Chem2 Ch.10

The normal boiling point of a liquid is 282 °C. What is the vapor pressure of this liquid at 220 °C? (∆Hvap = 28.5 kJ/mol)

0.4609 atm

What amount of heat (in kJ) is required to convert 12.6 g of an unknown solid (MM = 83.21 g/mol) at -5.00 °C to a liquid at 52.3 °C? (heat capacity of solid = 2.39 J/g･°C; heat capacity of liquid = 1.58 J/g･°C; ∆Hfus = 3.72 kJ/mol; Tf = 10.3°C)

1.86 kj

What amount of heat is required to vaporize 135.55 g of ethanol (C₂H₅OH)? (∆Hvap = 43.3 kJ/mol)

127kj

What quantity of heat is required to convert 25.0 g of ethanol (C₂H₅OH) at 23.0°C to a vapor at 78.3°C (its boiling point)? (heat capacity of ethanol = 2.46 J/g • C; ∆Hvap = 39.3 kJ/mol)

24.7kj

An experimental plot of ln(Pvap) vs. 1/T is obtained in lab for an unknown liquid. The slope of the best-fit line for the graph is -3260 K. What is the enthalpy of vaporization for the liquid in kJ/mol?

27.1 kj

Dispersion forces would be weakest in which of the following substances? A) He B) Ne C) Ar D) Kr E) Xe

A) He Dispersion forces arise from instantaneous changes in electron density, so compounds with fewer electrons tend to have weaker dispersion forces. Since He has the fewest electrons of the options given, it has the weakest dispersion forces.

Which of the following phase changes is endothermic? A) Sublimation B) Deposition C) Freezing D) Condensation

A) Sublimation An endothermic phase change is one that requires the input of energy to overcome intermolecular forces. Of these options, only sublimation requires the input of energy (to escape from the solid to the gas phase), meaning it is endothermic.

Which of the following would you expect to have the highest boiling point? A) H₂ B) NH₃ C) N₂ D) CH₄

B) NH₃ The boiling point is the temperature at which a substance will change from the liquid phase to the gas phase, so compounds with stronger intermolecular forces will have higher boiling points. NH₃ is a polar molecule and can form hydrogen bonds. The other molecules are nonpolar and their intermolecular forces are dispersion forces. Therefore, NH₃ has the strongest intermolecular forces and therefore the highest boiling point.

Which one of the following correctly ranks the compounds in order of lowest enthalpy of vaporization to highest enthalpy of vaporization based only in intermolecular forces? A) CH₃NH₂ < CH₃SH < CH₃CH₃ B) CH₃NH₂ < CH₃CH₃ < CH₃SH C) CH₃CH₃ < CH₃SH < CH₃NH₂ D) CH₃SH < CH₃NH₂ < CH₃CH₃

C) CH₃CH₃ < CH₃SH < CH₃NH₂ CH₃NH₂ would have the most intermolecular forces, with hydrogen bonding, dispersion forces and dipole-dipole interactions. CH₃SH would have the second most intermolecular forces, with dispersion forces and dipole-dipole interactions. CH₃CH₃ would have the fewest intermolecular forces with only dispersion forces. The stronger the intermolecular forces are, the higher the magnitude of the enthalpy of vaporization will be.

Rank the following compounds in order of lowest boiling point to highest boiling point based on intermolecular forces. A) HF < HCl < HBr < HI B) HI < HBr < HCl < HF C) HCl < HBr < HI < HF D) HBr < HI < HF < HCl

C) HCl < HBr < HI < HF The boiling point is the temperature at which a substance will change from the liquid phase to the gas phase, so compounds with stronger intermolecular forces will have higher boiling points. HF can form hydrogen bonds. Therefore, HF will have the strongest intermolecular forces and thus the highest boiling point. The other compounds are all polar and exhibit dipole-dipole and dispersion forces. Dispersion forces are higher for molecules with more electrons. HCl has 18 electrons, HBr has 36 electrons, HI has 54 electrons. HF only has 8 electrons, but its ability to form hydrogen bonds results in a higher boiling point.

Which of the following exhibits dipole-dipole intermolecular forces? A) BF₃ B) CO₂ C) NH₃ D) Br₂ E) CF₄

C) NH₃ Dipole-dipole forces are exhibited by any systems that contain polar compounds, or those with dipoles. By drawing the molecular structures of these options, the only option that has a net dipole moment is NH₃, meaning it will be the only option to exhibit dipole-dipole forces

Which one of the following phase changes would be exothermic? A) sublimation B) vaporization C) condensation D) melting

C) condensation An exothermic phase change is one in which molecules become closer together by giving up energy, resulting in a more condensed phase. Of these options, only condensation features molecules becoming more condensed (changing from gas to liquid), meaning it is exothermic.

