Which attraction is the strongest of the intermolecular forces acting between molecules in liquid water?
Hydrogen bonding
✓
Dispersion forces
Dipole-dipole attractions
Ion-dipole attractions
Correct Answer
Hydrogen bonding
Water molecules have H atoms bonded to highly electronegative O atoms, so each H is strongly attracted to a lone pair on a neighboring O; this hydrogen bonding is the strongest attraction between water molecules. Dispersion forces are present but are weak in a molecule as small as water. Ordinary dipole-dipole attractions are weaker than hydrogen bonds. Ion-dipole attractions require ions, and pure water contains essentially none.
Category: Intermolecular Forces
Question 2
Which halogen has the highest boiling point?
Br2
I2
✓
Cl2
F2
Correct Answer
I2
The halogens are nonpolar, so only dispersion forces hold their molecules together, and these grow stronger as molecules get larger and more polarizable; I2 is the largest and is a solid at room temperature. Br2 is a liquid with a lower boiling point than I2. Cl2 is a gas at room temperature. F2 is the smallest and has the lowest boiling point of the four.
Category: Intermolecular Forces
Question 3
Which compound has molecules that can hydrogen bond to one another?
CH3OCH3
CH4
CH3OH
✓
CH3Cl
Correct Answer
CH3OH
Methanol, CH3OH, has an H atom bonded directly to O, so its molecules hydrogen bond to each other. In CH3OCH3 every H is bonded to C, so it cannot donate a hydrogen bond to another molecule of itself. CH4 has only C-H bonds and is held by dispersion forces. In CH3Cl the H atoms are bonded to C, and Cl is not one of the atoms (N, O, F) that form hydrogen bonds.
Category: Intermolecular Forces
Question 4
What causes London dispersion forces between atoms or molecules?
Permanent dipoles in polar molecules
Full charges on positive and negative ions
Electrons shared between bonded atoms
Temporary dipoles from shifting electron clouds
✓
Correct Answer
Temporary dipoles from shifting electron clouds
Electron clouds move constantly, so at any instant an atom or molecule can have an uneven charge distribution that induces a dipole in its neighbor, producing a brief attraction. Permanent dipoles in polar molecules give rise to dipole-dipole attractions instead. Full charges on positive and negative ions produce ionic bonding. Electrons shared between bonded atoms form covalent bonds within a molecule.
Category: Intermolecular Forces
Question 5
Pentane and 2,2-dimethylpropane both have the formula C5H12, but pentane has the higher boiling point. Why?
Its extended shape gives more contact for dispersion forces
✓
Its molecules form hydrogen bonds with each other
Its molar mass is larger than that of the other isomer
Its covalent bonds are stronger within each molecule
Correct Answer
Its extended shape gives more contact for dispersion forces
Pentane's long chain can lie alongside neighboring molecules over a large area, while the compact, nearly spherical 2,2-dimethylpropane touches its neighbors over less surface, so dispersion forces are stronger in pentane. Hydrogen bonds cannot form because every H is bonded to C. The molar masses are equal because the formulas are identical. Boiling separates molecules from each other and does not break covalent bonds.
Category: Intermolecular Forces
Question 6
Which change lowers the viscosity of a liquid?
Strengthening its intermolecular forces
Raising its temperature
✓
Using a liquid with longer molecules
Cooling it toward its freezing point
Correct Answer
Raising its temperature
Higher temperature gives molecules more kinetic energy to overcome the attractions that resist flow, so warm syrup pours faster than cold syrup. Strengthening intermolecular forces makes molecules hold onto each other more, raising viscosity. Longer molecules tangle with one another and also raise viscosity. Cooling a liquid reduces molecular motion and makes it more viscous.
Category: Properties of Liquids
Question 7
Water in a narrow glass tube forms a meniscus that curves upward at the edges. What explains this shape?
Cohesive forces in the water exceed adhesive forces to the glass
The water has no surface tension inside the tube
Adhesive forces to the glass exceed cohesive forces in the water
✓
The glass repels water molecules near its surface
Correct Answer
Adhesive forces to the glass exceed cohesive forces in the water
Water molecules are attracted to the polar glass surface more strongly than to each other, so the water climbs the walls and forms a concave meniscus. When cohesive forces in the liquid exceed adhesive forces to the glass, as with mercury, the meniscus curves downward instead. Water in a tube still has surface tension. Glass attracts water molecules rather than repelling them.
