Transcript Chemical Principles Visualized: Seeing the Unseen

Chemical Principles
Visualized:
Seeing the unseen
David A. Katz
Department of Chemistry
Pima Community College
Tucson, AZ 85745, USA
Voice: 520-206-6044
Email: [email protected]
Web site: www.chymist.com
Chemistry is “hard”!
• Technical vocabulary
• Concepts and abstractions – difficult to relate to
everyday
• Difficult to visualize electrons, atoms, molecules,
reactions, etc…
• Cannot memorize information – must have some
degree of understanding
• Boring lectures (“chalk talks”) with a lot of
information (information overload)
• Requires math
Chemistry is Fun!
• Chemistry, as an experimental science, is not just an
intellectual pursuit, but, a hands-on (or “hands-in”)
science.
• Through chemistry we can create a wondrous range
of substances and materials with unique colors,
odors, and properties.
• None of the physical or natural sciences are as
creative as chemistry.
• Students, on the average, have little or no
concrete concepts or experiences of the
phenomena described in a college chemistry
course.
• Even with prep courses, typical instructors just
talk about chemistry and chemical reactions.
• Students cannot think in 3-D.
• Students have limited visualization skills
– Pictures may help
– Videos are better
– Live demonstrations and hands-on activities
in the classroom enhance the learning of
concepts.
Learning needs to be multisensory
Not this:
This:
• At the 14th International Conference on Chemical
Education, University of Queensland, Brisbane,
Australia, July 14-19, 1996, Roy Tasker, Bob Bucat, Ray
Sleet and Bill Chia, unveiled molecular-level
animations which are known as the VisChem project.
Tasker, R. & West, T. with Lockyer, L. & Harper, B. (2002). Chemistry
Molecular Level Construction Tool. Retrieved August 16, 2009 , from Learning
Designs Web site:
http://www.learningdesigns.uow.edu.au/tools/info/T4/index.html
• Modern textbooks are now employing macroto-micro diagrams and animations of
“molecules” are also available.
• These diagrams, often presented early in a
textbook, precede discussions of chemical
bonding and molecular geometries, as well as
the chemical interactions they may represent.
• Not only must students be trained in
understanding these diagrams, but they need
to experience the actual phenomena being
depicted.
These occur early in the textbook.
Little explanation is given.
Single particle
“atoms” are easy to
understand, water is
more difficult.
Concept is
good, but
students do not
understand the
“open”
structure of ice
as compared
with liquid
water
• This is better.
• It shows the student a
progression of
formula
representations.
• At this point in the
course, shapes have
little or no meaning to
students.
• Also, students are not
familiar with ball-andstick vs space filling
models.
Molecular Shapes
Using Modeling Clay and Toothpicks
• The shape of a molecule
plays an important role
in its reactivity.
• Students cannot think
in 3-D
• Manipulating “atoms”
into molecular shapes
formalizes VSEPR
• Teach shapes BEFORE
Lewis dot structures
Molecular Shapes
Modeling clay and toothpicks to build shapes
MX2 – linear, 180° bond angle
Characteristic of Periodic Table Group IIA
Molecular Shapes
MX3
triangular planar
(trigonal planar)
120° bond angle
Characteristic of Periodic
Table Group IIIA
Molecular Shapes
MX4
tetrahedral
109.5° bond angle
Characteristic of Periodic
Table Group IVA
Students must physically
form a 3-D structure
Molecular Shapes
Molecules with non-bonded electron pairs
Trigonal pyramid
107.5° bond angle
Characteristic of
Periodic Table Group VA
Bent
104.5° bond angle
Characteristic of Periodic
Table Group VIA
Students can build some models using The Molecular Level Visualization Tool
Roy Tasker, et. al., http://www.learningdesigns.uow.edu.au/tools/info/T4/index.html
LABORATORY EXPERIMENTS
and CLASS ACTIVITIES
The Scientific Method
1. Observation/Event
2. Hypothesis
3. Experiment
4. Communication/Publication
5. Research Grant
6. Experiment
7. Theory?
8. Verification/modification of theory
---------------------------9. Physical Law
The Scientific Method
Hypothesis and Experiment
4 cards
Each has a number on one side and a letter on the other
side.
Two letters showing, two numbers showing
Hypothesis: Any card with a vowel (A, E, I, O, U)
on one side has an even number (0, 2, 4, 6, 8)
on the other side.
Question: How many cards must we turn over to prove
(or disprove) the hypothesis?
Scientific Method
Mystery powders:
How to do an investigation
• 4 white powders:
•
•
•
•
Salt
Starch
Powdered sugar
Baking soda
• 3 liquids:
• Water (w)
• Vinegar (v)
• Iodine solution (i)
• Unknown mixtures of
2 or 3 powders
• Identify by properties
only.
The Electromagnetic Spectrum
 Rays
X-Rays
UV Light
Uranium glass
Tide laundry
detergent
Visible Light
An overhead projector
spectroscope
Holographic diffraction
grating
Slit and colored filters
Visible Light
An overhead projector
spectroscope
Holographic diffraction
grating
Slit and colored filters
The Electromagnetic Spectrum
Viewing spectra using holographic diffraction
grating (Flinn Scientific C-Spectra)
Hydrogen spectrum
Helium spectrum
The Electromagnetic Spectrum
How do we identify elements in space?
Build a spectroscope:
In class: Identify elements
using spectrum tubes
Homework: Find elements
in your environment
Colored Flames
Strontium – red
Lithium - red
Calcium – red/orange
Copper – green or blue
Barium – yellow-green
Potassium – violet
Sodium - yellow
Infrared Light
Microwaves
Radio Waves
The Visible Electromagnetic Spectrum
Optical Rotation
• An optically active compound can
rotate light
• Due to an asymmetrical carbon
atom (carbon bonded to 4 different
groups)
• Enantiomers: molecules are mirror
images of themselves
• Solutions of the D- isomer twists
the light clockwise; L-isomer twists
light counter-clockwise
Dextrose (d-glucose)
solution in polarized
light on an overhead
projector
Density
Indiana Jones – Raiders of the Lost Ark
Coke vs. Diet Coke
Hot and Cold
Separate water by density
HOT
COLD
COLD
HOT
Chemical Formulas and Nomenclature
Formula cards – polyatomic ions treated as single units
Chemical Reactions
What factors indicate a chemical reaction occurred?
The Synthesis of Zinc Iodide: Tracking
a Chemical Reaction
Test properties of powdered zinc
Test properties of iodine
Mix zinc and iodine in a petri dish
Place in a zip-lock bag
Add water
Filter resulting solution
Test properties of solution
Evaporate to dryness
Add water and test properties of
solution
Explain what happened
Write balanced equation
Chemical Reactions
Visualizing reaction stoichiometry
CH4 + O2  CO2 + H2O