Ammonia and hydrogen fluoride both have unusually high boiling points due to _____ A) dispersion forces. B) dipole-dipole interactions. C) hydrogen bonding. D) ion-dipole interactions. E) low molar masses.

C) hydrogen bonding. High boiling points are a consequence of strong intermolecular forces. Because ammonia (NH₃) and hydrogen fluoride (HF) both have high boiling points, they must have strong intermolecular forces. Each of these compounds contains an H atom directly bonded to one of the highly electronegative N, O, or F atoms, so their high boiling points result from their hydrogen bonding.

Which property typically indicates strong intermolecular forces are present in a liquid? A) low heat of vaporization B) low critical temperature C) low vapor pressure D) low boiling point E) low melting point

C) low vapor pressure Strong intermolecular forces in a liquid result in a greater attraction between the liquid molecules, meaning they are more likely to remain in the liquid phase. Of these options, only a low vapor pressure indicates strong intermolecular forces, as it implies that the liquid molecules are less likely to exit into the vapor (gas) phase.

Which of the following would exhibit hydrogen bonding? A) HCl B) CH₃CH₃ C) CF₂H₂ D) CH₃CH₂OH E) CH₃OCH₃

D) CH₃CH₂OH Hydrogen bonding results when a hydrogen atom is covalently bonded to the highly electronegative N, O, or F atoms, resulting in a large partial positive charge on the H atom. Since CH₃CH₂OH is the only option with H bonded directly to one of these atoms (O), it would exhibit hydrogen bonding.

Which of the following might be best described by the following set of properties? 1. white solid melting at 772 °C 2. solid does not conduct electricity 3. solid dissolves in water to form a conducting solution A) BN B) P₄S₃ C) Pb D) CaCl₂ E) Cannot be determined.

D) CaCl₂

What intermolecular forces are shared between CH₃CH₂CH₂F and CH₃CH₂CH₂OH? A) Dispersion forces only. B) Dipole-dipole forces only. C) Dispersion forces, dipole-dipole forces, and hydrogen bonding. D) Dispersion forces and dipole-dipole forces

D) Dispersion forces and dipole-dipole forces. Both molecules (as do all molecules) experience dispersion forces, and both have polar bonds (C-F in one case, C-O in the other). Only CH₃CH₂CH₂OH has the feature which can exhibit hydrogen bonding, a hydrogen atom attached to a highly electronegative atom such as O.

A certain solid substance that is hard, has a high melting point, and is nonconducting unless melted is most likely to be A) H₂S B) Cr C) I₂ D) NaCl E) CO₂

D) NaCl Hardness and a high melting point are indicative of very strong intermolecular forces, meaning that this substance is not held together by dispersion forces (like I₂ or CO₂) or dipole-dipole forces (like H₂S). Because the solid does not conduct electricity, it cannot be a metal like Cr. The remaining option, NaCl, is an ionic solid, which meets all of the properties mentioned in the question: due to its strong ion-ion intermolecular forces, NaCl forms a hard solid with a high melting point, with electrons localized to the individual ions (so it will not conduct electricity unless it is melted and separated into its individual ions).

What intermolecular forces are present between two molecules of CH₃CH₂SH? A) dispersion only B) dipole-dipole only C) hydrogen bonding only D) dispersion and dipole-dipole E) dispersion and hydrogen bonding

D) dispersion and dipole-dipole. CH₃CH₂SH is a polar molecule that will have dipole-dipole interactions between two molecules. Dispersion forces will also occur because all molecules have dispersion forces.

Which is expected to have the highest heat of vaporization? A) CH₄ B) SiH₄ C) NH₃ D) PH₃ E) H₂O

E) H₂O Heat of vaporization is the energy required to change a substance from a liquid to a gas, and so compounds with stronger intermolecular forces will have higher heats of vaporization. Two of these options exhibit hydrogen bonding (NH₃ and H₂O). The dipole created by the more electronegative O atom makes the intermolecular forces stronger in H₂O, meaning it will have the highest heat of vaporization.

Which is expected to have the lowest vapor pressure? A) CH₄ B) SiH₄ C) NH₃ D) PH₃ E) H₂O

E) H₂O Vapor pressure describes the amount of a liquid that can escape into the gas phase, so compounds with stronger intermolecular forces will have lower vapor pressures. Two of these options exhibit hydrogen bonding (NH₃ and H₂O). The dipole created by the more electronegative O atom makes the intermolecular forces stronger in H₂O, meaning it will have the lower vapor pressure.

Which of the following phase changes represents sublimation? A) gas ⟶ liquid B) liquid ⟶ gas C) liquid ⟶ solid D) solid ⟶ liquid E) solid ⟶ gas

E) solid ⟶ gas