Category: Properties of Liquids
Question 8
In a simple cubic unit cell, identical atoms touch one another along each edge. How does the edge length compare with the atomic radius?
Four times the radius
Equal to the radius
Half the radius
Twice the radius
✓
Correct Answer
Twice the radius
In a simple cubic cell an atom sits at each corner, and two corner atoms touch along an edge, so the edge spans one radius from each atom, or 2r. Four times the radius describes the face diagonal of a face-centered cubic cell or the body diagonal of a body-centered cubic cell. An edge equal to the radius would place the atoms' centers closer than touching allows. Half the radius is even shorter and impossible for touching atoms.
Category: Crystal Lattices
Question 9
Which liquid has the highest vapor pressure at 25 degrees Celsius?
Diethyl ether
✓
Ethanol
Water
Ethylene glycol
Correct Answer
Diethyl ether
Diethyl ether molecules cannot hydrogen bond to one another, so its intermolecular forces are weak and its vapor pressure is high; it boils at about 35 degrees Celsius. Ethanol hydrogen bonds through its single OH group and has a lower vapor pressure. Water forms an extensive hydrogen-bonded network, lowering its vapor pressure further. Ethylene glycol has two OH groups per molecule and the lowest vapor pressure of the four.
Category: Phase Transitions
Question 10
What is true of a liquid at its normal boiling point?
Its vapor pressure equals zero
Its vapor pressure equals 1 atm
✓
Its vapor pressure equals its critical pressure
It first begins to evaporate
Correct Answer
Its vapor pressure equals 1 atm
A liquid boils when its vapor pressure equals the external pressure, and the normal boiling point is the temperature at which that vapor pressure is exactly 1 atm. A vapor pressure of zero would mean no molecules escape at all. The critical pressure is reached only at the critical point, far above the normal boiling point for most substances. Evaporation happens at all temperatures, well below the boiling point.
Category: Phase Transitions
Question 11
The enthalpy of vaporization of water is 40.7 kJ/mol. How much heat is needed to vaporize 2.00 mol of water at its boiling point, to three significant figures?
20.4 kJ
40.7 kJ
81.4 kJ
✓
12.0 kJ
Correct Answer
81.4 kJ
Heat equals moles times enthalpy of vaporization: 2.00 mol x 40.7 kJ/mol = 81.4 kJ. 20.4 kJ divides by the moles instead of multiplying. 40.7 kJ is the heat for only one mole. 12.0 kJ uses the enthalpy of fusion of water (about 6.01 kJ/mol), which applies to melting, not boiling.
Category: Phase Transitions
Question 12
On a heating curve, the temperature of a solid stays constant while it melts even though heat is still being added. Why?
The added energy raises the particles' average speed
The solid stops absorbing heat once melting begins
The solid releases as much heat as it absorbs
The added energy overcomes attractions between particles
✓
Correct Answer
The added energy overcomes attractions between particles
During melting, the energy goes into loosening the attractions that hold particles in fixed positions, not into increasing their kinetic energy, so the temperature holds steady until all the solid has melted. Raising the particles' average speed would raise the temperature. The solid keeps absorbing heat throughout melting; that is the enthalpy of fusion. A solid that released as much heat as it absorbed would not change phase at all.
Category: Phase Transitions
Question 13
What is the direct change from a gas to a solid, as when frost forms on a cold window, called?
Deposition
✓
Sublimation
Condensation
Freezing
Correct Answer
Deposition
Deposition is the phase change from gas directly to solid without passing through the liquid state, and it is exothermic. Sublimation is the reverse process, solid directly to gas. Condensation is gas to liquid. Freezing is liquid to solid.
Category: Phase Transitions
Question 14
What does the triple point on a phase diagram represent?
