Chemical Principles Visualized:
Lecture demonstrationsand activities
David A. KatzChemist, educator, and consultant
Tucson, AZ 85745, USAVoice: 1-520-624-2207
Email: [email protected] site: www.chymist.com
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 chemistry course.
• 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.
Molecular ShapesUsing 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 ShapesModeling clay and toothpicks to build shapes
MX2 – linear, 180° bond angle Characteristic of Periodic Table Group IIA
Molecular Shapes
MX3triangular planar (trigonal planar)120° bond angle
Characteristic of Periodic Table Group IIIA
Molecular Shapes
MX4tetrahedral
109.5° bond angleCharacteristic of Periodic Table Group IVAStudents must physically form a 3-D structure
Molecular ShapesMolecules with non‐bonded electron pairs
Trigonal pyramid107.5° bond angle
Characteristic of Periodic Table Group VA
Bent104.5° bond angle
Characteristic of Periodic Table Group VIA
Visual Stoichiometry
CH4 + O2 CO2 + H2O (equation not balanced)
Visual Stoichiometry
H2SO4 + 2 NaOH Na2SO4 + 2 H2O
Density
Indiana Jones – Raiders of the Lost Ark
Coke vs. Diet Coke
Hot and ColdSeparate water by density
COLD HOT
HOT
COLD
Visible LightAn overhead projector spectroscope
Holographic diffraction grating (Flinn C-Spectra)
Slit and colored filters
Visible LightAn 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
Colored Flames
Strontium – redLithium ‐ redCalcium – red/orangeCopper – green or blueBarium – yellow‐greenPotassium – violetSodium ‐ yellow
lithium potassium
calcium barium
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(Note: A kit is available from Flinn Scientific)
Dextrose (d-glucose)solution in polarized light on an overhead projector
Iodine
1. Intermolecular forces using I2
1. Iodine vapor2. 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:Salting Effects
Mixture of 2‐propanol and water (15 mL of each)Add food colorAdd 7 g ammonium sulfate and shakeTest layers for conductivity
Reference: J. Chem. Educ, 87, 1332 (December 2010)
Intermolecular forces Drops of water on a coin
How many drops of water can you put on a coin? Why?
Intermolecular forces: Decrease in Volume
ethanol and water
Intermolecular forces: Decrease in Volume
ethanol water
Hydrogen BondingIncrease in Volume
HCl + NaOH NaCl + H2O
Hydrogen BondingIncrease in Volume
Hydrated H3O-
O-O distance 2.59 ÅHydrated OH-
O-O distance 2.50 Å
H2OO-O distance 2.82 Å
Source: Martin Chaplin,http://www1.Isbu.ac.uk/water/index.html
Intermolecular Forces:Which Will Evaporate First?
What factors affect evaporation?Spread these compounds on black chalkboards
Water methanol ethanol 2‐propanolEffect of molecular weight:
H2O = 18 CH3OH = 32 C2H5OH = 46 C3H8OH = 60
Effect of polarity
Acids and Bases
• Svante August Arrhenius (1859 –1927)– Acid produces hydrogen ions in water solution.
• Johannes Nicolaus Brønsted (1879‐1947) 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 pH 7 pH 14strong weak neutral weak strongacid acid base base
The pH scale according to the late Dr. Hubert Alyea, Princeton University
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
• Illustrate indicator colors using serial dilutions to observe color changes
Intermolecular ForcesWhy does a substance dissolve?
Course syllabi and experimentscan 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