1 Chapter 3 Chemical Compounds

2 Chemical Formulas; Molecular and Ionic Substances The chemical formula of a substance is a notation using atomic symbols with subscripts to show the relative proportions of atoms of the different elements in a substance. Consider the formula of aluminum oxide, Al 2 O 3. This formula implies that the compound is composed of aluminum atoms and oxygen atoms in the ratio 2:3.

3 A molecule is a definite group of atoms (two or more nonmetals) that are chemically bonded together – that is, tightly connected by attractive forces. Chemical Formulas; Molecular Substances Molecular substances A molecular substance is a substance that is composed of molecules, all of which are alike. A molecular formula gives the exact number of atoms of elements in a molecule.

4 Molecular Compounds

5 Naming Binary Molecular Compounds Binary molecular compounds consist of molecules that contain atoms of only two elements (nonmetals). In binary molecular compounds the more metallic element is listed first Increasing metallic character

6 Naming Binary Molecular Compounds Nonmetal + Nonmetal Prefixes mono =1hexa = 6 di = 2hepta = 7 tri = 3octa = 8 tetra = 4nona = 9 penta =5 deca = 10

7 Binary Molecular Compounds (prefix)name of the first nonmetal + (prefix)modified name of the second nonmetal Oxygen - ox + ide = oxide Chlorine - chlor + ide = chloride Fluorine – fluor + ide = fluoride Hydrogen – hydr + ide = hydride Nitrogen – nitr + ide = nitride Sulfur – sulf + ide = sulfide

8 Binary Molecular Compounds CO - carbon monoxide SO 2 – sulfur dioxide P 2 O 5 – diphosphorus pentoxide S 2 Cl 2 – disulfur dichloride N 2 O – dinitrogen monoxide use prefixes in front of both elements; exception mono is never used in front of the first element

9 Compounds that consist of ions Chemical Formulas; Ionic Substances Ionic substances An ion is an electrically charged particle obtained from an atom or chemically bonded group of atoms by adding or removing electrons. When an atom loses an electron it becomes a positively charged - cation When an atom gains an electron it becomes a negatively charged - anion

10 Formation of Ionic Compound, NaCl

11 NaCl “table salt”

12 The formula of an ionic compound is written by giving the smallest possible whole-number ratio of different ions in the substance. Chemical Formulas; Ionic Substances The formula unit of the substance is the group of atoms or ions explicitly symbolized by its formula. Sodium chloride NaCl, not Na 6 Cl 6

13 Naming Binary Ionic Compounds Metal + Nonmetal Name of the metal + modified name of a nonmetal Oxygen - ox + ide = oxide Chlorine - chlor + ide = chloride Fluorine – fluor + ide = fluoride Hydrogen – hydr + ide = hydride Nitrogen – nitr + ide = nitride Sulfur – sulf + ide = sulfide

14 Naming Binary Ionic Compounds Charges Group 1A – 1+ Group 2A – 2+ Group 3A – 3+ Group 5A – 3- Group 6A – 2- Group 7A – 1- Main group metallic elements have the charge equal to the group number. Most nonmetallic elements have the charge equal to the group number – 8 ( for example oxygen = - 2) We do not use prefixes for binary ionic compounds!!!

15 Charges on Some Common Monatomic Cations and Anions

16 Binary Compounds Metal + Nonmetal NaCl - sodium chloride KBr – potassium bromide LiI – lithium iodide MgBr 2 – magnesium bromide Na 2 O– sodium oxide Al 2 O 3 – aluminum oxide The total of charges = 0 Al 2 O 3 2 x (+3) + 3 x (-2) = AlO 23

17 Binary Compounds Metal + Nonmetal Transition Metals FeO - Fe 2 O 3 – CuO – Cu 2 O - iron (II) oxide iron (III) oxide copper (II) oxide copper (I) oxide

18 Mass and Moles of a Substance The molar mass of a substance is the mass of one mole of a substance. For all substances, molar mass, in grams per mole, is numerically equal to the formula weight in atomic mass units. That is, one mole of any element weighs its atomic mass in grams.

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20 Molar Mass of Compounds Calculate the molar mass of butane C 4 H 10 C = g/mol × 4 = g/mol H = 1.01 g/mol × 10 = g/mol C 4 H 10 = g/mol

21 Determining Chemical Formulas The percent composition of a compound is the mass percentage of each element in the compound. We define the mass percentage of “A” as the parts of “A” per hundred parts of the total, by mass. That is,

22 Mass Percentages from Formulas Let’s calculate the percent composition of butane, C 4 H moles of carbon × 12.01g/mol = g 10 moles of hydrogen × 1.01 g/mol = g Total mass of 1 mol of C 4 H 10 = g

23 C

24 Determining Chemical Formulas We can calculate the percent composition by mass of a compound if we know its formula. If we know the percent composition of a compound we can determine its empirical formula. An empirical formula (or simplest formula) for a compound is the formula of the substance in in which the ratio of atoms or ions gives the smallest possible integer value

25 Determining Chemical Formulas Benzoic acid is a white, crystalline powder used as a food preservative. The compound contains 68.8% C, 5.0% H, and 26.2% O by mass. What is its empirical formula? Cx HyOzCx HyOz In other words, give the smallest whole-number ratio of the subscripts (moles) in the formula

26 Determining Chemical Formulas For the purposes of this calculation, we will assume we have grams of benzoic acid. Since x, y, and z in our formula represent mole-mole ratios, we must first convert mass to moles. Cx HyOzCx HyOz

27 Determining Chemical Formulas

28 Determining Chemical Formulas The empirical formula is C 7 H 6 O 2. To calculate the ratio of moles: Multiply each number by 2 to get integers.

29 Determining Chemical Formulas Determining the molecular formula from the empirical formula. An empirical formula gives only the smallest whole-number ratio of atoms in a formula. The molecular formula could be a multiple of the empirical formula To determine the “true” molecular formula, we must know the molar mass of the compound.

30 Determining Chemical Formulas C6H6C6H6 C 2 H 6 C3H7O2C3H7O2

31 Determining Chemical Formulas A compound is 39.9% C, 6.70% H and 53.4% oxygen. The molecular weight of this compound is 60.0 g/mol. Determine the molecular formula of this compound. CxHyOzCxHyOz