standard state, and approximation conditions, but in student experiments it is often treated mainly as a value that determines the form of the fitted line.
When determining the enthalpy of vaporization, the slope is used primarily.

However, if the intercept is greatly shifted or the fitted line does not agree well with the measured points, there may have been problems with pressure measurement, temperature measurement, or insufficient attainment of equilibrium.

How to Interpret R2

The coefficient of determination R² of a Clausius-Clapeyron plot indicates how well the measured points fit the fitted line.
The closer R² is to 1, the better the linear relationship between lnP and 1/T is considered to be.

However, a high R² does not necessarily mean that there are no measurement errors.
If there are systematic errors in the thermometer or pressure gauge, the measured points may still lie on a straight line while the slope differs from the literature value.
Therefore, comparison of the enthalpy of vaporization with literature values is also important in addition to R².

What Is the Enthalpy of Vaporization?

The enthalpy of vaporization is the energy required to convert 1 mol of liquid into gas.
For liquid molecules to become gas, they must overcome the intermolecular interactions acting within the liquid.
Therefore, liquids with stronger intermolecular interactions tend to have larger enthalpies of vaporization.

For example, in liquids that form hydrogen bonds, the attractive forces between molecules are strong, so a large amount of energy may be required for vaporization.
In contrast, highly volatile liquids with weak intermolecular interactions tend to have smaller enthalpies of vaporization and higher vapor pressures.

Relationship Between Vapor Pressure and Intermolecular Interactions

Vapor pressure becomes larger when liquid molecules can escape into the gas phase more easily.
In liquids with weak intermolecular interactions, molecules are not strongly bound in the liquid, so they evaporate more readily and the vapor pressure becomes high.
Conversely, in liquids with strong intermolecular interactions, molecules are more strongly retained in the liquid and the vapor pressure becomes low.

In other words, liquids with higher vapor pressure tend to be more volatile and have lower boiling points.
Liquids with lower vapor pressure tend to be less volatile and have higher boiling points.

Relationship With Boiling Point

The boiling point of a liquid is the temperature at which its vapor pressure becomes equal to the external pressure.
When the external pressure is 1 atm, the temperature at which the vapor pressure reaches 1 atm is the normal boiling point.
A liquid with high vapor pressure reaches the external pressure at a lower temperature and therefore has a lower boiling point.
A liquid with low vapor pressure must be heated to a higher temperature before reaching the external pressure, so its boiling point is higher.

Error Caused by Temperature Measurement

Because the horizontal axis of a Clausius-Clapeyron plot is 1/T, errors in temperature measurement strongly affect the results.
If temperature is used in °C or conversion to K is incorrect, the graph slope cannot be determined correctly.
Errors in reading the thermometer or failure of the entire sample to reach the set temperature also become sources of error.

Vapor pressure is particularly sensitive to temperature, so even a small temperature difference may cause a large change in pressure.
Therefore, it is important to keep the temperature constant and measure only after sufficient thermal equilibrium has been reached.

Error Caused by Pressure Measurement

In vapor pressure measurement, pressure is measured using a pressure gauge, manometer, or similar device.
Calibration errors of the pressure gauge, reading errors, parallax in reading scales, air contamination in the apparatus, and errors in reading the height of a liquid column affect the measured vapor pressure.
Errors in pressure values shift lnP and therefore affect both the slope and intercept of the Clausius-Clapeyron plot.

Error Caused by Insufficient Attainment of Equilibrium

Vapor pressure is the pressure when a liquid and its vapor have reached equilibrium.
If pressure is read immediately after changing the temperature or while the sample has not yet stabilized sufficiently, the measured value may not reflect the true equilibrium vapor pressure.
Before equilibrium is reached, evaporation and condensation are still progressing and the pressure may be changing.

If the measured value changes with time, it is necessary to wait until the pressure stabilizes before reading it.
Insufficient attainment of equilibrium can cause variation and outliers in a Clausius-Clapeyron plot.

Error Caused by Air Contamination

In vapor pressure measurement, if noncondensable gases such as air remain in the apparatus, the measured pressure includes pressure from substances other than the vapor.
In this case, the measured value may become larger than the vapor pressure of the pure liquid.
Therefore, when handling vacuum systems or sealed systems, it is important to avoid air contamination.

