The Levels of Chemistry...20130804 2 The University of Manitoba CHEM 0900 Preparatory Chemistry...
Transcript of The Levels of Chemistry...20130804 2 The University of Manitoba CHEM 0900 Preparatory Chemistry...
2013-‐08-‐04
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Having Fun with the Routine: Lewis Structures, Oxidation States,
Nomenclature & Stoichiometry
François Gauvin ([email protected])
Department of experimental sciences
ChemEd 2013 Conference Waterloo (Ontario) CANADA
July 29th, 2013
Outline of presentation/workshop • Triangular and tetrahedral Concept maps • Preparatory and General Chemistry • Focus on
• Lewis structures drawing • Oxidation state determination • Nomenclature (a teaching approach proposal) • Stoichiometry calculations (… “in the kitchen”)
The Levels of Chemistry • The "Human" element • "Tetrahedral" chemistry
education • Relevance to our lives • Integration of content
and context
Mahaffy, P., Chem. Educ. Res. Pract., 2004, 5(3), 229-‐245 and Johnstone, A.H., Int. News. Chem. Educ., 1991, (36), 7-‐10.
Macroscale Level (Human Level; Observa2ons)
Nanoscale Level (Par2cle Level; Models)
Symbolic Level (Representa2on Level; Symbols, formulas,
& equa2ons)
CHEMISTRY: Ma@er and Changes
-‐ Periodic Table
-‐ Scien2fic Method -‐ Measurements
-‐ Elements -‐ Compounds
-‐ Ionic vs covalent
-‐ Chemical Amounts -‐ Molecular Structures
-‐ Kine2c Molecular Theory
The Levels of Chemistry
Adapted from: Mahaffy, P., Chem. Educ. Res. Pract., 2004, 5(3), 229-‐245 and Johnstone, A.H., Int. News. Chem. Educ., 1991, (36), 7-‐10.
Macroscale Level (Human Level; Observa2ons)
-‐ Mixtures and Proper2es of Solu2ons
-‐ Separa2on Techniques -‐ Chemical / Physical Proper2es
Nanoscale Level (Par2cle Level; Models)
-‐ Composi2on of MaPer -‐ Nuclear Chemistry -‐ Atomic Structure -‐ Chemical Bonding -‐ Molecular Structure
Symbolic Level (Representa2on Level;
Symbols, Formulas, & Equa2ons)
-‐ Chemical Symbols -‐ Chemical Formulas -‐ Nomenclature
-‐ Chemical Equa2ons
The Levels of Chemistry Ma@er
-‐ Elements:
-‐ Main Group Elements -‐ d-‐Block Elements -‐ Compounds:
-‐ Ionic versus Covalent -‐ Inorganic versus Organic
-‐ Carbon-‐Based Materials (Organic) -‐ Macromolecules:
-‐ Polymers and Proteins
Changes (Chemical Processes or Chemical Reac2ons)
-‐ Types of reac2ons:
-‐ Precipita2on, Acid/Base, Redox à Electrochemistry
-‐ Stoichiometry (amounts of maPer) -‐ Thermochemistry (energy)
-‐ Chemical Kine2cs (reac2on rates) -‐ Chemical Equilibrium
(in gas phase and in solu2on) -‐ Acids and bases à Buffers
-‐ Solubility equilibrium à Precipita2on -‐ Introduc2on to Thermodynamics
The Levels of Chemistry
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The University of Manitoba CHEM 0900 Preparatory Chemistry Course
Ø Non-credited, Pass or Fail course (≥ 60%) Ø For students with little or no background in chemistry ü To achieve prerequisites for 1st-year CHEM 1300/1310/1320
(mostly high school chemistry) ü For individuals who need a refresher course in chemistry
Ø No wet-laboratory component Ø In-class: Fall and Spring terms ü Enrollment: >100 in Fall and >50 in Spring
Ø Online: all year around (Fall, Winter, & Summer) ü Enrollment: >50 (and kept increasing)
CHEM 0900 OL Course format
Module 1: Let’s start from the beginning Ø (Basic concepts of science)
Module 2: What’s the "Matter"? Ø (Composition, structure, and properties)
Language of chemistry, Part 1: The Elements and the Periodic Table Module 3: How much or how many?
