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Chapter 12

CHEM 1000 Chapter Notes - Chapter 12: Close-Packing Of Equal Spheres, Bond Length, Ionic Compound


Department
Chemistry
Course Code
CHEM 1000
Professor
William Pietro
Chapter
12

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Chapter 12 - Intermolecular Forces: Liquids + Solids
Intermolecular forces occur when there are weak electrostatic
attractions between molecules. Electrostatic attractions occur when an
electrically positive region is attracted to a e. negative region.
Instantaneous dipole – is when electrons are concentrated in one region
at a given time (by chance). This causes charge separation, of – and + are
each side, creating a polar molecule (in a molecule that is USUALLY
nonpolar).
this is based on electron charge density, or probability of an e-
being at a region at a particular time.
Induced dipole – the instantaneous molecule induces a charge
separation in the molecule/atom beside it.
This is why solids + liquids have strong intermolecular forces, because
of dipoles.
Polarizability – the tendency for charge distribution to distort from its
normal shape in an atom/molecule. The greater this tendency, the more
polarizable. Polarizability increases with atomic or molecular size.
Because of e- being farther from the nucleus in larger molecules it
is not firmly held in place, and therefore can be easily displaced.
Because dispersion forces become stronger as polarizability
increases, and usually increases with MW, the melting and boiling
points also increase.
Electrons in elongated molecules are more displaced than are
those in small, compact, symmetrical molecules
Dipole-Dipole Interactions
In a polar molecule, they have permanent dipole moments and therefore
the molecules tend to line up with the + end directed toward the – ends
of neighboring dipoles. This ordering is what can cause molecules to
become solids + liquids at temps higher than expected.
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Hydrogen Bonding
In a hydrogen bond, the H is covalently bonded to a highly EN atom,
which attracts the H atoms only e- away from its nucleus towards the
highly EN it is bonded to. This leaves the H nucleus exposed and
attracted to a lone pair of e- on another highly EN atom in a neighboring
molecule. This is only possible with F, N, O. This occasionally happens
with Cl or S in a neighboring molecule, but is considered weak.
Intramolecular H bonding: H atom bonding to a high EN atom and
with another atom nearby in the same molecule.
12-2
Enthalpy of Vaporization
Vaporization or evaporation – is the passage of molecules from the
surface of the liquid into the gaseous or vapor state. This occurs more
readily with: 1. increased temp (more molecules have sufficient KE to
overcome IMF in the liquid),
2. increased surface area (more liquid molec. At surface)
3. decreased strength of intermolecular forces (the KE needed to
overcome IMF is less, more molecules have enough energy escape).
-> because the molecules lost thru vaporization are more energetic than
avg, the avg KE decreases with the remaining molecules and temp
decreases.
-> To vaporize a liquid at const. temp, we must replace excess energy
carried away by the vaporizing molecules by adding heat to the liquid.
Hvap is always +, the differences in enthalpies is based on IMF. Ex,
Condensation is the conversion of gas to liquid. Which is the reverse of
HVap.
Equation:
Vapor Pressure
The pressure exerted by vapor in the equilibrium with its liquid.
Occurs when both liquid and vapour are both present in a container.
Condensation + Vaporization is happening simultaneously. When
enough water is present and vapor is constant, they have reached
equilibrium. Liquids with high v.p. is considered volatile, liquids with
low v.p. is considered nonvolatile. The weaker the IMF, the more volatile
a liquid is. Vapor pressure depends on liquid and temp, not amounts.
V.p. increases with temp.
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