Unit 1-4
What amount of heat, in kJ, would be involved in condensing 15.9 g of CH₃OH? (∆Hvap = 38.0 kJ/mol)
-18.9 kJ Stoichiometry can be used. Since grams of CH₃OH was provided, that will be the starting point, with the molar mass and heat of vaporization (which is negative since condensation is an exothermic process) providing the conversion factors. 15.8 g CH3OH×1 mol CH3OH32.04 g CH3OH×−38.0 kJ 1 mol CH3OH=−18.7 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 one of the following statements would be true about vaporization? A) It is the phase change from solid to gas. B) It is an endothermic process. C) The liquid must release heat as it becomes a gas. D) None of the above statements are true.
B) It is an endothermic process. Vaporization is the phase change from a liquid to a gas, a process which requires the input of energy in order to overcome the attractive intermolecular forces. As such, it is an endothermic process.
What intermolecular forces are present between two molecules of CH₃CF₃? A) dispersion only B) dispersion and dipole-dipole C) dispersion, dipole-dipole, and hydrogen bonding D) dispersion and hydrogen bonding +
B) dispersion and dipole-dipole What intermolecular forces are present between two molecules of CH₃CF₃?
What phase change is represented by the transition from A to C? Normal pressure and temperature phase diagram A) fusion B) sublimation C) vaporization D) condensation E) solidification
B) sublimation Point A on this phase diagram falls within a region of high pressure (which will force the substance into a condensed phase) and low temperature (which will keep this substance from being able to overcome its intermolecular forces). Combined, these conditions cause the substance to be highly condensed, meaning it will be a solid. Point C on this phase diagram falls within a region of high temperature (which will allow this substance to overcome its intermolecular forces) and low pressure (which will not force the substance into a condensed phase). Combined, these conditions cause the substance to be in the gaseous phase. Therefore, the transition from point A to point C is the transition from the solid to the gas phase, which by definition is called sublimation.
What is represented by point D on the phase diagram? Normal pressure and temperature phase diagram A) melting point B) triple point C) boiling point D) sublimation point E) freezing point
B) triple point Point D on this phase diagram represents the pressure and temperature at which all three phases (solid, liquid, and gas) coexist. By definition, this is called the triple point.
Calculate the total number of atoms contained within this unit cell. A cube with 4 unit cells, and a dot at each corner and in the middle. A) 9 atoms B) 4 atoms C) 2 atoms D) 1 atom
C) 2 atoms 2 atoms. To calculate the number of atoms in the unit cell, multiply the number of atoms on vertices times the fraction of each atom that is within the unit cell. In a cubic unit cell, corners are 1/8 of an atom, edges are 1/4 of an atom, and faces are 1/2 of an atom. An atom in the center is fully in the unit cell and counts as a whole atom.
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.
Calculate the total number of atoms contained within this unit cell. A cube with 4 unit cells, and a dot at each corner. A) 8 atoms B) 4 atoms C) 2 atoms D) 1 atom
D) 1 atom 1 atom. To calculate the number of atoms in the unit cell, multiply the number of atoms on vertices times the fraction of each atom that is within the unit cell. In a cubic unit cell, corners are 1/8 of an atom, edges are 1/4 of an atom, and faces are 1/2 of an atom. An atom in the center is fully in the unit cell and counts as a whole atom.
Which term is a measure of the resistance to flow in a liquid? A) adhesion B) capillary action C) cohesion D) viscosity E) surface tension
D) viscosity Viscosity is defined as a liquid's resistance to flowing.
Which of the following liquid substances has the highest vapor pressure at its normal boiling point? A) CH₃OH (b.p. = 65°C) B) C₆H₆ (b.p. = 80°C) C) C₂H₅OH (b.p. = 78°C) D) H₂O (b.p. = 100°C) E) All would have same vapor pressure at their normal boiling points.
E) All would have same vapor pressure at their normal boiling points. A substance's normal boiling point is defined as the temperature at which its vapor pressure is equal to the external pressure. As such, all liquid substances at their normal boiling point will have the same vapor pressure, which is 1 atm at standard temperature and pressure.
