Transcript Review of Molecular Motion

Matter, Motion, and Entropy:
A Review
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CHM 102 Sinex
What is the difference between
an atomic and molecular
substance?
Atomic
single atom
He
Ar
Fe
Molecular
multiple atoms
N2
CO2
C6H12
Describe the types of motion
• Translational
whole atom or molecule changes its
location in three dimensional space
• Rotational
Motion
of whole
molecule
whole molecule spins around an axis in
three dimensional space
• Vibrational
 Motion within molecule
motion that changes the shape of the molecule –
stretching, bending, and rotation of bonds
Which type of motion contributes the most to
the entropy of a substance?
Translational motion
Which type of motion is the cause of collisions?
Translational motion
Which type of motion influences the orientation
for an effective collision?
Rotational motion
Types of Motion and
the States of Matter
Matter
State
Gas
Liquid
Solid
Type
Mode
Translation Rotation Vibration
Atomic
Yes
No
No
Molecular
Yes
Yes
Yes
Atomic
Yes
No
No
Molecular
Yes
Yes
Yes
Atomic
No
No
Yes
Molecular
No
No
Yes
Types of Vibrations
Vibration
Minimum How atoms
number of
are
atoms
connected
What
changes?
Stretching
2
Bond
length
Bending
3
Bond angle
4
Torsion
angle
Internal
rotation
In solids such as the carbon tetrahedron
from the diamond structure, how would
you describe the motion of the atoms?
As the carbon atoms vibrate, all the
bond lengths vary about an average
value.
Describe all the modes of vibration
in each of the molecules.
N2
stretching only
CO2
stretching, both symmetric and
asymmetric, and bending
C4H10
stretching, bending, and internal rotation
Which molecule has the highest entropy?
What hinders internal rotation
in a molecule?
• Double or triple bonds
• Bulky groups in certain positions on
some molecules
• Ring formation - cyclic molecules
• Sometimes hybrid orbitals (lone pairs
of electrons) on single bonds such as
hydrogen peroxide
• Molecules bound in a crystalline solid
Examples are shown on the following three sides!
Twisting vibration around double bond in ethene, C2H4
Click to view a
movie of the
vibration
Two methyl groups (-CH3) on biphenyl
Chime Guide
Click to view Chime
structure of lowest
energy state
Restricted internal rotation or twisting vibration in ring
C6H12
Click here for Chime
structure – you will
need to right click on
the Chime structure
and click on animation
Hydrogen peroxide, twisting motion along single bond
Click here for Chime
structure – you will
need to right click on
the Chime structure
and click on animation
H2O2 has a permanent dipole moment m = 2.0 D
Molecules in solid CO2 –
note the tight packing of molecules
How would larger
molecules ever
move around?
Chime structure
Hexane, C6H12, in
the solid state –
note the straight
chain arrangement
of the molecules
Spacefilled
From:
Roland Boese
per. comm.
Does the phase make a
difference in biphenyl?
solid
Both rings in the
same plane
Chime structure
gas
40o angle between rings
still allows pi clouds to interact
Chime structure
What is the cause of the entropy
of solid carbon monoxide?
Vibrations and arrangement of the molecules of CO
Why do solids have low entropies?
Stotal = Strans + Srot + Svib
Strans > Srot >> Svib
How do impurities in a solid influence
the solid’s entropy?
Impurities cause an increase in entropy.
Here is a crystal of the mineral zircon.
Click on the image, and radioactive decay
will occur. How does entropy change?
ZrSiO4
Source: http://www.esc.cam.ac.uk/movies
The decay particle causes a zirconium ion to recoil and it
disrupts the structure which slowly recovers. In the end,
the crystal structure becomes more disordered or DS > 0.
Proteins – long chains of amino acids
chain*
How does the entropy
change as the chain takes
the form of a helix?
coiling of 10 glycine chain
helix*
DS < 0 due to limited internal rotation
in hydrogen-bonded helix (secondary
structure of protein)
*Chime structures