Chapter 10 Liquids, Solids, and Intermolecular Forces

Chapter 10 Liquids, Solids, and Intermolecular Forces

The Three Phases of Matter (A Macroscopic Comparison) State of Matter Shape and volume Compressibility Ability to Flow Solid Retains its own shape and volume very low none Liquid Conforms to shape of container, but not to volume low moderate Gas Conforms to shape and volume of container high high

Molecular View of Phases of Matter Particles packed close together and are fixed in position (They may vibrate) Noncompressible Retain shape and volume when placed in a new container Do not flow

Molecular View of Phases of Matter Particles closely packed Particles have some ability to move around Noncompressible Take the shape of their container Flow, but don t have enough freedom to escape or expand

Molecular View of Phases of Matter Particles have complete freedom of motion There is a large amount of space between the particles

Kinetic Molecular Theory What state a material is in depends largely on two major factors: 1. the amount of kinetic energy the particles possess 2. the strength of attraction between the particles These two factors are in competition.

Gas phase particles

Attractive Forces Particles are attracted to each other by electrostatic forces. The strength of the attractive forces varies. The strength of the attractive forces depends on the kind(s) of particles. The stronger the attractive forces between the particles, the more they resist moving.

The Two Condensed States of Water

Phase Changes The amount of kinetic energy the particles have determines the state of matter. Solids melt when heated. Liquids boil when heated. Gases can be condensed. Liquids can be condensed.

Special Properties of Liquids Surface tension Viscosity Capillary Action

Surface Tension Surface tension -a property of liquids that results from the tendency of liquids to minimize their surface area To minimize their surface area, liquids form drops that are spherical.

Surface Tension

Viscosity, the resistance of a liquid to flow. Larger intermolecular attractions larger viscosity Higher temperature reduced viscosity

Capillary Action Capillary action - the ability of a liquid to flow up a thin tube against the influence of gravity Capillary action is the result of two forces working in conjunction, adhesive forces attract the outer liquid molecules to the tube s surface cohesive forces hold the liquid molecules together

Capillary Action Adhesive forces pull the surface liquid up the side of a tube, and the cohesive forces pull the interior liquid with it. The liquid rises up the tube until the force of gravity counteracts the capillary action forces. The narrower the tube diameter, the higher the liquid will rise up the tube.

Vaporization Molecules in a liquid are constantly in motion; some molecules have more kinetic energy than others. If these high energy molecules are at the surface, they may have enough energy to overcome the attractive forces Therefore the larger the surface area, the faster the rate of evaporation This will allow them to escape the liquid and become a vapor.

Dynamic Equilibrium In a closed container, once the rates of vaporization and condensation are equal, the total amount of vapor and liquid will not change. Evaporation and condensation are still occurring, but because they are opposite processes, there is no net gain or loss of either vapor or liquid.

Dynamic Equilibrium When two opposite processes reach the same rate so that there is no gain or loss of material, We call it a dynamic equilibrium This does not mean there are equal amounts of vapor and liquid it means that they are changing by equal amounts

Effect of Intermolecular Attraction on Evaporation and Condensation Weaker attractive forces less energy needed to vaporize Weaker attractive forces more energy will need to be removed from the vapor molecules before condensation Weak attractive forces the faster the evaporation Liquids that evaporate easily are said to be volatile. Liquids that do not evaporate easily are called nonvolatile.

Energetics of Vaporization The amount of heat energy required to vaporize one mole of the liquid is called the heat of vaporization, ΔHvap, or the enthalpy of vaporization.

Calculate the amount of heat needed to vaporize 90.0 g of C3H7OH at its boiling point (ΔHvap = 39.9 kj/mol) 39.9 kj + C3H8O (liquid) C3H8O (gas) g mol kj

Calculate the mass of water that can be vaporized with 155 kj of heat at 100 (ΔHvap = 40.7 kj/mol) 40.7 kj + H2O (liquid) H2O (gas) kj mol H 2 O g H 2 O

What happens when you heat up a liquid??

