Exam 5: General Chemistry

The primary chemical composition of kidney stones commonly includes Ca₃(PO₄)₂ (calcium phosphate), CaC₂O₄ (calcium oxalate), and MgNH₄PO₄ (magnesium ammonium phosphate).  The metal ions in these compounds are:

Passage Household cleaners commonly contain either ammonia (NH₃) or bleach (NaOCl) as the principal active ingredient.  Mixing ammonia-based and bleach-based cleaners can be hazardous, as the resulting reactions can form several toxic products.  One of these reactions forms the irritant dichloramine (NHCl₂) as follows: Reaction 1The overall reaction has a standard enthalpy change of −55 kJ/mol and a standard entropy change of −150 J/mol·K.  Partial thermodynamic data for formation of Reaction 1 reagents from their elements are shown in Table 1.Table 1  Partial Data for Enthalpies, Entropies, and Free Energies of Formation of Several Compounds at 25 °C After it is formed, dichloramine readily enters the gas phase, where it may be inhaled.  In the lungs, dichloramine reacts with water to produce hydrochloric acid and ammonia, as well as reactive oxygen species.  In small doses, these products cause mild irritation, but inhalation of large amounts of chloramines can lead to pneumonitis (inflammation of the lungs) and may require medical intervention. Adapted from Trogolo D, Arey JS. Benchmark thermochemistry of chloramines, bromamines, and bromochloramines: halogen oxidants stabilized by electron correlation. Phys Chem Chem Phys. 2015;17(5):3584-98. -If large amounts of ammonia and bleach were mixed, emergency responders could help stop the reaction by doing which of the following?

Passage Cyanide (CN⁻) is a powerful, rapid toxin that binds to the cellular respiratory enzyme cytochrome c oxidase.  The resulting enzyme-cyanide complex blocks the transfer of electrons to oxygen in the last step of the electron transport chain.  This results in cytotoxic anoxia, causing asphyxiation at the cellular level.Exposure to cyanide can occur from dietary sources, medical treatments, industrial applications, smoke inhalation (structural fires), and intentional use (suicide).  While dietary exposure from moderate consumption of naturally occurring plant compounds such as cyanogenic glycosides rarely results in acute adverse effects in humans, chronic effects have been observed from prolonged diets with significant levels of these compounds.  In the developed world, risk of significant cyanide exposure is most likely to result from smoke inhalation during building fires or from industrial/occupational exposures.The impact of a given exposure depends on the amount (dose, duration), the chemical composition of the cyanide compound (HCN, NaCN, KCN), and the route of exposure (inhalation, absorption, ingestion).  The ionic salts (NaCN, KCN) and free acid (HCN) forms of cyanide each have different acid-base properties that interact differently under the physiological conditions associated with each route of exposure.To better understand the toxicity of different forms of cyanide, animal models have been studied for comparison to known human exposure case studies.  Table 1 presents a summary of selected animal toxicity studies that examine exposure via ingestion.  The table provides the lethal dose to 50% of the population (LD₅₀) in milligrams of cyanide per kilogram of body weight (mg/kg).Table 1  Compilation of LD₅₀ Cyanide Values in Various Animal Species Based on animal models, one treatment developed for cyanide poisoning involves assisting the rhodanese enzyme in metabolizing CN⁻ to SCN⁻ via an intravenous solution of NaNO₂ and Na₂S₂O₃.  The Na₂S₂O₃ assists in the formation of a reactive S-SH (persulfide) bond in the enzyme on cysteine-247 (Reaction 1), which then reacts with the cyanide ion to form thiocyanate (Reaction 2). Adapted from: Hall AH, Isom GE, Rockwood GA. Toxicology of Cyanides and Cyanogens, Experimental, Applied and Clinical Aspects. John Wiley & Sons; 2015: 1-20; and Egekeze JO, Oehme FW. Cyanides and their toxicity: A literature review. Vet Q. 1980; 2(2): 104-114. -The base dissociation constant K_b for the hydrolysis of an alkali metal cyanide salt is given by the relation: where [OH^-],[HCN], and [CN^-] are the molar concentrations of the relevant chemical species.  What is the pH of an aqueous solution that is 0.001 M in KCN salt?  (Note:  K_a ≈ 1 × 10⁻⁹ for HCN, and pH ≠ pK_a in this solution.)

