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Having Fun with the Routine: Lewis Structures, Oxidation States,

Nomenclature & Stoichiometry

François Gauvin (fgauvin@ustboniface.ca)

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