Princeton WB GChem

MCAT SciENCE WoRKBOOK TABLE OF CONTENTS PERIODIC TABLE OF THE ELEMENTS BIOLOGY QuESTIONS AND PAssAGEs • • • •• •.•.• • SOLUTIONS 139 PHYSICS QuESTIONS AND pAS SAGES •• • .• • • • • .• 223 SoLUTIONS 353 GENERAL CHEMISTRY QuESTIONS AND PASSAGES 431 SoLUTIONS .• • •.• • .• • • • • • •.• 569 ORGANIC CHEMISTRY QuESTIONS AND PASSAGES 639 SoLUTIONS 731 MCAT SciENCE WoRKBOOK PASSAGE GUIDE BIOLOGY Chapter Number 10 MCAT Biological Sciences Review MCAT Biology Chapter Title Corresponding MCAT Science Workbook Passage Numbers Molecular Biology Microbiology Generalized Eukaryotic Cells Genetics and Evolution Nervous and Endocrine Systems Circulatory, Lymphatic, and Immune Systems Digestive and Excretory Systems Muscle and Skeletal Systems Respiratory and Skin Systems Reproductive Systems and Development 5, 12, 16, 18, 23,25, 27-32,55 13, 17, 19, 20, 21, 26, 34,35 1,2, 4,6, 7,10,24 33,56,62, 63,64, 65,66 8,9, 15,22,38,39,58,59,67-74 40,42,43,44,45, 46,47,61, 75 14,57, 76, 77,78 41,48,49,50,51,60, 79 52,53,54, 80 3, 11, 36,37,81, 82,83 MCAT Physical Sciences Review MCAT Physics Chapter Title Correspond;ng MCAT Science Workbook Passage Numbers Kinematics Mechanics I Mechanics II Mechanics Ill Fluids and Elasticity of Solids Electrostatics Electricity and Magnetism Oscillations and Waves Sound Light and Geometrical Optics 11 PHYSICS Chapter Number 10 3-9, 15-18 1Q-13, 19-24 31,32 25-30,33-39 14,40,45,46,48-55 42,43,56,57 41,44,47,58-60 GENERAL CHEMISTRY Chapter Number 10 MCAT Physical Sciences Review MCAT General Chemistry Chapter Title Corresponding MCAT Sqience Workbook Passage Numbers Atomic Structure Periodic Trends and Bonding Phases Gases Solutions Kinetics Equilibrium Acids and Bases Thermodynamics Redox and Electrochemistry 1-4,6,9-18,70 5, 7,19-28,43,72 35,57,59, 60,62, 63,65,66 40,61,64, 67 31,41, 42, 45,68, 69 33,44, 71 29,37,39,46 30,32,34,36,38,47-55 58, 73-78 8,56, 79-91 ORGANIC CHEMISTRY Chapter Number MCAT Biological Sciences Review MCAT Organic Chemistry Chapter Title Corresponding MCAT Science Workboo}f Passage Numbers Structure and Bonding Substitution and Elimination Reactions Electrophilic Addition Reactions Nucleophilic Addition/Cycloaddition Reactions Lab Techniques and Spectroscopy Biologically-Important Organic Chemistry 1,2, 4,5, 6, 7,8,9, 10 11, 12, 13, 14, 15, 16, 17, 18 19,20,21,22,23,24,25,26,27-30 31,32,33,34,35 36,37,38, 39,40,41,42,43 INTRODUCTION PERIODIC TABLE OF THE ELEMENTS r r He 4.0 H 1.0 Be B c Ll N 6.9 9.0 10.8 16.0 12 13 12.0 14 14.0 11 16 Na Mg AI Sl 23.0 24.3 27.0 28.1 15 p 31.0 19 20 21 22 24 26 27 28 29 Ca Sc: Ti v 2S K Cr Mo Fe Co Nl Cu 39.1 37 40.1 4.5.0 47.9 50.9 55.8 40 41 43 44 58.9 4.5 58.7 46 Kb Sr Zr Nb Mo Tc: Ku Rh 85 •.5 87.6 39 y 88.9 52.0 42 54.9 38 91.2 92.9 95.9 (98) 101.1 5.5 56 57 72 73 74 75 76 Cs 132.9 23 Ba La* ur Ta w Ke Os 137.3 138.9 178.5 180.9 10.5 Db (2621 183.9 186.2 107 Bb (262) 190.2 87 88 89 104 Fr Ka Act Kf (223) 226.0 227.0 (261) • t SCIENCE WORKBOOK s, _(263) 108 Hs (265) Ar 32.1 3.5 39.9 32 33 34 3.5 36 As Se Br Kr 63.5 69.7 72.6 74.9 79.0 79.9 83.8 47 48 49 50 51 Pd Ag 52 Cd lo So Sb Te 102.9 106.4 107.9 112.4 114.8 118.7 121.8 127.6 53 I 126.9 54 Xe 131.3 77 lr 192.2 78 79 80 81 82 83 81 209.0 85 Po At (209) (210) 86 Rn (222) 175.0 103 Pt Au Hg Tl Pb 195.1 197.0 200.6 204.4 207.2 84 109 Mt (267) 61 62 63 64 Sm Eu Gd 140.1 144.2 (145) 150.4 152.0 157.3 65 Tb 158.9 90 91· 92 93 94 95 96 97 Tb Pa Np Pu Am Cm Bk (243) (241} (247) (237) Cl Ge Pm 238.0 s 31 60 (231) 17 10 Ne 20.2 18 Ga Nd u F 19.0 30 Zn 65.4 59 Pr 140:9 58 Ce 232.0 MCAT 106 (244) 66 67 68 69 Dy 162.5 98 Ho Er Tm 164.9 167.3 168.9 70 Yb 173.0 cr 99 100 101 Es 102 Fm Md No Lr (251) (252) (257) (258) (259) (260) 7l Lu MCAT GENERAL CHEMISTRY PRACTICE QUESTIONS AND pAS SAGES 431 Passage (Questions 1-9) Which one of the following graphs best illustrates the relationship between the maximum kinetic energy of a photoelectron and the frequency of the radiation that · illuminates the metal? If an atom is irradiated with enough energy, electrons can be promoted to higher energy levels or ejected from the atom entirely An example of this latter phenomenon-electron ejection-is the photoelectric effect c A If a metal surface is illuminated with light of sufficient energy, an electron may absorb enough energy to free it from the metal entirely An electron ejected in this manner is called a photoelectron The kinetic energy of a photoelectron is equal to the energy of the absorbed photon minus the energy required to displace the electron from the atom The minimum amount of energy required to liberate an electron from the surface of a metal is called the metal's photoelectric work function, denoted q, Thus, the maximum amount of kinetic energy of the ejected electrons is equal to the difference between the energy of the irradiating photons, hf, and the metal's work function: KEmax D B KEmax fEquation Which of the following effects would result from increasing the intensity of the irradiating light (with A < A0 ) on a given metal surface? Different metals have different work functions, typically on the order of a few electron volts (eV); see Table The threshold frequency, / , is the frequency of the irradiating light below which no photoelectrons are produced The equation 'AJ0 = c determines the threshold