intermolecular forces
DESCRIPTION
Intermolecular Forces. Forces Between Molecules. Why are intermolecular forces important?. They determine the phase of a substance at room temperature. (Competition with kinetic energy.). Solids. Strong Intermolecular Forces. Gases. Weak Intermolecular Forces. - PowerPoint PPT PresentationTRANSCRIPT
Intermolecular Forces
Forces Between Molecules
Why are intermolecular forces important?
They determine the phase of a substance at room temperature. (Competition with kinetic energy.)
Solids
Strong Intermolecular Forces
Gases
Weak Intermolecular Forces
3 Types of Intermolecular Forces
Dispersion or Van der Waals Dipole-dipole
Hydrogen Bonds
Strongest Intermolecular Force
Hydrogen Bonds
Hydrogen Bonds
When do Hydrogen bonds occur?
Between molecules containing F, O, N bonded to a
H atom
Strongest hydrogen bonds?
Between molecules containing F-H bonds
Weakest hydrogen bonds?
Between molecules containing N-H bonds
What effect does H-bonding have on the properties of the
substance?
Hydrogen bonding leads to substantial increases in the
expected boiling point.
Weakest Intermolecular Force
Dispersion or Van der Waals forces
When do dispersion forces occur?
Occur between all molecules. Most important between
nonpolar molecules
Nonpolar
No Poles!The molecule is symmetric!
Cannot tell 1 end from the other.
2 Easy categories of Nonpolar Molecules
Monatomic Gases (Column 18)(kickballs)
&Diatomic Elements
(H2, N2, O2, F2, Cl2, Br2, I2)(footballs)
2 More Easy categories of Nonpolar Molecules
Hydrocarbons(CxHy)
&Small symmetric molecules(CO2, CS2, CF4, CBr4, etc.)
What are the diatomic elements?
H2, N2, O2, F2, Br2, Cl2, & I2
What can you say about dispersion forces as a
function of molecular size?
The larger the molecules the stronger the dispersion
forces!
When do dipole-dipole forces occur?
Between polar molecules.Permanent separation of charge.
1 side - (electron rich) vs. 1 side + (electron poor)
Polar
Has Poles!The molecule is NOT
symmetric!Can tell 1 end from the other.
Vapor
Gas phase of a substance that is normally a liquid at room
temperature.
Vapor Pressure
The partial pressure of a vapor above its liquid.
What does vapor pressure depend on? (Hint: see Table
H)
Vapor pressure depends on the temperature of the liquid.
What happens to the vapor pressure as the temperature of
the liquid is increased?
The vapor pressure always increases with temperature.
Does the vapor pressure depend on the amount of
liquid present?
No. A thimbleful is as good as an ocean.
What are some differences between evaporation &
boiling?
Evaporation occurs at all temperatures. Boiling occurs at a
definite temperature. Evaporation occurs at the surface.
Boiling occurs throughout the liquid.
Heat of fusion
Amount of energy required to convert 1 gram of a pure solid to
the liquid phase at its melting point.
Heat of vaporization
Amount of energy required to convert 1 gram of a pure liquid to the gas phase at its boiling
point.
What happens to the boiling point as the intermolecular
forces increase?
The stronger the intermolecular forces, the higher the boiling point.
What happens to the melting point as the intermolecular
forces increase?
The stronger the intermolecular forces, the higher the melting point.
What happens to the heat of fusion as the intermolecular
forces increase?
The stronger the intermolecular forces, the higher the heat of fusion.
What happens to the heat of vaporization as the
intermolecular forces increase?
The stronger the intermolecular forces, the
higher the heat of vaporization.
What happens to the evaporation rate as the intermolecular forces
increase?
The stronger the intermolecular forces, the
lower the evaporation rate.
What happens to the vapor pressure as the intermolecular
forces increase?
The stronger the intermolecular forces, the lower the vapor pressure.
Pre
ssu
re,
atm
Temperature, C
Relationship between IMF and physical properties?
IMF BP MP Hf HvEv.Rt V.P.
IMF BP MP Hf Hv EvRt VP
Boiling Point
Temperature at which the vapor pressure of a liquid is
equal to the external pressure.
Normal Boiling Point
Temperature at which the vapor pressure of a liquid is
equal to 1 atm or 101.3 kPa or 760 torr.
What happens to the boiling point when the external
pressure is reduced?
The boiling point is reduced.
Sublimation
Solid to gas
Deposition
Gas to solid
Vaporization
Liquid to Gas
Condensation
Gas to liquid
Fusion
Solid to liquidOr
Melting
Freezing
Liquid to solidCrystallizationSolidification
Which phase changes are endothermic?
Solid to liquidLiquid to GasSolid to Gas
G
L
S
Which phase changes are exothermic?
Gas to LiquidLiquid to SolidGas to Solid
G
L
S
Given 3 substances at room temperature: 1 solid, 1 liquid, & 1 gas, which has the strongest intermolecular forces?
Solid > Liquid > Gas
Describe the mp & bp of gases.
Low!
What happens to the temperature as heat is added to a substance at its melting or boiling point?
Nothing, until the phase change is complete. The heat
energy is going into the potential energy of the
system.
HF
HF
--------
Hydrogen Bonding!
Ne
Ne
--------
Van der Waals or dispersion forces
H-Cl
H-Cl
--------
Dipole-dipole forces
H-N
H-F
--------
Hydrogen Bonding
H H--
OH
H
-
-----------
----
---
Time
Tem
pera
tur
e
I II III IV V
Solid
Solid & Liquid
Liquid
Liquid & Gas
Gas
K.E. K.E. K.E.
K.E.↔K.E.↔P.E.↔
P.E.↔P.E.↔
P.E.P.E.Melt pt.
Boil pt.
Equation for a pure substance in a single phase being warmed up or cooled down.
Q = mCTQ = energy in Joulesm = mass in grams
C = specific heat in J/gT = temperature change
Equation for a substance melting or freezing at the freezing point.
Q = mHf
Q = energy in Joulesm = mass in gramsHf = heat of fusion
Equation for a substance boiling or condensing at the boiling point.
Q = mHv
Q = energy in Joulesm = mass in grams
Hv = heat of vaporization
Temperature of the ice-water equilibrium.
0C or 273KFreezing pointMelting point
Temperature of the steam-water equilibrium.
100C or 373KBoiling point
Condensation point