Some notes



Devotional
Oct 30,
The TA walk in lab will be
closed today for grading.
You may not want to sit in the
front row today
Unregistered clickers:
 1B37B39F
 1B462578
 1B3E85A0
Reactions go at different speeds

Cellulose + O2  CO2 + H2O



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Dead tree in forest
Wood in fire
Sawdust in explosion
Why?
What governs speed of a
reaction?


Definition: rate that reactants are
consumed, or products are produced
Factors:




Physical state of reactants
 Temp & Pressure
Collision rate
Energetic requirements
Entropic (organization) requirements
Why doesn’t your
diamond turn into a
lump of coal?
Downhill, but slow!
Effect of energy


The net energy released
(or consumed) does not
affect rates
The energy required to
reach the transition state
(“activation energy”) has
large effect

Transition state


The critical arrangement of atoms where the reacting system
“decides” whether or not to make products
Typically involves breaking and/or formation of bonds
Effect of
Entropy
Br + CH3Cl  CH3Br + Cl


Reactants frequently must have a certain orientation at
the transition state, or reaction will not occur
This corresponds to order in the transition state:
entropy of activation
Examples of Non-Reactive Collisions
Effect of
Entropy
Br + CH3Cl  CH3Br + Cl


Reactants frequently must have a certain orientation at
the transition state, or reaction will not occur
This corresponds to order in the transition state:
entropy of activation
Catalysts




Decrease energy or
increase entropy of
activation without
themselves being
consumed; speed up
rates
H2O2 decomposes
spontaneously
But, much faster in
presence of Br- (or
MnO2).
Catalyst makes new,
lower energy route
possible
Getting Help from Catalysts
Without catalyst
Need a spark
With catalyst
No spark needed
An example



H2
H 2 + O2
H2 + O2 w/
Pd/C
catalyst
Adding O2 allows
faster mixing
Pd surface stretches
the H-H bond, helping
it break and lowering
transition state energy
Equilibrium

Reactions go in both directions:
Reactants


Products
You’ve already seen this in the H2 + O2
reaction
This dynamic balance is called “chemical
equlibrium”
Meaning of Equilibrium


When forward and
reverse rates are equal,
amount of reactant &
product no longer
changes
Equilibrium is the state of
lowest energy, entropy;
everything moves toward
it
Collision Rates Limit Reactions
that take place in gases or liquids


Reactants must get close for electron clouds
to interact
What controls collision rate?



Temperature (because it is a reflection of
molecular speed)
Physical state
Pressure (for gases) or concentration (for liquids)
Some notes

The TA walk in lab will be
closed today for grading.

Unregistered clickers
from Monday’s Quiz:
 1B37B39F
 1B462578
 1B3E85A0
Devotional
Oct 30,
Chapters 21-22-23

Revisit the hypotheses of Chapter 12:
1: temperature at which change of state
takes place reflects the strength of forces
holding matter together
2: high temperature changes reflect
strong forces
Wave model of the atom provides explanations
for the observations of Chapter 12 and other observations
How do you identify metals using
the periodic table?
Nonmetals
Nonmetals
Metals
Metals
Quiz:
Which elements lose electrons
easily (low ionization energy)?
1.
2.
Metals
Non-metals
Quiz:
For a given row of elements, valence
electrons of metals are further away from
the nucleus.
1.
2.
True
False
Argon
Neon
Krypton
Xenon
Why do metals melt at high
temperatures?
MELTING
TEMPERATURE
BOILING
TEMPERATURE
BC
BC
(under 1 atm pressure)
State
At
Room
Temperature
doesn’t form solid
except under high
pressure!
-269
Gas
Hydrogen
-259
-253
Gas
Neon
-249
-246
Gas
Nitrogen
-210
-196
Gas
0
100
Liquid
Ethanol
-117
78.5
Liquid
Table salt
801
1413
Solid
Copper
1083
2567
Solid
Gold
1065
2807
Solid
Helium
Water
Why are metals dense?
DENSITY
g / cm3
Solid
Helium
Hydrogen
0.078
Neon
1.54
Nitrogen
Water
1.09
0.90 (0ºC)
Ethanol
1.3
Table salt
Copper
Gold
2.2
8.9
19.3
What other properties do metals have?






