AP Chapter 11. Intermolecular Forces / Liquids and Solids HW: 2 4 6 13 15 16 19 31 39 45 105. 11.1 – A Comparison of the States of Matter. The fundamental difference between states of matter is the distance between particles. Indefinite volume Indefinite shape
Intermolecular Forces / Liquids and Solids
HW: 2 4 6 13 15 16 19 31 39 45 105
The fundamental difference between states of matter is the distance between particles.
Rotational, Vibrational and Translational Particle Motion
Rotational and Vibrational
Because in the solid and liquid states particles are closer together, we refer to them as condensed phases.
The attractions between molecules are not nearly as strong as the intramolecular attractions (covalent bonds) that hold compounds together.
They are, however, strong enough to control physical properties such as boiling and melting points, vapor pressures, and viscosities.
Intermolecular forces between molecules are referred to as van der Waals forces.
The more polar (larger resultant dipole moment) the molecule, the higher is its boiling point because the attractions are stronger.
Hydrogen bonding arises in part from the high electronegativity of nitrogen, oxygen, and fluorine.
Because when hydrogen is bonded to one of those very electronegative elements, the hydrogen nucleus is exposed.
b. London Dispersion Forces(occur between nonpolar substances/atoms)-Induced Dipoles (not all books talk about these. Not a TRUE London Dispersion Force)
And then is attracted to dipole
While the electrons in the 1s orbital of helium would repel each other (and, therefore, tend to stay far away from each other), it does happen that they occasionally wind up on the same side of the atom. Causes an instantaneous dipole. The same can occur in any molecule…
Looks like this
At that instant, then, the helium atom is polar, with an excess of electrons on the left side and a shortage on the right side.
Another helium nearby, then, would have a dipole induced in it, as the electrons on the left side of helium atom 2 repel the electrons in the cloud on helium atom 1.
-A London Dispersion Force Occurs
-London dispersion forces, or dispersion forces, are attractions between an instantaneous dipole and an induced dipole.
(that’s why oil is thicker than water)
Why choose these two molecules to compare? Both have identical numbers of electrons, and if you made models you would find that the sizes were similar - as you can see in the diagrams. That means that the dispersion forces in both molecules should be much the same.
The higher boiling point of fluoromethane is due to the large permanent dipole on the molecule because of the high electronegativity of fluorine. However, even given the large permanent polarity of the molecule, the boiling point has only been increased by some 10°. (Dipole-Dipole Interactions don’t always make that much of a difference).
Here is another example showing the dominance of the dispersion forces. Trichloromethane, CHCl3, is a highly polar molecule because of the electronegativity of the three chlorines. There will be quite strong dipole-dipole attractions between one molecule and its neighbours.
On the other hand, tetrachloromethane, CCl4, is non-polar. The outside of the molecule is uniformly - in all directions. CCl4 has to rely only on dispersion forces
So which has the highest boiling point? CCl4 does, because it is a bigger molecule with more electrons. The increase in the dispersion forces more than compensates for the loss of dipole-dipole interactions.
The strength of the attractions between particles can greatly affect the properties of a substance or solution.
No molecules pull upward on the surface
Surface tension results from the net inward force experienced by the molecules on the surface of a liquid.
-Cohesion – Intermolecular attraction between liquid molecules
-Adhesion – Attraction between unlike molecules (ex – liquid and the tube)
Adhesion > Cohesion
Cohesion > Adhesion
As more molecules escape the liquid, the pressure they exert increases.
The liquid and vapor reach a state of dynamic equilibrium: liquid molecules evaporate and vapor molecules condense at the same rate. (in a closed container)
ln(Pvap) = -DHvap/(RT) + c
Sometimes shown without – and T2 and T1 reversed…
Heating / Cooling Curves:
q = msDT
q = m*1/M*DH
Phase diagrams display the state of a substance at various pressures and temperatures and the places where equilibria exist between phases.
It ends at the critical point (B); above this critical temperature and critical pressure the liquid and vapor are indistinguishable from each other.
Each point along this line is the boiling point of the substance at that pressure.
Carbon dioxide cannot exist in the liquid state at pressures below 5.11 atm; CO2 sublimes at normal pressures.
Because of the order in a crystal, we can focus on the repeating pattern of arrangement called the unit cell.
-Quickly cooled liquid->solid