Which of the following molecular structures has the most hydrophobic character?

Passage Students were asked to bring items from home to serve as examples for different types of reactions.  The instructor provided additional reactants, solvents, heat, or a catalyst to demonstrate reactions with the household items.The students brought aluminum foil, table salt, sugar, baking soda, vinegar, toilet bowl cleaner, hydrogen peroxide, and antacid tablets.Experiment 1Reactions were performed using the household items.  The reaction conditions and student observations are recorded in Table 1.Table 1  Reaction Conditions and Visual Observations Experiment 2The reactions that were observed to produce gas bubbles in Experiment 1 were repeated, and samples of the gases produced were collected.  Each gas sample was tested by holding a burning (glowing) wooden splint within the reaction vessel to determine if the reaction evolved O₂, H₂, or CO₂.  A glowing splint placed in O₂ will burn more brightly, one placed in H₂ will ignite the gas, and one placed in CO₂ will be extinguished.  The results of the gas tests are recorded in Table 2.Table 2  Glowing Splint Gas Test Observations -Based on the information in the passage, what is the identity of the gas formed from the decomposition of H₂O₂?

The compressibility of a real gas and an ideal gas at a temperature of 200 K is shown as a function of pressure in the graph below. Compared to the volume of the ideal gas at 350 atm, the volume of an equimolar amount of the real gas under identical conditions is:

Passage The radiopharmaceutical 2-deoxy-2-(¹⁸F)fluoro-D-glucose (abbreviated as ¹⁸F-FDG) is a modified form of D-glucose in which the hydroxyl group on the 2-carbon of the molecule has been replaced with a positron-emitting ¹⁸F radionuclide.  Synthesis of ¹⁸F-FDG is accomplished in several steps involving both nuclear and chemical reactions, as shown in Figure 1. Figure 1  Synthetic scheme for 2-deoxy-2-(¹⁸F)fluoro-D-glucoseInitially, a target of ¹⁸O-enriched water (¹⁸OH₂) is bombarded by protons using a cyclotron to achieve a (p-n) nuclear reaction in which a proton hitting the ¹⁸O nucleus is absorbed by the nucleus, causing the ejection of a neutron and forming an ¹⁸F nucleus.  Following separation of the resulting ¹⁸F⁻ anions from the residual ¹⁸OH₂ via an ion exchange column, ¹⁸F⁻ is used in a nucleophilic substitution reaction with an acetylated mannose triflate substrate.  Subsequent hydrolysis to remove the acetyl protecting groups yields ¹⁸F-FDG.Because ¹⁸F-FDG is an analog of D-glucose, it is readily taken into cells by the same metabolic processes as D-glucose.  Once inside the cells, the positron emissions of the ¹⁸F radionuclide can be detected by positron emission tomography (PET) scans to produce diagnostic medical images of objects such as tumors.  Emissions from the ¹⁸F radionuclide subsequently diminish over time via radioactive decay (Figure 2). Figure 2  Nuclear decay of the ¹⁸F radionuclideBecause of limitations in the spatial resolution of PET scanners, when an object (such as a tumor) is smaller than 30 mm in diameter, the measured specific uptake value (SUV) of ¹⁸F-FDG accumulated within the object is lower than the actual SUV.  The recovery coefficient (the ratio of the measured SUV to the actual SUV) shows that the measured SUV decreases as the size of the object decreases below the 30-mm threshold (Figure 3). Figure 3  Recovery coefficient as a function of object size in a PET scan with a spatial resolution of 7 mm using ordered subsets expectation maximization (OSEM) image processing Adapted from: S. Yu. "Review of F-FDG Synthesis and Quality Control." Biomed Imaging Interv J. ©2006 Department of Biomedical Imaging, Faculty of Medicine, University of Malaysa; and P. E. Kinahan, et al., "Positron emission tomography-computed tomography standardized uptake values in clinical practice and assessing response to therapy." Semin Ultrasound CT MR. ©2010 Elsevier. -Suppose that the (p-n) nuclear reaction described in the passage was attempted but resulted in the ¹⁸O nucleus absorbing a proton without ejecting a neutron.  The resulting nucleus formed would be an isotope of:

At the end of the actinium nuclear decay series, polonium-215 decays in an alternating sequence of alpha and beta decays.  What are the nuclei produced in the nuclear decay sequence listed in order of formation from first to last?

