Chemistry: Intermolecular Forces and Physical Properties
Connecting particle attractions to boiling point, solubility, vapor pressure, and more
Chemistry: Intermolecular Forces and Physical Properties
Connecting particle attractions to boiling point, solubility, vapor pressure, and more
Chemistry - Grade 9-12
- 1
Identify the strongest intermolecular force present in a sample of liquid water, H2O, and explain why it occurs.
Hydrogen bonding occurs when hydrogen is bonded to nitrogen, oxygen, or fluorine.
The strongest intermolecular force in liquid water is hydrogen bonding. It occurs because hydrogen atoms bonded to oxygen are attracted to lone pairs on oxygen atoms in nearby water molecules. - 2
Identify the strongest intermolecular force present between molecules of carbon dioxide, CO2. Explain why CO2 does not have dipole-dipole attractions even though each C=O bond is polar.
The strongest intermolecular force between CO2 molecules is London dispersion forces. CO2 has polar bonds, but the molecule is linear and symmetrical, so the bond dipoles cancel and the molecule is nonpolar. - 3
Rank the following substances from lowest boiling point to highest boiling point: CH4, CH3Cl, CH3OH. Explain your ranking using intermolecular forces.
Stronger intermolecular forces usually lead to higher boiling points.
The order from lowest to highest boiling point is CH4, CH3Cl, CH3OH. CH4 is nonpolar and has only London dispersion forces, CH3Cl is polar and has dipole-dipole forces, and CH3OH can form hydrogen bonds, which are strongest. - 4
Explain why ethanol, C2H5OH, has a higher boiling point than dimethyl ether, CH3OCH3, even though they have the same molecular formula, C2H6O.
Look for an O-H, N-H, or F-H bond in the molecule.
Ethanol has a higher boiling point because ethanol molecules can form hydrogen bonds through the O-H group. Dimethyl ether has polar bonds but no hydrogen bonded to oxygen, so it cannot hydrogen bond to itself as strongly. - 5
A student says, "All molecules with polar bonds are polar molecules." Explain why this statement is not always correct, using CCl4 as an example.
The statement is not always correct because molecular shape matters. CCl4 has polar C-Cl bonds, but its tetrahedral shape is symmetrical, so the bond dipoles cancel and the molecule is nonpolar overall. - 6
Compare the boiling points of F2 and Br2. Both are nonpolar diatomic molecules. Which one has the higher boiling point, and why?
For nonpolar substances, larger molar mass and more electrons usually mean stronger dispersion forces.
Br2 has the higher boiling point. Both substances have London dispersion forces, but Br2 has more electrons and a larger electron cloud, making its dispersion forces stronger than those in F2. - 7
Explain why water has a much higher surface tension than hexane, C6H14.
Water has a much higher surface tension because water molecules form strong hydrogen bonds with each other. Hexane is nonpolar and has only London dispersion forces, so its molecules do not attract each other as strongly. - 8
A liquid has a high vapor pressure at room temperature. What does this tell you about the strength of its intermolecular forces? Explain.
Vapor pressure depends on how easily particles escape from the liquid surface.
A high vapor pressure means the liquid has relatively weak intermolecular forces. More molecules can escape from the liquid into the gas phase because they are not held together strongly. - 9
Predict which substance is more soluble in water: sodium chloride, NaCl, or iodine, I2. Explain your answer using particle attractions.
Like dissolves like means polar and ionic substances tend to dissolve well in polar solvents.
Sodium chloride is more soluble in water because water is polar and can attract Na+ and Cl- ions through ion-dipole attractions. Iodine is nonpolar, so it does not interact strongly with polar water molecules. - 10
Oil and water separate into two layers when mixed. Explain this observation using polarity and intermolecular forces.
Oil and water separate because water is polar and forms strong hydrogen bonds with itself, while oil is mostly nonpolar and interacts mainly through London dispersion forces. The two liquids do not form strong attractions with each other, so they are immiscible. - 11
Which liquid would you expect to be more viscous at room temperature: glycerol, C3H8O3, or propane, C3H8? Explain your reasoning.
Viscosity increases when molecules attract each other strongly or have shapes that make movement difficult.
Glycerol is more viscous because it has three O-H groups and can form many hydrogen bonds between molecules. Propane is nonpolar and has only London dispersion forces, so its molecules flow past each other more easily. - 12
Rank HCl, HBr, and HI from lowest boiling point to highest boiling point. All three are polar molecules, but hydrogen bonding is not present. Explain your ranking.
The order from lowest to highest boiling point is HCl, HBr, HI. Although all three molecules have dipole-dipole attractions, the larger molecules have more electrons and stronger London dispersion forces, so HI has the highest boiling point. - 13
A mystery liquid has a boiling point of 80 degrees C, while another similar-sized liquid has a boiling point of 10 degrees C. Which liquid likely has stronger intermolecular forces? Explain.
Boiling requires particles to overcome attractions to nearby particles.
The liquid with a boiling point of 80 degrees C likely has stronger intermolecular forces. More energy is needed to separate its particles and change it from a liquid to a gas. - 14
Explain why ammonia, NH3, has a higher boiling point than phosphine, PH3, even though PH3 has a greater molar mass.
Ammonia has a higher boiling point because NH3 molecules can form hydrogen bonds due to N-H bonds and a lone pair on nitrogen. PH3 is larger, but it does not form significant hydrogen bonds, so its intermolecular forces are weaker overall. - 15
A student is comparing two molecules: molecule A is nonpolar with a molar mass of 58 g/mol, and molecule B is polar with a molar mass of 30 g/mol. The student claims molecule B must always have the higher boiling point because it is polar. Evaluate the claim.
Boiling point depends on all intermolecular forces, not just one type.
The claim is not always correct. Polarity can increase boiling point through dipole-dipole forces, but molar mass and electron cloud size also affect London dispersion forces. A larger nonpolar molecule can sometimes have a higher boiling point than a smaller polar molecule if its dispersion forces are strong enough.