Transcript Lecture 12
Chemistry 103 Lecture 12 Outline I. Covalent Bonding Lewis Dot Diagrams/Nomenclature (in review) Bond/Molecular Polarity II. Counting in Chemistry (CH8) The Mole/Avogadro’s Number Ionic vs. Covalent NaCl (sodium chloride) CH4 (methane) Learning Check Name each of these compounds. A. SO3 B. MnCl2 C. (NH4)3PO4 D. Cu2CO3 E. N2O4 4 Bonds and reality Ionic (e-transfer) NaCl Covalent (e- shared) H2 Bonds and reality Ionic (e-transfer) NaCl uneven e- sharing HCl covalent (e- shared) H2 Even vs. Uneven sharing of bonding electrons Copyright © 2005 by Pearson Education, Inc. Publishing as Benjamin Cummings Bond Polarity Bond Polarity: a measure of the degree of inequality in the sharing of electrons between two atoms in a chemical bond The positive end (or pole) in a polar bond is represented + and the negative pole -. The negative end is toward the atom with the higher electronegativity Example: HCl H Cl Polar v. Nonpolar Bonds Polar covalent bonds (example: HCl) A covalent bond in which there is unequal sharing of electrons between two atoms. Nonpolar covalent bonds (example: H2) A covalent bond in which there is equal sharing of electrons between two atoms Electronegativity A measure of the relative attraction that an atom has for the shared electrons in a bond Electrons are pulled closer to the atom with the higher electronegativity H Cl- Trends in Electronegativity Electronegativity Trends in electronegativity Electronegativity values increase from left to right on the periodic table Electronegativity values increase from the bottom to the top of the periodic table Range of Bond Types Some Rules of Thumb about Bond Polarity If a bond is between two of the same kinds of atoms: It will be NONpolar If a bond is between two different atoms: It will be polar to some degree (more so the further apart the two atoms are on the periodic table) An important exception: C-H bonds are nonpolar Some Rules of Thumb about Bond Polarity RULES: Electronegativity difference between the two elements in a bond is 0.0 to 0.4 nonpolar covalent. (your book’s convention) Electronegativity difference between the two elements that make a bond is 1.8 or greater, bond considered ionic Polar Bonds? Example: NaF Example: O2 Example: NH3 Example: HCl Molecular Polarity A measure of the degree of inequality in the attraction of bonding electrons to various locations within a molecule Polar Molecule: A molecule in which there is an unsymmetrical distribution of charge Why do we care if the molecule is polar? Polar molecules dissolve in polar solvents (like water or blood) Nonpolar molecules dissolve in nonpolar solvents (like hexane or cell membranes) Molecular Shape and Molecular Polarity For a molecule to be polar: 1. It must contain polar bonds 2. The molecular geometry must not cancel out the effect of the polar bonds (through vector addition) Polar Bonds and Nonpolar Molecules For example, the bond dipoles in CO2 cancel each other because CO2 is linear. Polar Bonds and the molecule is not linear and the Polar Molecules Inbondwater, dipoles do not cancel each other. Therefore, water is a polar molecule. Chemical Quantities Atomic Mass Atomic mass is the Mass of a single atom in atomic mass units (amu) Mass of an atom compared to a 12C atom Number below the symbol of an element Periodic Table and Atomic Mass Ag has atomic mass = 107.9 amu S has atomic mass = 32.07 amu C has atomic mass = 12.01 amu Molecular and Formula Mass Atomic mass unit (amu) = the unit of mass for atoms Molecular mass is the sum of the masses of all atoms in a molecule Formula mass is the sum of the masses of all atoms in a formula unit of an ionic compound (or molecule - used interchangeably) Formula Mass The formula mass is The mass in amu of a compound The sum of the atomic masses of the elements in a formula Calculating Formula Mass To calculate formula mass of Na2SO4, Multiply the atomic mass of each element by its subscript Total the masses 2 Na x 22.99 amu = 45.98 amu 1 Na 1 S x 32.07 amu = 32.07 amu 1S 4 O x 16.00 amu = 64.00 amu 1O Formula mass Na2SO4 142.05 amu Atomic masses in Grams 107.9g of Ag. How many Ag atoms? 