Error Caused by Impurities in the Sample

If impurities are present in the sample, the vapor pressure may deviate from the value for the pure substance.
If a low-volatility impurity is present, the vapor pressure of the liquid may decrease.
Conversely, if a highly volatile impurity is mixed in, the measured pressure may increase.

If water or another solvent is mixed into the sample, the measured vapor pressure may represent that of the mixture rather than that of the target liquid alone.
Therefore, sample purity is an important point for discussion.

Discussion When Deviations Occur on the Low- or High-Temperature Side

In a Clausius-Clapeyron plot, measured points only on the low- or high-temperature side may deviate from the fitted line.
On the low-temperature side, vapor pressure is small, so the relative error of pressure measurement tends to become large.
On the high-temperature side, temperature control may become more difficult, or bubbles, boiling, and large pressure changes in the apparatus may occur.

Checking which temperature range contains the deviating points makes it possible to discuss the source of error more specifically.

When the Clausius-Clapeyron Plot Is Not Linear

If the Clausius-Clapeyron plot is not linear, possible causes include measurement error, a wide temperature range, limitations of the approximation that assumes constant enthalpy of vaporization, impurities in the sample, and insufficient attainment of equilibrium.
Incorrect pressure or temperature units may also greatly reduce linearity.

In a report, do not simply write that “the plot was not linear.”
Check which points deviate, whether the temperature conversion and pressure units are correct, and whether the measurement conditions were stable.

When the Enthalpy of Vaporization Is Larger Than the Literature Value

If the experimentally determined enthalpy of vaporization is larger than the literature value, this means that the absolute value of the slope of the Clausius-Clapeyron plot was estimated too large.
Possible causes include reading the low-temperature pressure too low, reading the high-temperature pressure too high, or having a systematic error in temperature measurement.

When the Enthalpy of Vaporization Is Smaller Than the Literature Value

If the experimentally determined enthalpy of vaporization is smaller than the literature value, this means that the absolute value of the slope of the Clausius-Clapeyron plot was estimated too small.
If the pressure change over the temperature range is measured as too small, or if air contamination causes the low-temperature pressure to be overestimated, the slope tends to become less steep.

Handling of Units

In a Clausius-Clapeyron plot, temperature is handled in K and pressure is handled using a consistent unit.
When calculating lnP, mixing different pressure units partway through causes the values to become inconsistent.
However, if the same pressure unit is used throughout, differences in pressure units mainly affect the intercept and may have little effect on the slope.

When determining the enthalpy of vaporization, attention must also be paid to the units of the gas constant R.
If R = 8.314 J mol⁻¹ K⁻¹ is used, ΔH_vap is obtained in J/mol.
To convert to kJ/mol, divide by 1000.

Calculation of Error Rate

When comparing the experimentally determined enthalpy of vaporization with a literature value, the error rate can be calculated to express the difference quantitatively.
The error rate is obtained by dividing the difference between the experimental and literature values by the literature value and expressing it as a percentage.

Error rate (%) = |Experimental value − Literature value| ÷ Literature value × 100

Simply showing the error rate is not sufficient as a discussion.
Explain how temperature measurement, pressure measurement, insufficient attainment of equilibrium, air contamination, sample impurities, and unit conversion shifted the value and in which direction.

When the Results Can Be Considered Good

Vapor pressure measurement results can be considered good when vapor pressure consistently increases with increasing temperature, the Clausius-Clapeyron plot is close to linear, R² is high, and the determined enthalpy of vaporization does not greatly contradict the literature value.
It is also important that temperature and pressure be sufficiently stable before measurement.

Example Discussion When the Experiment Did Not Go Well

If vapor pressure measurement does not go well, the causes are considered from results such as vapor pressure not increasing monotonically with temperature, the Clausius-Clapeyron plot not becoming linear, the enthalpy of vaporization differing greatly from the literature value, or large variation among measured points.
Organizing the causes separately into temperature measurement, pressure measurement, attainment of equilibrium, air contamination, sample purity, and unit conversion makes the discussion easier.

How to Write Points for Improvement

In a discussion of vapor pressure measurement, including points for improvement in addition to sources of error makes the report easier to organize.
Improvements are easier to write when divided into temperature control, pressure measurement, attainment of equilibrium, sample and apparatus management, and data analysis.