Boiling As a liquid is heated, its temperature rises and the molecules move past each other more vigorously. Once the temperature reaches the boiling point, the molecules have sufficient energy to overcome the attractions that hold them in contact with other molecules and the liquid boils.

ΔT

What happens when you heat up a solid??

Melting As a solid is heated, its temperature rises and the molecules vibrate more vigorously. Once the temperature reaches the melting point, the molecules have sufficient energy to overcome some of the attractions that hold them in position and the solid melts.

ΔT

Energetics of Melting The amount of heat energy required to melt one mole of the solid is called the Heat of Fusion, ΔHfus or the enthalpy of fusion Generally much less than ΔHvap

How much heat energy is required to raise the temperature of 1.0 mol of water from -25ºC to 125ºC??

Quantitative Aspects of Phase Changes Within a phase, a change in heat is accompanied by a change in temperature which is associated with a change in average kinetic energy of the molecules. q = ( quantity of matter )(molar heat capacity)( T) J = g x x ºC J g ºC

Quantitative Aspects of Phase Changes During a phase change, a change in heat occurs at a constant temperature, which is associated with a change in average rotational and translational energy of the molecules, as the average distance between molecules changes. q = ( quantity of matter )(enthalpy of the phase change) kj = mol x kj mol

Heating Curve of Water

Segment 1 Heating 1.00 mole of ice at 25.0 C up to the melting point, 0.0 C q = mass x Cs x ΔT mass of 1.00 mole of ice = 18.0 g Cs = 2.09 J/g C

Segment 2 Melting 1.00 mole of ice at the melting point, 0.0 C q = n ΔHfus n = 1.00 mole of ice ΔHfus = 6.02 kj/mol

Segment 3 Heating 1.00 mole of water at 0.0 C up to the boiling point, 100.0 C q = mass x Cs x ΔT mass of 1.00 mole of water = 18.0 g Cs = 4.18 J/g C

Segment 4 Boiling 1.00 mole of water at the boiling point, 100.0 C q = n ΔHvap n = 1.00 mole of water ΔHvap = 40.7 kj/mol

Segment 5 Heating 1.00 mole of steam at 100.0 C up to 125.0 C q = mass x Cs x ΔT mass of 1.00 mole of water = 18.0 g Cs = 2.01 J/g C

Heating Curve of Water 0.941 6.02 7.52 40.7 0.904 56.085 56.1 kj

Attractive Forces Particles are attracted to each other by electrostatic forces The strength of the attractive forces depends on the kind(s) of particles The stronger the attractive forces between the particles, the more they resist moving The strength of the attractions between particles of a substance determines its state.

Kinds of Attractive Forces Dispersion Forces between Molecules Temporary polarity in molecules due to unequal electron distribution Dipole Dipole Attractions between Molecules Permanent polarity in molecules due to their structure Hydrogen Bonds between Molecules An especially strong dipole dipole attraction resulting from the attachment of H to an extremely electronegative atom Ion Dipole Attractions - Not Intermolecular Between mixtures of ionic compounds and polar compounds (esp. aqueous solutions)

Some molecules are considered nonpolar because of the atoms which they contain and the arrangement of these atoms in space. CH4 BH3 C2H2 CO2 Nonpolarized electron clouds But these molecules can all be condensed.

Origin of Instantaneous Dipoles δδδδ+ The δδ+ charge attracts electrons. The δδ- charge repels electrons.

Size of the Induced Dipole The magnitude of the induced dipole depends on several factors: Polarizability of the electrons Volume of the electron cloud larger molar mass more electrons larger electron cloud increased polarizability stronger attractions Larger molecules have more electrons, leading to increased polarizability.

Size of the Induced Dipole Shape of the molecule more surface-to-surface contact larger induced dipole stronger attraction Molecules that are flat have more surface interaction than spherical ones.