wavelength, that is, the wavelength of the irradiating light above which no photoelectrons are produced It is easy to show that A0 = (1240 eV-nm)/(j) Metal P (eV) A The kinetic energy of the ejected electrons will increase due to the increased energy of the incident photons B The kinetic energy of the ejected electrons will be the same as that of photoelectrons ejected by lower intensity light, but more electrons will be expelled C The threshold frequency will increase due to the increased energy of the incident photons D No effect should be observed; the work function is a constant for a given metal, so the photoelectrons are unaffected by changing light intensities A0 (nm) Cesium (Cs) 1.94 639 Rubidium (Rb) 2.13 582 Potassium (K) 2.25 551 Sodium (Na) 2.29 541 Lithium (Li) 2.46 504 Copper (Cu) 4.70 264 A sample of cesium is illuminated by a source of monochromatic light If the electrons that are ejected from the sample have a maximum kinetic energy of 0.11 eV, what is the frequency of the incident light? Table Work Functions and Threshold Wavelengths A B C D Electrons that are not provided with enough energy to be either elevated to a higher energy level or ejected remain in their ground state That is, an electron cannot absorb a quantum of energy unless it is large enough to produce one of the two above effects For atoms in the gas phase, q, is the first ionization energy, Ir (Note: Planck's constant= h = 4.1 X 10-u eV-s = 6.6 X 10-34 J-s, and eV = 1.6 X 10- 19 J.) MCAT SCIENCE WORKBOOK 432 4.0 5.0 4.0 5.0 x 10 14 x 10 14 x 10 16 x 10 16 Hz Hz Hz Hz A sample containing a mixture of rubidium and cesium is irradiated with light of wavelength of approximately 550 nm Which of the following best explains the effect? The amount of energy required to remove an electron from an atom increases from left to right across a row in the periodic table Which of the following accompanies this trend? A A decrease in atomic radius and a decrease electronegati vity B A decrease in atomic radius and an increase electronegati vity C An increase in atomic radius and a decrease electronegati vity D An increase in atomic radius and an increase electronegati vity A No electrons are expelled from either metal because the incoming radiation has insufficient energy B Electrons are expelled from both metals and have equivalent kinetic energies C Electrons are expelled from both metals, but those originating in the rubidium have greater kinetic energy D Electrons are expelled from both metals, but those originating in the cesium have greater kinetic energy in in in Based on the work functions given in Table I, it can be inferred that which one of the following elements is the most reactive? S Which of the following best explains why the work function for sodium is less than that for lithium? A Copper B Potassium A Sodium is more electronegative than lithium B The threshold frequency for sodium is greater than that for li thi urn C The valence electron of lithium is less tightly bound than that of sodium due to decreased nuclear shielding in the lithium atom D The valence electron of sodium is less tightly bound than that of lithium due to increased nuclear shielding in the sodium atom c Cesium D Lithium In a photoelectric-effect experiment, a sample of copper is illuminated with ultraviolet light of wavelength 250 nm, and x photoelectrons per second are emitted If the copper is instead illuminated with green light of wavelength 500 nm, how many photoelectrons should be emitted per second? An electron falls from the 4s subs hell to the 3p subshell Which of the following is most likely to occur? A An absorption of energy and the appearance of bright band in the atomic spectrum B An emission of energy and the appearance of bright band in the atomic spectrum C An emission of energy and the appearance of dark band in the atomic spectrum D An absorption of energy and the appearance of dark band in the atomic spectrum in a A B tx C X D 2x a a a 433 GENERAL CHEMISTRY Passage (Questions 1-6) As a result of a nuclear reaction, a nucleus undergoes a · change in its charge Which of the following is · necessarily true? When an unstable nucleus spontaneously decomposes by emitting protons, neutrons, or electrons, it is said to have undergone radioactive decay A B C D The disappearance of radioactive isotopes in a sample is generally predicted by the half-life of the element in the sample The half-life is the time it takes for half of the radioactive isotopes in the sample to disappear The half-life of a radioactive element is independent of the size of the sample Atomic mass has increased Atomic mass has decreased Nuclear binding energy has increased The atom has been transformed into a different element If an isotope of magnesium undergoes beta decay, then the daughter nuclide will be an isotope of: Two of the more common forms of radioactive decay are alpha decay, in which the nucleus emits two protons and two neutrons, and beta decay, in which the nucleus converts a neutron into a proton and emits a particle that is equivalent to an electron A neon B sodium c aluminum D silicon How many half-lives would need to elapse before 87.5% of a given sample of a radioisotope has decomposed? Which of the following graphs most closely represents the amount remaining of a sample undergoing