Improvements to Temperature Control

• Keep the temperature constant using a thermostatic bath or similar equipment
• Measure only after the entire sample has reached the set temperature
• Calibrate the thermometer
• Convert temperature from °C to K for analysis
• Record temperature fluctuations during measurement

Improvements to Pressure Measurement

• Calibrate the pressure gauge
• Read the pressure only after it becomes stable
• Read the liquid-column height or scale accurately
• Use consistent pressure units
• Confirm measured values multiple times

Improvements to the Apparatus and Sample

• Remove as much air as possible from the apparatus
• Check for leaks
• Confirm sample purity
• Avoid contamination by water or volatile impurities
• Allow sufficient equilibration time after changing the temperature

Improvements to Data Analysis

• Calculate 1/T and lnP correctly
• Check the equation of the fitted line and R²
• Check the cause of outliers
• Determine ΔH_vap from the slope
• Match the units of R and ΔH_vap
• Check whether the temperature range is the same as that used for the literature value

Difference Between a Superficial Discussion and a Good Discussion

In a discussion of vapor pressure measurement, simply writing that “vapor pressure increased as temperature rose” or “the graph became linear” results in a superficial discussion.
Relating temperature, molecular motion, intermolecular interactions, the Clausius-Clapeyron equation, the slope, the enthalpy of vaporization, and sources of error produces a more persuasive discussion.

Examples of Expressions That Can Be Used in Reports

The following expressions can be used when writing the results and discussion of vapor pressure measurement.
Adjust the necessary parts according to your own experimental results.

• Vapor pressure is the pressure of the gas phase when a liquid and its vapor are in equilibrium.
• As the temperature rose, the thermal motion of the liquid molecules became more active and the vapor pressure increased.
• According to the Clausius-Clapeyron equation, a linear relationship exists between lnP and 1/T.
• The slope of the fitted line corresponds to −ΔH_vap/R.
• The enthalpy of vaporization can be determined by multiplying the slope by −R.
• The enthalpy of vaporization is the energy required to convert 1 mol of liquid into gas.
• Liquids with stronger intermolecular interactions tend to have larger enthalpies of vaporization.
• Temperature-measurement error affects the value of 1/T and the slope of the fitted line.
• Pressure-measurement error directly affects lnP and shifts the calculated enthalpy of vaporization.
• If measurement is performed before equilibrium is reached, the true equilibrium vapor pressure cannot be reflected.

Points to Check When Discussing Vapor Pressure Measurement

Checking the following points before writing the report makes the discussion easier to write.

• Have you converted the measured temperature to K?
• Have you used consistent pressure units?
• Have you calculated 1/T and lnP correctly?
• Have you created a Clausius-Clapeyron plot?
• Have you explained the meaning of the slope of the fitted line?
• Have you determined the enthalpy of vaporization from the slope?
• Have you matched the units of R and ΔH_vap?
• Have you explained why vapor pressure increases as temperature rises?
• Have you related intermolecular interactions to the enthalpy of vaporization?
• Have you discussed insufficient attainment of equilibrium?
• Have you considered air contamination and sample impurities as sources of error?
• Do the points for improvement correspond to the sources of error?

Summary

Vapor pressure is the pressure of the gas phase when a liquid and its vapor are in equilibrium.
As the temperature rises, the thermal motion of liquid molecules becomes more active and more molecules can move from the liquid into the gas phase, so the vapor pressure increases.
Liquids with higher vapor pressure are more volatile and generally tend to have lower boiling points.

In a Clausius-Clapeyron plot, 1/T is placed on the horizontal axis and lnP on the vertical axis.
If the enthalpy of vaporization can be regarded as approximately constant within the measured range, the graph becomes linear and the slope of the fitted line corresponds to −ΔH_vap/R.
Therefore, the enthalpy of vaporization can be determined from the slope.

In a report, do not simply write that “the plot became linear.”
Discuss the relationship between temperature and vapor pressure, the meaning of the slope, the enthalpy of vaporization, intermolecular interactions, and sources of error in relation to one another.
Temperature measurement, pressure measurement, insufficient attainment of equilibrium, air contamination, sample impurities, and unit conversion greatly affect both the linearity of the Clausius-Clapeyron plot and the value of the enthalpy of vaporization.