Passage The tendency of a chemical species to undergo reduction during an electrochemical reaction is indicated by the standard potential E° for the reaction.  As represented generally by Reaction 1, E° measures the potential Z at which element X with oxidation number N accepts N electrons and is reduced to its elemental state.X(N) + Ne⁻ → X(0),    E° = Z voltsReaction 1In a galvanic cell, E° > 0 indicates a spontaneous reaction.  Therefore, chemical species with higher positive E° values are reduced more easily than those with lower E° values.Using E° as a basis of comparison, the relative thermodynamic stabilities of different chemical species involving the same element at different oxidation states can be presented graphically using a Frost diagram.  For a series of compounds containing element X, a Frost diagram plots the value of NE° for each compound against the corresponding oxidation number N of element X within the species.  Frost diagrams for several species containing manganese or chlorine are shown in Figure 1. Figure 1  Frost diagrams for manganese and chlorineOn a Frost diagram, NE° is proportional to the standard Gibbs free energy ΔG° according to the relationshipΔG° = −FNE° = −nFE°Equation 1where n is the number of moles of electrons transferred during the electrochemical process and F is the Faraday constant.  Free energy considerations also show that a species is prone to disproportionation if its position on the Frost diagram lies above a line connecting the points of two adjacent species, as seen for species B in Figure 2. Figure 2  General Frost diagram for an element forming species A, B, C, and D.The slope of a line segment joining two species on a Frost diagram is equal to the standard reduction potential for the couple.  A greater slope indicates a higher corresponding reduction potential.  As a result, in Figure 2, the reduction potential for B to A is lower than that for D to C. -Iodine reacts with ClO₃⁻ under acidic conditions according to the reaction shown below.5 ClO₃⁻ + 3 I₂ + 3 H₂O  →  5 Cl⁻ + 6 IO₃⁻ + 6 H⁺Does I₂ act as the oxidizing agent or reducing agent in the reaction?

Some models of rechargeable hearing aids use lithium-ion batteries composed of a carbon (graphite) anode and a cobalt oxide-based cathode.  Reaction 1 shows the half-reaction that occurs at the anode during battery discharge, and Reaction 2 shows the balanced net reaction.LiC₆ → Li⁺ + e⁻ + C₆Reaction 1LiC₆ + CoO₂ → LiCoO₂ + C₆Reaction 2When the hearing aid is operating, what half-reaction occurs at the cathode?

Passage Living organisms that require oxygen to respire can build up hydrogen peroxide (H₂O₂) as a byproduct of respiration.  Because hydrogen peroxide can cause oxidative damage to cells, the amount of H₂O₂ in cells must be closely regulated.  Although H₂O₂ decomposes spontaneously, a catalyst is needed for the reaction to occur at a sufficiently rapid rate suitable for biological purposes.  In biological systems, an enzyme called a catalase accelerates the catalytic decomposition of H₂O₂ into water and oxygen (Reaction 1). Reaction 1In a laboratory setting, several different inorganic compounds can also serve as catalysts for H₂O₂ decomposition, allowing researchers to compare how different catalysts affect the efficiency of the reaction.  To compare Fe(NO₃)₃ (a metal homogeneous catalyst) and NaI (a halogen homogeneous catalyst), researchers measured the amount of heat evolved after placing each catalyst in a fresh solution of 3% hydrogen peroxide.To perform the measurements, the researchers placed 50 mL of 3% H₂O₂ in an insulated coffee cup to be used as a calorimeter, as shown in Figure 1.  Before adding any catalyst, the initial, baseline temperature of the H₂O₂ solution was determined by recording the temperature every 30 seconds for 5 minutes.  Then 10 mL of 0.10 M Fe(NO₃)₃(aq) catalyst was added to the H₂O₂ solution.  The temperature was recorded every 30 seconds for another 15 minutes.  The experiment was then repeated using 10 mL of 0.50 M sodium iodide in 0.01 M NaOH as the catalyst.  The results for each reaction are shown in Figure 2. Figure 1  Coffee cup calorimeter Figure 2  Temperature vs. time measurements for reactions catalyzed by Fe(NO₃)₃ and NaI Adapted from: C. Marzzacco, "The effect of a change in catalyst on the enthalpy of decomposition of hydrogen peroxide." Chem13 News Magazine. ©2008 University of Waterloo. -The experiment was repeated with 3.0 g of solid MnO₂ particles added as an alternative catalyst.  When MnO₂ was ground to a fine powder, the temperature rose more quickly than when the particles were not ground.  The most likely reason for this is that with heterogeneous catalysts:

