Exam 5: General Chemistry

Passage Bone growth and remodeling during fracture healing are often slow and have potentially poor outcomes, including malunion or nonunion at the fracture site.  Magnesium bone implants can aid in bone formation and safely biodegrade in the body according to the following reaction:Mg(s) + 2H₂O(l) → Mg(OH)₂(s) + H₂(g)Reaction 1  Corrosion of magnesium metal in aqueous mediumHowever, magnesium metal tends to degrade too quickly and forms hydrogen gas pockets in the body.  Corrosion is slowed as the metal naturally forms a protective layer of magnesium hydroxide in a process called passivation.  Magnesium hydroxide is less soluble than magnesium metal, and its dissolution is described by the endothermic reaction: Reaction 2  Solubility equilibrium of Mg(OH)₂ in water at 25°CAlthough passivation slows down the corrosion rate, it must be slowed even further to adequately facilitate bone reformation.Researchers are interested in the differences in the corrosion rates of magnesium metal protected by different biocompatible coatings.  They compared 10-mm magnesium disks coated with compounds of various cation and anion combinations, with solubility results shown in Table 1.  The coated disks were immersed in simulated body fluid (SBF) at room temperature for 50 hours.Table 1  Solubility for Biocompatible Inorganic Salts Given in mmol/100mL of Pure Water at 25°C Adapted from Huan ZG, Leeflang MA, Zhou J, Fratila-apachitei LE, Duszczyk J. In vitro degradation behavior and cytocompatibility of Mg-Zn-Zr alloys. J Mater Sci Mater Med. 2010;21(9):2623-35. -If researchers wanted to use a coating material that is more soluble in pure water than Mg(OH)₂ but less soluble than MgF₂, which of the following would be a good option?

Passage Gastric acid is one of many acids important to biological function in the human body.  Production of the main component of gastric acid, hydrochloric acid (HCl), is accomplished in the stomach by the parietal cells, as shown in Figure 1. Figure 1  Formation of gastric acid in the stomachFacilitated by the carbonate dehydratase enzyme (1), the parietal cells utilize the interaction between aqueous carbon dioxide and water in the blood as a source of bicarbonate ions and acidic H⁺ ions (protons).  The H⁺ ions produced in this process are then exchanged for potassium ions through the cell membrane by a proton pump, the H⁺/K⁺-ATPase (2).  In conjunction with this proton exchange, bicarbonate ions are also exchanged for chloride ions from the interstitial fluid.  As a result, H⁺ and Cl⁻ ions are concentrated in the lumen of the stomach to form aqueous hydrochloric acid.Because the production of the acid proceeds with a drop in pH from 7.0 in the cell cytosol to 1.0 in the stomach lumen, the process occurs against a concentration gradient and is endergonic, requiring the input of energy supplied by the hydrolysis of ATP.  Ignoring effects from ion charge, the free energy change for ion transport across the cell membrane (ΔG_t) can be estimated by Equation 1: ∆Gt=kRT logCfCiEquation 1where R is the gas constant (8.32 J/mol·K), k is a logarithm conversion constant, T is the absolute temperature, and C_i and C_f are the initial and final molar concentrations across the gradient, respectively.Although dilute, the gastric (hydrochloric) acid released into the lumen yields a strongly acidic environment crucial to protein digestion and the destruction of harmful micro-organisms.  A comparison of its acid dissociation constant (K_a) to those of other biologically relevant organic acids is given in Table 1.Table 1  Comparison of Strengths of Selected 0.10 M Acids at 25°C Adapted from Koolman J, Roehm KH. Color Atlas of Biochemistry. Thieme; 2011: 268-271; and Lehninger AL, Nelson DL, Cox MM. Lehninger Principles of Biochemistry. Macmillan; 2005: 397-398, 419, 520. -What happens to the pOH of the gastric acid entering the duodenum (pH 6.0) as a result of the introduction of bicarbonate ions from incoming pancreatic secretions?