Ø ("Amounts" of matter) Language of Chemistry, Part 2: Binary acids & compounds
Module 4: Let’s not overreact! Ø (Description of chemical reactivity)
Language of chemistry, Part 3: Oxoacids and polyatomic ions Module 5: The third state of matter:
Ø “It’s a gas!"
Focus on Lewis Structures, Oxidation States, Nomenclature, and Stoichiometry
A) Lewis Structures • Show how valence electrons are arranged on, or between,
atoms in a molecule or an ion. • Lewis Structures lead to Valence Bond Theory and hybridization • There is currently a debate about how to teach bonding Valence Bond theory versus Molecular Orbitals Theory J. Chem. Educ.: 2011, 88, 860-862 and 2012, 89, 567-568
• This was also discussed at the
• Not to be discussed today!
A) Lewis Structures • Show how valence electrons are arranged on,
or between, atoms in a molecule or an ion.
• Bonding (or shared) pairs (bp) between atoms versus Lone pairs (lp) on atoms.
• In covalent bonds, atoms share pairs of electrons to reach a noble-gas Valence shell configuration (i.e. 8 Valence Electrons): OCTET RULE.
• Lewis structures are extensively used in Organic Chemistry.
Guidelines for Drawing Lewis Structures
(from common textbooks) 1. Find the total number of valence electrons in the molecule
or ion (adding electrons for each negative charge, or removing electrons for each positive charge, on ions).
2. Draw a skeleton structure using chemical symbols for the atoms and lines in-between (–) for each bonding pair (bp) of electrons. Put least electronegative atom(s) as center(s).
3. Satisfy the Octet Rule for each atom, by placing lone pairs (lp) of leftover electrons, starting with the terminal atoms.
4. All remaining electrons should be placed around central atom; first as lone pairs, and then as lone electrons.
5. Rearrange the bonding using lp’s to form multiple bonds to central atom in order to obey the Octet rule for every atom.
Despite simple guidelines, we often see all kinds of Lewis Structure drawings
For SCl2 For NO+
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Other examples
For CO For SO32–
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Successful Examples
For SCl2
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Successful Examples For NO+ Write the Lewis Structure of the following
species: a) F2 b) N2 (isolectronic with NO+, CO, CN–) c) O2 (limits of Lewis Structure models) d) HCl e) H2O f) O3
LEWIS STRUCTURE EXERCISES
Resonance Occurs when more than one valid Lewis structure can be written for a particular molecule or ion.
Another example: SO2
Resonance hybrids are then obtained. The actual structure is an average of the resonance structures. Resonance contributes to stabilize the total energy of the molecule.
Write ALL the possible Lewis Structure of each following species (central atom is boldfaced): a) CO3
2-
b) NO2-
c) N2O (one N is central) d) OCN-
RESONANCE STRUCTURE EXERCISES
Write ALL the possible Lewis Structure of each following species (central atom is boldfaced): a) CO3
2-
b) NO2-
c) N2O (one N is central) d) OCN-
Can any resonance structure of these species have a dominant character?
RESONANCE STRUCTURE EXERCISES Formal Charges Formal Charge: In a molecule or ion, resultant charge
on an atom if all shared electrons are equally shared. Lone pair electrons are assigned (or “belong”) to the
atom on which they are found. Half of the bonding electrons (bonding pairs) are
assigned to each of the two atoms that are linked together.
The sum of formal charges on a molecule is equal to zero
The sum of formal charges on a polyatomic ion is equal to the charge of that ion.
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Formal Charge Calculations
FC should always be kept as low as possible on each atom if they cannot be brought to a value of zero.
( )( )ionor moleculein atomon e Valence
atom neutralon e Valence(FC) charge Formal-
-
∑∑
−
=
Find the dominant resonance structure of the following species: a) N2O (one N is central) b) OCN-
FORMAL CHARGES EXERCISES