Chemistry II Unit 5b Practice Test

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1 Practice for Unit 5b Exam Unit5Practicetest2013.odt Chemistry II Unit 5b Practice Test Reading: This material is covered in chapter 5 and chapter 12 in your book. Your notes and your molecular drawings are also an important resource to help you prepare for the exam 1. Describe how the following bonds are formed. Describe the unique physical properties that each of these types of bonds give each of these substances and explain why these bonds lead to those properties. ionic bonds covalent bonds metallic bonds 2. Explain why solutions of ionic compounds will conduct electricity, but ionic solids will not conduct electricity. 3. Explain why metals are good conductors of electricity. Include a description of metallic bonding in your discussion. 4. Explain why molecular substances will not conduct electricity either as solids or in solutions. 5. Write the names and draw Lewis diagrams for the following molecules. What do all of these molecules have in common? (Describe 3 things.) For the following molecules: H 2 O 2 N 2 F 2 Cl 2 a. Draw the 3D Lewis diagrams. Identify the shape, geometry and bond angles for each molecule. b. Identify each bond as single, double or triple. c. Identify any polar bonds. Label the appropriate charges on your Lewis Diagram. d. Identify the molecule as polar (dipole) or non polar. 6. H 2 O 7. C 2 H 2 8. C 2 H 4 9. HC 2 H 3 O CH 3 OH 11. HCN 12. PH CS Explain why diamonds and graphite have much higher melting points than any molecular compounds. What is the special name that we give these solids? 15. Use words and create models to describe how the following intermolecular forces work. Rank them in strength from weakest to strongest. hydrogen bonding dispersion force dipole interaction 16. Draw 3 dimensional structures and use them along with the electronegativity to determine whether they are dipoles, non polar, ionic or metallic substances. Identify the forces that would be the most important among particles of each of these compounds. H 2 S NaCl CCl 4 CH 2 O N 2 CaCl 2 CO 2 NH 3 HCl HCN Kr Cl 2 O C 2 H 6

2 Practice for Unit 5b Exam Unit5Practicetest2013.odt 17. Predict whether each of the substances in #16 would conduct electricity when dissolved in water. Assume that all of them will dissolve in water. Explain why the conductors conduct. 18. Rank the following compounds by least to greatest melting point. Explain how you determined this ranking: NaCl graphite water ammonia (NH 3 ) fluorine (F 2 ) nitrogen (N 2 ) carbon dioxide (CO 2 ) CH 3 Cl 19. Explain the relationship between the formation and breaking of bonds and when energy is taken in or given off. 20. Methanol, CH 3 OH, is used to make formaldehyde, CH 2 O, which is used in embalming fluids. The molecules of these substances have close to the same atoms and about the same molecular mass, so why is methanol a liquid at room temperature and formaldehyde a gas? 21. Carbon disulfide, CS 2, which is used to make rayon, is composed of nonpolar molecules that are similar to carbon dioxide molecules, CO 2. Unlike carbon dioxide, carbon disulfide is liquid at room temperature. Why? 22. Explain the difference between an atom, a molecule, a network solid and an ion. 23. Equal amounts of water, gasoline (C 8 H 18 ) diesel fuel (C 12 H 23 ) are spilled on the garage floor. A day later, only the diesel fuel remains on the floor, the other liquids have evaporated. a. Identify the most important intermolecular force in each of the liquids. b. Use the observations above to determine which of the 3 liquids has the strongest intermolecular forces. Explain what would cause the force to be stronger than in the other molecules. 24. Milk bottles are often made of polyethylene plastic, which is composed of extremely large, nonpolar molecules. What type of attraction holds these molecules together in the solid form? In general, would you expect this type of attraction to be stronger or weaker than the hydrogen bonds that hold water molecules together? Because polyethylene is a solid at room temperature and water is a liquid, the attractions between polyethylene molecules must be stronger than those between water molecules. What makes the attractions between polyethylene molecules so strong? 25. In the liquid nitrogen demonstration, we observed the behavior of 2 liquids that have very weak intermolecular forces. What observations verify that these intermolecular forces are much weaker than the intermolecular forces in water?

3 Practice for Unit 5b Exam Unit5Practicetest2013.odt Solutions to Unit 5 Practice Test 1. Ionic Bonds are formed when one atom gives electrons away (forming a positive ion) to another atom (which forms a negative ion). The 2 ions are held to each other by the attractive force between positive and negative charges. Covalent Bonds are formed when 2 atoms share pairs of electrons. The 2 atoms are held to each other because both atoms are attracted to the other atom's electrons in order to have completely filled energy levels. Metallic Bonds -- are formed because metal atoms will always tend to give away their valence electrons, leaving positively charged metal ions surrounded by free electrons. The positively charged metal ions are held together by their attraction to the free electrons. 2. Solutions of ionic compound will conduct electricity because when ionic compounds are dissolved in water, the individual ions separate and are able to move freely in the solution and carry charge. In a solid ionic compound, the ions are not free to move and thus, cannot carry the charge. 3. Metals are good conductors of electricity, because metallic bonds consist of positive metal ions surrounded by free moving electrons. These free moving electrons can carry electrical charge which means that the substance can conduct electricity. 4. Molecular substances are formed when elements bond to each other by sharing electrons in covalent bonds. Since no electrons are transferred in a covalent bond, none of the atoms in a molecular compound have a charge. If there are no charges present in molecular compounds they cannot conduct electricity since conducting electricity requires charges to move. 5. hydrogen oxygen nitrogen fluorine chlorine All of these molecules have linear shapes and are gases at room temperature

