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Intermolecular Forces and
Liquids and Solids
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
A phase is a homogeneous part of the system in contact with other parts of the system but separated from them by a well-defined boundary.
2 Phases
Solid phase - ice
Liquid phase - water
11.1
Intermolecular Forces
11.2
Intermolecular forces are attractive forces between molecules.
Intramolecular forces hold atoms together in a molecule.
Intermolecular vs Intramolecular
• 41 kJ to vaporize 1 mole of water (inter)
• 930 kJ to break all O-H bonds in 1 mole of water (intra)
Generally, intermolecular forces are much weaker than intramolecular forces.
“Measure” of intermolecular force
boiling point
melting point
Hvap
Hfus
Hsub
Intermolecular Forces
Dipole-Dipole Forces
Attractive forces between polar molecules
Orientation of Polar Molecules in a Solid
11.2
Intermolecular Forces
Ion-Dipole Forces
Attractive forces between an ion and a polar molecule
11.2
Ion-Dipole Interaction
11.2
Intermolecular ForcesDispersion Forces
Attractive forces that arise as a result of temporary dipoles induced in atoms or molecules
11.2
ion-induced dipole interaction
dipole-induced dipole interaction
Intermolecular ForcesDispersion Forces Continued
11.2
Polarizability is the ease with which the electron distribution in the atom or molecule can be distorted.
Polarizability increases with:
• greater number of electrons
• more diffuse electron cloud
Dispersion forces usually increase with molar mass.
SO
O
What type(s) of intermolecular forces exist between each of the following molecules?
HBrHBr is a polar molecule: dipole-dipole forces. There are also dispersion forces between HBr molecules.
CH4
CH4 is nonpolar: dispersion forces.
SO2
SO2 is a polar molecule: dipole-dipole forces. There are also dispersion forces between SO2 molecules.
11.2
Intermolecular ForcesHydrogen Bond
11.2
The hydrogen bond is a special dipole-dipole interaction between they hydrogen atom in a polar N-H, O-H, or F-H bond and an electronegative O, N, or F atom.
A H…B A H…Aor
A & B are N, O, or F
Hydrogen Bond
11.2
Properties of Liquids
Surface tension is the amount of energy required to stretch or increase the surface of a liquid by a unit area.
Strong intermolecular
forces
High surface tension
11.3
Properties of Liquids
Cohesion is the intermolecular attraction between like molecules
11.3
Adhesion is an attraction between unlike molecules
Adhesion
Cohesion
Properties of Liquids
Viscosity is a measure of a fluid’s resistance to flow.
11.3
Strong intermolecular
forces
High viscosity
Maximum Density40C
Ice is less dense than water
Density of Water
11.3
Water is a Unique Substance
Types of Crystals
Ionic Crystals• Lattice points occupied by cations and anions• Held together by electrostatic attraction• Hard, brittle, high melting point• Poor conductor of heat and electricity
CsCl ZnS CaF2
11.6
Types of Crystals
Covalent Crystals• Lattice points occupied by atoms• Held together by covalent bonds• Hard, high melting point• Poor conductor of heat and electricity
11.6diamond graphite
carbonatoms
Types of Crystals
Molecular Crystals• Lattice points occupied by molecules• Held together by intermolecular forces• Soft, low melting point• Poor conductor of heat and electricity
11.6
Types of Crystals
Metallic Crystals• Lattice points occupied by metal atoms• Held together by metallic bonds• Soft to hard, low to high melting point• Good conductors of heat and electricity
11.6
Cross Section of a Metallic Crystal
nucleus &inner shell e-
mobile “sea”of e-
Types of Crystals
11.6
An amorphous solid does not possess a well-defined arrangement and long-range molecular order.
A glass is an optically transparent fusion product of inorganic materials that has cooled to a rigid state without crystallizing
Crystallinequartz (SiO2)
Non-crystallinequartz glass 11.7
The boiling point is the temperature at which the (equilibrium) vapor pressure of a liquid is equal to the external pressure.
The normal boiling point is the temperature at which a liquid boils when the external pressure is 1 atm.
11.8
Mel
ting
11.8F
reez
ing
H2O (s) H2O (l)
The melting point of a solid or the freezing point of a liquid is the temperature at which the solid and liquid phases coexist in equilibrium
PHASE TRANSISTIONSPHASE TRANSISTIONSALSO CALLED CHANGES OF STATE, HAPPENS BY CHANGING THE TEMPERATURE
AND/OR PRESSURE OF A SUBSTANCE.