A radioactive atom decays by 5 alpha, 3 beta minus, and 2 gamma emissions to yield ²¹¹Po.  What was the original nucleus?

Passage Students conducted an experiment to determine the changes in enthalpy ΔH°, entropy ΔS°, and Gibbs free energy ΔG° resulting from the dissociation of acetic acid (CH₃COOH) in water.  The students first determined the acid dissociation constant K_a for the reaction by measuring the pH of a 1 M acetic acid solution at various temperatures over a narrow range of less than 10 °C.To perform the pH measurements, the acetic acid solution was initially cooled to 0 °C using an ice bath and then warmed with gentle heating to room temperature with continuous stirring.  The pH was recorded at intervals of 1 °C.  From the pH data, the value of K_a at each temperature T was calculated.  The experimental results over the selected temperature range are shown in Table 1.Table 1  Student Data for the pH and K_a of Acetic Acid with Increasing Temperature To evaluate ΔH° and ΔS°, the students constructed a van 't Hoff plot (Figure 1), which is a graph of the natural log of the dissociation constant (ln K_a) vs. the reciprocal absolute temperature (1/T). Figure 1  Linear van 't Hoff plot relating the dissociation constant K_a of acetic acid to temperature TThe relationship between K_a, T, ΔH°, and ΔS° in the van 't Hoff plot was then described using a linear fit of the data following the form of Equation 1, in which R is the gas constant (8.3 J∙mol⁻¹∙K⁻¹). Adapted from: C. Rezsnyak "Determination of Thermodynamic Values (S°, H°, and G°) from the Dissociation of a Weak Acid" World Journal of Chemical Education. ©2017 Science and Education Publishing. -During the dissociation of acetic acid at the temperatures tested in Table 1, the molecular disorder within the reaction vessel:

Passage Copper plays a vital role as a trace mineral for biological processes.  The amount of dietary copper needed by an adult each day is about 30 μg/kg of body weight, and blood plasma normally has a copper concentration of 1 mg/L.Copper is an essential element, and toxic effects are not common.  However, given sufficient exposure to high levels by inhalation, absorption, or ingestion from sources such as industrial hazards or contaminated groundwater, copper toxicity can occur.Salts that are more soluble tend to exhibit higher toxicity when ingested, but the solubilities of copper salts vary and are also temperature-dependent, as shown by selected examples in Figure 1. Figure 1  The solubility of selected copper salts in distilled water with increasing temperatureSome copper salts have solubilities much lower than those shown in Figure 1.  For example, CuF₂ has a solubility of only 0.075 g per 100 g of water (at 25 °C).  This indicates that CuF₂ dissociates in water only to a small extent (Reaction 1) and that the equilibrium favors the undissociated salt, giving CuF₂ a solubility product constant of K_sp = 1.6 × 10⁻⁶. Reaction 1Compound-specific properties can also impact solubility.  In CuF₂, the dissociated F⁻ ion is mildly basic (pK_b = 10.8), causing the equilibrium to be influenced by the pH of the solution.Specific characteristics of copper can also play a role.  For example, copper ions in solution express an affinity to form coordinate bonds with available lone-pair electrons of nitrogen atoms in amines (Reaction 2). Reaction 2If excess amine ligands are present, coordinate bonds like that shown in Reaction 2 will continue to form until each Cu²⁺ ion in solution is coordinated with four nitrogen atoms.  This acquisition of copper ions by ligands in solution can also impact the solubility equilibrium of copper salts by forming soluble coordination complexes. -For the separation of a saturated aqueous mixture of CuF₂ (K_sp = 1.6 × 10⁻⁶) and BaF₂ (K_sp = 3.0 × 10⁻⁶), a lab technician chose to use the common ion effect to precipitate CuF₂ from the solution rather than BaF₂.  Which of the following statements does NOT explain why precipitating CuF₂ is the better choice for the separation procedure?