Classified as network-type solids
Generally have:
 High Melting T
 High Boiling T
 High Density
Conduct both heat and electricity
Malleable – flatten into thin sheets
Opaque – can’t see through even
thin sheets
Reflective (Metallic Luster) – shiny
ONE BONDING MODEL EXPLAINS THEM ALL – THE METALLIC BOND
Which electrons form bonds?
1.
2.
3.
The outer or valence
electrons
the inner electrons with
lowest energies
All electrons participate
equally in bonding.
What happens if
“metal molecules” get
bigger and bigger?
Orbitals
Atomic
Molecular
Li2
↑
antibonding
↑
2s1 2s1
↑↓
Li3
↑ ↑ ↑
↑
2s1 2s1 2s1
↑↓
Li4
↑ ↑ ↑ ↑
2s1 2s1 2s1 2s1
↑↓
↑↓
bonding
Look at the progression of
molecular orbitals
when more atomic
orbitals become involved.
Notice that in each
molecule, there are empty
orbitals.
What would be the pattern of
orbitals if you had a “molecule”
with 1023 atoms?
(about 27 g of Al, 200 g of Hg)
From atoms to molecules to
metals
LiN(metal)
N a very large number
unfilled
levels
↑ ↑ ↑ ↑ ↑ ↑ ↑ ↑
2s1 2s1 2s1 2s1 2s1 2s1 2s1 2s1
=
filled
levels
N atomic orbitals
N molecular orbitals
very very closely spaced together
Many closely spaced molecular orbitals gives
rise to a continuous energy band



Metals have few valence
electrons compared to number
of orbitals
Electrons want to be in low
energy states – “filled” levels
Levels still exist (and easily
available) even when there are
no electrons in them – “empty”
levels
Where are valence electrons in metals?




Molecular orbitals extend
through entire piece of metal
(we say they are
delocalized)
Lots of available energy
levels for electrons
Small amounts of energy
can move electrons between
orbitals
Electrons not tightly attached
to any particular atom – “Sea
of Electrons”
The energy band with mobile electrons explains all metallic properties!
Properties that arise from the Energy
Band and Mobile Electrons
High melting temperatures – nuclei surrounded by electron sea
melting requires the breaking of strong attractive interactions
Electrical conductivity – mobile charge carriers are the electrons
Thermal conductivity – electrons can absorb/give up heat easily; transport
it away because of electron mobility
Malleability – electrons serve as lubricant, allowing layers of nuclei to
slide past one another
l20_metal masher.swf
Properties arising from the Energy Band
and Mobile Electrons
Opacity – Metals can absorb all colors of light
animation: l20_photon.swf
Luster and Reflectivity – also tied to existence
of many available energy
levels.
What is an ALLOY? Why do they form?
Alloy = A combination of two or more metals to
make a new metal


Examples:
 Copper + Zinc → Brass
 Copper + Tin → Bronze
 Gold + Nickel (± Palladium, Zinc) → White Gold
 Iron + Carbon → Steel
 Iron + Chromium (± others) → Stainless Steel
Alloys form because metal atoms all have:


Few valence electrons
Low ionization energies
Classification of the Elements
Metals
Size a constraint for making alloys: very similar or very different favored
Are the properties of alloys
the same as pure metals?
NOT QUITE



Alloys not as good conductors
of electricity and heat
Alloys often melt at a lower
temperature
Alloys may be less malleable
than pure metals
low-melting solder
used to hold copper pipe
together is an alloy
What is a Semi-conductor?


Not quite metal or non-metal
Have some properties of metals



Conduct electricity under certain conditions
Solids with high melting points
Widely used in computers and other electronic
devices

Particularly, Si and Ge
Higher ionization energies and
more valence electrons
give rise to change in the
energy level structure
The electrical conductivity of semiconductors and metals
with Temperature is very different.
Semiconductors are
poor conductors at low temp
They are
better
conductors
at high temp
but still not as
good as metals
Resistivity: measure of the resistance to flow of current;
a high number means bad conductor
What’s the basis for differences from metals?
Si (3s23p2)
Ge (4s24p2)
Compared to metals:
Semiconductors have a relatively large number of valence electrons,
held with reasonable tightness, relatively close to the nucleus
More electrons, different energies and closer distance to
the nucleus…. Atomic orbitals interact differently.
BAND
MO’s of semiconductors have one major difference.
BAND
BAND
GAP
Energy Band Splits in Two
NO LEVELS IN GAP!!
Valence Band filled –
no electrical conductivity
possible at low temperatures
BAND
Remember, conductivity requires
mobile electrons with
empty energy levels to move into.
BAND
BAND
BAND
Electrical Conductivity of Semiconductors at High
Temperatures….
Some electrons get kicked upstairs at high temperatures
gaining access to empty levels AND
creating “holes” down below
Light Emitting Diodes – An application of
BAND
semiconductors
Electrical energy (from a battery) can also kick
electrons upstairs into conduction band.
BAND
When electrons fall back
downstairs they emit photons!
Photon color emitted by diode
Is related to energy of band gap
You have 3 LED’s. One gives off red light, one blue light and one
green light.
Which energy structure corresponds to the green LED?
1.
2.
3.
LED1
LED2
LED3
33%
33%
ENERGY
33%
10
0
of
5
LED 1 LED 2 LED 3
Answer Now
1
2
3