SOLID TO LIQUID: MELTINGMELTINGLIQUID TO SOLID: FREEZINGFREEZING
GAS TO LIQUID: CONDENSATIONCONDENSATIONLIQUID TO GAS: EVAPORATIONEVAPORATION
SOLID TO GAS: SUBLIMATIONSUBLIMATIONGAS TO SOLID: DEPOSITIONDEPOSITION
Intermolecular ForcesIntermolecular ForcesLondon Dispersion Forces:London Dispersion Forces: Also called Induced
dipole forces. A dipole is a separation of charge. An instantaneous dipole is created within the atom or molecule via the instantaneous movement of the electrons around the nucleus. All molecules have LDF.
Dipole-Dipole Forces:Dipole-Dipole Forces: The attractive force between molecules due to the existence of an overall dipole moment. Polar molecules have d-d forces.
Hydrogen Bonding:Hydrogen Bonding: The attractive force between a highly electronegative atom of one molecule with the hydrogen on another molecule also containing a very electronegative atom. N, O, F are the electronegative atoms.
Interrogating interactions
Why can bugs walk on water?
Why green strawberry plastic baskets float on water despite the 1” open square “holes”??
Properties dependent on the Intermolecular ForcesProperties dependent on the Intermolecular Forces
SURFACE TENSION:SURFACE TENSION: describes the resistance that a liquid has to an increase in its own surface area.
• Intermolecular forces create surface tension – an invisible “skin” holding the molecules together
• Force must be applied to break the “skin”
Properties dependent on the Intermolecular Forces
EVAPORATION: Evaporation of a liquid occurs when the
average kinetic energy present within the liquid is greater than the intermolecular forces responsible for holding the substance in its liquid state. When the particles have enough kinetic energy to overcome these attractive forces, the particles will escape from the surface to become a gas.
Energy must be added to a system to overcome the attractive forces that are exerted among liquid
molecules. When an equal quantity of vapor condenses to a liquid, an equal amount of energy is
released.
WHAT HAPPENS DURING BOILING?? Describe in terms of…
1)Molecular movement in liquids vs. gases?2)Energy transfer
3)Forces (attractive and intermolecular forces)
Explain the physical process of boiling.Explain the physical process of boiling.At room temperature the water molecules have
enough energy to allow the particles to move past each other but not enough to escape the surface tension.
As the temperature of water increases, the heat energy (from the burner) is transferred to kinetic energy (for the molecules) leading to an increase in the molecular motion of the molecules. This action results in an increase in the vapor pressure above the surface of the liquid.
When the vapor pressure of the water equals the external pressure, boiling begins. Now a sufficient amount of the molecules have enough energy to resist the attractive forces. Bubbles of vapor are formed throughout the liquid and these bubbles rise to the surface to escape.
Properties dependent on the Intermolecular Forces• VAPOR PRESSURE: When a liquid
evaporates in a closed container, the gaseous vapor that forms at the surface of the liquid eventually establishes an equilibrium with the particles remaining in the liquid state. Equilibrium is established when the rate of evaporation is equal to the rate of condensation.
• A VOLATILE substance evaporates readily, has a low surface tension, and a high vapor pressure at ambient temperature. The Intermolecular forces are weak.
• A NONVOLATILE substance requires a large amount of energy to evaporate, has a high surface tension, and a low vapor pressure. The intermolecular forces are strong.
THE DISSOLVING PROCESS
Intermolecular Forces: “Like dissolves like”
Miscible / Immiscible: Two liquids are miscible in each other if they readily mix to form a uniform
solution. Two immiscible liquids will always separate out into two distinct layers.
HYDRATES HYGROSCOPIC DELIQUESCENT
DESICCANTSEFFLORESCENCEHYDROPHOBIC / HYDROPHILLIC
THE DICTIONARY GAME:
PROPERTIES ASSOCIATED WITH WATERPROPERTIES ASSOCIATED WITH WATERHYDRATES: Solids that contain water molecules as part of
their crystalline structure. The water in the hydrate is known as the water of hydration or the water of crystallization.
HYGROSCOPIC: A substance is hygroscopic if it readily absorbs water from the atmosphere and forms a hydrate.
DELIQUESCENT: A substance is deliquescent if it absorbs water from the air until it forms a solution.
DESICCANTS: Compounds that absorb water and are used as drying agents.
EFFLORESCENCE: The process by which crystalline materials spontaneously lose water when exposed to air.
HYDROPHOBIC / HYDROPHILLIC: Hydrophobic refers to nonpolar substances interacting with water. Hydrophillic
(water loving) is in reference to the interactions polar substances have with water.
Some Physical Properties of WaterSome Physical Properties of Water Water is colorless, odorless, and tasteless. The normal boiling point is 100oC and the normal
melting point is 0oC. The heat of vaporization (Hvap) is 2259 J/g or 540
cal/g and the heat of fusion (Hfus) is 335 J/g or 80 cal/g.
