polarity of molecules. electronegativity the pull an atom has for the electrons it shares with...
TRANSCRIPT
Polarity of Molecules
Electronegativity
The pull an atom has for the electrons it shares with another atom in a bond.
Electronegativity is a periodic trendAs atomic radius increases and number of
electron shells increases, the nucleus of an atom has less of a pull on its outermost electrons
Periodic Table with Electronegativies
increases
decreases
Polar Bond
A polar covalent bond is when there is a partial separation of charge
One atom pulls the electrons closer to itself and has a partial negative charge.
The atom that has the electrons farther away has a partial positive charge
Model Electronegativity of central atom
Electronegativity of outer atom(s)
Subtraction Total
Type of Bond
3.0 3.0 0 Non-polar Covalent
N N
Two atoms sharing equally
N N
Each nitrogen atom has an electronegativity of 3.0
They pull evenly on the shared electrons
The electrons are not closer to one or the other of the atoms
This is a non-polar covalent bond
Atoms sharing almost equally
Electronegativities: H = 2.1 C = 2.5
The carbon pulls on the electrons slightly more, pulling them slightly towards the carbon
Put the difference isn’t enough to create a polar bond
This is a non-polar covalent bond
C HH
H
H
Sharing unevenly
Electronegativities: H = 2.1 C = 2.5 O = 3.5
The carbon-hydrogen difference isn’t great enough to create partial charges
But the oxygen atoms pulls significantly harder on the electrons than the carbon does. This does create a polar covalent bond
This is a polar covalent bond
C OH
H
Showing Partial Charges
There are two ways to show the partial separation of chargesUse of “” for “partial” Use of an arrow pointing towards the partial
negative atom with a “plus” tail at the partial positive atom
C OH
H
+ -C OH
H
Ionic Bonds
Ionic bonds occur when the electronegativies of two atoms are so different that they can’t even share unevenly…one atom just takes them from the other
How to determine bond type
Find the electronegativies of the two atoms in the bond
Find the absolute value of the difference of their valuesIf the difference is 0.4 or less, it’s a non-polar
covalent bondIf the difference is greater than 0.4 but less than
1.4, it’s a polar covalent bondIf the difference is greater than 1.4, it’s an ionic
bond
Let’s Practice
Example:If the bond
is polar, draw the polarity arrow
C – H
O—Cl
F—F
C—Cl
Let’s Practice
Example:If the bond
is polar, draw the polarity arrow
C – H
O—Cl
F—F
C—Cl
2.5 – 2.1 = 0.4 non-polar
3.5 – 3.0 = 0.5 polar
4.0 – 4.0 = 0.0 non-polar
2.5 – 3.0 = - 0.5 polar
Polar Bonds versus Polar Molecules
Not every molecule with a polar bond is polar itselfIf the polar bonds cancel out then the molecule
is overall non-polar.
The polar bonds cancel out.No net dipole
The polar bonds do not cancel out.
Net dipole
The Importance of VSEPR
You must think about a molecule in 3-D (according to VSEPR theory) to determine if it is polar or not!
Water drawn this way shows all the polar bonds canceling out. But water drawn in
the correct VSEPR structure, bent, shows the polar bonds don’t cancel out!
Net dipole
H O H
O H H
Let’s Practice
Example:Is NH3 a
polar molecule?
Let’s Practice
Example:Is NH3 a
polar molecule?
NH H
HElectronegativities:N = 3.0H = 2.1Difference = 0.9 Polar bonds
VSEPR shape = Trigonal pyramidal
Net dipole
Yes, NH3 is polar
Intermolecular Forces
Intra- versus Inter-molecular Forces
So far this chapter has been discussing intramolecular forcesIntramolecular forces = forces within the
molecule (chemical bonds)
Now let’s talk about intermolecular forcesIntermolecular forces = forces between
separate molecules
Breaking Intramolecular forces
Breaking of intramolecular forces (within the molecule) is a chemical change2 H2 + O2 2 H2O
Bonds are broken within the molecules and new bonds are formed to form new molecules
Breaking Intermolecular forces
Breaking of intermolecular forces (between separate molecules) is a physical changeBreaking glass is breaking the intermolecular
connections between the glass molecules to separate it into multiple pieces.
Boiling water is breaking the intermolecular forces in liquid water to allow the molecules to separate and be individual gas molecules.
London Dispersion Forces
All molecules have electrons.
Electrons move around the nuclei. They could momentarily all “gang up” on one side
This lop-sidedness of electrons creates a partial negative charge in one area and a partial positive charge in another.
+ Positively charged nucleus - Negatively charged electron
+-
-
-
-
Electrons are fairly evenly dispersed.
+--
- -As electrons move, they “gang up” on one side.
+
-
London Dispersion Forces
Once the electrons have “ganged up” and created a partial separation of charges, the molecule is now temporarily polar.
The positive area of one temporarily polar molecule can be attracted to the negative area of another molecule.