Chemical Reactions
Visualizing reaction stoichiometry
CH4 + 2 O2  CO2 + 2 H2O

The Activity Series
Group I
Group II
Group III
Transition elements
Group IV
Hydrogen
Group IB (jewelry and tooth fillings)
Intermolecular forces
Drops of water on a coin
How many drops of water can you put on a
coin? Why?
1. Intermolecular forces using I2
1. Iodine vapor
2. Iodine-hexane:
Nonpolar interactions
(London forces)
2. Intermolecular forces using I2
Dipole - Induced dipole
3. Intermolecular forces using I2
Ion – induced dipole
4. Intermolecular forces using I2
Solubility preference:
Like dissolves like
Hexane
layer
Water
layer
Intermolecular Forces:
Which Will Evaporate First?
What factors affect evaporation?
Spread these compounds on black chalkboards
Water
methanol
Effect of molecular weight:
H2O = 18
CH3OH = 32
Effect of polarity
ethanol
2-propanol
C2H5OH = 46
C3H8OH = 60
Visualizing Equilibrium
One
student
to a 150-mL
the
Students
startswitches
with 2 containers
ofbeaker
coloredwhile
water,
second
student
uses
a 400-mL
beaker,
and continues
2 400-mL
beakers,
and
2 500-mL
graduated
cylinders
the process
Acids and Bases
• Svante August Arrhenius (1859 –1927)
– Acid produces hydrogen ions in
water solution.
• Johannes Nicolaus Brønsted (18791947) and Thomas Martin Lowry
(1874-1936)
– An acid-base reaction consists of
the transfer of a proton (or
hydrogen ion) from an acid to a
base
pH
• First introduced by Danish chemist Søren Peder Lauritz
Sørensen (1868-1939), the head of the Carlsberg
Laboratory’s Chemical Department, in 1909
• pH means ‘the power of hydrogen’.
• Each value of pH means the H+ concentration changes by a
factor of 10
• As the H+ concentration decreases, the OH- concentration
increases
pH 1
strong
acid
weak
acid
pH 7
neutral
weak
base
The pH scale according to the late Dr. Hubert Alyea, Princeton University
pH 14
strong
base
pH values
for some
common
substances
Acids, Bases, and pH
• Acids, bases, and pH
using red cabbage paper
– Buffers for reference
– Solutions of household
products to spot the
paper
– 5 -10 mL of solution can
serve 100 students
• Illustrate indicator
colors using serial
dilutions of strong acids
and bases to observe
color changes
Colloids
• Tyndall effect
Why is the sky blue?
Normal sky color
Pale blue sky near horizon
Course syllabi and experiments
can be found at
http://www.chymist.com
On the left-hand menu, click on
Compleat Chymical Demonstrator
or
Magic Into Science
or, for course information and experiments,
Pima Chem Courses
then click on appropriate course link:
Chem 121, Chem 125, Chem 130,
Chem 151, or Chem 152