Which of the following straight-chain hydrocarbons will have the highest viscosity at 298 K? A) C₅H₁₂ B) C₆H₁₄ C) C₇H₁₆ D) C₈H₁₈ E) C₉H₂₀
E) C₉H₂₀ Viscosity is defined as a liquid's resistance to flowing, so compounds with stronger intermolecular forces will have higher viscosities. Since these options are all nonpolar molecules, C₉H₂₀, which has the most electrons to be dispersed and therefore the strongest intermolecular forces will have the highest viscosity.
What amount of heat, in kJ, is required to vaporize 129.20 g of ethanol (C₂H₅OH)? (∆Hvap = 43.3 kJ/mol)
121 kJ Stoichiometry can be used. Since grams of C₂H₅OH was provided, that will be the starting point, with the molar mass and heat of vaporization (which is positive since vaporization is an endothermic process) providing the conversion factors. 115.00 g C2H5OH×1 mol C2H5OH46.07 g C2H5OH×43.3 kJ 1 mol C2H5OH=108 kJ
What quantity of heat, in kJ, is required to convert 15.0 g of ethanol (C₂H₅OH) at 23.0°C to a vapor at 78.3°C (its boiling point)? (specific heat capacity of ethanol = 2.46 J/g • C; ∆Hvap = 39.3 kJ/mol)
14.8 kJ Heating liquid ethanol at 23.0 °C to a vapor at 78.3 °C requires two steps: (1) heating the liquid ethanol to its boiling point (78.3 °C), and (2) converting the liquid into a vapor. The total amount of heat required is the sum of the heat required for each step. q1=50.0 g C2H5OH(l)×2.46 J g C2H5OH(l)⋅∘C×(78.3−23.0)∘C=6800 J×1 kJ1000 J=6.80 kJq2=50.0 g C2H5OH×1 mol C2H5OH46.07 g C2H5OH×39.3 kJ1 mol C2H5OH=42.7 kJ qtotal=q1+q2=6.80 kJ+42.7 kJ=49.5 kJ
What amount of heat, in kJ, is required to convert 1.10 g of water at 67.0 °C to 1.10 g of steam at 100.0 °C? (specific heat capacity of water = 4.184 J/g • °C; ∆Hvap = 40.7 kJ/mol)
2.64 kJ Heating liquid water at 67.0 °C to steam at 100.0 °C requires two steps: (1) heating the liquid water to its boiling point (100.0 °C), and (2) converting the liquid into steam. The total amount of heat required is the sum of the heat required for each step. q1=1.00 g H2O(l)×4.184 J g H2O(l)⋅∘C×(100.0−67.0)∘C=140. J×1 kJ1000 J=0.140 kJq2=1.00 g H2O×1 mol H2O18.02 g H2O×40.7 kJ1 mol H2O=2.26 kJ qtotal=q1+q2=2.40 kJ
A metal crystallizes with a face-centered cubic lattice. The edge of the unit cell is 442 pm. What is the diameter (in pm) of the metal atom?
313 pm The relation between the atomic radius (r) and the unit cell length (a) depends on the type of crystal lattice. For a face-centered cubic lattice: a=(2√2)rr=a(2√2)=442 pm(2√2)=156.3 pm d=2r=2(156.3 pm)=313 pm
What is the expected value for the heat of sublimation of acetic acid, in kJ/mol, if its heat of fusion is 10.8 kJ/mol and its heat of vaporization is 24.3 kJ/mol?