+ - + -
Strength of London Dispersion Forces
Electrons can gang-up and cause a non-polar molecule to be temporarily polar
The electrons will move again, returning the molecule back to non-polar
The polarity was temporary, therefore the molecule cannot always form LDF.
London Dispersion Forces are the weakest of the intermolecular forces because molecules can’t form it all the time.
Strength of London Dispersion Forces
Larger molecules have more electrons
The more electrons that gang-up, the larger the partial negative charge.
The larger the molecule, the stronger the London Dispersion Forces
Larger molecules have stronger London Dispersion Forces than smaller molecules.
All molecules have electrons…all molecules can have London Dispersion Forces
Dipole Forces
Polar molecules have permanent partial separation of charge.
The positive area of one polar molecule can be attracted to the negative area of another molecule.
+ - + -
Strength of Dipole Forces
Polar molecules always have a partial separation of charge.
Polar molecules always have the ability to form attractions with opposite charges
Dipole forces are stronger than London Dispersion Forces
Hydrogen Bonding
Hydrogen has 1 proton and 1 electron.There are no “inner” electrons. It bonds with the only
one it has.When that electron is shared unevenly (a polar
bond) with another atom, the electron is farther from the hydrogen proton than usual.This happens when Hydrogen bonds with Nitrogen,
Oxygen or FluorineThis creates a very strong dipole (separation of
charges) since there’s no other electrons around the hydrogen proton to counter-act the proton’s positive charge.
Strength of Hydrogen Bond
Hydrogen has no inner electrons to counter-act the proton’s charge
It’s an extreme example of polar bonding with the hydrogen having a large positive charge.
This very positively-charged hydrogen is highly attracted to a lone pair of electrons on another atom.
This is the strongest of all the intermolecular forces.
Hydrogen Bond
N
H H
N
H H
Hydrogen bond
Intermolecular Forces & Properties
IMF’s and Properties
IMF’s are Intermolecular ForcesLondon Dispersion ForcesDipole interactionsHydrogen bonding
The number and strength of the intermolecular forces affect the properties of the substance.
It takes energy to break IMF’sEnergy is released when new IMF’s are
formed
IMF’s and Changes in State
Some IMF’s are broken to go from solid liquid. All the rest are broken to go from liquid gas.
Breaking IMF’s requires energy.
The stronger the IMF’s, the more energy is required to melt, evaporate or boil.
The stronger the IMF’s are, the higher the melting and boiling point
Water
Water is a very small moleculeIn general small molecules have low melting and
boiling pointsBased on it’s size, water should be a gas under
normal conditionsHowever, because water is polar and can form
dipole interactions and hydrogen bonding, it’s melting point is much higher
This is very important because we need liquid water to exist!
IMF’s and Viscosity
Viscosity is the resistance to flowMolasses is much more viscous than
water
Larger molecules and molecules with high IMF’s become inter-twined and “stick” together more
The more the molecules “stick” together, the higher the viscosity
Solubility
In order from something to be dissolved, the solute and solvent must break the IMF’s they form within itself
They must then form new IMF’s with each other
Solubility
- +
- +
- + - +- +
Solvent, water (polar)
+
-- + Solute, sugar (polar)
Water particles break some intermolecular forces with other water molecules (to allow them to spread out) and begin to form new ones with the sugar molecules.
Solubility
Solvent, water (polar)
+
-- + Solute, sugar (polar)
As new IMF’s are formed, the solvent “carries off” the solute—this is “dissolving”
- +
- +
- +- + - +
Solubility
If the energy needed to break old IMF’s is much greater than the energy released when the new ones are formed, the process won’t occurAn exception to this is if more energy is added
somehow (such as heating)
Oil & Water
Water has London Dispersion, Dipole and hydrogen bonding. That takes a lot of energy to break
Water can only form London Dispersion with the oil. That doesn’t release much energy
Much more energy is required to break apart the water than is released when water and oil combine.
Water is polar and can hydrogen bond, Oil is non-polar.
Therefore, oil and water don’t mix!
Surface Tension
Surface tension is the resistance of a liquid to spread out.This is seen with water on a freshly waxed car
The higher the IMF’s in the liquid, the more the molecules “stick” together.
The more the molecules “stick” together, the less they want to spread out.
The higher the IMF’s, the higher the surface tension.
Soap & Water
Soap has a polar head with a non-polar tail
The polar portion can interact with water (polar) and the non-polar portion can interact with the dirt and grease (non-polar).
Polar head
Non-polar tailSoap
Soap & Water
The soap surrounds the “dirt” and the outside of the this Micelle can interact with the water.
The water now doesn’t “see” the non-polar dirt.
Dirt
Soap & Surface Tension
The soap disturbs the water molecules’ ability to form IMF’s and “stick” together.
This means that the surface tension of water is lower when soap is added.
The lower surface tension allows the water to spread over the dirty dishes.
What did you learn about soap?
Soap
Inter-molecular forces
Inter-molecular forces
Works based on
Molecular Geometry
Molecular Geometry
Bonding types &
Structures
Bonding types &
Structures
Determined by
Determined by