Multiple Choice
Which of the following correctly describes how to calculate the enthalpy change (ΔH) of a reaction using bond energies?
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8. Thermochemistry
Thermochemical Equations
Multiple Choice
Given the following vapor pressure data for a liquid metal at different temperatures, which method would you use to determine the enthalpy of vaporization (ΔH_vap)?Temperature (K): 400, 420, 440, 460Vapor Pressure (atm): 0.12, 0.22, 0.38, 0.65Choose the most appropriate approach:
A
Plot vapor pressure versus Temperature and use the intercept to calculate ΔH_vap.
B
Plot ln(Temperature) versus vapor pressure and use the slope to calculate ΔH_vap.
C
Plot ln(vapor pressure) versus 1/Temperature and use the slope to calculate ΔH_vap.
D
Plot Temperature versus vapor pressure and use the area under the curve to calculate ΔH_vap.
Verified step by step guidance1
Recognize that the enthalpy of vaporization (\( \Delta H_{vap} \)) can be determined using the Clausius-Clapeyron equation, which relates vapor pressure and temperature:
\[ \ln P = -\frac{\Delta H_{vap}}{R} \cdot \frac{1}{T} + C \]
where \( P \) is the vapor pressure, \( T \) is the temperature in Kelvin, \( R \) is the gas constant, and \( C \) is a constant related to entropy.
To find \( \Delta H_{vap} \), rearrange the equation to the form of a straight line: plotting \( \ln P \) (natural logarithm of vapor pressure) on the y-axis versus \( \frac{1}{T} \) (inverse temperature) on the x-axis will yield a straight line.
The slope of this line is equal to \( -\frac{\Delta H_{vap}}{R} \). By determining the slope from the plot, you can calculate \( \Delta H_{vap} \) using:
\[ \Delta H_{vap} = - \text{slope} \times R \]
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