Based on the Pauli exclusion principle, which electron subshell configurations cannot exist?3s 2p⁷6d  4f¹⁴

Passage The primary structure of a protein is a series of amino acids that are connected in a long chain via C−N peptide bonds (Figure 1). Figure 1  Peptide bonds within a protein segment (Note:  Rⁿ = amino acid side chains)Protein chain folding is guided by multiple intramolecular interactions that can occur between two peptide bonds, between a peptide bond and a side chain, or between two side chains, as shown in the protein in Figure 2.  Although these interactions are intramolecular because they are within the same molecule, they act effectively like intermolecular interactions because proteins are such large molecules. Figure 2  Intramolecular interactions between the peptide chain and amino acid side chains within a hypothetical proteinBecause intramolecular interactions play a significant role in maintaining the three-dimensional folded structure of a protein, having a greater number of interactions or stronger interactions will result in increased protein stability.  The energy required to break these interactions within a protein is related to free energy, ΔG.  The greater the ΔG value, the more stable the protein.To find the overall contribution of particular intramolecular interactions to protein stability, researchers made several protein variants in which one amino acid is replaced by another to change the noncovalent interactions between the side chains.  The difference in free energy between the variant and the original protein, Δ(ΔG), indicates the change in overall stability of the protein.  A negative Δ(ΔG) means the variant protein is less stable than the original.Table 1 shows the results of the amino acid substitution experiments for a protein with the approximate energy difference when each amino acid is changed.Table 1  Change in Free Energy Based on Changing Intramolecular Interactions Caused by Amino Acid Substitutions Adapted from: C. N. Pace, J. M. Scholtz, G. R. Grimsley, "Forces stabilizing proteins." FEBS Lett. ©2014 Wiley. -Assuming the amino acid substitutions found in Table 1 occur on the protein surface, which substitution would cause the smallest change in the water solubility of the protein?

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. -Based on the van 't Hoff plot shown in Figure 1, the dissociation of acetic acid is:

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 -Which of the following is the reaction for the decomposition of calcium carbonate based on the information in Tables 1 and 2?

Passage Depending on the active compound's specific method of action, sunscreens are broadly classified into two groups: organic and inorganic.  Organic sunscreens typically contain aromatic compounds that absorb ultraviolet (UV) radiation. The absorbed energy from the UV radiation causes these compounds to enter electronically excited states which then relax back to the ground state and harmlessly re-emit the absorbed radiation as heat.  Alternatively, inorganic sunscreens contain metallic nanoparticles that serve to reflect, scatter, and absorb incident radiation.  The mechanism for UV protection of each group is shown in Figure 1. Figure 1  Method of action for organic and inorganic sunscreensThe effectiveness of sunscreens is traditionally evaluated based on their ability to prevent reddening of the skin, as indicated by their sun protection factor (SPF).  However, studies have shown that additional deleterious effects, such as the suppression of T cell-mediated immune responses, can occur at lower UV doses than erythema, indicating the need for new metrics to judge sunscreen efficacy.  For this reason, researchers conducted a study to compare the protective ability of six sunscreens against photo-isomerization of the photoreceptor trans-urocanic acid (UCA) to its cis form, a process believed to be linked to UV-induced immunosuppression.  Table 1 lists the sunscreens tested and their relevant characteristics.Table 1  Results from Tested Sunscreens In vivo experiments were conducted on two groups of human volunteers.  Participants in the acute exposure group received a single UV dose of 100 mJ/cm² whereas participants in the chronic exposure group received the same dosage once daily over four consecutive days.  Each sunscreen was applied to a different spot on the volunteers' back prior to irradiation.  A portion of skin without sunscreen was also irradiated to serve as a control.  After irradiation, the sunscreens were removed and the skin was wiped with filter papers.  Filters were washed with potassium hydroxide, and the aqueous solution was analyzed for UCA using high-pressure liquid chromatography.  Figure 2 shows the mean percentages of cis-UCA formed upon irradiation for each exposure type and sunscreen tested. Figure 2  Percentage of UCA in cis configuration upon UV exposure with various sunscreens Adapted from Van der molen RG, Out-luiting C, Driller H, Claas FH, Koerten HK, Mommaas AM. Broad-spectrum sunscreens offer protection against urocanic acid photoisomerization by artificial ultraviolet radiation in human skin. J Invest Dermatol. 2000;115(3):421-6. -Zinc, titanium, iron, and aluminum are all ingredients found in inorganic sunscreens.  Which of the following choices correctly lists the atoms in order of increasing atomic radius?