4 Practice for Unit 5b Exam Unit5Practicetest2013.odt C O

5 Practice for Unit 5b Exam Unit5Practicetest2013.odt Diamonds and graphite are both composed of carbon atoms that are covalently bonded to each other. These type of solids are called network solids The covalent bonds hold the particles of the substance together and are much stronger forces than the Van Der Waals forces that hold the particles together in molecular compounds. Because these covalent bonds are so much stronger than any dispersion force, dipole interaction or hydrogen bond, the melting points or the amount of energy to overcome these forces is much higher than any molecular compounds. 15. In order from weakest to strongest: Dispersion Forces - are formed when the electrons surrounding molecules move to one end of the molecule. This creates unbalanced charges, positive charge in the region that is deficient in electrons and negative charge in the region that the electrons have moved to. This creates a temporary dipole with a positive and negative end to the molecule which is attracted to the oppositely charged end of another molecule. This attraction is what is known as dispersion force. Dispersion forces are the weakest of intermolecular forces, but become stronger in molecules that have a larger number of electrons. Dipole Interactions - result from the attraction between the positive and negatively charged ends of polar molecules. These forces are stronger than dispersion forces because they are between permanent dipoles as opposed to the temporary dipoles that have dispersion forces. Hydrogen Bonding - occurs when a very polar molecule contains a hydrogen atom attached directly to a very electronegative atom, (O, N, F, Cl) which has an unshared electron pair. The hydrogen atom becomes a bare proton when the electronegative atom takes away its electrons and this bare proton forms a bond with the unshared electron pair in the electronegative atom on another molecule. 16.

6 Practice for Unit 5b Exam Unit5Practicetest2013.odt 17. Conductors: NaCl - It is an ionic compound. Ionic compounds separate into + and - ions when they are dissolved in water. CaCl 2 - Same reason as NaCl NH 3 - When dissolved in water it forms NH 4 +1 ions and OH -1 ions. HCl - It is an acid. When dissolved in water, it forms H 3O +1 and Cl -1 ions. Non Conductors: All of the rest of the compounds are non conductors because they are molecular compounds that do not form ions when dissolved in water. Charged particles must be present for the conduction of electrical current. 18. nitrogen < flourine < carbon dioxide < CH 3Cl < ammonia < water < NaCl < graphite The melting point of substances increases with the increase in the strength of the forces holding the particles of a substance together. Nitrogen, fluorine and carbon dioxide are all non polar. This is why they are listed as having the lowest melting points. The only intermolecular forces in non polar molecules are dispersion forces, which increase in strength as the number of electrons increase. Nitrogen has the least number of electrons followed by fluorine, with carbon dioxide with the greatest number of electrons among the non polar molecules. CH 3Cl has the next lowest melting point. It is a dipole, but does not hydrogen bond, so it will have a lower melting point than ammonia or water. Ammonia and water both hydrogen bond. Ammonia's bonds are less polar than water's bonds because of the lower electronegativity of nitrogen compared with oxygen, which means that ammonia will have a lower melting point than water. NaCl is an ionic compound. Ionic bonds are stronger than any of the Van der Waals forces that are holding the particles together for the other substances but the ionic bonds are weaker than the covalent bonds that hold the carbon atoms together in graphite, therefore NaCl has a higher melting point than all of the substances except for graphite. 19. A bond is simply a force that holds two atoms together. In order to break a bond energy must be put into the bond to overcome the force holding the two atoms together. Since breaking a bond takes in energy, forming a bond, which is the opposite action, would release energy. 20. Because of the O H bond in methanol, the attractions between CH 3OH molecules are hydrogen bonds. The hydrogen atoms in CH 2O molecules are bonded to the carbon atom, not the oxygen atom, so there is no hydrogen bonding for formaldehyde. The C O bond in each formaldehyde molecule is polar, and when there is only one polar bond in a molecule, the molecule is polar. Therefore, CH 2O molecules are held together by dipole-dipole attractions. For molecules of about the same size, hydrogen bonds are stronger than dipole-dipole attractions. The stronger hydrogen bonds between CH 3OH molecules raise its boiling point above room temperature, making it a liquid. 21. Both of these substances have nonpolar molecules held together by London forces. Because the CS 2 molecules are larger than CO 2 molecules, they have stronger London forces that raise carbon disulfide s boiling point to above room temperature.

7 Practice for Unit 5b Exam Unit5Practicetest2013.odt 22. An atom is the smallest particle of an element that still has all of the properties of that element. A molecule is a group of atoms bonded together which is the smallest particle of a chemical compound that still has all of the properties of that compound. An ion is an atom or group of atoms that does not have a neutral charge. A network solid is a substance that is not made of ions or molecules, but is made of atoms that are all held together in one single solid piece of matter by covalent bonds. Examples of network solids include diamonds and graphite, which are both made of bonded together carbon atoms. 23. a. Both C 8H 18 and C 12H 23 have non polar covalent bonds, which means that they will not form dipoles. The only forces present in these liquids would be dispersion forces. Water has hydrogen bonding because it is a polar molecule and has hydrogen bonded to oxygen, which is the second most electronegative element. b. Since all 3 liquids are at the same temperature and have approximately the same surface area, the difference in rate of evaporation must be due to the stronger intermolecular forces in diesel fuel. Even though water has hydrogen bonding, the greater number of electrons in diesel molecules gives them a stronger dispersion force. 24. Nonpolar molecules, such as polyethylene, are attracted to each other by London forces,which we expect to be generally weaker than hydrogen bonds. Because the strength of London forces increases with increased size of molecules, and because polyethylene molecules are extremely large, in this case, the London forces for polyethylene are actually stronger than the hydrogen bonds between water molecules. 25. Both liquid nitrogen and liquid natural gas are non polar liquids. When they are poured on the floor, both liquids appear to rapidly spread out rather than clump together like water does as it did in the demonstration where a full cup of water was able to hold 50+ additional pennies..