14.1 Molecular Substances14.1 Molecular SubstancesIntermolecular ForcesIntermolecular Forces

►Forces between molecules determine the Forces between molecules determine the melting and boiling point of substances.melting and boiling point of substances.

► Ionic compounds have high melting and Ionic compounds have high melting and boiling points due to their electrostatic boiling points due to their electrostatic attraction.attraction.

►Molecular substances are held together Molecular substances are held together by one of three forces.by one of three forces. DispersionDispersion DipoleDipole Hydrogen BondHydrogen Bond

Dispersion ForcesDispersion Forces►Dispersion forces, also called Dispersion forces, also called London London

ForcesForces, are a result of the temporary , are a result of the temporary location arrangement of the electrons.location arrangement of the electrons.

►Electrons have a probability of being Electrons have a probability of being located on one side of the atom.located on one side of the atom.

►This side of the atom becomes This side of the atom becomes temporarily charged negative while temporarily charged negative while the other side becomes positive.the other side becomes positive.

►This is a temporary situationThis is a temporary situation►This is the weakest of the forcesThis is the weakest of the forces

Temporary DipoleTemporary Dipole

Dipole ForcesDipole Forces►Polar molecules have a permanent Polar molecules have a permanent

asymmetrical distribution of electrons.asymmetrical distribution of electrons.►Although the total molecule is neutral, Although the total molecule is neutral,

it has a positive and negative side.it has a positive and negative side.►The opposite attraction between +/- The opposite attraction between +/-

sides of the molecule is permanent sides of the molecule is permanent and thus will cause polar substances and thus will cause polar substances to have a higher melting and boiling to have a higher melting and boiling point than non-polar substances.point than non-polar substances.

►Dipole forces are the next strongest.Dipole forces are the next strongest.

Forces Between Polar Forces Between Polar MoleculesMolecules

Hydrogen BondingHydrogen Bonding►The strongest intermolecular force is a The strongest intermolecular force is a

subset of dipole forces.subset of dipole forces.► If hydrogen is bonded to oxygen, If hydrogen is bonded to oxygen,

nitrogen, or fluorine the molecule has nitrogen, or fluorine the molecule has the strongest of polarities.the strongest of polarities.

►This is called a hydrogen bond.This is called a hydrogen bond.►The hydrogen is highly positive and The hydrogen is highly positive and

the negative side is similarly negative.the negative side is similarly negative.►These substances have the highest These substances have the highest

melting and boiling points.melting and boiling points.

Hydrogen Bonding in WaterHydrogen Bonding in Water

Unique Properties of WaterUnique Properties of Water► Survey► Water is unique in that it is the only natural substance that is found in all Water is unique in that it is the only natural substance that is found in all

three states -- liquid, solid (ice), and gas (steam) -- at the temperatures three states -- liquid, solid (ice), and gas (steam) -- at the temperatures normally found on Earth.normally found on Earth.

► Water freezes at 32Water freezes at 32oo Fahrenheit (F) and boils at 212 Fahrenheit (F) and boils at 212oo F. Water is unusual in F. Water is unusual in that the solid form, ice, is less dense than the liquid form, which is why ice that the solid form, ice, is less dense than the liquid form, which is why ice floats.floats.

► Water has a high specific heat index. This means that water can absorb a lot Water has a high specific heat index. This means that water can absorb a lot of heat before it begins to get hot. The high specific heat index of water also of heat before it begins to get hot. The high specific heat index of water also helps regulate the rate at which air changes temperature, which is why the helps regulate the rate at which air changes temperature, which is why the temperature change between seasons is gradual rather than sudden, temperature change between seasons is gradual rather than sudden, especially near the oceans.especially near the oceans.

► Water has a very high surface tension. In other words, water is sticky and Water has a very high surface tension. In other words, water is sticky and elastic, and tends to clump together in drops rather than spread out in a thin elastic, and tends to clump together in drops rather than spread out in a thin film. Surface tension is responsible for capillary action, which allows water film. Surface tension is responsible for capillary action, which allows water (and its dissolved substances) to move through the roots of plants and (and its dissolved substances) to move through the roots of plants and through the tiny blood vessels in our bodies.through the tiny blood vessels in our bodies.

HomeworkHomework►Terms Chapter 14Terms Chapter 14►Come tomorrow with a unique water Come tomorrow with a unique water

factfact►Pg 514 [1-3, 7,8,10-13]Pg 514 [1-3, 7,8,10-13]

Heating CurvesHeating Curves►A heating Curve is a graphical representation A heating Curve is a graphical representation

of the phase change of a substance.of the phase change of a substance.

Heating CurvesHeating Curves►A to B is heating A to B is heating

the solidthe solid►B to C is melting B to C is melting

the solidthe solid►C to D is heating C to D is heating

the liquidthe liquid►D to E is D to E is

vaporizing the vaporizing the liquidliquid

►E to F is heating E to F is heating the gasthe gas

Heating CurvesHeating Curves►The amount of The amount of

energy from B to C energy from B to C and from D to E is and from D to E is dependent on the dependent on the intermolecular intermolecular forces between forces between atoms or molecules.atoms or molecules.

►The slope of AB or The slope of AB or CD or EF depends CD or EF depends on the specific heat on the specific heat of the substance.of the substance.

Evaporation & Vapor Evaporation & Vapor PressurePressure

►Evaporation is a natural process in Evaporation is a natural process in which a liquid converts to a gas in an which a liquid converts to a gas in an endothermic process.endothermic process.

►HH22O(l) O(l) → H→ H22O(g) O(g) ∆H = +∆H = +►Evaporation is a cooling processEvaporation is a cooling process►The energy is supplied from the The energy is supplied from the

environment.environment.►Fun Fact! Lake Tahoe evaporates 335 Fun Fact! Lake Tahoe evaporates 335

million of water dailymillion of water daily

► If we can contain evaporation by enclosing our If we can contain evaporation by enclosing our liquid source (think of placing a lid on a pot) a liquid source (think of placing a lid on a pot) a pressure would begin to build within the air pressure would begin to build within the air space.space.

► Eventually the pressure will cause Eventually the pressure will cause condensation at the same rate as evaporation.condensation at the same rate as evaporation.

► The pressure this happens at is called the The pressure this happens at is called the vapor pressure of water.vapor pressure of water.

► At vapor pressure, Vp, the system is at At vapor pressure, Vp, the system is at dynamic equilibrium.dynamic equilibrium.

► This pressure is above atmospheric pressureThis pressure is above atmospheric pressure► Total Pressure = Vp + 1 atm.Total Pressure = Vp + 1 atm.

Boiling PointBoiling Point►The first bubbles to form are the air The first bubbles to form are the air

molecules (Nmolecules (N22, O, O22) coming out of the ) coming out of the solution.solution.

►Boiling occurs when the pressure formed Boiling occurs when the pressure formed within an air bubble is great enough to within an air bubble is great enough to sustain the inward pressure of the water.sustain the inward pressure of the water.

►This occurs when the vapor pressure of This occurs when the vapor pressure of the bubble is equal to atmospheric the bubble is equal to atmospheric pressure.pressure.

Boiling PointsBoiling Points►B.P. B.P.

decreases decreases with altitude with altitude because atm because atm pressure pressure decreases.decreases.

►You have to You have to cook food cook food different at different at altitude!altitude!