Chapter 12 Structure and Shape

Transcription

1 Free Study Guide for Cracolice Peters Introductory Chemistry: An Active Learning Approach Second Edition Chapter 12 Structure and Shape Chapter 12Assignment A: Lewis Diagrams Chapter 11 includes a description of a covalent bond between two atoms. In this chapter you will learn how two or more of these bonds are arranged around atoms in molecules and ions. A Lewis diagram is a representation of a molecule or ion that provides as many atoms as possible with eight valence electrons. Lewis diagrams give useful depictions of molecules or ions and also provide an accurate accounting for valence electrons. Your sole objective in this assignment is to learn to draw Lewis diagrams of polyatomic molecules or ions. Study Strategy Section Focus on Goal 1 as you study. Pay particular attention to the Drawing a Lewis Diagram summary, and then practice, practice, practice. Lewis Diagram Recognition Exercises 110 and Questions, Exercises, and Problems 19. Check your answers with those at the end of the chapter. If your instructor recommends the Active Learning, do Questions, Exercises, and Problems 19. Chapter 12Assignment B: Molecular Geometry and Polarity Lewis diagrams give us some idea about the structure of molecules, but they can be misleading. A Lewis diagram is simply an accounting device to keep track of the valence electrons. It often represents a three-dimensional molecule on a two dimensional piece of paper. The physical and chemical properties of compounds are largely the result of the twoand three-dimensional shapes (geometries) and polarities of their molecules. Look for these big ideas: 1) A bond angle is the angle between any two bonds formed by the same atom. 73

2 Study Guide for Introductory Chemistry: An Active Learning Approach 2) Electron-pair geometry describe the arrangement of two, three, or four pairs of electrons, either shared or unshared, around a central atom. 3) Molecular geometry describes the arrangement of two, three, or four atoms around a central atom to which they are all bonded. 4) Electron-pair and molecular geometries are said to be linear, angular (bent), bent (angular), trigonal planar, tetrahedral, or trigonal pyramidal. 5) Electron-pair, and therefore molecular, geometry may be predicted by the valence shell electron-pair repulsion (VSEPR) theory. 6) The contribution to the molecular geometry of a multiple bond is the same as if it were a single bond. 7) The polarity of a molecule may be predicted from its shape and the polarity of its bonds. Some molecules with polar bonds are nonpolar because of their molecular geometries. Study Strategy Sections Focus on Goals 25 as you study. The key is learning the names of the shapes that correspond to each molecular and electron-pair geometry and being able to form a mental image of those shapes. Questions, Exercises, and Problems Check your answers with those at the end of the chapter. If your instructor recommends the Active Learning, do Questions, Exercises, and Problems Chapter 12Assignment C: Some rganic Compounds (ptional) About 95% of all chemical compounds that have been characterized are classified as organic compounds compounds based upon the carbon atom. This section gives a quick overview of some organic compounds made from carbon, hydrogen, and oxygen. The main ideas in this section are: 1) Because carbon atoms bond to each other forming long chains, there is immense variety in the number and shapes of carbon compounds. 2) ydrocarbons are made of carbon and hydrogen. The alkanes are a hydrocarbon family with all single bonds. 3) Alkenes are hydrocarbons with carbon-carbon double bonds; alkynes are hydrocarbons with carbon-carbon triple bonds. 74

