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Identify two common observations indicating some solids, such as dry ice and mothballs, have vapor pressures sufficient to sublime?
What is the relationship between the intermolecular forces in a liquid and its vapor pressure?
The vapor pressure of a liquid decreases as the strength of its intermolecular forces increases.
What is the relationship between the intermolecular forces in a solid and its melting temperature?
Why does spilled gasoline evaporate more rapidly on a hot day than on a cold day?
As the temperature increases, the average kinetic energy of the molecules of gasoline increases and so a greater fraction of molecules have sufficient energy to escape from the liquid than at lower temperatures.
Carbon tetrachloride, CCl 4 , was once used as a dry cleaning solvent, but is no longer used because it is carcinogenic. At 57.8 °C, the vapor pressure of CCl 4 is 54.0 kPa, and its enthalpy of vaporization is 33.05 kJ/mol. Use this information to estimate the normal boiling point for CCl 4 .
When is the boiling point of a liquid equal to its normal boiling point?
When the pressure of gas above the liquid is exactly 1 atm
How does the boiling of a liquid differ from its evaporation?
Use the information in [link] to estimate the boiling point of water in Denver when the atmospheric pressure is 83.3 kPa.
approximately 95 °C
A syringe at a temperature of 20 °C is filled with liquid ether in such a way that there is no space for any vapor. If the temperature is kept constant and the plunger is withdrawn to create a volume that can be occupied by vapor, what would be the approximate pressure of the vapor produced?
Explain the following observations:
(a) It takes longer to cook an egg in Ft. Davis, Texas (altitude, 5000 feet above sea level) than it does in Boston (at sea level).
(b) Perspiring is a mechanism for cooling the body.
(a) At 5000 feet, the atmospheric pressure is lower than at sea level, and water will therefore boil at a lower temperature. This lower temperature will cause the physical and chemical changes involved in cooking the egg to proceed more slowly, and a longer time is required to fully cook the egg. (b) As long as the air surrounding the body contains less water vapor than the maximum that air can hold at that temperature, perspiration will evaporate, thereby cooling the body by removing the heat of vaporization required to vaporize the water.
The enthalpy of vaporization of water is larger than its enthalpy of fusion. Explain why.
Explain why the molar enthalpies of vaporization of the following substances increase in the order CH 4 <C 2 H 6 <C 3 H 8 , even though the type of IMF (dispersion) is the same.
Dispersion forces increase with molecular mass or size. As the number of atoms composing the molecules in this homologous series increases, so does the extent of intermolecular attraction via dispersion forces and, consequently, the energy required to overcome these forces and vaporize the liquids.
Explain why the enthalpies of vaporization of the following substances increase in the order CH 4 <NH 3 <H 2 O, even though all three substances have approximately the same molar mass.
The enthalpy of vaporization of CO 2 ( l ) is 9.8 kJ/mol. Would you expect the enthalpy of vaporization of CS 2 ( l ) to be 28 kJ/mol, 9.8 kJ/mol, or −8.4 kJ/mol? Discuss the plausibility of each of these answers.
The boiling point of CS 2 is higher than that of CO 2 partially because of the higher molecular weight of CS 2 ; consequently, the attractive forces are stronger in CS 2 . It would be expected, therefore, that the heat of vaporization would be greater than that of 9.8 kJ/mol for CO 2 . A value of 28 kJ/mol would seem reasonable. A value of −8.4 kJ/mol would indicate a release of energy upon vaporization, which is clearly implausible.
The hydrogen fluoride molecule, HF, is more polar than a water molecule, H 2 O (for example, has a greater dipole moment), yet the molar enthalpy of vaporization for liquid hydrogen fluoride is lesser than that for water. Explain.
Ethyl chloride (boiling point, 13 °C) is used as a local anesthetic. When the liquid is sprayed on the skin, it cools the skin enough to freeze and numb it. Explain the cooling effect of liquid ethyl chloride.
The thermal energy (heat) needed to evaporate the liquid is removed from the skin.
Which contains the compounds listed correctly in order of increasing boiling points?
(a) N 2 <CS 2 <H 2 O<KCl
(b) H 2 O<N 2 <CS 2 <KCl
(c) N 2 <KCl<CS 2 <H 2 O
(d) CS 2 <N 2 <KCl<H 2 O
(e) KCl<H 2 O<CS 2 <N 2
How much heat is required to convert 422 g of liquid H 2 O at 23.5 °C into steam at 150 °C?
1130 kJ
Evaporation of sweat requires energy and thus take excess heat away from the body. Some of the water that you drink may eventually be converted into sweat and evaporate. If you drink a 20-ounce bottle of water that had been in the refrigerator at 3.8 °C, how much heat is needed to convert all of that water into sweat and then to vapor?? (Note: Your body temperature is 36.6 °C. For the purpose of solving this problem, assume that the thermal properties of sweat are the same as for water.)
Titanium tetrachloride, TiCl 4 , has a melting point of −23.2 °C and has a Δ H fusion = 9.37 kJ/mol.
(a) How much energy is required to melt 263.1 g TiCl 4 ?
(b) For TiCl 4 , which will likely have the larger magnitude: Δ H fusion or Δ H vaporization ? Explain your reasoning.
(a) 13.0 kJ; (b) It is likely that the heat of vaporization will have a larger magnitude since in the case of vaporization the intermolecular interactions have to be completely overcome, while melting weakens or destroys only some of them.
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