Passage Although acetylsalicylic acid (aspirin) is a well-known and widely used analgesic and anti-inflammatory agent, copper(II) acetylsalicylate, a copper complex formed from aspirin and copper, has also been found to exhibit some distinctive and potentially beneficial pharmacological properties.  Formation of copper(II) acetylsalicylate is achieved when the conjugate base of aspirin, acetylsalicylate (C₉H₇O₄⁻), binds to Cu²⁺ ions in solution, as outlined in Reaction 1.4 NaC₉H₇O₄(aq) + 2 CuSO₄(aq) → Cu₂(C₉H₇O₄)₄(s) + 2 Na₂SO₄(aq)Reaction 1Using Reaction 1, a researcher synthesized a sample of copper(II) acetylsalicylate for analysis.  A small volume of 0.50 M aqueous copper(II) sulfate (molar mass = 159.6) was mixed into 100 mL of filtered, saturated aqueous sodium acetylsalicylate (molar mass = 202.2) at room temperature and slowly stirred.  Formation of copper(II) acetylsalicylate (molar mass = 843.7) was immediately observed as a bright blue precipitate.  The precipitate was then isolated by filtration and dried, affording the product complex in high yield. Adapted from: Seguel GV, Rivas BL, Contreras JG. Synthesis and characterization of copper complexes: interaction of metalliccarboxylates with 1,3-thiazole ligand. Journal of the Chilean Chemical Society. 1970; 58(4):1967. -If the solubility product constant K_sp for NaC₉H₇O₄ is estimated to be 34.9, what is the approximate acetylsalicylate ion concentration in the saturated NaC₉H₇O₄ solution used for the reaction?

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 results in Figure 2 would most likely NOT be affected by which of the following changes to the experiment?

Which of the following compounds can participate in hydrogen bonding with methanol (CH₃OH)?NH₃HFCH₄

Passage Water is unique in that all three of its phases exist over a small range of pressures and temperatures, as shown in Figure 1. Figure 1  Phase diagram for waterIn the liquid phase, water dissolves and transports molecules, and aids in the biochemical reactions necessary to support life.  For this reason, scientists believe liquid-phase water is essential to the existence of extraterrestrial life.  Although it has been known for some time that water is present in both the gas and ice phases on Mars, the question remains as to whether there are regions where liquid-phase water can stably exist.  Estimates of Mars' surface temperature and pressure generated from satellite readings and global climate models, as well as observations made by Mars rovers, reveal that surface conditions on the planet can vary greatly.Table 1  Temperature and Pressure of Various Locations on Mars Despite the extremely low pressures and temperatures on Mars, satellite images have repeatedly found evidence of flowing liquid water in several regions during the late Martian summer season.  These regions, called recurring slope lineae (RSL), are hypothesized to be thermodynamically stable under certain Martian conditions due to the presence of dissolved salts.  A simple understanding of this hypothesis is afforded by Raoult law, which states that the vapor pressure and freezing point of an ideal solution are lowered as the number of moles of dissolved solute increases.  Moreover, this effect also impacts the solution boiling point (the point at which the vapor pressure of the solution becomes equal to the ambient pressure).The briny-water hypothesis was confirmed in 2015 when spectral data from the Compact Reconnaissance Imaging Spectrometer onboard the Mars Reconnaissance Orbiter revealed the presence of hydrated perchlorates in the same regions and times that RSL were observed.  Studies are already underway to determine how common such briny water flows are and whether the conditions within them are suitable for microbial life. -Which of the following molecules is unable to form hydrogen bonds with water?