The emission line spectra of hydrogen, lithium, sulfur, and selenium for the visible region of the spectrum are shown below. If a sample contains more than one of these elements, which of the following spectra would correspond to a sample containing both S and Se?

Passage Hyperbaric oxygenation therapy involves placing a patient in a chamber with oxygen-rich air at greater than atmospheric pressure.  Normal air has approximately 78% N₂, 21% O₂, and trace amounts of other gases.  The pressure within the hyperbaric chamber is typically measured in terms of ATA, or atmospheres absolute (1 ATA is the average atmospheric pressure at sea level, or 1 atm).  A hyperbaric chamber set at 2.5 ATA would be the average atmospheric pressure at sea level (1 ATA) plus the additional pressure applied to the chamber (1.5 ATA).There are two main types of hyperbaric chambers: monoplace chambers and multiplace chambers.  Monoplace chambers accommodate a single person.  The air in these chambers is 100% oxygen and can be raised to a maximum pressure of 3 ATA.  Monoplace chambers are used as therapy for conditions where oxygen supply has been depleted, such as carbon monoxide poisoning.  They are also used when a higher concentration of oxygen in plasma is needed, such as in wound healing.Multiplace chambers are large enough to house at least four people.  The air in these chambers is typically a mixture of oxygen and nitrogen, but the patient breathes pure oxygen through a mask.  Some multiplace chambers can endure maximum pressures of 6 ATA and are typically used to eliminate intravascular air bubbles that can occur from decompression sickness or embolisms.  The high pressure from the multiplace chamber results in a decrease in volume of any air pockets within the body, including the sinuses, inner ear, and unwanted air bubbles in the arteries or veins.Patients report that their ears "pop" during the pressurization process, similar to being on an airplane in flight.  In addition, they report that although there is an initial sense of temperature increasing, the chamber's temperature stays relatively stable during both the pressurization and the depressurization processes. -In a hyperbaric chamber, the volume of intravascular air bubbles decreases as pressure increases.  Which of the following gas laws best explains this behavior?

The boiling point of CH₃CH₂Br (bromoethane) is 38.4 °C whereas the boiling point of CH₃CH₂I (iodoethane) is 71.5 °C.  Which of the following best explains the difference between the boiling points of the two alkyl halides?

Passage Dmitri Mendeleev is credited with grouping the elements by atomic mass and observing that similar properties emerge periodically.  Henry Moseley then determined that a fundamental property, which Ernest Rutherford later called the atomic number, is more important than atomic mass for ordering the elements.As the periodic table took shape, scientists noted certain properties and trends.  For example, Rayleigh and Ramsay observed that some elements are unreactive and gave them the name noble gases.  But it was not until Niels Bohr described his model of the atom that scientists understood why noble gases are stable.Another important trend was discovered by Linus Pauling.  He was the first to identify electronegativity as a way to describe bonds that are neither completely ionic nor completely covalent.  Pauling assigned fluorine the highest electronegativity value of 4.0 because of its strong tendency to attract electrons when bonded to other atoms.  All other atoms were assigned values relative to fluorine.  This became known as the Pauling electronegativity scale.Scientists identified other trends based on x-ray crystallography and reactivity studies.  Figure 1 shows the first ionization energy of the elements, and Figure 2 shows electron affinity. Figure 1  First ionization energy of elements Figure 2  Electron affinity of several elements (larger negative values indicate greater electron affinity) -The acid strength of hydrogen halides is ranked from strongest to weakest as follows:H−I > H−Br > H−Cl >> H−FWhich of the following best explains why H−F is not a strong acid?

Shiny nickel metal granules added to orange liquid bromine mixed in alcohol produced a combination reaction yielding a blue-green solution of nickel(II) bromide.  In the reaction between Ni and Br₂:

If the hydroxide concentration in common duct bile is 1 × 10⁻⁶ M, what is the pH of the bile?