radioactive decay over time? A 2.5 B c 3.5 D c A The difference between the actual mass of a nucleus and the sum of the masses of the protons and neutrons · that form it is called the: A B C D t- t- D B t- r- If a nucleus has undergone radioactive decay by emitting three alpha particles, by how much has its atomic number decreased? A B c D 12 MCAT SciENCE WoRKBOOK 434 mass defect mass number atomic weight critical mass s Questions through 20 are NOT based on a descriptive passage The outermost shell of electrons in the noble gases (with the exception of helium) have which of the following electron configurations? A ns np B ns np c ns np D ns np How many orbitals can occupy the 5fsubshell? A B c Compared to the electronegativity of iodine, the electronegativity of chlorine is: 10 D 14 A greater, because chlorine has a greater nuclear charge which translates into a greater force exerted on electrons B less, because chlorine has a smaller nuclear charge which translates into a smaller force exerted on electrons C greater, because decreased nuclear shielding allows for a stronger pull on the valence electrons D less, because increased nuclear shielding results in a weaker pull on the valence electrons Which of the following could be the electron configuration of a boron atom in an excited state? A ls2 2s 2p B ls 2s 2p C ls 2s 2p' D ls 2s 2p2 Which of the following gives the ground-state electron configu.ration of a calcium atom once it acquires an electric charge of +2? Which of the following elements would be expected to exhibit the greatest ionization energy? A [Ar] B [Ar] 4s C {Ar] 4s 3tfl D {Ar] 4s 4p A Potassium B Calcium c Scandium D Titanium Which of the following is true of an electron in an excited state? Compared to the atomic radius of calcium, the atomic radius of gallium is: A It has absorbed a photon, and its energy has A larger, because increased electron charge requires that the same force be distributed over a greater number of electrons B smaller, because gallium gives up more electrons, thereby decreasing its size C larger, because its additional electrons increases the atomic volume D smaller, because increased nuclear charge causes electrons to be held more tightly • increased B It has absorbed a photon, and its energy has decreased c It has emitted a photon, and its energy has increased D It has emitted a photon, and its energy has decreased 435 GENERAL CHEMISTRY 15 A compound is found to contain 64% silver, 8% nitrogen, All of the following are examples of polar molecules EXCEPT: and 28% oxygen by mass What is the empirical formula of this compound? A co B HF c D A AgN0 B AgN0 C Ag 3NO D Ag N03 H 20 CC1 • 10 How many electrons are shared in the bond between 16 What is the percent mass composition of H 2S0 4? sodium and chlorine in a molecule of NaCl? A A B C D B c D 13- particle is emitted by a lithium nucleus resulting nucleus will be an isotope of: 11 A The D oxygen, oxygen, oxygen, oxygen, 14% 24% 33% 48% sulfur, sulfur, sulfur, sulfur, 1% 1% 2% 2% hydrogen hydrogen hydrogen hydrogen 17 The oxidation states of sulfur in H2SO4 and H 2S0 are, respectively: A helium B lithium c 85% 75% 65% 50% A B c ·beryllium boron D +2 +4 +4 +6 and +4 and +2 and +6 and_+4 12 Carbon-14 has a half-life of 5700 years Approximately what percent of a carbon-14 sample will remain after 2000 years? If two moles of Cu 2S are consumed in the reaction above, how many moles of solid copper are produced? A 22% B 35% A B c c 52% D 78% D 13 A sample of oxygen contains 2.5 moles of0 molecules 19 What is the mass of the sample? A B 20 g 40 g c 60 g 80 g D If one mole of Ag is produced in the reaction above, how many grams of oxygen gas are produced? 14 What is the total mass of the hydrogen atoms contained A B 2g 4g c 8g D 16 g in moles of glucose (C H120 )? A B c D MCAT 3g 12 g 36 g 48 g SCIENCE WORKBOOK 20 Which of the following describes the orbital geometry of an sp hybridized atom? A B C D 436 Linear Trigonal planar Tetrahedral Trigonal bipyramid Knowing that the transition metal chromium may assume the oxidation states +2, +3, +4, +5, or +6, a researcher attempted to identify the oxidation state of chromium in an oxide of unknown formula: Cr"'O, The compound was completely dissolved in water and the pH of the resulting solution was measured to be 6.0 Among the following, the formula for the experimental CrmOn molecule is most likely: passage (Questions 1·5) The transition metals are characterized by valence electrons that lie within the d subshell Those positioned in the third period of the periodic table, like chromium and iron, undergo serial filling of orbitals in the 3d subshell and have the general electron configuration [Ar] sx 3dY where (Ar] represents the electron configuration for the noble gas argon, xis the number of electrons in the s subshell andy is the number of electrons in the 3d subshell A B C D For iron in the +3 oxidation state (Fe 3-t) the electron configuration is [Ar] 3d5 • In transforming from a neutral to a +3 state, the iron atom first loses two electrons from the 4s subshell and then an electron from the 3d subshell Cr0 2• Cr03' Cr0 • Cr 20 The electronic configuration [Ar] s 3d~ corresponds to a neutral atom of the element: In general, transition metals may assume multiple oxidation states Although metals tend to