35.1 kJ/mol The heat of sublimation is the heat required to change from a solid to a gas. For a given substance, the expected heat of sublimation can be determined by adding the heat of fusion (the heat required to change from a solid to a liquid) to the heat of vaporization (the heat required to change from a liquid to a gas). ΔHsub=ΔHfus+ΔHvap=10.8 kJ/mol+24.3 kJ/mol=35.1 kJ/mol
What amount of heat (in kJ) is required to convert 15.0 g of an unknown liquid (MM = 83.21 g/mol) at 19.2 °C to a gas at 93.5 °C? (specific heat capacity of liquid = 1.58 J/g･°C; specific heat capacity of gas = 0.932 J/g･°C; ∆Hvap = 22.5 kJ/mol; normal boiling point, Tb = 57.3°C)
5.47 kJ First, you need to find the energy required to change from 19.2 °C to 57.3 °C.q=mcΔT=(10.1g)(1.58J/g⋅°C)(57.3°C−19.2°C)=607.9998Jq=mcΔT=(10.1g)(1.58J/g⋅°C)(57.3°C−19.2°C)=607.9998J Then, you need to calculate the energy required for the liquid to become a gas. Convert the grams into moles. q=mHvap=(10.1g83.21g/mol)(22500J)=2731.5Jq=mHvap=(10.1g83.21g/mol)(22500J)=2731.5J Then, you need to find the energy required to change from 57.3 C to 93.5 C.q=mcΔT=(10.1g)(0.932J/g⋅°C)(93.5°C−57.3°C)=340.75784Jq=mcΔT=(10.1g)(0.932J/g⋅°C)(93.5°C−57.3°C)=340.75784J Finally, add all of these values together and divide by 1000 to get an answer in Joules.607.9998J+2731.5J+340.75784J=3680.25764J=3.68kJ
Barium metal (d = 3.51 g/cm³) has a body-centered cubic structure. Calculate the edge length of a unit cell in pm. (1 m = 10¹² pm)
506 pm The density (d) of a unit cell depends on the unit cell volume (V), the molecular weight of the atoms (MW), Avogadro's number (N), and the number of atoms in the unit cell: d=(# atoms/unit cell)⋅MWNA⋅1Vcd=(# atoms/unit cell)⋅MWNA⋅1Vc To determine the edge length of a unit cell (a), first determine the unit cell volume (V). Vc=(# atoms/unit cell)⋅MWNA⋅1dVc=(# atoms/unit cell)⋅MWNA⋅1d Next, determine the number of atoms in a body-centered cubic cell. A body-centered cubic unit cell has 1/8 of an atom at each of its eight corners and one atom in the center of the cell: (8 corners/unit cell×18 atom/corner)+(1 atom/unit cell)=2 atoms/unit cell(8 corners/unit cell×18 atom/corner)+(1 atom/unit cell)=2 atoms/unit cell Now determine the unit cell volume: Vc=(2 atoms)⋅137.33gmol/6.022×1023atomsmol⋅13.51g/cm3=1.299×10−22 cm3a=(Vc)13=(1.299×10−22 cm3)13=5.06×10−8 cm×1010 pm1 cm=506 pm
Chromium metal crystallizes as a body-centered cubic lattice. If the atomic radius of Cr is 1.25 × 10⁻⁸ cm, what is the density of Cr metal in g/cm³?
7.15 g/cm^3 The density (d) of a unit cell depends on the unit cell volume (V), the molecular weight of the atoms (MW), Avogadro's number (N), and the number of atoms in the unit cell: d=(# atoms/unit cell)⋅MWNA⋅1Vcd=(# atoms/unit cell)⋅MWNA⋅1Vc To determine the unit cell volume (V), first determine the unit cell edge length (a) based on the atomic radius (r). For a body-centered cubic cell: a=4r√3=(4)(1.25×10−8 cm)√3=2.887×10−8 cmVc=a3=(2.887×10−8 cm)3=2.406×10−23 cm3a=4r3=(4)(1.25×10−8 cm)3=2.887×10−8 cmVc=a3=(2.887×10−8 cm)3=2.406×10−23 cm3 Next, determine the number of atoms in a body-centered cubic cell. A body-centered cubic unit cell has 1/8 of an atom at each of its eight corners and one atom in the center of the cell: (8 corners/unit cell×18 atom/corner)+(1 atom/unit cell)=2 atoms/unit cell(8 corners/unit cell×18 atom/corner)+(1 atom/unit cell)=2 atoms/unit cell Now solve for the density:d=(2 atoms)(52.00g/mol/6.022×1023atoms/mol)⋅12.406×10−23 cm3=7.18gcm3
A metal has a body-centered cubic lattice with a unit cell edge length of 2.866 Å (1 Å = 10⁻¹⁰ m). The density of the metal is 7.87 g/cm³. What is the mass of an atom of this metal in grams? (1 m = 10¹² pm).