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. -Which of the following products is formed from ¹⁸F-FDG following a positron emission?  (Note: Assume any anions produced acquire hydrogen ions from the aqueous physiological environment.)

The bond enthalpies for six selected chemical bonds are shown below. For a reaction in which 2 moles of H₂(g) reacts with 1 mole of O₂(g) to form 2 moles of H₂O(g), what is the heat of the reaction?

Passage Amino acids and copper both serve vital roles in many biological processes of living organisms.  Beyond their separate biological roles, however, chemists have also discovered that amino acids are able to function as chelates with Cu²⁺ ions to form blue complexes with potential medicinal applications.  One such complex, copper(II) glycinate, has shown to be effective against some oral diseases.  The reaction to form copper(II) glycinate is shown in Figure 1. Figure 1  Synthesis of two isomers of copper(II) glycinateTwo different isomers can form depending on the reaction conditions.  Mixing hot aqueous glycine (70 °C) with a hot solution of copper(II) acetate dissolved in 50% aqueous ethanol, followed by cooling the mixture on ice, yields a precipitate of only the cis isomer.  If a suspension of the cis isomer is held at boiling for an hour or longer in a solution of glycine and copper(II) acetate, the trans isomer is obtained.Because of the geometric differences between the two isomers, the stretching vibrations of the central Cu-N and Cu-O bonds in each isomer are different.  Symmetric and asymmetric bond stretching give four possible vibrational modes for each isomer, as shown in Figure 2. Figure 2  Vibrational modes of the coordinate bonds in copper(II) glycinateIf the stretching of the bonds during vibration causes a transient, unequal distribution of electron density between the atoms within the bond, an oscillating dipole will form.  Bonds with dipoles that are not cancelled out by another dipole in the molecule will absorb infrared light at particular frequencies (often expressed in wave numbers) and are designated as IR-active.  As a result, the vibrational differences between the two isomers can be analyzed by far-infrared (far-IR) spectroscopy, as shown in Figure 3. Figure 3  Far-IR spectra of the cis and trans isomers of copper(II) glycinateBoth isomers show a skeletal absorbance (unrelated to coordinate bond vibration) near 385 cm⁻¹.  Additional absorbance bands are seen that specifically relate to the vibrational modes of the bonds in each isomer. -What are the formal charges of the oxygen atoms in the acetate ion of the Cu(C₂H₃O₂)₂ used during the reaction if the effects from resonance are excluded?

If a channel of a particular cell preferentially transmits smaller ionic species across the membrane, given the following biologically relevant, isoelectronic ions, the channel is LEAST likely to transmit:

Passage Bone growth and remodeling during fracture healing are often slow and have potentially poor outcomes, including malunion or nonunion at the fracture site.  Magnesium bone implants can aid in bone formation and safely biodegrade in the body according to the following reaction:Mg(s) + 2H₂O(l) → Mg(OH)₂(s) + H₂(g)Reaction 1  Corrosion of magnesium metal in aqueous mediumHowever, magnesium metal tends to degrade too quickly and forms hydrogen gas pockets in the body.  Corrosion is slowed as the metal naturally forms a protective layer of magnesium hydroxide in a process called passivation.  Magnesium hydroxide is less soluble than magnesium metal, and its dissolution is described by the endothermic reaction: Reaction 2  Solubility equilibrium of Mg(OH)₂ in water at 25°CAlthough passivation slows down the corrosion rate, it must be slowed even further to adequately facilitate bone reformation.Researchers are interested in the differences in the corrosion rates of magnesium metal protected by different biocompatible coatings.  They compared 10-mm magnesium disks coated with compounds of various cation and anion combinations, with solubility results shown in Table 1.  The coated disks were immersed in simulated body fluid (SBF) at room temperature for 50 hours.Table 1  Solubility for Biocompatible Inorganic Salts Given in mmol/100mL of Pure Water at 25°C Adapted from Huan ZG, Leeflang MA, Zhou J, Fratila-apachitei LE, Duszczyk J. In vitro degradation behavior and cytocompatibility of Mg-Zn-Zr alloys. J Mater Sci Mater Med. 2010;21(9):2623-35. -Magnesium hydroxide begins to precipitate onto the magnesium metal surface when which of the following expressions is true?