form basic oxides (e.g., Na 20) and nonmetals tend to form acidic oxides (e.g., S0 3), transition metal oxides may be acidic or basic depending on the oxidation state of the metal For example, Mn is • highly acidic while MnO is basic A manganese B technetium c D This behavior of the oxides, or anhydrides as they are also known, may be better understood in terms of the elemental hydroxides Reaction of the two manganese oxides above with water takes place as follows: chromium vanadium Based upon closed-shell and half-closed shell considerations, which of the following metals is the LEAST reactive? A B C D Li(s) K(s) Mg(s) Cu(s) Compounds I and II, two oxides of the same transition The difference in behavior between the two oxides and their hydroxides results from the relative sizes of the M-0 electronegativity difference (6.XM_0 ) When AXM-o is less than AXa-H' the 0-H bond has ionic character, and the proton may dissociate; consequently, the metal hydroxide is an acid This type of behavior is observed in metal anhydrides in which the metal has a high oxidation state (usually greater than +5) metal, are placed separately into two vessels of aqueous media Compound I produces an acidic solution, and Compound II produces a basic solution It can be concluded that the metal-oxide bonding in: On the other hand, AXM-o is greater than AXo-w the M-OH bond has ionic character, and the hydroxide may dissociate; accordingly, the metal hydroxide is basic GeneraHy, metal anhydrides are basic when the oxidation state of the metal is less than or equal to +4 Compound II is more covalent D Compound I is more covalent, and the bonding in Compound II is more ionic A Compounds I and II are nonpolar B Compound I i~ more polar than is the bonding in Compound II C Compound I is more ionic, and the bonding in Which of the follo~ing would NOT be predicted to be paramagnetic based on the electronic configurations given? A B C D 437 Sc 2+ Mn2-t Co 3-t Zn 1-t - [Ar] (Ar] (Ar] [Ar] 3d 3d5 3d6 3d 10 GENERAL CHEMISTRY A The Lewis dot structure for ozone is: Passage 72 + - ·a·/ C The second diagram in the passage indicated that the energy (space) between vibrational energy levels is much less than that between electronic energy levels Therefore, if the energy gap between the first two electronic energy levels corresponds to UV photons, then the energy gap between the first two vibrational energy levels must correspond to photons of less energy Infrared photons have several hundred times less energy than UV photons ~o · Therefore, the geometry of is trigonal planar, but its shape is bent A The boiling point temperature of a liquid is that temperature at which the vapor pressure of the liquid is equal to the external pressure Since the external pressure in the stratosphere is very low, water, and all other liquids, will boil at a much lower temperature that at standard pressure (water has a VP of torr at 5°C): choice A is correct D A Bohr atom is any atom or ion that has only one electron Choices A and B have no electrons, and choice C has two electrons The Be + ion has one electron The total pressure equals the sum of all of the partial pressures If the total pressure of all the gases in the stratosphere is torr, then there is no way that ozone can have a partial pressure greater than this (choice B is impossible) Lowering the pressure over a substance will never substantially increase the melting point of that substance (choice C is impossible) Evidently, gaseous reactions occur at these low pressures- this passage has been written about them (choice D is eliminated) C When the molecule goes from the first excited (v 2) vibrational energy level to the ground state (v l} energy level, it decreases in energy (according to they axis!} by releasing energy to the surroundings (choices A and Dare true} In all sciences, the terms energy (or reactivity} and stability are inverses of one another Therefore, a molecule which decreases in energy increases in stability (choice B is OK} The energy required to break the covalent bond, called the bond dissociation energy, is the energy required to go from the occupied vibrational energy level to v oo Less energy is required to go from the v = to v = oo than to go from the v to v = oo Therefore, the vibrational transition v to v = increases the energy required to break the bond = = = C The rate law only depends upon the reactants in the rate·determining (slow) step of the overall reaction - the third reaction step takes hours, while all of the others take minutes or seconds (choices A, B, and Dare all eliminated} The exponent in (the order of} the rate expression reflects the nu~ber of those types of molecules that must come together in the rate determining step Since two ClO are reacting together, this rate law must be second order = A Even without ever seeing a Morse potential diagram, you should know that the stability of any system is inversely proportional to its energy (choice A) Morse potential diagrams indicate that as the energy of a vibrational energy level gets bigger, the change in length of the oscillating bond (the length of the horizontal vibrational line) increases (choice B is wrong} Furthermore, the spacing between these lines gets smaller and smaller as you move up to higher levels (choice Cis wrong) Finally, choice D