9.26 x 10^-23 g The density (d) of a unit cell depends on the unit cell volume (V), the molecular weight of the atoms (MW), Avogadro's number (N), and the number of atoms in the unit cell: d=(# atomsunit cell)⋅MWNA⋅1Vcd=(# atoms/unit cell)⋅MW/NA⋅1/Vc Rearrange this equation to solve for the molar mass, and omit Avogadro's number to directly calculate the mass of one atom instead of the mass of one mole of this metal: Mass of one atom=(d)(Vc)/(# atomsunit cell)Mass of one atom=(d)(Vc)(# atoms/unit cell) Density has units of g/cm³, so first use the unit cell edge length (a) to determine the unit cell volume (V) in cm³: a=2.866 Å×1 cm108 Å=2.866×10−8 cmVc=(2.866×10−8 cm)3=2.354×10−23 cm3a=2.866 Å×1 cm108 Å=2.866×10−8 cmVc=(2.866×10−8 cm)3=2.354×10−23 cm3 Next, determine the number of atoms in a body-centered cubic cell. A body-centered cubic unit cell has 1/8 of an atom at each of its eight corners and one atom in the center of the cell: (8 corners/unit cell×18 atom/corner)+(1 atom/unit cell)=2 atoms/unit cell(8 corners/unit cell×18 atom/corner)+(1 atom/unit cell)=2 atoms/unit cell Now solve for the mass of a single atom of this metal: Mass of one atom=(7.87gcm3)(2.354×10−23 cm3)2 atoms=9.26×10−23g/atom
Which one of the following would have the largest dispersion forces? A) CH₃CH₂SH B) CH₃NH₂ C) CH₄ D) CH₃CH₃
A) CH₃CH₂SH CH₃CH₂SH will have the largest dispersion forces because it is the largest molecule. Dispersion forces are from instantaneous dipoles which form more often when there are more electrons present. This also scales with mass and a more massive molecule will generally have more protons and therefore more electrons.
Which of the following should exhibit the highest viscosity at 298 K? A) HOCH₂CH₂OH B) CH₃OCH₃ C) CH₃OH D) CH₃Br E) CH₂Cl₂
A) HOCH₂CH₂OH Viscosity is defined as a liquid's resistance to flowing, so compounds with stronger intermolecular forces will have higher viscosities. Due to its two hydrogen bonding sites, HOCH₂CH₂OH has the strongest intermolecular forces of the options listed, meaning it will have the highest viscosity.
Which of the following would you expect to have the lowest boiling point? A) H₂ B) F₂ C) HF D) H₂S
A) H₂ The boiling point is the temperature at which a substance will change from the liquid phase to the gas phase, so compounds with weaker intermolecular forces will have lower boiling points. H₂ is a nonpolar molecule exhibiting only dispersions forces, which are weaker for molecules with fewer electrons. H₂ has the fewest electrons of all the molecules. Therefore, H₂ has the weakest intermolecular forces and the lowest boiling point.
Which of the following statements is incorrect? A) Molecular solids typically have high melting points. B) The binding forces in a molecular solid include London dispersion forces. C) Ionic solids usually have high melting points. D) Ionic solids are insulators. E) All of these statements are correct.
A) Molecular solids typically have high melting points. Molecular solids are held together by dispersion forces, which are the weakest type of intermolecular force. As a result, the molecules in a molecular solid will not be held together tightly and can therefore melt at relatively low temperatures, meaning they will not have high melting points.
What type of packing is shown in this unit cell? A cube with 4 unit cells, and a dot at each corner. A) Simple Cubic B) Body Centered Cubic C) Face Centered Cubic D) Hexagonal Closest Packed
A) Simple Cubic This unit cell has simple (primitive) cubic packing with atoms on each of the corners (vertices).
Which combination of properties would be expected to lead to the most viscous liquid? A) Strong intermolecular forces, low temperature B) Strong intermolecular forces, high temperature C) Weak intermolecular forces, low temperature D) Weak intermolecular forces, high temperature.
A) Strong intermolecular forces, low temperature Viscosity is defined as a liquid's resistance to flowing, so compounds with stronger intermolecular forces will have higher viscosities. High temperature provides molecules with enough energy to overcome attractive forces, therefore, low temperatures lead to more viscous liquids.
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 represents the intermolecular forces present in CO₂? A) dispersion only B) dipole-dipole only C) hydrogen bonding only D) dispersion and dipole-dipole E) dispersion and hydrogen bonding
A) dispersion only CO₂ does not have any hydrogen atoms, so it cannot exhibit hydrogen bonding. While CO₂ has two internal dipoles, they are of equal magnitude and point in opposite directions, meaning they cancel each other out and the molecule has no net dipole moment, so it cannot exhibit dipole-dipole forces. The only intermolecular forces it will exhibit are dispersion forces, which are exhibited by all species with electrons.
