Gen Chem Diagnostic 1A
Which of the following atoms/ions has the greatest radius? A. Br- B. Kr C. Rb+ D. Sr2+
A. All of these atoms and ions are isoelectronic and therefore have the same amount of electron-electron repulsion. Br- has the fewest number of protons, so its electrons feel the least attractive force, making it largest in size.
The first three electron energy levels of a hydrogen atom are given as: E1 = -13.6 eV E2 = -3.4 eV E2 = -3.4 eV If an electron is in the E2 level, which of the following will occur spontaneously? A. The electron transitions to E1, and a photon of 10.2 eV is emitted. B. The electron transitions to E3, and a photon of 1.9 eV is emitted. C. The electron transitions to E1, and a photon of 10.2 eV is absorbed. D. The electron transitions to E3, and a photon of 1.9 eV is absorbed.
A. Hydrogen has only one electron, so if it is in the second energy level it must be in an excited state. The electron will therefore transition from a high energy level to a lower one, in this case E2 to E1, to reach a ground state configuration (eliminate choices B and D). Since the electron is moving closer to the nucleus, a photon will be emitted during the process (eliminate choice C). The difference between the two levels is 10.2 eV, making choice A correct.
Given the enthalpies of formation for NO and NO2 are 90 kJ/mol and 33 kJ/mol, respectively, what is the reaction enthalpy for 2 NO(g) + O2(g) ? 2 NO2(g)? A. -114 kJ/mol B. -57 kJ/mol C. -24 kJ/mol D. Inadequate information provided to answer the question
A. The reaction enthalpy can be calculated by taking the sum of the enthalpies of formation of the products and subtracting from it the sum of the enthalpies of formation of the reactants after multiplying each by their stoichiometric coefficients. Thus we find that the reaction enthalpy is (2 × 33 kJ/mol) - (2 × 90 kJ/mol) = -114 kJ/mol (choice A is correct). Note that we define the enthalpy of formation of elements in their standard state as zero.
If a neutral atom loses two electrons, which of the following would be true of the resulting species? A. It would be attracted to a positive plate because the resulting species would be negatively charged. B. It would be attracted to a negative plate because the resulting species would be positively charged. C. It would be attracted to neither a positive or negative plate, because the resulting species would be neutral. D. It would be attracted to neither a positive or negative plate, because electrons have a much smaller mass compared to protons.
B. A neutral atom losing two electrons would become positively charged (eliminate choices A and C). Mass has nothing to do with the sign of a charge (eliminate choice D). Opposite charges attract, so a positively charged ion will be attracted to a negative plate, making choice B correct.
Which bond is most polar? A. C—Si B. H—Cl C. N—O D. F—F
B. Polarity is determined by the difference in electronegativity between the atoms sharing electrons in a bond. Electronegativity generally increases from the bottom left to the top right corners of the periodic table. Elements that are farther from each other on the periodic table have larger differences in electronegativity, while elements that are closer to each other have smaller differences. Even though fluorine is the most electronegative element, a F—F bond is nonpolar because there is no difference in electronegativity between the two atoms (eliminate choice D). Since the C—Si and N—O pairs of elements are right next to each other on the table, they are only slightly polar (eliminate choices A and C), while hydrogen and chlorine are far from each other, yielding the most uneven sharing of electrons in the bond.
A chemist wishes to determine the energy of a chlorine-chlorine bond. She knows the strength of a H—Cl bond and a H—H bond are 433 kJ/mol and 436 kJ/mol, respectively. Given the following reaction, what is the strength of a chlorine-chlorine bond?H2(g) + Cl2(g) → 2 HCl(g) ΔH° = -187 kJ/mol A. 187 kJ/mol B. 243 kJ/mol C. 676 kJ/mol D. 1056 kJ/mol
B. The Cl-Cl bond energy can be determining by utilizing the following equation: change in enthalpy= sum (bonds broken-bonds formed) Therefore, the dissociation energy of a Cl—Cl bond is 243 kJ/mol.
Which of the following is true about the amount of shielding the highest energy electrons of calcium and arsenic experience? A. The electrons of calcium have a greater amount of shielding than the electrons of arsenic. B. The electrons of calcium have a lesser amount of shielding than the electrons of arsenic. C. The electrons of calcium have the same amount of shielding as the electrons of arsenic. D. The amount of shielding between the electrons of calcium and of arsenic can only be determined experimentally.