is incorrect because a catalyst decreases the activation energy of a reaction, but it never affects the thermodynamics (energy) of the overall process D This question is just testing if you remember what a catalyst does and does not A catalyst works by lowering the activation energy of the rate· determining step (choice A is true) A catalyst does not affect the overall thermodynamics (energy} of the catalyzed reaction (choice B is true} Finally, a catalyst only has a net effect on a reaction that is not at equilibrium, and in this case, the catalyst just increases the rate at which the system reaches equilibrium Therefore, at equilibrium, the [0 3] has to be much less because the catalyst Cl is just working to get the system to equilibrium by destroying ozone (choice C is true} MCAT SCIENCE WORKBOOK = 624 The passage suggests that the greenhouse effect of clouds ends up warming the surface at night (last paragraph) Choice B is neither correct nor suggested by the passage B This is a deceiving question because it doesn't really require you to consider a specific Morse potential diagram For example, choices A and C, which are identical, suggest that the C=O bond is weaker than the C-0 bond We should all know that a double bond is always stronger than a single bond (choices A and C are eliminated) Furthermore, choice D contradicts the passage - the passage says that all covalent bonds str:etch and bend The answer is choice B, which says that the average bond length of the C=O bond is less than the C-0 bond; we don't need a ~orse potential diagram to tell us this The more bonding electrons that are shared between two atoms, the greater the bond strength, and the shorter the bond B By definition, when the net heat (energy) flux is into the system, q is positive (choice B) Remember, 6.E = q + w (so q = 6.E- w), so if the system is absorbing energy, tiE is positive, thus q must be positive because no work is being done in this case C The key to getting this one is if you realize that just as much energy is being absorbed by the Earth as is being radiated to space, and that each individual IR/microwave photon has less energy than each individual visible photon Based upon this, more IR/ microwave photons must be emitted by the Earth because each photon carries less energy than each visible photon (choice C) Note that while visible photons are only absorbed half of the day (during daylight hours), the Earth is constantly emitting IR/ ·microwaves in an attempt to cool off (choice D) B Vibrational modes are the oscillatory movement of covalent bonds In order to have a vibrating covalent bond, you need a covalent bond to vibrate! Neon, a noble gas, is a monatomic gas than never forms any covalent bonds (choice B) [The only noble gases that form any covalent bonds are krypton and xenon (and this occurs only under very exotic conditions).] C The question tells us that E = constant x 'r Therefore, as T gets larger, E gets much, much larger B The Lewis dot structure for nitrous oxide, N20 is: •• + N=N=O S A We have all experienced the fact that we lose body heat faster the colder the air is in contact with our skin This is the sensible heat flux talked about in this passage Therefore, as the difference between the surface and air temperatures gets larger, so too should the heat flux q (choices B and C are eliminated) Furthermore, when there is no difference between surface and air temperature, there should be no net heat flux (choice A) While choice D is also an acceptable Lewis structure, it cannot be the correct answer because the question tells us that N20 is not symmetric Passage 73 B This question requires the understanding of the diagrams in the passage Foremost, the passage says that without the greenhouse effect of clouds and C0 , the temperature of the surface would be -20°C Life as we know it requires liquid water to metabolize and grow Therefore choice A is OK Furthermore, clouds reflect 25% of the sunlight reaching the Earth Therefore, during the day, clouds end up cooling the surface, an effect we are all very familiar with (choice C)! Obviously, C0 must interact with IR and/or microwaves because it is a greenhouse gas B Latent heat is the heat in water vapor that was absorbed during evaporation Therefore, a calculation of the latent heat flux requires knowledge of the amount of heat required to evaporate water: 6.Hvaporil.lltion (choice B) B This question 'is asking us to nothing more than to figure out how C0 and H20 are alike While water is a polar molecule (meaning that it has a permanent dipole moment) that experiences hydrogen bonding, carbon dioxide is a nonpolar molecule that has van der Waals forces (choices A, C, and D are eliminated) The only way carbon dioxide and water are alike is that they both have polar covalent bonds Recall that C0 is not polar overall because it is a linear, symmetric molecule (choice B) 625 GENERAL CHEMISTRY SOLUTIONS Passage 74 D Since ll.Hrxn = Hf{products)- Hf(reac•anu>'we have the following for the reaction of hydrogen and bromine to make hydrogen bromine: B All of these choices sound good; A C, and D are true statements However, only the entropy of simple, chemically pure elements is zero at 0.0 K (choice B) The question tells us that Hf(HBr) = -36 kJ/mol Hf for a pure element in its natural phase