What intermolecular forces are present between two molecules of CF₃CF₃? A) dispersion only B) dipole-dipole only C) dispersion and dipole-dipole D) dispersion and hydrogen bonding E) dipole-dipole and hydrogen bonding
A) dispersion only Two molecules of CF₃CF₃ will have dispersion forces present because these forces exist between all molecules. This molecule is nonpolar and will therefore not have any dipole-dipole interactions.
Which best explains why the trend in noble gas boiling points increases down the group? A) increasing dispersion interactions B) increasing dipole-dipole interactions C) increasing ion-dipole interactions D) increasing hydrogen bonding interactions E) increasing ion-ion interactions +
A) increasing dispersion interactions Higher boiling points are a consequence of stronger intermolecular forces. The noble gases all exist as single atoms in their ground state, so they do not exhibit ion, dipole, or hydrogen-bonding intermolecular forces, but only exhibit dispersion forces. The strength of dispersion forces increases with a greater number of electrons. As you go down the group of noble gases, the number of electrons in each atom increases, leading to an increase in dispersion forces and therefore an increase in boiling point.
What is represented by A on the phase diagram? Normal pressure and temperature phase diagram A) solid B) liquid C) gas D) cannot determine without exact pressure and temperature
A) solid Point A on this phase diagram falls within a region of high pressure (which will force the substance into a condensed phase) and low temperature (which will keep this substance from being able to overcome its intermolecular forces). Combined, these conditions cause the substance to be highly condensed, meaning it will be a solid.
Calculate the total number of atoms contained within this unit cell. A cube with 4 unit cells, and a dot at each corner and in the middle and on the faces. A) 14 atoms B) 4 atoms C) 2 atoms D) 1 atom
B) 4 atoms 4 atoms. To calculate the number of atoms in the unit cell, multiply the number of atoms on vertices times the fraction of each atom that is within the unit cell. In a cubic unit cell, corners are 1/8 of an atom, edges are 1/4 of an atom, and faces are 1/2 of an atom. An atom in the center is fully in the unit cell and counts as a whole atom.
What type of packing is shown in this unit cell? A cube with 4 unit cells, and a dot at each corner and in the middle. A) Simple Cubic B) Body Centered Cubic C) Face Centered Cubic D) Hexagonal Closest Packed
B) Body Centered Cubic Body-centered cubic. This is a cubic unit cell with atoms on every corner of the unit cell and an atom in the center of the cell.
Which of the following substances is a liquid at room temperature? A) CH₃CH₃ B) CH₃OH C) CH₄ D) HCl E) CF₄
B) CH₃OH The phase of a chemical compound depends on the strength of its intermolecular forces, with strong intermolecular forces resulting in greater attractions and a greater tendency to be in the liquid or solid phase. All of these options exhibit dispersion forces, and some exhibit dipole-dipole forces, but CH₃OH is the only compound that exhibits hydrogen bonding (due to its internal O-H bond), meaning it has the strongest intermolecular forces and is most likely to be a liquid.
Which of the following pairs correctly ranks the viscosity of the compounds? A) H₂O < CCl₄ B) C₅H₁₂ < C₆H₁₄ C) C₂H₅OH < C₂H₆ D) C₈H₁₈ < C₂H₆
B) C₅H₁₂ < C₆H₁₄ Viscosity is defined as a liquid's resistance to flowing, so compounds with stronger intermolecular forces will have higher viscosity. Only C₅H₁₂ < C₆H₁₄ correctly ranks compounds in relation to which has the stronger intermolecular forces.
Which of the following properties is not typical of metallic solids? A) High coordination number of atoms in the lattice. B) High vapor pressure. C) High electrical conductivity. D) High thermal conductivity. E) All of the above are true statements.
B) High vapor pressure. Metallic solids are held together by metallic bonds, which are relatively strong. As a result, the metal atoms are unlikely to escape into the gaseous phase, meaning they will not have high vapor pressures.
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 best explains why the meniscus of mercury in a glass tube is convex? A) The greater attraction of mercury to the glass than to itself. B) The lower attraction of mercury to the glass than to itself. C) The low surface tension of mercury. D) The high density of mercury compared to water. E) The repulsion between mercury and the glass.