B. The amount of shielding that the highest energy electrons of an atom feels is determined by the number of filled shells in that atom (inner core electrons), as well as subshells within an energy level (eliminate choice D). Because calcium and arsenic are in the same period (row), they have the same number of inner core electrons. However, the highest energy electrons in arsenic are in the p subshell and are shielded by the electrons in the s subshell. Since the highest energy electrons in calcium are in the s subshell, there is more shielding for arsenic, eliminating choices A and C.
A graduate student estimates the chemical energy in a solution before and after a reaction. The chemical potential energy following the reaction is significantly reduced and the student hypothesizes that the unaccounted energy transitioned to thermal energy. Which of the following laws of thermodynamics best explains this observation? A. Zeroth law B. First law C. Second law D. Third law
B. The first law of thermodynamics stipulates that energy is always conserved and in this example, it is simply changing form from chemical energy to thermal energy (choice B is correct). The zeroth law states that two systems at equilibrium with a third are in equilibrium with each other (choice A is incorrect). The second law states that all processes proceed toward disorder (choice C is incorrect) and the third law states that a perfect crystal possesses no entropy at absolute zero (choice D is incorrect).
Which of the following compounds is least likely to be soluble in water? A. NH3 B. CS2 C. CH3OH D. LiF
B. The rule of thumb for solubility is "like dissolves like," meaning polar solutes are soluble in polar solvents, and likewise for nonpolar things. Since water is a polar molecule that has hydrogen bonds as intermolecular forces, other hydrogen-bonding compounds like NH3 and CH3OH should be very soluble in water (choices A and C are wrong). Since LiF is an ionic compound that dissociates into charged particles in solution, it will need a polar solvent to dissolve (choice D is wrong). CS2, however, is a nonpolar compound and is the least soluble in water, making it the best choice (choice B is correct).
4 CO2(g) + 2 H2O(g) → 2 C2H2(g) + 5 O2(g) Given the following reactions, what is the enthalpy for the reaction above? C2H2(g) + 2 H2(g) → C2H6(g) ΔH° = -94.5 kJ/mol 2 C2H6(g) + 7 O2(g) → 4 CO2(g) + 6 H2O(g) ΔH° = -566 kJ/mol 2 H2O(g) → 2 H2(g) + O2(g) ΔH° = 142.4 kJ/mol A. -519 kJ/mol B. 470 kJ/mol C. 519 kJ/mol D. 1036 kJ/mol
B. This question can be solved utilizing Hess's Law, which allows for determination of the reaction enthalpy by summing the enthalpies of a series of other reactions. In order for the given reactions to be added to give the desired equation, we must reverse all three equations and multiply the first and third equations by two. This results in ΔH° values for the three equations of 189 kJ/mol, 566 kJ/mol, and -284.8 kJ/mol, respectively. These can be summed to give the reaction enthalpy of 470 kJ/mol (choice B is correct).
Which of the following ions will align itself parallel to the poles of a magnetic field? A. Ca2+ B. V3+ C. Cr6+ D. Se2
B. V3+ Any atom or ion that aligns itself parallel to a magnetic field is defined as paramagnetic, and all such species have unpaired electrons in their orbitals. Therefore, eliminate as incorrect any choice that has only completely filled orbital subshells. Ca2+ (Z = 20) has a closed shell noble gas configuration because it is isoelectronic with argon, having 18 e-s (eliminate choice A). V3+ (Z = 23) has 20 e-s, two of which are unpaired in two 3d orbitals since transition metals lose their s electrons first when forming cations (choice B is correct). Both Cr6+ (Z = 24) and Se2- (Z = 34) are also ions with closed shell noble gas configurations with 18 e-s and 36 e-s, respectively. As such, they like Ca2+ will also be diamagnetic with all electrons paired (eliminate choices C and D).
Rank the following species by increasing boiling point. I. CH3CHO II. F2 III. CH3OH IV. KBr A. I < II < III < IV B. II < I < III < IV C. III < I < IV < II D. II < I < IV < III
B. When comparing molecules, the weaker intermolecular forces occur between molecules with a lesser magnitude of charge. The charge (or partial charge) is due to polarity, which occurs due to differences in electronegativity. KBr will have ionic forces, the strongest option, because of the full charges on the K+ and Br- ions (choice B is correct). F2 is a nonpolar molecule and will form instantaneous-induced dipoles (also known as London dispersion forces), the weakest of all intermolecular forces (choices A and C are wrong). CH3OH and CH3CHO are both polar molecules, but methanol is able to hydrogen-bond due to the -OH group, while weaker dipole-dipole forces hold together the CH3CHO.