The vapor pressure of water at 20oC is 17.5 torr; this is relatively low when compared to volatile ethyl alcohol (43.9 torr) and very volatile ethyl ether (442.2 torr)
The density of water at 4.0oC is 1.0 g/mL; the density of ice at 0oC is 0.917 g/mL.
The specific heat of water is 1.0 cal/g oC or 4.184 J/g oC.
The Unusual Properties of WaterThe Unusual Properties of Water Water co-exists in all three states of matter naturally
on earth. The only common substance is a liquid at STP. As a solid, it is less dense than its liquid form, that is
“Ice floats”. Most substances contract upon solidifying.
It has a very high Heat Capacity. It stores a large amount of energy with very little atomic or molecular motion.
It requires a lot of heat energy (enthalpy) to change states.
It has a high boiling point for such a low molecular weight compound.
It is a universal solvent, as a good dissolving medium a large number of substances are soluble in water.
What do you and water have in common??
Everyone is unique. What makes you so UnUsUal?? Name one thing that makes you different. How are you different than others??http://www.humanmetrics.com/cgi-win/jtypes2.asp
Relate this to chemistry. Why are you different? What determines your characteristics?
Relate this to water: What mAkEs WaTeR so UnUsUal?? How does its chemical make-up determine these characteristics?
Why is Water so unusual?Why is Water so unusual?The fundamental explanation for water’s unusual properties relates to
the polarity of its bonds. Polarity describes the partial charge associated with a bond or molecule. A polar bond or molecule has a charge distribution present (one end positively charged and the other end negatively charged) while a nonpolar bond or molecule has no distinct charge distribution (neutral).
Water is composed of two polar covalent O-H bonds (the difference in electronegativity is 1.4) arranged in a “bent” molecular geometry. Each bond has a dipole moment pointing in an overall similar direction leading to the existence of an overall dipole moment. The oxygen atom pulls the pair of electrons closer towards itself (making it partially negative) and further from the hydrogen atoms (making them partially positive).
-
+This charge distribution allows the partially positive hydrogen atoms
from one molecule to be attracted to the partially negative oxygen atom of another molecule. This strong interlocking network between neighboring molecules is called HYDROGEN BONDINGHYDROGEN BONDING. The ability to form strong hydrogen bonds is the main reason for water’s unusual properties.
Why does ice float in water?Why does ice float in water?
Ice floats in its own liquid due to the intermolecular force, hydrogen bonding. As water freezes, the molecular motion of the molecules slow down and the partial positive end (hydrogen) of one water molecule is attracted to the partial negative end (oxygen) of another water molecule. Combine this event with the bent shape of water and the molecules become arranged in a 3-D hexagonal array. This array creates pockets of vacuum (empty space) in the lattice structure as well as a decrease in the number of molecules per unit volume. The mass is directly related to the number of molecules therefore, in the solid state, since there are less particles then there must be less mass per unit volume therefore the solid is less dense than the liquid.
Explain if ice will float in ethyl alcohol Explain if ice will float in ethyl alcohol (d = 0.789 g/L)?(d = 0.789 g/L)?
Ice would not float in pure ethyl alcohol because the density of water is 1.000 g/mL which is greater than 0.789 g/mL for ethyl
alcohol. Yet since ethyl alcohol also undergoes a small degree of hydrogen
bonding, the sinking effect is not as dramatic as it would be with a nonpolar
substance.
Why does water have a relatively high boiling Why does water have a relatively high boiling point?point?
Water has a relatively high boiling point because of the amount of intermolecular forces present. Water experiences LDF (London Dispersion Forces) and d-d (dipole-dipole) forces, along with the additional attractive force, Hydrogen bonding. A large amount of heat energy is required to break all of these forces in order for a phase transition to occur, thus the high boiling point.
Water as a universal solventWater as a universal solvent• Water is called the universal solvent because of its ability to
dissolve many substances. The general solubility rule is “like dissolves like”. Since water is a polar molecule it will dissolve other polar substances as well as ionic compounds. Water will not dissolve or mix with nonpolar substances therefore water is immiscible in nonpolar substances.
• Description of how water dissolves an ionic salt (like NaCl) on the molecular level?
Although the attractive force from the partial charge of a single polar molecule is not as strong as the charge from an ion, it is plausible that a multitude of polar molecules could react on a single ion effectively. The positive end (H+) of several water molecules are attracted to the negative end of the salt crystal (Cl-) while the negative end of several water molecules (O2-) are attracted to the positive end of the crystal (Na+). The ionic bonds of the crystal are weakened by the solvating effect of the water molecules and the ions break away from the bulk crystal. The large number of water molecules in the container prevent the salt ions from re-combining.
