CHEMISTRY 1A Fall 2010 EXAM 3 Key CHAPTERS 7 (part), 8, 9 & 10 (part) You might find the following useful.

Transcription

1 CHEMISTRY 1A Fall 2010 EXAM 3 Key CHAPTERS 7 (part), 8, 9 & 10 (part) You might find the following useful. 1

2 For each of the following, write the word, words, or number in each blank that best completes each sentence. (2 points each) 1. The third ionization energy is the energy necessary to remove one mole of electrons from one mole of isolated and gaseous 2 ions to form one mole of isolated and gaseous 3 ions. 2. Isomers are compounds that have the same molecular formula but different molecular structures. 3. A(n) trans isomer is a structure that has like groups on different carbons (which are linked by a double bond) and on different sides of the double bond. 4. A(n) saturated fat is a triglyceride with single bonds between all of the carbon atoms. 5. Hydrogenation is a process by which hydrogen is added to an unsaturated triglyceride to convert double bonds to single bonds. This can be done by combining the unsaturated triglyceride with hydrogen gas and a platinum catalyst. 6. A nonpolar molecule (or a portion of a molecule or polyatomic ion) that is not expected to mix with water is called hydrophobic. 7. Partial pressure is the portion of the total pressure that one gas in a mixture of gases contributes. Assuming ideal gas character, the partial pressure of any gas in a mixture is the pressure that the gas would yield if it were alone in the container. 8. Of the two ionic bonds, Al -Br (in AlBr 3 ) or Na-F (in NaF), the Al-Br bond has the most covalent character. 9. Draw a reasonable Lewis structure for each of the following, use it to predict whether the molecule is polar or nonpolar, and explain why. (4 points each) a. CH 3 COF b. IF 3 Asymmetrical distribution of polar bonds - polar Asymmetrical distribution of polar bonds - polar 2

3 10. Identify each of the following triglycerides as saturated or unsaturated. Which is more likely to be a solid at room temperature, and which is more likely to be a liquid? (4 points) Saturated Solid Unsaturated Liquid 11. Methanol, CH 3 OH, is used as an antifreeze, solvent, fuel, and to denature ethanol. Menthol, which acts as a local anesthetic is commonly used to relieve minor sour sthoat pain. Which compound would you expect to be more soluble in water? Explain your answer.(4 points) methanol menthol Methanol has a larger percentage of its structure that is polar and is therefore more soluble in water than menthol. In fact, butanoic methanol is miscible with water (will mix with it in any proportion). The much higher proportion of the menthol structure that is nonpolar makes it only slightly soluble in water. 12. Write abbreviated electron configurations mercury, Hg, and the mercury(ii) ion, Hg 2+, and explain why the mercury loses the electrons that it does when it forms mercury(ii). (6 points) Hg [Xe] 4f 14 5d 10 6s 2 Hg 2+ [Xe] 4f 14 5d 10 The two electrons in the 6s orbital are in the largest orbital. Therefore, they have their electron clouds farthest from the nucleus, so they are attracted the least to the nucleus. Therefore, it takes less energy to remove them. 3

4 13. There are disagreements among chemists as to how Lewis structures should be drawn. Some feel that we should try to minimized formal charges, others feel we should emphasize the octet rule, and others feel that for most of the uses for Lewis structures, it doesn t make any difference. As an example, consider the prediction of the molecular geometry and molecular polarity of sulfur trioxide, SO 3. (8 points) a. Draw a reasonable Lewis structure for SO 3 that minimizes formal charges. This structure does not have resonance. Draw the geometric sketch, including representative bond angles, predicted from this structure. Predict whether, based on this structure, the molecule is polar or nonpolar. nonpolar (symmetrical distribution of polar bonds) b. Draw a reasonable Lewis structure for SO 3 that has eight total electrons around each of the atoms. Identify all formal charges. This structure has resonance, so draw all the resonance structures and the resonance hybrid. Draw the geometric sketch, including representative bond angles, predicted from this structure. Predict whether, based on this structure, the molecule is polar or nonpolar. nonpolar (symmetrical distribution of polar bonds) 4

5 14. For each of the following, write the name of the type of attraction holding these particles in the solid and liquid form. Indicate the formula in each pair that represents the substance that you would expect to have the stronger attractions among particles and which would have the higher melting point and boiling point. (3 points each) a. Methanol, CH 3 OH type of attraction hydrogen bond and London forces or silicon dioxide, SiO 2 type of attraction covalent bond stronger attractions? SiO 2 higher melting and boiling points? SiO 2 b. Propanone (acetone), CH 3 COCH 3 type of attraction dipole-dipole and London forces or benzoic acid, C 6 H 5 CO 2 H type of attraction hydrogen bond and London forces stronger attractions? C 6 H 5 CO 2 H higher melting point and boiling point? C 6 H 5 CO 2 H c. propanone (acetone), CH 3 COCH 3 type of attraction Dipole-dipole and London forces or methyl ethyl ketone, CH 3 COCH 2 CH 3 type of attraction Dipole-dipole and London forces stronger attraction? CH 3 COCH 2 CH 3 higher melting and boiling points? CH 3 COCH 2 CH 3 5

