APCE Intermolecular Forces & Properties 1 — Questions and Answers
Question 1: Which type of intermolecular force is responsible for the attraction between polar molecules?
- London dispersion forces.
- Hydrogen bonding.
- Dipole-dipole interactions. (Correct answer)
- Ionic bonds.
Correct answer: Dipole-dipole interactions.
Dipole-dipole interactions are attractive forces that occur between the permanent dipoles of polar molecules. The partially positive end of one polar molecule is attracted to the partially negative end of an adjacent polar molecule. These forces are stronger than London dispersion forces but generally weaker than hydrogen bonds.
Question 2: What is the strongest type of intermolecular force in water?
- Dipole-dipole interactions.
- London dispersion forces.
- Hydrogen bonding. (Correct answer)
- Ionic bonding.
Correct answer: Hydrogen bonding.
Water (H₂O) exhibits hydrogen bonding because it contains hydrogen atoms directly bonded to a highly electronegative oxygen atom. Hydrogen bonds are a particularly strong type of dipole-dipole interaction, occurring when hydrogen is bonded to fluorine, oxygen, or nitrogen. These strong attractions are responsible for many of water's unique properties, including its relatively high boiling point.
Question 3: What is the effect of increasing intermolecular forces on the boiling point of a substance?
- The boiling point decreases.
- The boiling point increases. (Correct answer)
- The boiling point remains the same.
- The boiling point becomes unpredictable.
Correct answer: The boiling point increases.
Boiling point is the temperature at which a liquid's vapor pressure equals the surrounding atmospheric pressure, allowing it to transition into a gas. To overcome the attractive forces holding liquid molecules together and allow them to escape into the gas phase, energy is required. Stronger intermolecular forces mean more energy is needed to separate the molecules, thus resulting in a higher boiling point.
Question 4: Which of the following substances would have the highest boiling point?
- H₂O (Water). (Correct answer)
- H₂ (Hydrogen gas).
- N₂ (Nitrogen gas).
- Cl₂ (Chlorine gas).
Correct answer: H₂O (Water).
Water (H₂O) has the highest boiling point among the given options because it exhibits strong hydrogen bonding, a particularly potent type of intermolecular force. H₂, N₂, and Cl₂ are nonpolar or weakly polar molecules that primarily rely on weaker London dispersion forces for attraction. The extensive hydrogen bonding in water requires significantly more energy to overcome, leading to its much higher boiling point.
Question 5: What is the primary factor that affects the strength of London dispersion forces?
- The polarity of the molecule.
- The size of the molecule and the number of electrons. (Correct answer)
- The type of bond the molecule has.
- The temperature of the substance.
Correct answer: The size of the molecule and the number of electrons.
London dispersion forces (LDFs) arise from temporary, induced dipoles caused by the instantaneous movement of electrons. The strength of LDFs increases with the number of electrons in a molecule because a larger electron cloud is more easily distorted (more polarizable). This means larger molecules with more electrons will experience stronger London dispersion forces.
Question 6: What type of intermolecular force exists between nonpolar molecules?
- Dipole-dipole interactions.
- London dispersion forces. (Correct answer)
- Hydrogen bonding.
- Ionic bonding.
Correct answer: London dispersion forces.
London dispersion forces are the only type of intermolecular force present between nonpolar molecules. While present in all molecules, they are the dominant force when permanent dipoles or hydrogen bonds are absent. These forces arise from temporary, instantaneous dipoles created by the random movement of electrons, inducing temporary dipoles in neighboring molecules.
Question 7: What effect does hydrogen bonding have on the boiling point of a substance?
- It lowers the boiling point.
- It increases the boiling point. (Correct answer)
- It has no effect on the boiling point.
- It increases the freezing point.
Correct answer: It increases the boiling point.
Hydrogen bonding is a particularly strong type of intermolecular force. When present, these strong attractions require a significant amount of energy to overcome for molecules to transition from the liquid to the gaseous state. Therefore, substances exhibiting hydrogen bonding typically have much higher boiling points compared to similar molecules that only have weaker dipole-dipole or London dispersion forces.
Question 8: Which of the following is an example of a molecule that exhibits hydrogen bonding?
- N₂ (Nitrogen gas).
- H₂O (Water). (Correct answer)
- CO₂ (Carbon dioxide).
- CH₄ (Methane).
Correct answer: H₂O (Water).
Hydrogen bonding occurs when a hydrogen atom is covalently bonded to a highly electronegative atom (fluorine, oxygen, or nitrogen) and is attracted to another electronegative atom in a different molecule. Water (H₂O) fits this criterion perfectly, with hydrogen atoms bonded to oxygen, allowing for extensive hydrogen bonding networks. N₂, CO₂, and CH₄ do not have hydrogen bonded to F, O, or N.
Question 9: Which type of intermolecular force is most responsible for the high boiling point of water?
- London dispersion forces.
- Dipole-dipole interactions.
- Hydrogen bonding. (Correct answer)
- Ionic bonds.
Correct answer: Hydrogen bonding.
Water's exceptionally high boiling point, compared to other hydrides of similar molecular weight, is primarily attributed to the extensive network of hydrogen bonds it forms. These strong intermolecular attractions require a large amount of thermal energy to break, allowing water to remain liquid over a wide temperature range vital for life. While dipole-dipole and London dispersion forces are also present, hydrogen bonding is by far the dominant factor.
APCE Intermolecular Forces & Properties 1
Which type of intermolecular force is responsible for the attraction between polar molecules?