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) CH 4 (methane)

4 Learning Check Name each of these compounds. A. SO 3 B. MnCl 2 C. (NH 4 ) 3 PO 4 D. Cu 2 CO 3 E. N 2 O 4

Bonds and reality Ionic Covalent (e-transfer) (e- shared) NaCl H 2

Bonds and reality Ionic uneven e- sharing covalent (e-transfer) (e- shared) NaCl HClH 2

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: H 2 )  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   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: O 2 Example: NH 3 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 CO 2 cancel each other because CO 2 is linear.

Polar Bonds and Polar Molecules In water, the molecule is not linear and the bond 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 12 C atom Number below the symbol of an element

Periodic Table and Atomic Mass Ag has atomic mass = amu C has atomic mass = amu S has atomic mass = 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 Na 2 SO 4, Multiply the atomic mass of each element by its subscript Total the masses 2 Na x amu = amu Formula mass 1 Na Na 2 SO 4 1 S x amu = amu amu 1 S 4 O x amu = amu 1 O

Atomic masses in Grams 107.9g of Ag. How many Ag atoms? 12.01g of C. How many atoms of C? 32.07g of S. How many S atoms?

The “gram” mass of certain elements from the Periodic Table

Counting Particles by Mass Experimentally know conversion: 1 amu = x 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 = x 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 = x 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) x g amu = x C atoms

The MOLE Chemists’ counting unit x “anything”

Avogadro’s Number Solution = x 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 x can be written as an equality and two conversion factors. Equality: 1 mol = x particles Conversion Factors: x particles and 1 mol___ 1 mol x particles Avogadro’s Number

Calculations with Avogadro’s Number A Tums tablet has x 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 2 H 2 ? How many moles of “C” atom are in 3.0 moles of C 2 H 2 ? How many individual “C” atoms are in 3.0 moles of C 2 H 2 ?

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

Give the molar mass for: A. 1 mol K atoms =39.10 g B.1 mol Sn atoms =118.7 g Solution

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 CaCl 2. ElementNumber of Moles Atomic MassTotal Mass Ca g/mol g Cl g/mol g CaCl g

Molar Mass of K 3 PO 4 Calculate the molar mass of K 3 PO 4. ElementNumber of Moles Atomic MassTotal Mass in K 3 PO 4 K g/mol g P g/mol g O g/mol g K 3 PO g

Some One-Mole Quantities g g g g g

Molar mass factors are used to convert between the grams of a substance and the number of moles. Calculations Using Molar Mass Grams Molar mass factor Moles

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 CH 4 used in gas cook tops and gas heaters. Molar mass: 1 mol CH 4 = g Conversion factors: g CH 4 and 1 mol CH 4 1 mol CH g CH 4 Molar Mass Factors

Aluminum is often used to build lightweight bicycle frames. How many grams of Al are in 3.00 mol Al? Moles to Grams

Aluminum is often used to build lightweight bicycle frames. How many individual Al atoms are in 3.00 mol Al? Moles to Individual Particles

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 (C 9 H 8 O 4 ) in 52.1 g of aspirin (C 9 H 8 O 4 ).