What do Lady and the Tramp and Bromine and Cyclohexane Have in Common?
When you are in love, you would rather spend time with that someone who likes spaghetti as much as you do.
Introduction
Background:
What are the trends we’ve explored on the periodic table? How have these trends related to what we saw
concerning reactivity and conductivity in the ‘Oh, oobee-doo’ lab? Keep in mind that this will involve
discussing ionization and electronegativity in detail, explaining as well how these forces change going
across the period and down the group. What must happen when two compounds react in a double
replacement reaction? Why did a precipitate form; can trends be used to explain why a precipitate formed
in some cases but not others? How do we explain why differences in reactivity were observed when
halogens reacted with halide salts (single replacement reactions)?
What issues are you we exploring in the Spaghetti lab? Again, each aspect of this exercise was designed to
explore surface tension (including the processes of adhesion and cohesion) solubility, conductivity, melting
point and volatility. Make sure each of these issues are thoroughly explored. What do we know about why
these interactions occur the way they do? Again, keep in mind that we are now looking at VSEPR and how
the positioning of electron pairs and atoms impact molecular behavior.
Research Rationale:
What are the rationales behind each of the lab activities you completed? What were we expected to see and
why expect to see anything at all?
1. Why compare how many drops of water you can position on a penny? Why repeat the exercise
with isopropyl alcohol, oil and hexane? What was the point of conducting a second experiment
after having rubbed detergent on the surface of the penny?
2. What about the procedures involving a glass slide and wax paper?
3. A third section involved the dissolution of a number of halide salts, sugar and organic acids in
water, methanol and cyclohexane. What do think is the rationale behind the decision to use these
particular compounds in this exercise?
Results and Observations:
You designed a number of tables that were used during the course of your classroom presentations. Please
include them at this point in the lab write-up.
For example:
Penny Demonstration
Substance
Without detergent
With Detergents
Water
Isopropyl Alcohol
Cooking oil
Cyclohexane
Substance
Benzoic Acid
Magnesium
Chloride
Stearic Acid
Sugar
Copper (II)
Chloride
Chemical
Formula
Volatility
+, -
Melting
Point
Dissolves in
water
Dissolves in
Methanol
Dissolves
in hexane
Conductivity
Zinc Chloride
Sodium
Chloride
Discussion:
During the course of the presentations, the goals were to identify patterns in the data and explain any
observed differences using what we know about VSEPR.
1. How can the molecular geometry of water, alcohol, oil and hexane help us understand the
differences in surface tension you observed.
2. Use what you know about VSEPR to explain the nature of the patterns. How does VSPER account
for differences in melting point and volatility. How does one explain why NaCl or ZnCl2 dissolves
in water but not hexane. What about the other ionic compounds (salts) and molecular acids and
their solvation patterns in water, alcohol and hexane? What about conductivity? Can VSEPR be
used to explain the differences you may have seen in the degree to which the substances you
observed conducted electricity?
Final Headache:
Boiling point was a physical property that was not explored in this investigation. Liquids may change to a
vapor at temperatures below their boiling points through the process of evaporation. Evaporation is a
surface phenomenon in which molecules located near the liquid's edge, not contained by enough liquid
pressure on that side, escape into the surroundings as vapor. On the other hand, boiling is a process in
which molecules anywhere in the liquid escape, resulting in the formation of vapor bubbles within the
liquid.
Generally speaking, the stronger the intermolecular forces, the more energy it takes to overcome these
interactions and to cause the substance to boil. Therefore, molecules with strong bonds have high boiling
points, and molecules with weak bonds have lower boiling points.
Consider the following graph:
Take a look at the first series of compounds on the above table (CH4, NH3, HF, H2O) and consider what
you know about periodic trends (electronegativity), VSEPR and the different kinds of Van Del Waal forces.
How would you explain the differences in boiling points between these four molecular compounds?