is 0.0 kJ/mol at standard conditions So HHHll = kJ/mol However, the natural phase of bromine under standard conditions is liquid, not gas - so in this question Hf(Brl> 7: 0.0 kJ/mol Therefore, unless we are told what the Hr(Brll for gas phase bromine is, we cannot calculate tJ H rxn A The reaction H2 + produces moles of H 20 We use 11 Go rxn = Go r (praducu> - Go r creactantsr Since pure elements in their standard state have G0 r = 0, !J.Gorxn = 2G r(water) = -457.2 kJ·mol- 1• C A positive ASu.n means that the entropy of the products is greater than that of the reactants, i.e., you •re getting more disorganized Keeping in mind that gases typically have much more entropy than liquids or solids, choices A and B are processes with large negative ASrxn' While diamond has less entropy than graphite (choice D has a slight positive ASu.a>• this choice pales in comparison to the process of converting solid carbon dioxide to carbon dioxide gas Choice C is the best choice D First of all, an equilibrium constant must always be positive (so choices A and B are eliminated) Evidently, the reaction of methane and oxygen occurs spontaneously and essentially goes to completion Since K [products] I {reactants], and very little reactants remain once the reaction is initiated (that's why this reaction is tested as a potential rocket fuel), the {products] are much, much greater at equilibrium than the [reactants] Therefore, K must be very large = C The question tells us that oxidizing agents can be recognized because they have two or more highly electronegative atoms directly bonded together Choices A and B only have one highly electronegative atom each (Br and N) Considering that S is not a highly electronegative atom and that the structure for S0 is O=S=O, sulfur dioxide is not an oxidizing agent by this definition Hydrogen peroxide is a powerful oxidizing agent because its structure is H-0-0-H; the two oxygen atoms are directly bonded together (choice C) This rule is very useful; you might want to keep it in mind! D Since the scientist used only one gram of each set of reactants, the most exothermic reaction will be the one that is the optimum rocket fuel, #(4 ) Passage 75 B If the conversion of heat to electrical energy was complete (100% efficient), then the temperature of the intake and outflow tertiary water should be the same It is not (choice A is eliminated) The reaction inside all nuclear reactions is fission, not fusion (choice C is eliminated) Choice Dis backwards Electric turbines are used to convert mechanical energy (the spinning propeller driven by the pressure of the steam) to electrical energy C If each H atom has an oxidation state of + 1, the total of the oxidation states of the two N atoms must be -4 (so that the molecule has a net oxidation state of zero) Therefore, each N atom has an oxidation state of -2 The vapor pressure for pure water is 760 torr At higher temperatures, the vapor pressure of water is much greater than this (choice B) Recall that water does not boil until the vapor pressure equals the external pressure As long as the pressure over the water inside the pipes is greater than the water's vapor pressure, it will remain a liquid Unfortunately, the vapor pressure of the primary coolant at the Chernobyl nuclear reactor did exceed the pressure in the system, and it boiled The subsequent pressure buildup (steam takes up more than 100 times mo~ space than liquid water) ruptured the pipes and blew the top of the reactor core right off1 MCAT SciENCE WoRKBOOK II 626 1· :.~' l A This reaction is a oxidation-reduction (redox) reaction because the oxidation state of the sodium and hydrogen atoms go from to + l and + to 0, respectively Hint: the production of hydrogen gas is almost always a dead give-away that a chemical reaction is a redox reaction (choice A) Choices B and C are eliminated because these reactions not involve the change in oxidation states of any of the participating atoms A combustion reaction is a subclass of redox reactions in which oxygen gas is converted to carbon dioxide and/or water (choice D is not applicable here) A Before passing by the turbine, the steam has some amount of internal energy As it passes through the turbine, some of its internal energy is converted into electrical energy and transported away (as electricity) Therefore, after the steam has passed through the turbine, it has to have less internal energy than when it started: AE is negative B If F (Z = 9) were to alpha decay the daughter nucleus would be N (Z = - = 7), and the mass number would decrease from A = 19 to A = 19 - = 15 (this eliminates A) If 15 decayed by positron emission, the daughter nucleus would have Z = 8- l 7, and the mass number, A = 15, would remain unchanged (this eliminates C) Finally, if uo experienced electron capture, the net effect is the same as positron emission: the atomic number would decrease by l (to = 7, which is N), and the mass number(= 15) would remain the same (this eliminates D) The correct answer must be B If 14C beta decays, its atomic number will increase by to + = 7, which is nitrogen, but the mass number would stay at 14 That is, 14C beta decays to 14 N, not to 15 N = D The heat content of a substance is mass·C,; AT The mass of 1.0 L of water is 1.0 kg or 1,000 grams (recall that each mL of water