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Intermolecular Forces & Properties Questions
Hydrogen bonding is the strongest intermolecular force (typically 5–40 kJ/mol), arising when H is bonded to N, O, or F and interacts with a lone pair on an electronegative atom.
Q.
London dispersion forces increase with:
Increasing bond strength
Increasing molecular size and polarizability
Decreasing molecular mass
Higher temperature
Explanation:
London dispersion forces arise from instantaneous dipoles and increase with electron count (molecular size/polarizability). Larger molecules have stronger dispersion forces.
Q.
Hydrogen bonding can occur when hydrogen is covalently bonded to:
N, O, or F
Cl, Br, or I
Any atom
C, Si, or Ge
Explanation:
Hydrogen bonds form when H is bonded to a strongly electronegative, small atom: nitrogen, oxygen, or fluorine.
Q.
Water has a higher boiling point than H₂S primarily because of:
Larger molecular mass
Higher molecular symmetry
Hydrogen bonding
Covalent network structure
Explanation:
Water forms strong hydrogen bonds, while H₂S only experiences weaker dipole-dipole and dispersion forces because sulfur is less electronegative than oxygen.
Q.
Which of the following has the highest boiling point?
H₂Se
H₂O
H₂S
H₂Te
Explanation:
H₂O forms extensive hydrogen bonds, giving it a boiling point (100°C) much higher than H₂S, H₂Se, or H₂Te despite its lower molar mass.
Q.
Which type of intermolecular force is present in ALL molecules, polar or nonpolar?
Hydrogen bonding
Dipole-dipole interactions
London dispersion forces
Ion-dipole interactions
Explanation:
London dispersion forces arise from instantaneous fluctuations in electron density and are universal — present in all atoms and molecules.
Q.
Which of the following molecules would have the strongest London dispersion forces?
F₂
I₂
Cl₂
Br₂
Explanation:
Among the halogens (F₂, Cl₂, Br₂, I₂), iodine (I₂) has the largest and most polarizable electron cloud, leading to the strongest dispersion forces.
Q.
Surface tension in liquids is caused by:
Outward pressure from the gas phase
Net inward force on surface molecules due to unbalanced attractions
Hydrogen bonding only
Ionic interactions only
Explanation:
Molecules at the surface have fewer neighboring molecules, so they experience a net inward pull. This unbalanced force creates surface tension.
Q.
Viscosity of a liquid increases with:
Smaller molecular size
Higher temperature
Weaker intermolecular forces
Stronger intermolecular forces and larger molecular size
Explanation:
Stronger intermolecular forces and larger molecular size increase viscosity by making it harder for molecules to slide past each other.
Q.
The vapor pressure of a liquid is determined by:
Surface area only
Volume of the container only
Atomic mass only
Strength of intermolecular forces and temperature
Explanation:
Vapor pressure depends on the strength of intermolecular forces and temperature. Weaker forces and higher temperatures increase vapor pressure.
Q.
Which of the following is NOT an intermolecular force?
Dipole-dipole interaction
Ionic bond
London dispersion force
Hydrogen bond
Explanation:
Ionic bonds are intramolecular (they occur within an ionic compound). London dispersion, dipole-dipole, and hydrogen bonds are all intermolecular forces.
Q.
Molecules of a gas are characterized by:
Strong intermolecular forces and fixed positions
High density and low compressibility
Ordered crystalline arrangement
Weak intermolecular forces and large distances between molecules
Explanation:
In a gas, intermolecular forces are negligible and molecules are far apart, moving rapidly and randomly.
Q.
The critical temperature of a gas is:
The boiling point of a liquid
The temperature above which a gas cannot be liquefied by pressure alone
The temperature at which a gas becomes an ideal gas
The freezing point of a gas
Explanation:
Above the critical temperature, a gas cannot be liquefied regardless of pressure. It represents the temperature at which the gas and liquid phases become indistinguishable.
Q.
Which of the following liquids would have the highest vapor pressure at room temperature?
Ethanol
Water
Ethylene glycol
Diethyl ether
Explanation:
Higher vapor pressure = weaker intermolecular forces. Diethyl ether has only weak dipole-dipole and dispersion forces (no hydrogen bonding), so it is more volatile than water, ethanol, or ethylene glycol (which form H-bonds).
Q.
Adhesive forces are:
Forces within a molecule
Forces between molecules of the same substance
Forces between molecules of different substances
Nuclear forces
Explanation:
Adhesive forces are attractive forces between molecules of DIFFERENT substances (e.g., water and glass). Cohesive forces are between identical molecules.
Q.
Capillary action in a narrow tube is due to:
Only cohesive forces
Atmospheric pressure alone
Combined effect of adhesive and cohesive forces
Gravity alone
Explanation:
Capillary action results from the combination of adhesive forces (between liquid and tube) and cohesive forces (within the liquid). When adhesion > cohesion (as with water in glass), the liquid rises.
Q.
Mercury does not wet glass because:
Cohesive forces exceed adhesive forces with glass
Its density is too low
Its boiling point is too low
It has no intermolecular forces
Explanation:
Mercury's cohesive forces are stronger than its adhesive forces with glass, so it forms a convex meniscus and is pushed down in a capillary tube.
Q.
Which of the following exhibits hydrogen bonding?
HCl
HBr
HI
HF
Explanation:
HF can form hydrogen bonds because H is attached to highly electronegative F. HCl, HBr, and HI have progressively weaker hydrogen bonding (essentially none).
Q.
Ion-dipole interactions are important in:
Pure covalent crystals
Metallic solids
Nonpolar gases
Solutions of ionic compounds in polar solvents
Explanation:
Ion-dipole interactions occur between an ion and a polar molecule — for example, Na⁺ surrounded by water molecules in solution. These interactions are key to dissolution of ionic salts in water.
Q.
Which of the following has the lowest boiling point?
SnH₄
SiH₄
CH₄
GeH₄
Explanation:
Small, nonpolar molecules have only weak London dispersion forces. Among CH₄, SiH₄, GeH₄, SnH₄, methane is smallest and has the weakest dispersion forces — lowest boiling point.
Q.
Which of the following statements about hydrogen bonding is FALSE?
Hydrogen bonding explains the structure of DNA and proteins
Hydrogen bonding affects boiling points of water and ammonia
Hydrogen bonds are as strong as covalent bonds
Hydrogen bonds form when H is bonded to N, O, or F
Explanation:
Hydrogen bonds are much weaker than covalent bonds (5–40 kJ/mol vs 150–1000+ kJ/mol). They are attractions between molecules, not true chemical bonds in the covalent sense.
Q.
The anomalous high boiling point of water compared to other hydrides is primarily due to:
Water has weak intermolecular forces
Each water molecule can form up to four hydrogen bonds
Water is nonpolar
Water has a very high molecular mass
Explanation:
Water forms up to 4 hydrogen bonds per molecule (two as donor, two as acceptor), resulting in a highly extensive and strong hydrogen-bonding network.
Q.
Which has the highest boiling point among the following?
I₂
F₂
Cl₂
Br₂
Explanation:
Larger nonpolar molecules have stronger London dispersion forces. I₂ has the largest electron cloud and highest molecular mass among F₂, Cl₂, Br₂, I₂, giving the strongest dispersion forces and highest boiling point.
Q.
The meniscus of water in a glass tube is concave because:
Cohesive forces are stronger than adhesive forces
Water has a low surface tension
Gravity pulls water upward
Adhesive forces with glass are stronger than cohesive forces
Explanation:
When adhesion (water-glass) is stronger than cohesion (water-water), water is pulled up along the walls of the tube, producing a concave meniscus.