Conditions beyond which liquid and gas look the same
Conditions where solid, liquid and gas coexist
✓
Conditions where the solid melts at 1 atm
Conditions where the liquid boils at 1 atm
Correct Answer
Conditions where solid, liquid and gas coexist
The triple point is the single temperature and pressure at which the solid, liquid and gas phases are all in equilibrium, where the three phase boundary lines meet. The conditions beyond which liquid and gas cannot be told apart define the critical point. Melting at 1 atm is the normal melting point. Boiling at 1 atm is the normal boiling point.
Category: Phase Diagrams
Question 15
On the phase diagram of water, the line between the solid and liquid regions slopes slightly to the left as pressure rises. What does this show?
Raising the pressure raises the melting point
Ice cannot melt at pressures above 1 atm
Raising the pressure lowers the melting point
✓
Pressure has no effect on the melting point
Correct Answer
Raising the pressure lowers the melting point
Ice is less dense than liquid water, so squeezing ice favors the denser liquid and lowers the melting point, giving the solid-liquid line a negative slope. Raising the melting point with pressure is the usual behavior for most substances, whose solids are denser than their liquids. Ice does melt above 1 atm, at slightly lower temperatures. A line with any slope means pressure does affect the melting point.
Category: Phase Diagrams
Question 16
At 1 atm, solid carbon dioxide (dry ice) changes directly into a gas instead of melting. What feature of its phase diagram explains this?
Its critical point lies below 1 atm
It has no liquid phase at any pressure
Its solid-liquid line slopes to the left
Its triple point lies above 1 atm
✓
Correct Answer
Its triple point lies above 1 atm
The triple point of CO2 is about 5.1 atm, and liquid CO2 cannot exist at pressures below the triple point, so at 1 atm the solid passes straight to gas. The critical point of CO2 is far above 1 atm (about 73 atm). Liquid CO2 does exist at pressures above the triple point, as in CO2 fire extinguishers. The solid-liquid line of CO2 slopes to the right, like most substances.
Category: Phase Diagrams
Question 17
What is a substance called when it is held above both its critical temperature and its critical pressure?
Supercritical fluid
✓
Saturated vapor
Amorphous solid
Plasma
Correct Answer
Supercritical fluid
Above the critical temperature and pressure, the liquid and gas phases can no longer be distinguished, and the substance is a supercritical fluid with properties of both; supercritical CO2 is used to remove caffeine from coffee. A saturated vapor is a gas in equilibrium with its liquid, which cannot exist above the critical temperature. An amorphous solid is a solid without long-range order. Plasma is a state of ionized gas at very high temperatures, not the region beyond the critical point.
Category: Phase Diagrams
Question 18
Diamond, in which each carbon atom is bonded to four others throughout the entire crystal, is classified as which type of solid?
Molecular solid
Covalent network solid
✓
Ionic solid
Metallic solid
Correct Answer
Covalent network solid
In diamond, covalent bonds extend through the whole crystal with no separate molecules, which makes it a covalent network solid that is very hard and has a very high melting point. A molecular solid consists of discrete molecules held by intermolecular forces. An ionic solid is made of cations and anions, and diamond contains no ions. A metallic solid has delocalized electrons and conducts electricity, unlike diamond.
Category: Types of Solids
Question 19
Which type of solid conducts electricity well as a solid and can be hammered into thin sheets?
Ionic solid
Covalent network solid
Metallic solid
✓
Molecular solid
Correct Answer
Metallic solid
Metallic solids contain positive metal ions surrounded by delocalized valence electrons that carry current and let layers of atoms slide without the solid shattering. An ionic solid conducts only when melted or dissolved and is brittle. A covalent network solid such as diamond is hard and usually does not conduct. A molecular solid is soft and does not conduct electricity.
Category: Types of Solids
Question 20
How many atoms belong to one face-centered cubic unit cell?
2
1
14
4
✓
Correct Answer
4
A face-centered cubic cell has eight corner atoms each shared by eight cells (8 x 1/8 = 1) and six face atoms each shared by two cells (6 x 1/2 = 3), for a total of 4. Two atoms per cell describes a body-centered cubic cell. One atom per cell describes a simple cubic cell. Fourteen counts every atom position without accounting for sharing between neighboring cells.
Category: Crystal Lattices