Chapter 3: Molecules, Compounds, and Chemical Equations

Transcription

1 Chapter 3: Molecules, Compounds, and Chemical Equations 2. Chemical Bonds a. Ionic bonds are formed when a metal atom transfers an electron to a nonmetal and the two ions become attracted i. This results in the formation of a lattice in the solid phase b. Covalent bonds are formed when two nonmetals shares electrons i. This bond is stable because it lowers the potential energy of both atoms 3. Representing Compounds: Chemical Formulas and Molecular Models a. Chemical formulas can be divided into three types: i. Empirical formula gives the relative number of atoms of each element in a compound 1. The empirical formula for hydrogen peroxide is HO ii. Molecular formula gives the actual number of atoms of each element in a molecule of a compound 1. The molecular formula for hydrogen peroxide is H 2 O 2 iii. Structural formula uses lines to represent the covalent bonds and shows how atoms in a molecule are bonded to each other 1. The structural formula for hydrogen peroxide is:* b. There are two ways that we can represent molecular models: i. Ball-and-stick model represents atoms as balls and chemical bonds as sticks ii. Space-filling molecular model atoms fill the space between them to closely represent our best estimates for how a molecule might appear 1

2 4. At Atomic-Level View of Elements and Compounds a. Vocabulary: i. Atomic elements those that exist in nature with single atoms as their basic units (for example neon) ii. Molecular elements do not normally exist in nature with single atoms as their basic units 1. Think of the diatomic molecules, H 2, N 2, O 2, F 2, Cl 2, Br 2, and I 2 and the polyatomic molecules P 4 and S 8 iii. Molecular compounds usually composed of two or more covalently bonded nonmetals (for example H 2 O) iv. Ionic compounds composed of cations and anions bound together by ionic bonds (for example NaCl)* b. Homework Problems: i. Page 109 #6, 8, Ionic Compounds: Formulas and Names a. There are three rules for writing formulas for ionic compounds i. They contain positive and negative ions 2

3 ii. The sum of the charges of the cations must always equal the sum of the charges of the anions iii. The formula reflects the smallest whole-number ratio of ions* b. For naming binary compounds that contain a metal that forms only one cation (for example group 1 or 2 metals see table 3.2 in book), it is the name of the cation (metal) followed by the base name of the anion (nonmetal) plus ide i. For example KCl is named potassium chloride 1. See table 3.3 for common anions* c. For naming binary compounds that contain a metal that forms more than one cation (for example transition metals see table 3.4 in book), it is the name of the cation followed by (charge of cation in roman numerals) followed by the base name of the anion plus ide i. For example CuO is named copper(ii) oxide* d. Naming ionic compounds that contain polyatomic ions follows the same way as the previous compounds, except the name of the polyatomic ion is used whenever it occurs i. For example NH 4 NO 3 is ammonium nitrate 1. See table 3.5 for common polyatomic ions* e. For naming hydrates (compounds containing water), you name them the same way as above then follow the name with prefixhydrate i. For example BaCl 2 6H 2 0 is barium chloride hexahydrate 1. Common prefixes are in the margin of page 86 of book f. Homework Problems: i. Page 109 #12, 14, 16, 18, 20, 22, 24, Molecular Compounds: Formulas and Names a. Remember that molecular compounds form between two or more nonmetals. To name binary (two-element) molecular compounds you write the prefix attached to the name of the first element followed by another prefix attached to the base name of the second element plus ide 3

4 i. For example N 2 O is dinitrogen monoxide* b. Acids are molecular compounds that release hydrogen ions (H + ) when dissolved in water. To name binary acids, it is hydro attached to the base name of the nonmetal plus ic followed by acid i. For example HCl is hydrochloric acid c. Oxyacids contain hydrogen and an oxyanion (an anion containing a nonmetal and oxygen). For naming these acids there are two options: i. If the oxyanion ends in ate, you write the base name of oxyanion plus ic followed by acid 1. For example HNO 3 is nitric acid ii. If the oxyanion ends in ite, you write the base name of oxyanion plus ous followed by acid 1. For example H 2 SO 3 is sulfurous acid d. Homework Problems: i. Page 110 #28, 30, Formula Mass and the Mole Concept for Compounds a. The formula mass is the average mass of a molecule i. Another name for the molecular weight of a molecule* b. The molar mass of a compound is the mass in grams of 1 mol of its molecules or formula units i. So the molar mass of calcium nitrate is g/mol* c. Homework Problems: i. Page 110 #36, 38, 40, Composition of Compounds a. The Mass percent composition (or mass percent) or an element is that element s percentage of the compound s total mass i. With element X the formula is: 4

5 b. Usually we want to know the mass in gram of a constituent element in a given mass of the compound; to do this we need a conversion factor from the chemical formula* c. Homework Problems i. Page 110 #44, 46, 48, 54, 55 d. Determining the Percentage Composition of a Mixture Lab 9. Determining a Chemical Formula from Experimental Data a. We know how to calculate the mass percent from a chemical formula, but can we do the reverse? i. Yes we can! We just get the empirical formula by following the following steps: 1. Write the given masses of each element present in the sample 2. Convert each mass to moles 3. Write a pseudoformula for the formula 4. Divide all subscripts in the formula by the smallest subscript 5. If the subscripts are not whole numbers, multiply all subscripts by a small whole number to get whole number subscripts** b. We can also calculate the molecular formula from the empirical formula if we care given the molar mass of the compound* c. Using the products of combustion we can find the empirical formula of the original hydrocarbon* d. Homework Problems i. Page 111 #58, 60, 64, Writing and Balancing Chemical Equations* a. Homework Problems i. Page 112 #70, 72, 76, Organic Compounds a. Organic compounds come from living things (i.e. sugar) while inorganic compounds come from the Earth (i.e. salt) 5

6 b. There are some common hydrocarbon names that you might want to be familiar with (see table 3.7): Name Molecular Model Common Uses Methane CH 4 Primary component of natural gas Propane C 3 H 8 LP gas for grills and outdoor stoves n-butane C 4 H 10 Common fuel for lighters n-pentane C 5 H 12 Component of gasoline Ethene C 2 H 4 Ripening agent in fruit Ethyne C 2 H 2 Fuel for welding torches c. Homework Problems: i. Page 112 #80 *Determining the Formula of a Hydrate Lab All chapter 3 homework problems: #6, 8, 10, 12, 14, 16, 18, 20, 22, 24, 26, 28, 30, 32, 36, 38, 40, 42, 44, 46, 48, 54, 55, 58, 60, 64, 66, 70, 72, 76, 78, 80 (32) Review problems: #81, 84, 87, 90, 93 6