3 Chapter 12 Structure and Shape 4) Compounds with the same molecular formula but different molecular structures are called isomers. 5) If a hydrogen atom in a hydrocarbon, C 4, for example, is replaced by a hydroxyl group,, the resulting molecule is an alcohol, C 3. An alcohol may be thought of as a water molecule () in which one hydrogen atom is replaced by a hydrocarbon group (C 3 ). 6) An ether may be thought of as a water molecule () in which both hydrogen atoms are replaced by hydrocarbon groups (C 3 C 3 ). Alcohols and ethers with the same number of carbon atoms are isomers. 7) If a hydrogen atom in a hydrocarbon is replaced by a carboxyl group, C, the resulting molecule is a carboxylic acid. The most common carboxylic acid is acetic acid, written as C or C 3 C. Study Strategy Section Focus on Goals 68 as you study. Be able to define and recognize examples of each of the following: organic compound, hydrocarbon, alcohol, ether, carboxylic acid. Watch for the hidden performance goal, Define the term and recognize isomers. Questions, Exercises, and Problems Check your answers with those at the end of the chapter. If your instructor recommends the Active Learning, do Questions, Exercises, and Problems Chapter 12Assignment D: Summary and Review You can frequently draw more than one Lewis diagram that satisfies the octet rule for a given substance. Sometimes these additional diagrams are correct for other real substances, and sometimes they are not. You cannot tell if a compound exists just because it has an acceptable Lewis diagram; you also need laboratory evidence. owever, there are some generalizations that guide you to diagrams that are most apt to be correct. They are summarized in the textbook. Most importantly, remember that (a) carbon atoms tend to bond to one another, (b) molecules with a nonmetal (X), oxygen, and hydrogen usually have the arrangement X, and (c) hydrogen is always a terminal atom. When you must predict geometries, keep in mind that there are two types of geometry, electron-pair geometry and molecular geometry. Electron-pair geometry describes the orientation of the electron pairs around a central atom; molecular geometry describes the orientation of atoms bonded to a central atom. The two geometries are the same only if there are no lone pair electrons around the central atom. 75

4 Study Guide for Introductory Chemistry: An Active Learning Approach You can remember the single-bonded oxygen organic compounds as water molecules with a hydrocarbon group replacing hydrogen: water alcohol R ether R R where R is a hydrocarbon group that completes the Lewis structure. Also memorize the carboxyl group, C, that characterizes carboxylic acids. Review your lecture and textbook notes. the Chapter in Review and the Key Terms and Concepts, and read the Study ints and Pitfalls to Avoid. Take Questions, Exercises, and Problems Include Questions 3538 if assigned by your instructor. Check your answers with those at the end of the chapter. If your instructor recommends the Active Learning, do Questions, Exercises, and Problems Include Questions 2529 if assigned by your instructor. the chapter summary test that follows. Check your answers with those at the end of this assignment. Chapter 12 Sample Test Instructions: Draw Lewis diagrams for each substance in Questions 18. You may use a clean periodic table. 1) F 2 2) N 3 3) Se 4 2 (Se, Z = 34) 4) N 3 76

5 Chapter 12 Structure and Shape 5) 3 P 4 6) C 3 4 7) C 3 7 F 8) C 3 C 2 C For Questions 912, draw three-dimensional ball-and-stick sketches and predict the electron-pair and molecular geometry of the substances from Questions 14. Electron-pair Geometry Molecular Geometry 9) F 2 10) N 3 11) Se ) N 3 For Questions 1314, draw Lewis diagrams and 3-D sketches for the substance indicated, and state whether it is nonpolar or polar. If polar, indicate the electropositive and electronegative regions on the sketch. 13) BF 2 14) BeBr 2 15) Write the Lewis structures of all the organic compounds having the formula C 2 6. Identify the alcohols and ethers. 77

6 Study Guide for Introductory Chemistry: An Active Learning Approach s to Chapter 12 Sample Test 1) : F : : F : 2).. N 3) 2 Se 4) N 5) P 6) C C C or C C C 7) C C C F : or C C C : F : 78

7 Chapter 12 Structure and Shape 8) C C C Electron-pair Geometry Molecular Geometry 9) F 2 tetrahedral bent (angular) 10) N 3 tetrahedral trigonal pyramidal 11) Se ) N 3 tetrahedral trigonal planar tetrahedral trigonal planar 13) d (F end) : F : : F : \ / B d + ( end) 14) : Br Be Br : nonpolar 15) C C alcohol C C ether 79