has a mass of one gram) The C,P of water equals 1.0 call (gram·°C), and AT equals 8°C (= 31 °C- 23°C) Therefore, the heat content of l.O L = 1000 grams of tertiary coolant water is 8,000 cal Passage 76 D The compressed refrigerant will release more energy to the environment when the environment is cooler (so choice A is eliminated) The passage tells us that the compressed gas has to be allowed to release its heat to the surroundings Therefore, you not want the gas to be insulated from the environment (choice B) Again, the compressed gas has to be allowed to release its heat to the surroundings Thus the compressed gas should be allowed to equilibrate with the surroundings (choice C is eliminated, choice D is the best) D As the temperature of a substance increases, the number of intermolecular forces decreases (choice A is just not true) Furthermore, as the temperature of a substance increases, it expands, so the spacing between molecules increases (choice B is not true) The passage indicates that the primary coolant water is pressurized so it won •t boil (choice C is eliminated) As one increases the temperature of a substance, the molecular motion (average kinetic energy) of the molecules increases (choice D is the best answer) C The passage indicates that the refrigerator, just like all other heat engines (system in which work and heat are converted· to one another and then back again), is not completely efficient Therefore, energy (in the form of electrical current) must be periodically added to keep the cycle going So while the cooling inside the refrigerator is counteracting the heat released by the compression coils, over three days, the additional energy is getting pumped into the closet from the electrical outlet causes the temperature to go up S A Based upon the infoqnation in the question, we can write: H+ + -oH ~ H20 + heat, AHr~n q = < (exothermic) Increasing the temperature will shift the reaction to the left, increasing [H•] and [-OH] Since the pH is nothing more than -log[H+], at high temperatures, the pH of water is less than seven [All K' s are a function of temperature (choice B is eliminated) Pressure does not affect reactions that not have gaseous products or reactants (choice D is incorrect).] 627 GENERAL CHEMISTRY SOLUTIONS D This question is easy if you remember that expanding gases cool, and compressing gases warm When compressed air escapes to the low pressure out here, 760 torr, it must cool (choice A is OK) When air in pressurized, it's compressed, and it has to heat up (choice B is OK) The pressure at any depth in the ocean is greater than the surface pressure Therefore, as a gas bubble rises through the water column, it is exposed to less pressure, and it expands -and cools (choice C is OK) The boiling point of a liquid is that temperature at which the vapor pressure of the liquid equals the external pressure 'A glass of water at room temperature will boil if the external pressure can be reduced to 24 torr (placed under a vacuum) As defined by A H vaponzauon , the vaporization of a liquid to a gas is an endothermic process Therefore, when liquid water evaporates (or boils), the remaining liquid is chilled (choice D is backwards) The body takes advantage of this chilling effect during perspiration Passage 77 A Initially, as pressure is raised, the volume will decrease Thus, Vinicial > Vfinal' On the graph, Vis - V Positive, so it must be defined (here) as V 111111a1 final' B If the diameter is doubled, then the area increases by a factor of 22 If the force is doubled, then the pressure - which is force per area - will become 2/4 = 1/2 as great = C As the pressure of a gas is raised at constant temperature, there comes a point at which many mQlecules no longer have sufficient kinetic energy to remain in the gas phase At this point, the gas begins to condense into liquid D Adiabatic (from the Greek: a, not+ diabatos, passable) walls are defined as barriers that not permit the transfer of heat Thus any heat produced by a system will remain in the system C CF 2Cl is not an ion, so it cannot experience ionic forces (choice A is eliminated) In order to experience hydrogen bonding, the molecule must at least have one hydrogen atom (choice B is eliminated) CF2Cl is a slightly polar molecule since fluorine is more electronegative than chlorine Therefore, considering that dipole forces are stronger than van der Waals interactions, the best choice is C C Equilibrium D is in solution (liquid phase) so changes in pressure will have little effect on it Of the remaining gas reactions, only Equilibrium C has a net change in the number of gas molecules, so this equilibrium will be affected most by compression and the corresponding increase in pressure A This question is asking us to identify an endothermic process Freezing (liquid to solid), condensation (gas to liquid), and deposition (gas to solid) are all exothermic processes Sublimation (solid to gas) requires the absorption of energy from the environment C From the ideal gas law, PV =nRT, we see that P is proportional to T if Vis held constant Thus, Twill double if P is doubled A The change of the internal energy of a gas may be written as A E = q + w = heat + work Therefore, the compressor can increase the E of a gas by doing work on that gas (choice A) Choice C is a fancy