B) The lower attraction of mercury to the glass than to itself. A convex meniscus in a glass tube is the result of high surface tension, which causes the substance (mercury) to be more attracted to itself than to the glass, limiting the total surface area.
Which of the following substances would have the highest vapor pressure at 298 K? A) propanol (b.p. = 97°C) B) acetone (b.p. = 56°C) C) water (b.p. = 100°C) D) benzene (b.p. = 80°C) E) chloroform (b.p. = 61°C)
B) acetone (b.p. = 56°C) Vapor pressure describes the tendency of a substance to change from the liquid to the gas phase, so substances with weaker intermolecular forces will have higher vapor pressures. A substance's boiling point is also indicative of the strength of the intermolecular forces, with weaker intermolecular forces resulting in lower boiling points. Because acetone has the lowest boiling point of the options given, it has the weakest intermolecular forces, and will therefore have the highest vapor pressure.
What is represented by B on the phase diagram? Normal pressure and temperature phase diagram A) solid B) liquid C) gas D) cannot determine without exact pressure and temperature
B) liquid Point B on this phase diagram falls within a region of high pressure (which will force the substance into a condensed phase) and high temperature (which will take allow this substance to overcome its intermolecular forces). These forces balance to give the moderately condensed liquid phase.
What type of packing is shown in this unit cell? A cube with 4 unit cells, and a dot at each corner and in the middle and on the faces. A) Simple Cubic B) Body Centered Cubic C) Face Centered Cubic D) Hexagonal Closest Packed
C) Face Centered Cubic Face centered cubic. This is a cubic unit cell with atoms on every corner of the unit cell and an atom on each face.
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 substances would not exhibit any hydrogen bonding interactions in a pure substance? A) H₂O B) HF C) N(CH₃)₃ D) CH₃CH₂OH E) CH₃NH₂
C) N(CH₃)₃ 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 N(CH₃)₃ is the only option without H bonded directly to N, O, or F (here, there are only C-H bonds), it would not exhibit hydrogen bonding.
Which of the following exhibits dipole-dipole intermolecular forces in a pure substance? 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 of the compounds below is an example of a network solid? A) C₂₅H₂₅(s) B) NaCl(s) C) SiO₂(s) D) S₈(s) E) MgO(s)
C) SiO₂(s) A network solid is one in which all of the atoms are held together by a network of covalent bonds, with the subscripts in the molecular formula providing the ratio of atoms within the solid. Of these options, only SiO₂(s) is capable of forming a network solid, with each Si atom covalently bound to four O atoms. The other options are either held together by ionic bonds (NaCl and MgO) or by intermolecular forces (C₂₅H₂₅ and S₈).
When the temperature of a liquid increases what happens to the viscosity and surface tension of the liquid? A) Viscosity increases, surface tension increases. B) Viscosity increases, surface tension decreases. C) Viscosity decreases, surface tension decreases. D) Viscosity decreases, surface tension increases.
C) Viscosity decreases, surface tension decreases. Viscosity is defined as a liquid's resistance to flowing, and surface tension is defined as the amount of energy required to increase the surface area of a liquid. Compounds with stronger intermolecular forces will have higher viscosity and surface tension. Increasing the temperature will provide molecules with enough energy to overcome the attractive forces. Therefore, an increase in temperature results in lower viscosity and surface tension.
Which of the factors contributes to a low viscosity of a liquid? A) high boiling point B) hydrogen bonding C) compact molecular shape D) low temperature E) high molar mass
C) compact molecular shape Viscosity is defined as a liquid's resistance to flowing, so compounds with weaker intermolecular forces will have lower viscosities. Of these options, only having a compact molecular shape results in weaker intermolecular forces, so this will result in the lowest viscosity.
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.
What intermolecular forces are present between two molecules of CH₃OCH₂CH₃? A) dispersion only B) dipole-dipole only C) dispersion and dipole-dipole D) dispersion and hydrogen bonding E) dipole-dipole and hydrogen bonding
C) dispersion and dipole-dipole Two molecules of CH₃OCH₂CH₃ will have dipole-dipole interactions because CH₃OCH₂CH₃ is a polar molecule. Dispersion forces will also be present because these forces exist between all molecules.
