Chapter 13.5, Problem 1.4ACP

### Chemistry & Chemical Reactivity

10th Edition
John C. Kotz + 3 others
ISBN: 9781337399074

Chapter
Section

### Chemistry & Chemical Reactivity

10th Edition
John C. Kotz + 3 others
ISBN: 9781337399074
Textbook Problem

# The vapor pressure of pure heptane is 361.5 mm Hg at 75.0 °C and its normal boiling point is 98.4 °C. Use the Clausius-Clapeyron equation (page 512) to determine the enthalpy of vaporization of heptane.

Interpretation Introduction

Interpretation: The enthalpy of vaporization of heptane has to be determined using Clausius-Clapeyron equation.

Concept introduction:

• Enthalpy of vaporization (ΔHvap ) is the amount of energy required to change one mole of a substance from the liquid phase to the gas phase at constant temperature and pressure.
• Clausius-Clapeyron equation is used to estimate the vapor pressure at another temperature, if the vapor pressure is known at some temperature, and if the enthalpy of vaporization is known.

ln(P2P1)=ΔHvapR(1T2-1T1)

ΔHvap is the enthalpy of vaporization.

R is the gas constant (8.3145Jmol1K1 )

P1andP2 are the vapor pressures at two temperatures T1andT2.

Explanation

Given data:

Vaporâ€‰pressureâ€‰ofâ€‰heptaneâ€‰=â€‰361.5â€‰mmâ€‰HgT1â€‰=â€‰â€‰â€‰98.4Â°Câ€‰=â€‰371.55â€‰Kâ€‰â€‰T2â€‰=â€‰75Â°Câ€‰=â€‰348.15â€‰K

Clausius-Clapeyron equation is used to estimate the vapor pressure at another temperature, if the vapor pressure is known at some temperature, and if the enthalpy of vaporization is known.

Â Â Â Â Â Â Â Â Â Â Â Â lnâ€‰â€‰(P2P1)â€‰â€‰=â€‰â€‰Î”HvapRâ€‰(1T2â€‰-â€‰1T1)

Substituting the given values of pressure and temperatures in the above equation,

Â Â Â Â Â Â Â Â Â Â Â Â lnâ€‰â€‰(361

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