The combustion of methane is given by the reaction: CH4 + 2 O2 ? CO2 + 2 H2O If 20 moles of methane and 20 moles of oxygen are initially present, how many moles of carbon dioxide are produced? A. 1 B. 10 C. 20 D. 30
B. When initial amounts of the two reactants are given, this usually indicates a limiting reactant problem. The first step is determining which of the two reactants is the limiting reactant. Here, let's choose one (e.g. methane). If we have 20 moles of methane, we will need twice as much (or 40 moles) of oxygen to obtain a stoichiometric amount because the ratio of oxygen to methane is 2:1. However, the problem statement says that we only have 20 moles of oxygen present, indicating that we do not have enough present. This means that oxygen is the limiting reactant. Conversely, had we chosen to start with oxygen, the reasoning is slightly different. If we have 20 moles of oxygen, we will need half as much (or 10 moles) of methane to obtain a stoichiometric amount, because the ratio of methane to oxygen is 1:2. Since the problem statement says that we have 20 moles of methane present, that reactant is in excess, thus making oxygen limiting. Now that we have concluded that oxygen is limiting, we still have to solve the problem. All yield questions should be performed using the limiting reactant. If we have 20 moles of oxygen, we generate half as much (2:1 ratio) of carbon dioxide. Thus, we will generate only 10 moles of carbon dioxide, making choice B the correct answer.
Which of the following elements is the most electronegative? A. Chlorine B. Bromine C. Oxygen
C. Electronegativity, which is the ability of an atom to attract electrons to itself in a covalent bond, is inversely related to atomic radius. Therefore, electronegativity increases as you move from the bottom left to the top right of the periodic table. Also remember the relationship F > O > N > Cl > Br > I > S > C ≈ H.
Which of the following statements is true about two neutral isotopes of the same element? A. They have a different number of electrons. B. They have a different number of electrons and protons. C. They have a different number of neutrons. D. They have a different number of neutrons and protons.
C. Isotopes, by definition, have the same atomic number, but different mass number. Because the number of protons has to be the same, the number of neutrons has to be different making choice C correct. Looking at the other choices, they cannot have a different number of electrons because if they have the same number of protons, they must have the same number of electrons to remain neutral (eliminate choice A). If they had a different number of protons, they cannot be isotopes (eliminate choices B and D).
Which of the following represents the ground state electron configuration for silver? A. [Kr] 5s2 5d9 B. [Kr] 5s2 4d9 C. [Kr] 5s1 4d10 D. [Kr] 5s1 3f14 4d10
C. The f subshell appears for the first time in the fourth energy level, so 3f orbitals do not exist (eliminate choice D). The d subshell in any given energy level is one principle quantum number behind the s subshell. Therefore, 5s is followed by 4d, not 5d (eliminate choice A). Since completely filled d subshells are more stable for transition metals, we observe that one s electron is promoted into the d subshell in the copper family, of which silver is a member. This means choice C is better than choice B.
Which of the following correctly describes the molecular shape of AsCl3? A. Bent B. Trigonal planar C. Trigonal pyramidal D. Tetrahedral
C. The molecular shape is determined by the total number of electron groups around the central atom (bonds and lone electron pairs) which determine the bond angles, as well as the total number of atoms bonded to the central atom. Since AsCl3 has three atoms attached to it, it cannot have a bent or tetrahedral shape, which require two or four atoms, respectively, bonded to the central atom (eliminate choices A and D). A molecule can only have a trigonal planar shape if it has three bonded atoms with no lone pairs. Since the central arsenic atom has three bonds and a lone pair, its geometry must be three dimensional (eliminate choice B).
A researcher investigates an endothermic reaction found to be non-spontaneous in a temperature-independent manner. Which of the following is the reaction being studied? A. 2 H2O(g) → 2 H2(g) + O2(g) B. CO2(s) → CO2(l) C. N2(g) + 2 O2(g) → 2 NO2(g) D. N2O4(g) → 2 NO2(g)
C. The reaction described is non-spontaneous in a temperature-independent manner. As it is endothermic, this indicates that the reaction must have a negative ΔS given ΔG = ΔH - TΔS. The only reaction displaying a negative change in entropy is N2(g) + 2 O2(g) → 2 NO2(g) because the total number of gaseous moles decreases (choice C is correct, and choices A and D are not). In addition, choice B can be eliminated since as a substance changes phase from solid to liquid to gas, the particles are more disordered, so entropy increases.