Which quantity is not determined from a heating or cooling curve for a substance? A) boiling point B) heat of vaporization C) triple point D) melting point E) heat of fusion
C) triple point A heating curve demonstrates the energy changes and temperatures required to change a substance from one phase to another. Because it does not address the pressure of the system, it cannot provide the triple point, which is defined as the temperature and pressure at which all three phases (solid, liquid, gas) coexist.
What phase change is represented by the transition from B to C? Normal pressure and temperature phase diagram A) fusion B) sublimation C) vaporization D) condensation E) solidification
C) vaporization Point B on this phase diagram falls within a region of high pressure (which will force the substance into a condensed phase) and high temperature (which will allow this substance to overcome its intermolecular forces). These forces balance to give the moderately condensed liquid phase. Point C on this phase diagram falls within a region of high temperature (which will allow this substance to overcome its intermolecular forces) and low pressure (which will not force the substance into a condensed phase). Combined, these conditions cause the substance to be in the gaseous phase. Therefore, the transition from point B to point C is the transition from the liquid to the gas phase, which by definition is called vaporization.
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.
In a molecule of HI, what intermolecular forces are present? 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 HI does not have any hydrogen atoms bound to N, O or F, so it cannot exhibit hydrogen bonding. Because HI is a heteronuclear diatomic molecule, it has a dipole, meaning it will exhibit dipole-dipole forces. Further, all compounds with electrons exhibit dispersion forces, so HI will exhibit both dipole-dipole and dispersion forces.
Identify the type or types of intermolecular forces present between molecules of the compound pictured below: Carbon skeletal structure with 1 Nitrogen A) dispersion only B) dipole-dipole C) hydrogen bonding D) dispersion and dipole-dipole only E) dispersion, dipole-dipole, and hydrogen bonding
D) dispersion and dipole-dipole only This molecule is polar. Therefore, dispersion forces and dipole-dipole forces will be present. There are no hydrogens covalently bound to the nitrogen. Therefore, no hydrogen bonding will be present.
You are given a small bar of an unknown metal, M. You find the density of the metal to be 10.5 g/cm³. An X-ray diffraction experiment measures the edge of the unit cell as 409 pm. Assuming that the metal crystallizes in a face-centered cubic lattice, what metal is M most likely to be? A) Pt B) Pb C) Rh D) Ag E) None of these.
D) Ag The density (d) of a unit cell depends on the unit cell volume (V), the molecular weight of the atoms (MW), Avogadro's number (N), and the number of atoms in the unit cell: d=(# atoms/unit cell)⋅MWNA⋅1Vcd=(# atoms/unit cell)⋅MWNA⋅1Vc Solving for the molar mass will give the identity of the unknown metal. MW=(d)(NA)(Vc)(# atomsunit cell)MW=(d)(NA)(Vc)(# atomsunit cell) First, convert the unit cell volume into the same units as the volume in the density. Vc=(409 pm)3=6.84×107 pm3×1 cm31030 pm3=6.84×10−23 cm3Vc=(409 pm)3=6.84×107 pm3×1 cm31030 pm3=6.84×10−23 cm3 Next, determine the number of atoms in a face-centered cubic cell. A face-centered cubic unit cell has 1/2 of an atom at each of its six faces. (6 faces/unit cell×12 atom/face)+(8 corners/unit cell×18 atom/corner)=4 atoms/unit cell(6 faces/unit cell×12 atom/face)+(8 corners/unit cell×18 atom/corner)=4 atoms/unit cell Now solve for the molar mass of the unknown metal: MW=(10.5g/cm^3)(6.022×1023atoms/mol)(6.84×10−23 cm3)4 atoms=108g/molMW=(10.5gcm^3)(6.022×10^23atoms/mol)(6.84×10^−23 cm3)4 atoms=108g/mol The atom with a molar mass of approximately 108 g/mol is silver (Ag).
Which one of the following correctly ranks the compounds in order of lowest boiling point to highest boiling point based only in intermolecular forces? A) CH₃CH₂OH < CH₃OCH₃ < CH₃CH₂CH₃ B) CH₃OCH₃ < CH₃CH₂CH₃ < CH₃CH₂OH C) CH₃CH₂CH₃ < CH₃CH₂OH < CH₃OCH₃ D) CH₃CH₂CH₃ < CH₃OCH₃ < CH₃CH₂OH
D) CH₃CH₂CH₃ < CH₃OCH₃ < CH₃CH₂OH The stronger the intermolecular forces, the higher the boiling point will be. CH₃CH₂OH would have the most intermolecular forces, with hydrogen bonding, dipole-dipole interactions, and dispersion forces. CH₃OCH₃ would have the second most intermolecular forces, with dispersion forces and dipole-dipole interactions. CH₃CH₂CH₃ would have the fewest intermolecular forces, with only dispersion forces.
