Transcript 3 molecules
Chapter 3 Molecules and Compounds Molecules and Compounds Chemical Formulas 1:1 CO 1:2 H2O 1:3 NH3 2:3 1:4 Al2O3 CH4 etc C + 4 H = CH4 Molecular model Molecular Models Ethanol Molecular Models Some compounds are IONIC - electrons are TRANSFERRED from a metal to a nonmetal; the compound is held together ELECTROSTATICALLY Coulomb’s Law: force = k (n+)(n-)/d2 Some compounds are COVALENT - electrons are SHARED between two atoms e + + e E. g. carbon dioxide This commonly occurs for two or more NONMETALS The Covalent Bond Ions • An atom or group of atoms with a net charge caused by the net loss or gain of electrons • CATION = positively charged ion • ANION = negatively charged ion Formation of Cations & Anions Predicting Whether an Atom Will Form a Cation or Anion in Order to Make an Ionic Compound Metals LOSE electrons to form CATIONS, nonmetals GAIN electrons to form ANIONS Valence Electrons in Ionic Compounds • The A-group (representative) elements follow the OCTET RULE; they obtain an inert gas valence (outer) shell that contains 8 electrons • Metals - lose # electrons = group number e.g. Ca Ca2+ + 2e- (Ar outer shell) • Nonmetals - gain electrons = 8 - group # e.g. N + 3e N3- (Ne outer shell) Valence Electrons for Covalent Compounds • Covalent compounds form between two or more nonmetals • In this case the nonmetals can either LOSE all of their valence electrons, or • GAIN enough electrons to obtain an OCTET Examples SO3 - oxygen (VIA) gains 8-6 = 2 O2- ion forms - sulfur (VIA) loses all 6 S6+ ion forms NOTE: There is a rule that states that oxygen is ALWAYS -2. These rules are coming up! Example 3.3 - Predicting Ion Charges When Forming Ionic Compounds • Metals lose electrons, Nonmetals gain them • Al = group IIIA metal, so LOSES 3 ELECTRONS Al Al3+ + 3e• S = group VIA nonmetal, so GAINS 8 - 6 = 2 ELECTRONS S + 2e S2- Polyatomic Ions • Contains 2 or more atoms COVALENTLY bonded, and the complete unit contains a net charge, e.g. nitrate, NO 3 Polyatomic Ion Examples 2 2 3 NO2 , CO3 , SO4 , PO4 NO2- ion CO32- ion Ionic Compound = Metal + Nonmetal or a Metal + Polyatomic Ion • Compound held together electrostatically • Very strong forces hold the lattice together, so ionic cmpd’s have very high melting points NaCl crystal lattice m.p. = 800 oC Predicting Formulas of Ionic Compounds Balance positive and negative charges to produce a neutral molecule Ca2+ + Cl Ca2+ + CO32- Ca2+ + PO43- Al3+ + 2 O Oxidation Numbers • A number assigned to each element in a compound in order to keep track of the electrons during a reaction Mg2+ = +2 Cl- = -1 O2- = -2 N3- = -3 Rules for Assigning Oxidation Numbers (Chap. 5, p. 207) Rules higher up take precedence over lower rules 1 The O.N. for an atom in its pure, uncombined state = 0. 2 The sum of the O.N.’s for a neutral molecule = 0. For a polyatomic ion, the sum = charge. Rules cont’d 3 Group IA = +1 Group IIA = +2 4 H = +1 UNLESS combined with IA or IIA, then = -1 5 Oxygen = -2 6 For binary ionic compounds only Group VA = -3 Group VIA = -2 Group VIIA = -1 Examples • P4 • Al2O3 • • • • MnO4 NaH Na2SO3 Mg3N2 Chemical Nomenclature Examples (More Detail in Lab) • Ionic Compounds NaCl Al2S3 FeSO4 KClO3 sodium chloride aluminum sulfide iron(II) sulfate potassium chlorate • Covalent Compounds SO2 P2O5 N2O sulfur