way of saying that the gas has been allowed to expand, a process that reduces the internal energy of the gas (choice C is eliminated) The compressor converts electrical energy into mechanical energy (choice D is wrong) Moreover, there is no such term as pressure energy MCAT SCIENCE WORKBOOK 628 Passage 78 A Since the metal was initially submerged in boiling water, Tini«iat • the battery output (Vout) is constant Apparently, Vout only drops off when ruse gets very large Choice A is incorrect because it states that as ruse gets bigger, vout should get bigger This just does happen- contrary to the belief of some, no battery just keeps going and going and going Choice B is incorrect because it suggests that from day one, the voltage should drop steadily to zero - this is the equation of a straight line, contrary to the given plot Choice C is incorrect because at the instant the battery is first used, when ru,e is very small, the value of -K/rusc would be so large that the battery would have a huge, negative voltage for awhile If this wasn't bad enough, at some later time, as the value of -K/Tuse became small, the voltage would approach a constant voltage that would be maintained forever In short, choice C is just not realistic The answer is D If we assume Keto be very small, for some stretch of time, ruse' the value of -Ke(ruac) would be small and thus Vout basically equals E ~:eu for most of the cell's life But eventually, the cubic function of ruse grows so large that the "drainage" term, -Kc(rus) , becomes significant At this point, V rapidly decreases to zero- just like a real cell (choice D) Passage 90 l C The voltage of a cell equals the sum of the voltage of the reduction and the oxidation reaction Therefore, if the reductive reaction (Mn0 to Mn 2•), has a potential of -+:_1.23 volts and the oxidation reaction (Zn to Zn 2+ under basic conditions) has a potential of +0.44 volts, then total cell potential is must be about 1.7 volts A Throw out choice D right away; the passage tells us that this is an alkaline battery -the cell is operating under basic conditions The rest require a little more thought The overall reaction in the alkaline dry cell is: 0111 Mn0 (s) + Zn(s) + 4·H•(paste) -7 Mn 2•(paste) + Zn •(paste) + 2· H20(paste) B Choice A is not true Most batteries would explode if you tried to recharge them due to the internal production of hydrogen gas Choice C is backwards: oxidation occurs at the anode (remember your vowels) Choice D is also backwards: the anode is often consumed in an electrochemical cell (and the cathode usually grows due to metal deposition) As the cell operates, it consumes acid Therefore, the pH of the charged cell must be lower than the pH of the expended one 637 GENERAL CHEMISTRY SOLUTIONS C Choices A, B, and D are devices that have actually utilized solar cells to produce power However, consider a flashlight that derives its power from solar cells-it probably wouldn't sell very welL Doesn't it violate common sense to try to power a device with light when the device is designed to function when it's dark? Flashlights could easily be designed to use solar cells in conjunction with rechargeable batteries, but not alone D The student's experiment indicated that nitric acid was very corrosive because of the extra oxidative kick bestowed by the nitrate ion (conjugate base) The key to this question is to find another strong acid that has a conjugate base which is a powerful oxidizing agent in its own right Remember that an oxidizing agent is a molecule which has two or more highly electronegative atoms bound together Choices A and C fail in this respect-HF isn't even a strong acid by definition Phosphoric acid is not a strong acid either, and the phosphate group is not a strong oxidizing agent (P has only a moderate electronegativity) Perchloric acid is a strong acid, and the perchlorate ion, Clo -, is a powerful oxidizing agent Passage 91 A A metal with a high reduction potential will not be easily oxidized by the H+ in the acid The student's data indicated that choices B, C, and D were more reactive with acids than choice A C Choices A and B are correct, nitric acid is more corrosive than sulfuric and hydrochloric acid because the nitrate ion has a higher reduction potential (is a better oxidizing agent) than H+ Choice D is also correct because the ions present in Bath A and Bath B were unable (had a lower reduction potential) to oxidize lead, silver, copper, and mercury-the student observed no reaction If choice C was correct, sulfuric acid would be more corrosive that HCI; this was not observed in the data set D Evidently, the ordorless, colorless gas was hydrogen Choices A, B, and C are incorrect because ammonia, sulfur dioxide and nitrogen are colorless gases N0 is an acrid, orange-brown gas B Choices A, C, and D would be serious errors Since the metal samples were the limiting reagent, not the acids themselves, the concentration of the acids is irrelevant A In order for a molecule to have a tetrahedral or square planar geometry (choices B and C), that molecule must consist of a central atom plus four surrounding atoms! The nitrate ion has a central nitrogen atom surrounded by three oxygen atomsthe only possible geometry for this arrangement is trigonal planar or trigonal pyramid MCAT SCIENCE WORKBOOK 638