Which of the following would exhibit hydrogen bonding in a pure substance? 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 solids would be expected to display the highest melting point? A) NaCl B) SrBr₂ C) CBr₄ D) MgCl₂ E) C₁₂H₂₂O₁₁
D) MgCl₂ Compounds with high melting points have strong intermolecular forces, such as ion-ion forces. Of these options, NaCl, SrBr₂, and MgCl₂ are ionic solids, whereas the other options are held together by weaker intermolecular forces such as dispersion forces and dipole-dipole forces. When comparing the strength of ion-ion forces among ionic solids, those that contain smaller ions with higher charges are held together more strongly. Both SrBr₂ and MgCl₂ have ions with a 2+ charge (whereas the ions in NaCl both have single charges), but the Mg and Cl are smaller ions than Sr and Br, respectively, as they are located closer to the top of the periodic table. As a result, MgCl₂ will have the strongest ion-ion intermolecular forces, and as such will exhibit the highest melting point.
Which of the following properties does NOT impact the vapor pressure of a particular liquid in a closed container? A) Amount of hydrogen bonding in the liquid B) The temperature of the liquid C) The surface tension of the liquid D) The volume of liquid E) The viscosity of the liquid
D) The volume of liquid Of the options given, the volume of the liquid in the container does not affect the vapor pressure inside the container since it does not have any effect on the intermolecular forces. The other options make implications about the relative strength of the intermolecular forces, which will have an impact on the vapor pressure.
Which of the following phases does a sample of water exist in at its triple point? A) gas B) liquid C) solid D) all of these E) none of these
D) all of these The triple point on a phase diagram is defined as the temperature and pressure at which all three phases (solid, liquid, and gas) coexist.
The predominant intermolecular force between molecules of I₂ is _____ A) ionic bonds. B) dipole-dipole interactions. C) ion-dipole interactions. D) dispersion forces. E) covalent bonds.
D) dispersion forces. I₂ is a homonuclear diatomic molecule, meaning that it cannot be ionic and cannot have a dipole. Therefore, the only possible intermolecular forces it can experience are dispersion forces, which are experienced by any species with electrons.
Which of the following substances would have the greatest dispersion forces? A) H₂ B) O₂ C) Cl₂ D) Br₂ E) I₂
E) I₂ Dispersion forces arise from instantaneous changes in electron density, so compounds with more electrons tend to have greater dispersion forces. Since I₂ has the most electrons of the options given, it has the greatest dispersion forces.
Identify the type or types of intermolecular forces present between molecules of the compound pictured below: 3 carbons attached to 7 Hydrogens and 1 Nitrogen which is attached to 2 hydrogens and has 1 lone pair A) dispersion only B) dipole-dipole C) hydrogen bonding D) dispersion and dipole-dipole only E) dispersion, dipole-dipole, and hydrogen bonding
E) dispersion, dipole-dipole, and hydrogen bonding This molecule is polar, and it has a hydrogen covalently bonded to a nitrogen. Therefore, dispersion forces, dipole-dipole forces, and hydrogen bonding are all present.
Identify the type or types of intermolecular forces present between molecules of the compound pictured below: Carbon skeletal structure with 1 Nitrogen bonded to 1 Hydrogen A) dispersion only B) dipole-dipole C) hydrogen bonding D) dispersion and dipole-dipole only E) dispersion, dipole-dipole, and hydrogen bonding
E) dispersion, dipole-dipole, and hydrogen bonding This molecule is polar, and it has a hydrogen covalently bonded to a nitrogen. Therefore, dispersion forces, dipole-dipole forces, and hydrogen bonding are all present.
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 Sublimation is defined as the phase change from a solid to a gas.
Which term is a measure of the energy required to increase the surface area of a liquid? A) adhesion B) capillary action C) cohesion D) viscosity E) surface tension
E) surface tension Surface tension is defined as the amount of energy required to increase the surface area of a liquid.
