Chapter 11: Problem 46
Carbon disulfide, \(\mathrm{CS}_{2}\), is a volatile, flammable liquid. It has a vapor pressure of \(400.0 \mathrm{mmHg}\) at \(28.0^{\circ} \mathrm{C}\) and \(760.0\) \(\mathrm{mmHg}\) at \(46.5^{\circ} \mathrm{C}\). What is the heat of vaporization of this substance?
Short Answer
Expert verified
The heat of vaporization is approximately 29.18 kJ/mol.
Step by step solution
01
Understanding Clausius-Clapeyron Equation
The Clausius-Clapeyron equation relates the vapor pressure of a substance with its temperature and heat of vaporization, \(\Delta H_{vap}\). The equation is given by:\[\ln \left(\frac{P_2}{P_1}\right) = \frac{\Delta H_{vap}}{R} \left(\frac{1}{T_1} - \frac{1}{T_2}\right)\]where \(P_1\) and \(P_2\) are the vapor pressures at temperatures \(T_1\) and \(T_2\), respectively, and \(R\) is the ideal gas constant, \(8.314\, \text{J/mol·K}\).
02
Convert Temperatures to Kelvin
We need to convert the given temperatures from Celsius to Kelvin. Remember, \(K = ^{\circ}C + 273.15\).- For \(28.0^{\circ}C\):\[ T_1 = 28.0 + 273.15 = 301.15\, \text{K} \]- For \(46.5^{\circ}C\):\[ T_2 = 46.5 + 273.15 = 319.65\, \text{K} \]
03
Substitute Values into Clausius-Clapeyron Equation
Substitute the known values into the Clausius-Clapeyron equation:\[\ln \left(\frac{760.0}{400.0}\right) = \frac{\Delta H_{vap}}{8.314} \left(\frac{1}{301.15} - \frac{1}{319.65}\right)\]
04
Solve for Heat of Vaporization
Calculate \(\ln \left(\frac{760.0}{400.0}\right)\) and then solve for \(\Delta H_{vap}\):- Find the natural log:\[\ln \left(\frac{760.0}{400.0}\right) = \ln (1.9) \approx 0.6419\]- Calculate the temperature difference in reciprocals:\[\frac{1}{301.15} - \frac{1}{319.65} \approx 0.05448 - 0.05265 = 0.00183\, \text{K}^{-1}\]- Substitute these into the equation:\[0.6419 = \frac{\Delta H_{vap}}{8.314} \times 0.00183\]- Solve for \(\Delta H_{vap}\):\[\Delta H_{vap} = \frac{0.6419 \times 8.314}{0.00183} \approx 29178.5\, \text{J/mol} \approx 29.18\, \text{kJ/mol}\]
05
Final Step: Conclusion
The heat of vaporization of carbon disulfide is approximately \(29.18\, \text{kJ/mol}\).
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Key Concepts
These are the key concepts you need to understand to accurately answer the question.
Clausius-Clapeyron Equation
The Clausius-Clapeyron equation plays a crucial role in understanding the relationship between vapor pressure, temperature, and the heat of vaporization of a substance. This equation is essential for predicting how the vapor pressure of a liquid changes as its temperature changes.
The equation is expressed as:
The equation is expressed as:
- \( \ln \left(\frac{P_2}{P_1}\right) = \frac{\Delta H_{\text{vap}}}{R} \left(\frac{1}{T_1} - \frac{1}{T_2}\right) \)
- \(P_1\) and \(P_2\) are the vapor pressures at the absolute temperatures \(T_1\) and \(T_2\), respectively.
- \( \Delta H_{\text{vap}} \) is the heat of vaporization.
- \( R \) is the ideal gas constant.
Vapor Pressure
Vapor pressure is a fundamental concept in physical chemistry. It is the pressure exerted by a vapor in equilibrium with its liquid at a given temperature.
This means that:
This means that:
- When a liquid is in a closed container, molecules evaporate into the space above.
- Over time, a balance is reached where the number of molecules leaving the liquid equals those returning, creating equilibrium.
- The pressure exerted by these vapor molecules is the vapor pressure.
Ideal Gas Constant
The ideal gas constant, denoted as \( R \), is a crucial part of many equations in chemistry and physics. It acts as a bridge between different physical properties and is an inherent part in the Clausius-Clapeyron equation.
The value of \( R \) is:
In context:
The value of \( R \) is:
- 8.314 J/mol·K
In context:
- When using the Clausius-Clapeyron equation, \( R \) helps quantify the relationship between temperature, pressure, and the heat involved in phase changes.
- The unit J/mol·K represents that it relates energy in joules to the amount of substance in moles at a particular temperature in Kelvin.