Passage The alkaline-earth metals are a highly reactive family of metals occupying Group 2 (IIA) of the periodic table.  In their elemental form, the alkaline-earth metals have a shiny, silvery-white to slightly yellowish appearance.  The alkaline-earth metals exhibit some similarities to the chemistry of the alkali metals (Group 1) but are a little less reactive.Atoms of the alkaline-earth metals (M) have only two valence electrons and a low first and second ionization energy.  Consequently, these metals readily lose both electrons to produce cations with a +2 charge (M⁺²) during reactions with nonmetals.  As a result, the alkaline-earth metals do not exist in their elemental metallic form in nature.  Instead, the Group 2 elements are found primarily as metal oxides, which form by reacting oxygen (Reaction 1). Similarly, elemental samples of alkaline-earth metals will react with elemental halogens (X₂) to form metal halides (Reaction 2), all of which are ionic except for BeCl₂. Except for beryllium, the alkaline-earth metals react with water to produce very basic metal hydroxide solutions and hydrogen gas (Reaction 3). The most difficult alkaline-earth metal to study is radium, because it is a rare radioactive element that is produced from the nuclear decay of uranium.  The most common natural isotope, radium-226, has a half-life of 1,600 years and decays by alpha emission into radon-222 (Reaction 4). -Which of the following diagrams correctly labels the activation energy E_a for the reaction shown in Figure 1?

Which statements correctly describe electrolysis?  During electrolysis:an electrochemical decomposition reaction occurs.electric current is supplied as the energy to perform an endergonic reaction.the electrochemical cell operates with a negative ΔG.electric current flows in a nonspontaneous direction within an electrochemical cell.

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 -When sugar is burned completely, the following unbalanced combustion reaction takes place:C₁₂H₂₂O₁₁ + O₂(g) → CO₂(g) + H₂O(g) + heatIf the reaction is balanced, it has a ratio of:

Passage The primary structure of a protein is a series of amino acids that are connected in a long chain via C−N peptide bonds (Figure 1). Figure 1  Peptide bonds within a protein segment (Note:  Rⁿ = amino acid side chains)Protein chain folding is guided by multiple intramolecular interactions that can occur between two peptide bonds, between a peptide bond and a side chain, or between two side chains, as shown in the protein in Figure 2.  Although these interactions are intramolecular because they are within the same molecule, they act effectively like intermolecular interactions because proteins are such large molecules. Figure 2  Intramolecular interactions between the peptide chain and amino acid side chains within a hypothetical proteinBecause intramolecular interactions play a significant role in maintaining the three-dimensional folded structure of a protein, having a greater number of interactions or stronger interactions will result in increased protein stability.  The energy required to break these interactions within a protein is related to free energy, ΔG.  The greater the ΔG value, the more stable the protein.To find the overall contribution of particular intramolecular interactions to protein stability, researchers made several protein variants in which one amino acid is replaced by another to change the noncovalent interactions between the side chains.  The difference in free energy between the variant and the original protein, Δ(ΔG), indicates the change in overall stability of the protein.  A negative Δ(ΔG) means the variant protein is less stable than the original.Table 1 shows the results of the amino acid substitution experiments for a protein with the approximate energy difference when each amino acid is changed.Table 1  Change in Free Energy Based on Changing Intramolecular Interactions Caused by Amino Acid Substitutions Adapted from: C. N. Pace, J. M. Scholtz, G. R. Grimsley, "Forces stabilizing proteins." FEBS Lett. ©2014 Wiley. -How will the extent of van der Waals interactions be affected in the hydrophobic region IV in Figure 2 if a protein variant is made that replaces valine with alanine in this region?