6 15. With reference to the valence bond model, explain how bromine atoms are able to form five bonds and one lone pair in BrF 5. also explain why these six electron groups are arranged in an octahedral electron group geometry with bond angles of 90 and 180. (8 points) Only the highest energy electrons participate in bonding. Covalent bonds form to pair unpaired electrons. Therefore, two electrons must be promoted from paired electrosn in 4p orbitals to empty 4d orbitals. To explain the geometry and bond angles, we assume the the 4s, three 4p, and two 4d orbitals blend to form six sp 3 d 2 hybrid orbitals, which are arranged in an octahedral geometry with angles of 90 and 180. Because the bonds form from the overlap of sp 3 d 2 hygrid orbitals of the bromine with sp 3 hybrid orbitals of the fluorine, the geometry of the molecule reflects the geometry of the sp 3 d 2 hybrid orbitals. For each of the following numerical problems, be sure to show your work and put a box around your answer. (6 points each) 16. The bulbs used for fluorescent lights have a mercury gas pressure of 1.07 Pa at 40 C. How many milligrams of liquid mercury must evaporate at 40 C to yield this pressure in a L fluorescent bulb? g 1.07 Pa 1.39 L 3 g PVM 1 mol 1 kpa 10 mg PV = RT g = = 3 M RT L kpa = mg 10 Pa 1 g 313 K K mol 17. Air bags in cars inflate when sodium azide, NaN 3, decomposes to generate nitrogen gas and sodium metal. How many grams of NaN 3 must react to generate 112 L N 2 at 121 kpa and 305 K? 2NaN 3 (s) 2Na(s) + 3N 2 (g)? g NaN = 112 L N K mol 121 kpa 2 mol NaN g NaN L kpa 305 K 3 mol N2 1 mol NaN3 = 232 g NaN 3 6

7 18. Consider the following Lewis Structure for the resonance hybrid for nitric acid, HNO 3. (8 points) a. What is the hybridization for the nitrogen atom? sp 2 b. What is the hybridization for the top and right oxygen atoms? sp 2 c. What is the hybridization for the left oxygen atom? sp 3 d. Write a description of the bonding, stating whether each bond is sigma, pi, or part of a delocalized pi system and by stating which atomic orbitals overlap to form the bonds. 1 sigma O-H bond due to sp 3-1s overlap 1 sigma O-N bond due to sp 3 -sp 2 overlap 2 sigma N-O bonds due to sp 2 -sp 2 overlap 1 delocalized pi system due to 3 p orbitals overlapping, one on the nitrogen atom, one on the top and right oxygen atoms e. What is the name of the electron group geometry around the left O? tetrahedral f. What is the name of the molecular geometry around the left O? bent g. What is the name of the electron group geometry around the N? trigonal planar h. Draw a sketch with bond angles. Answer the following short answer questions. (6 points each) 19. Consider the following Lewis Structure for C 2 H 4. Explain why one of the bonds in the double bond is weaker than the other. The p orbitals, which form the pi bond by parallel overlap, overlap less than the sp 2 atomic orbitals, which form the sigma bond by end-on overlap. Less overlap leads to less of an increase in negative charge between the nuclei, less of an increase in +/- attraction, and therefore, less stabilization of the molecule. 7

8 20. Explain why chlorine, Cl, atoms are smaller than magnesium, Mg, atoms. Cl 1s 2 2s 2 2p 6 3s 2 3p 5 Mg 1s 2 2s 2 2p 6 3s 2 They both have their highest energy electrons in the n=3 principal energy level, but the 3p electrons of chlorine atoms feel a +7 effective charge and the 3s electrons of magnesium atoms feel a +2 effective charge. All other factors being equal, higher effective charge will lead to greater attraction for the electrons, causing the electron cloud to contract. This makes chlorine atoms smaller than magnesium atoms. 21. Describe how ozone is produced in the air we breathe, and explain why the highest concentrations of ozone in the air we breathe are found in large industrial cities with lots of cars and lots of sun. The explanation why the highest concentrations of O 3 in the air we breathe are found in large industrial cities with lots of cars and lots of sun begins with a description of the source of nitrogen oxides. Any time air (which contains nitrogen and oxygen) is heated to high temperature (as occurs in the cylinders of our cars and in many industrial processes), nitrogen oxides are formed (NO and NO 2 ). N 2 (g) O 2 (g) 2NO(g) 2NO(g) O 2 (g) 2NO 2 (g) Nitrogen dioxide, NO 2, is a red-brown gas that contributes to the brown haze associated with smog. The radiant energy that passes through the air on sunny days can supply the energy necessary to break covalent bonds between nitrogen atoms and oxygen atoms in NO 2 molecules, converting NO 2 molecules into NO molecules and oxygen atoms. Remember that the shorter the wavelength of light is, the higher the energy. Radiant energy of wavelengths less than 400 nm has enough energy to break N O bonds in NO 2 molecules, but radiant energy with wavelengths longer than 400 nm does not supply enough energy to separate the atoms. The oxygen atoms react with oxygen molecules to form ozone molecules. O(g) O 2 (g) O 3 (g) 8