Init 3/1/2011 by Daniel Raymond Barnes
. . . explain diffusion
. . . explain how salt dissolves in water
. . . explain why some materials conduct
electricity and others don’t.
conductor
insulator
At present, this power point is just a bare bones
outline.
I will flesh it out with pictures and animations when
time permits.
Diffusion: use drop of food coloring in beaker to show
effect of random molecular motion. This helps explain
the dissolving process, which helps explain why salt
water conducts electricity.
Covalent vs Metallic bonds: compare & contrast water
molecule with chunk of iron. They both involve
sharing of electrons, but only in the metal are the
shared electrons allowed to wander all over the place,
hence, its conductivity.
Molecular motion in still air vs. Molecular motion in
windy air vs. Molecular motion in still water vs.
Molecular motion in water in a current vs. Electron
motion in non-conducting metal vs. Electron motion in
conducting metal. Balance vs. Bias. Voltage drives
electric current.
Testing of samples for conductivity, pt I (solids only):
1. Screwdriver shaft (This this thing provide energy? No. Just
a path for the flow of electron traffic. It’s the road, not the
gasoline.)
2. Screwdriver handle
3. Aluminum foil
4. Plastic spoon
5. Glass beaker
6. Washcloth
7. Wooden ruler
Routing of electron flow through conductivity apparatus,
including
*discussion of plastic insulation vs copper wire (protecting
humans from electrocution AND protecting house wiring from
shorts)
*light bulb interlude (button, black donut, collar, W filament)
*irony of carbon, a “nonmetal”, being a conductor.
Testing of samples for conductivity, pt II:
1.
2.
3.
4.
5.
6.
7.
8.
9.
Tap water
Distilled water
Salt (cr)
Salt water (salt + distilled water)
Sugar water (sugar + distilled water)
Vinegar
Ammonia (aq)
Oil
HCl
Q1: What are particles doing
when a metal is conducting
electrical current?
Al3+
Al3+
Al3+
Al3+
Al3+
Al3+
Al3+
Al3+
Al3+
Al3+
Al3+
Al3+
Al3+
Al3+
Al3+
Al3+
Al3+
Al3+
Al3+
Al3+
Al3+
Al3+
Al3+
Al3+
Al3+
Al3+
Al3+
Al3+
Al3+
Al3+
Al3+
Al3+
Al3+
Al3+
Al3+
Al3+
Al3+
Al3+
Al3+
Al3+
Q1: What are particles doing when a metal
is conducting electrical current?
Positive metallic ions are stuck in place, in a crystal pattern,
due to metallic bonding.
Metallic cations may be stuck in place, but electrons are free to
wander.
When electric current is flowing,
Positive and negative charges move relative to one another.
(A car moves relative to the road, even though the road is not
moving relative to the earth.)
When a voltage is applied, electron wandering becomes
imbalanced.
Q2: Why did the insulators not conduct?
Insulators tend to be made of nonmetal elements. For instance,
plastics, like wire insulation, are made of carbon and hydrogen.
Nonmetal atoms have high electronegativities & high ionization
energies, so they don’t allow their electrons to wander.
Electrons on a nonmetal atom stay home, orbiting their nucleus.
Q3: What are the particles doing when an
electrolyte solution is conducting current?
Cl
-
Cl
+
Na
Na
Cl
-
Cl
+
Na
Na
Cl
-
Cl
+
Na
Na
Q3: What are the particles doing when an
electrolyte solution is conducting current?
An electrolyte solution includes positive and negative ions
wandering around in the solvent.
(The solvent is usually water, so most electrolyte solutions are
“aqueous” solutions.)
The positive electrode can pull electrons off of negative ions.
Such electrons then travel through the wires and light bulbs,
making the light bulbs light up.
The electrons then are given away by the negative electrode to
positive ions.
electronegativity
+
+
2.1
3.5
-
3.5
-2.1
1.4
Cl-
Na+
Cl-
Na+
Cl-
Na+
Cl-
Na+
Cl-
Na+
Cl-
N
Na+
Cl-
Na+
Cl-
Na+
Cl-
Na+
Cl-
Na+
Cl-
Na+
C
Cl-
Na+
Cl-
Na+
Cl-
Na+
Cl-
Na+
Cl-
Na+
Cl-
N
Na+
Cl-
Na+
Cl-
Na+
Cl-
Na+
Cl-
Na+
Cl-
Na+
C
Cl-
Na+
Cl-
Na+
Cl-
Na+
Cl-
Na+
Cl-
Na+
Cl-
N
Na+
Cl-
Na+
Cl-
Na+
Cl-
Na+
Cl-
Na+
Cl-
Na+
C
Cl-
Na+
Cl-
Na+
Cl-
Na+
Cl-
Na+
Cl-
Na+
Cl-
N
Na+
Cl-
Na+
Cl-
Na+
Cl-
Na+
Cl-
Na+
Cl-
Na+
C
Cl-
Na+
Cl-
Na+
Cl-
Na+
Cl-
Na+
Cl-
Na+
Cl-
N
Explanation of how water dissolves the ionic and polar
materials that it does dissolve. Invocation of polarity
of covalent bond between H & O in H2O.
+
Cl-
Na+
Cl-
Na+
Cl-
Na+
Cl-
Cl-
Na+
Cl-
Na+
Na+
Cl-
Na+
Cl-
Na+
-
Idea of an “electrolyte” as a substance that breaks into
ions in water.
Comparisson of acetic acid vs. hydrochloric acid:
Weak molecules are strong electrolytes/strong acids.
Strong molecules are weak electrolytes/weak acids.
Show dissociation equations. HCl has single arrow,
CH3COOH has double arrow.
NaCl(cr)  Na+(aq) + Cl-(aq)
STRONG electrolyte
HCl(g) + H2O(l)  H3O+(aq) + Cl-(aq)
CH3COOH(aq) ↔ CH3
COO-(aq)
+
STRONG electrolyte
H+(aq)
weak
electrolyte