32.07g of S. How many S atoms? 12.01g of C. How many atoms of C? The “gram” mass of certain elements from the Periodic Table Counting Particles by Mass Experimentally know conversion: 1 amu = 1.66053 x 10-24 g Let’s say we want to use the atomic mass number in the periodic table in a way useful for experimentation in the lab - grams units Counting Particles by Mass 1 amu = 1.66053 x 10-24 g C (ave atom) 12.01g - how many atoms of C are in this gram quantity using the conversion above? Counting Particles by Mass 1 amu = 1.66053 x 10-24 g C (ave atom) 12.01g - how many atoms of C are in this gram quantity using the conversion above? 12.01g C x ( 1 amu___ ) x ( 1 ave C atom) 1.66053x10-24 g 12.01 amu = 6.022 x 1023 C atoms The MOLE Chemists’ counting unit 6.022 x 1023 “anything” Avogadro’s Number Solution = 6.022 x 1023 Avogadro’s number is equal to 1 mole Makes working with large numbers easier Familiar Collection Terms A collection term states a specific number of items. 1 dozen donuts = 12 donuts 1 ream of paper = 500 sheets 1 case = 24 cans Copyright © 2005 by Pearson Education, Inc. Publishing as Benjamin Cummings Avogadro’s Number Avogadro’s number 6.022 x 1023 can be written as an equality and two conversion factors. Equality: 1 mol = 6.022 x 1023 particles Conversion Factors: 6.022 x 1023 particles 1 mol and 1 mol___ 6.022 x 1023 particles Calculations with Avogadro’s Number A Tums tablet has 7.224 x 1023 atoms of Ca. How many moles is this? Subscripts State Atoms and Moles 1 mole aspirin 9 mol C 8 mol H 4 mol O Parts of the Whole How many “C” atoms are in 125 molecules of C 2H 2? How many moles of “C” atom are in 3.0 moles of C2H2? How many individual “C” atoms are in 3.0 moles of C2H2? Molar Mass from Periodic Table Molar mass • Is the atomic mass expressed in grams Molar Mass The molar mass • Is the mass of one mole of an element or compound • Is the atomic mass expressed in grams Copyright © 2005 by Pearson Education, Inc. Publishing as Benjamin Cummings Solution Give the molar mass for: A. 1 mol K atoms = 39.10 g B. 1 mol Sn atoms = 118.7 g Molar Mass of a Compound The molar mass of a compound is the sum of the molar masses of the elements in the formula. Example: Calculate the molar mass of CaCl2. Element Number Atomic Mass of Moles Ca 1 40.08 g/mol 40.08 g Cl 2 35.45 g/mol 70.90 g CaCl2 Total Mass 110.98 g Molar Mass of K3PO4 Calculate the molar mass of K3PO4. Element Number Atomic Mass of Moles Total Mass in K3PO4 K 3 39.10 g/mol 117.3 g P 1 30.97 g/mol 30.97 g O 4 16.00 g/mol 64.00 g K3PO4 212.3 g Some One-Mole Quantities 32.07 g 55.85 g 58.44 g 294.20 g 342.30 g Calculations Using Molar Mass Molar mass factors are used to convert between the grams of a substance and the number of moles. Grams Molar mass factor Moles Molar Mass Factors Molar mass conversion factors: • Are written from molar mass • Relate grams and moles of an element or compound. Example: Write molar mass factors for methane CH4 used in gas cook tops and gas heaters. Molar mass: 1 mol CH4 = 16.04 g Conversion factors: 16.04 g CH4 1 mol CH4 and 1 mol CH4 16.04 g CH4 Moles to Grams Aluminum is often used to build lightweight bicycle frames. How many grams of Al are in 3.00 mol Al? Moles to Individual Particles Aluminum is often used to build lightweight bicycle frames. How many individual Al atoms are in 3.00 mol Al? Subscripts State Atoms and Moles 1mole aspirin 9 mol C 8 mol H 4 mol O Learning Check Calculate the number of moles of aspirin (C9H8O4) in 52.1 g of aspirin (C9H8O4).