Effect of Molecular Size on Magnitude of Dispersion Force As the molar mass increases, the number of electrons increases. Therefore, the strength of the dispersion forces increases. The stronger the attractive forces between the molecules, the higher the boiling point. Gas Radius Molar Mass B.P.(K) He 31 4 4.2 Ne 38 20 27 Ar 71 40 87 Kr 88 84 120 Xe 108 131 165 Rn 120 222 211

Properties of Straight Chain Alkanes NonPolar Molecules

Effect of Molecular Shape on Size of Dispersion Force 2,2-dimethylpropane molar mass=72.15 b.p = 9.5 ºC 2-methylbutane molar mass=72.15 b.p = 27.9 ºC n-pentane molar mass=72.15 b.p = 36.1 ºC A larger surface-to-surface contact between molecules results in stronger dispersion force attractions and a higher boiling point.

Kinds of Attractive Forces Dispersion Forces between Molecules Temporary polarity in molecules due to unequal electron distribution Dipole Dipole Attractions between Molecules Permanent polarity in molecules due to their structure Hydrogen Bonds between Molecules An especially strong dipole dipole attraction resulting from the attachment of H to an extremely electronegative atom Ion Dipole Attractions - Not Intermolecular Between mixtures of ionic compounds and polar compounds (esp. aqueous solutions)

Some molecules are inherently polar because of the atoms which they contain and the arrangement of these atoms in space. H2O NH3 CH2O HCl δ δ+ A crude representation of a polar molecule

Dipole Dipole Attractions Polar molecules have a permanent dipole because of bond polarity and shape 1) dipole moment 2) as well as the always present induced dipole The permanent dipole adds to the attractive forces between the molecules

Effect of Dipole Dipole Attraction on Boiling and Melting Points Name Formula Molar mass Structure Structure b.p. m.p. formaldehyde CH2O 30.03-19.5º -92º H H C H ethane C2H6 30.07-88º -172º H C H H

Determine if dipole dipole attractions occur between CH2Cl2 molecules Formula Lewis Structure Bond Polarity Molecule Polarity Cl C 3.0 2.5 = 0.5 polar 4 bonding areas no lone pairs tetrahedral shape C H 2.5 2.1 = 0.4 nonpolar polar molecule; therefore dipole dipole attractions do exist

Hydrogen Bonding When a very electronegative atom is bonded to hydrogen, it strongly pulls the bonding electrons toward it: O H, N H, F H Because hydrogen has no other electrons, when its electron is pulled away, the nucleus becomes deshielded, exposing the H proton. The exposed proton acts as a very strong center of positive charge.

H-Bonding in Water

Effect of Hydrogen-Bonding on Boiling and Melting Points Name Formula Molar mass Structure Structure b.p. m.p. ethanol C2H6O 46.07 78.2º -114.1º dimethyl ether C2H6O 46.07-22º -138.5º

H-Bonds Very strong intermolecular attractive forces Stronger than dipole dipole or dispersion forces Substances that can hydrogen bond will have higher boiling points and melting points than similar substances that cannot. But hydrogen bonds are not nearly as strong as chemical bonds.

One of these compounds is a liquid at room temperature (the others are gases). Which one and why? MM = 30.03 Polar No H-Bonds MM = 34.03 Polar No H-Bonds MM = 34.02 Polar H-Bonds -19ºC -78ºC +150ºC b.p. Because only hydrogen peroxide has the additional very strong H- bond additional attractions, its intermolecular attractions will be the strongest. We therefore expect hydrogen peroxide to be the liquid.

Hierarchy of Intermolecular Forces Molecules containing O-H, N-H, or F-H Bonds H-bonding Polar Molecules All Molecules Dipole forces Dispersion forces

Comparison of Intermolecular Forces Dispersion forces: H2, b.p. -253ºC weak attractions between molecules Dipole-dipole forces: HCl, b.p. -85ºC strong attractions between molecules Hydrogen bonding: HF, b.p. +20 ºC very strong attractions between molecules

Ion Dipole Attractions - Not Intermolecular Between mixtures of ionic compounds and polar compounds (esp. aqueous solutions)

Non-Bonding (Inter-Molecular) Forces 0.05-40.0 kj/mol 5-25 kj/mol 10-40 kj/mol 40-600 kj/mol

Bonding Molecular Forces

Types of Crystalline Solids

Types of Crystalline Solids