Name GSI KEY Chemistry 1A Fall 2008 Midterm Exam 1 September 16 2008 90 min closed book Name SID GSI Name The test consists of 4 short answer questions and 15 multiple choice questions Put your written answers in the boxes provided Answers outside the boxes may not be considered in grading Write your name on every page of the exam Question Points Question A 2 12 Question B 3 10 Question C 4 12 Question D 5 6 T K T C 273 15 Multiple Choice 6 30 1 atm 760 mm Hg 760 torr 1 bar Total Score Useful Equations and Constants Page PV nRT n2 P a 2 V nb nRT V KE 1 2 mv 2 Ptotal PA PB XAPA XBPB N0 6 02214 x 1023 mol 1 R 0 0821 L atm K 1 mol 1 70 Page 1 of 7 Name GSI A Isomers There are three containers in the lab that are all labeled contains pure C3H4 Using your new found knowledge of structures you determine that the containers could contain these three molecules methylacetylene 1 2 propadiene and cyclopropene Based on the structures below and the data presented you can determine which substance is in each container 1 2 propadiene methylacetylene cyclopropene H H H C H H C C C C H C H H H C C C H H H The substances in Containers 1 and 3 are soluble in water but the substance in Container 2 is not What is the material in Container 2 circle one methylacetylene 1 2 propadiene cyclopropene Explain your choice in terms of molecular structure and macroscopic properties Solubility increases with the strength of intermolecular forces In general like will dissolve like 1 2 propadiene is non polar so would not dissolve in water because water is polar Methylacetylene and cyclopropene are slightly polar due to their geometry so they dissolve in water To determine the relative boiling points of the substances you gently heat the substance in Container 1 Instead of turning into vapor it reacts to form a stringy mess Which substance would you think would be the most unstable circle one methylacetylene 1 2 propadiene cyclopropene Explain your choice in terms of molecular structure and macroscopic properties The least stable molecule of the ones pictured is cyclopropene The carbon atoms that have 3 groups attached would ideally like to have bond angles of 120 but instead is closer to 60 This would make the molecule tend to be reactive opening the ring up to relieve the strain When the other two substances are boiled it was determined that the substance in Container 3 boiled at a higher temperature than the substance in Container 2 Which substance would you think would be in Container 3 methylacetylene 1 2 propadiene cyclopropene Explain your choice in terms of molecular structure and macroscopic properties Boiling point increases with the strength of intermolecular forces Methylacetylene is slightly polar due to its asymmetric geometry Methyacetylene would be able to have dipole dipole interactions with other methylacetylene molecules This would make the int forces slightly stronger than that of 1 2 propadiene molecules Page 2 of 7 Name GSI B Models of bonding Consider the properties of sulfur oxides name formula measured bond length sulfur monoxide SO 148 pm sulfur dioxide SO2 143 pm Draw the best structure for SO S O Draw two structures for SO2 one that obeys the octet rule and one that minimizes formal charge S O S O O S O O O Given your structures and the experimental data which model is more valid circle one minimize formal charge satisfy the octet rule Explain your reasoning The bond length data indicates that SO and SO2 are not that different structurally SO definitely has a double bond like O2 so SO2 probably has two double bonds as well This indicates that the structure on the right shown above is more stable in which the formal charge is minimized The structure on the left has a net 1 5 bonds for the S O linkage You would expect that bond length to be longer than a S O Other answers that are chemically logical are accepted Page 3 of 7 Name GSI C Gas Models A gas is trapped in a cylinder with a piston as shown in the drawing The piston is free to move up and down The gas pressure is 1 0 atm the volume is 500 mL and the temperature is 300 K a Draw the cylinder after the gas is heated to 600 K In your drawing include the new position of the arrow on the pressure gauge the position of the piston the reading on the thermometer arrows indicating speed on the molecules bigger arrows mean moving faster smaller arrows mean moving slower b At 600K the pressure inside the cylinder is less than 1 0 atm greater than 1 0 atm equal to 1 0 atm c Use the motions and collisions of the gas molecules to explain the value of the pressure inside the cylinder after the sample stabilizes at 600K Pressure depends on the frequency and the strength of the collisions The piston can freely move to maintain 1 atm at the higher temperature The pressure remains the same because the effect of the molecules traveling faster is canceled out by the dilution The collisions deliver more momentum but less frequently because there are less molecules per unit volume d What is the volume of the gas at 600 K Show your work 500mL V2 V1 V2 V2 1000 mL T1 T2 300 K 600 K Page 4 of 7 Name GSI D Bonding in solids Sugar crystals are hard and not flexible but shatter easily Kevlar is hard and flexible but it does not shatter Sucrose C12H22O11 Kevlar Why are sugar and Kevlar both hard solids The strength of intermolecular interactions will influence the hardness of solids Both sugar and Kevlar are polar and capable of hydrogen bonding with other molecules The hydrogen bonding increases the hardness of the material Why is sugar brittle and Kevlar flexible A crystal of sucrose molecules would be held together by hydrogen bonding When pressure is applied the crystal can break between the molecules The molecules are H bonded all along the chain In Kevlar the long chains can bend but breaking is harder because poking through the material would mean breaking a chemical bond instead of intermolecular attraction Page 5 of 7 Name GSI Multiple Choice Circle the best answer and bubble your answers on in your scantron form 1 Suppose you have a mixture of magnesium chloride MgCl2 s and silicon dioxide SiO2 s How can you separate one from the other a Add water The MgCl2 will dissolve b Add water The SiO2 will dissolve c Heat The MgCl2 will evaporate d Heat The SiO2 will evaporate e None of the above 2 Number of different molecules with the formula C2H6O a 0 b 1 c 2 d 3 e 4 3 Highest effective nuclear charge a Li b C c F d Si e Cl 4 Most polar bond a H C b …
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