dioxide diphosphorus pentaoxide dinitrogen oxide The Mole - The mole is the chemist’s counting unit pair = 2 Dozen = 12 Gross = 144 Ream = 500 Avogadro’s Number (NA) = 6.022 X 1023 Where Does Avogadro’s Number Come From? • By definition, 12C = 12.000 amu • How many particles does it take to have 12.000 grams of 12C ? • NA = 6.022 X 1023 (as determined by experiment) Significance of the Mole Mass in amu’s Mass in grams/mole NA of carbon atoms weighs NA of iron atoms weighs Molar Mass - the mass in grams of one mole of any element • Molar mass of sodium (Na) = mass of 1 mol of Na atoms = 22.99 g/mol = mass of 6.022 X 1023 Na atoms • Molar mass of lead (Pb) = mass of 1 mol of Pb atoms = 207.2 g/mol = mass of 6.022 X 1023 Pb atoms Mass Moles Conversion Moles to Mass Mass to Moles moles • grams = grams 1 mole grams • 1 mole = moles grams Molar mass 1 / Molar mass Example 3.6 - Mass to Moles How many moles are represented by 125 g of silicon, an element used in semiconductors? Example 3.7 - Moles to Mass What mass, in grams, is equivalent to 2.50 mol of lead (Pb) ? Mole Calculation Using Density The graduated cylinder in the photograph contains 25.0 cm3 of Hg. If the density of Hg = 13.534 g/cm3 at 25 oC, how many moles of Hg are in the cylinder? How many atoms of Hg are there? Molar Mass of a Compound Sum up the molar masses of each atom in the compound HC2H3O2 Example 3.9 - Molar Mass & Moles You have 16.5 g of the common compound oxalic acid, H2C2O4. Calculate 1. The number of moles 2. The number of molecules 3. The number of C atoms 4. The mass of one molecule Other Fun Stuff 46.07 g contains 2(12.01) = 24.02 g of carbon 1(16.00) = 16.00 g of oxygen 6(1.008) = 6.05 g of hydrogen C2H5OH MW = 46.07 1 molecule contains 2 carbon atoms 1 oxygen atom 6 hydrogen atoms 1 mole contains 2 moles of carbon atoms 1 mole of oxygen atoms 6 moles hydrogen atoms Conversion factors for C2H5OH • 2(12.01) g C/ 46.07 g C2H5OH OR 24.02 g C/ 46.07 g C2H5OH • 6 moles H/ mole C2H5OH • 1 mole oxygen/ 2 moles C More Problems How many grams of Na are there in 200. g of Na2CO3 ? How many moles of oxygen are there in 25.0 mol of SO2 ? More Problems How many aluminum atoms are there in 150. g of Al2O3 ? How many oxygen atoms are there in 500. mL of a 30.0 % solution of H2SO4 with a density of 1.250 g/cm3 ? (MW = 98.1) Percent Composition from a Known Formula NH3 MW = 17.03 g/mol %N= %H= Empirical & Molecular Formulas Empirical = simplest ratio of atoms in the molecules Molecular = actual ratio Calculating Empirical Formulas Formulas of unknown compounds are determined from the percent composition of each element by mass. Assume 100 g and divide by atomic weight Divide by fewest number of moles Calculating Molecular Formulas The molecular weight must be known. It is obtained from a separate experiment Benzene empirical formula = CH formula weight = 12.01 + 1.008 = 13.018 If the MW = 78.11, then what is the molecular formula? Example 3.10 Eugenol is the active component of oil of cloves. It has a MW of 164.2 g/mol and is 73.14 %C and 7.37 %H; the remainder is oxygen. What are the empirical and molecular formulas? Another example Vanillin is a common flavoring agent. It has a molar mass of 152 g/mol and is 63.15 %C and 5.30 %H; the rest is oxygen. What are the empirical and molecular formulas?