Of the following valence shell electron configurations, which is associated with the greatest stability for an atom? A. ns2np4 B. ns2np5 C. ns2np6 D. ns2np7
C. This is the valence shell electron configuration that correlates with an octet configuration. An octet configuration (eight electrons in the outermost shell) produces great stability.
Which of the following would have the biggest decrease in entropy for a gas in a piston cylinder? A. Compression B. Doing work on the gas C. Cooling D. Deposition
D. An increase in entropy means an increase in randomness, or a decrease in order. Therefore, this question is asking for the change that would make the system most ordered. Compression and doing work on the gas are synonymous, so choices A and B can be eliminated. Cooling the piston cylinder would slow the movements of the gas, causing the volume to decrease and bringing molecules closer together. This definitely decreases entropy and could perhaps even increase intermolecular forces to the point that the gas might condense, decreasing entropy even more. Deposition is the process of going from a vapor to a solid, so since this phase change from the least ordered to most ordered phase is made explicit, choice D is a better answer than choice C.
Which of the following would have the highest energy? A. Radio waves, because they have longer wavelengths than gamma rays B. Radio waves, because they have higher frequencies than gamma rays C. Gamma rays, because they have longer wavelengths than radio waves D. Gamma rays, because they have higher frequencies than radio waves
D. Gamma rays have the highest energy in the electromagnetic spectrum (eliminate A and B). High energy is synonymous with short wavelength and high frequency because E = hf and f = c/λ, making choice D the correct answer.
Which of the following statements about solids is true? A. The intramolecular bonds of a molecular solid are the same as its intermolecular forces. B. Ionic solids have a lower melting point than molecular solids. C. Network solids have greater electrical conductivity than metallic solids. D. Unlike network solids, metallic solids are ductile.
D. Metallic solids are unique in that their covalently-bound lattice of nuclei and inner shell electrons are surrounded by a "sea" of valence electrons. Thus, they are excellent conductors of electricity and heat (eliminate choice C). They are also malleable and ductile. The intramolecular bonds of an ionic solid and of a network solid are the same as their respective intermolecular forces, but molecular solids have covalent intramolecular forces and van der Waals intermolecular forces (hydrogen bonds, dipole-dipole forces, or London dispersion forces), making choice A incorrect. Ionic solids have stronger intermolecular forces than molecular solids, thus they have a higher melting point (eliminate choice B).
What is the hybridization of the central atom in PCl3? A. p B. sp C. sp2 D. sp3
D. Phosphorus is the central atom in this compound and will have three bonds to Cl and one lone pair of electrons in the Lewis dot structure. It will therefore need four hybrid orbitals in this compound. As a result, we expect one s and three p atomic orbitals to combine to give four sp3 hybrid orbitals.
An intern in a laboratory is asked to clean up after a new graduate student in the lab who was making a HEPES buffered solution. The intern finds a white solid powder left on the counter from the buffer preparation and is uncertain how to dispose of it. Which of the following is the most likely identity of this white powder? A. Na B. H2SO4 C. C3H6O D. NaC3H5O3
D. The intern was disposing of a white powder (solid), thus we are looking for the compound that will have the strongest intermolecular forces (and therefore is most likely to be solid). H2SO4 (sulfuric acid) is capable of hydrogen bonding, C3H6O (acetone) is capable of dipole-dipole interaction, and NaC3H5O3 (sodium lactate) is an ionic compound. Ionic interactions are stronger than any other form of IMF, making NaC3H5O3 the answer (choices B and C are incorrect, D is the correct answer). While sodium is a metal, pure sodium would not appear as a white powder, nor would it be added to any traditional solution given its volatile reactivity when added to water (choice A is incorrect).
Which of the following ions has the largest ionization energy? A. Sr+ B. Ca+ C. Mg+ D. Be+
D. The ionization energy periodic trend is opposite the trend for atomic radius. Atoms and ions with valence electrons close to the nucleus require more energy to remove them than atoms/ions with valence electrons farther from the nucleus. Since these ions are in the same group on the periodic table, the smallest one (Be) will have the greatest ionization energy.