Intermolecular Forces

Tro's Introductory Chemistry, Chapter 123  Many of the phenomena we observe are related to interactions between molecules that do not involve a chemical reaction.  Your taste and smell organs work because molecules in the thing you are sensing interact with the receptor molecule sites in your tongue and nose.  In this chapter, we examine the physical interactions between molecules and the factors that effect and influence them.

Tro's Introductory Chemistry, Chapter 124  Matter can be classified as solid, liquid, or gas based on what properties it exhibits. Fixed = Keeps shape when placed in a container. Indefinite = Takes the shape of the container.

Tro's Introductory Chemistry, Chapter 125  The atoms or molecules have different structures in solids, liquids, and gases, leading to different properties.

Tro's Introductory Chemistry, Chapter 126  Low densities compared to solids and liquids.  Fluid.  The material exhibits a smooth, continuous flow as it moves.  Take the shape of their container(s).  Expand to fill their container(s).  Can be compressed into a smaller volume.

Tro's Introductory Chemistry, Chapter 127  In the gas state, the particles have complete freedom from each other.  The particles are constantly flying around, bumping into each other and their container(s).  In the gas state, there is a lot of empty space between the particles.  On average.

Tro's Introductory Chemistry, Chapter 128  Because there is a lot of empty space, the particles can be squeezed closer together. Therefore, gases are compressible.  Because the particles are not held in close contact and are moving freely, gases expand to fill and take the shape of their container(s), and will flow.

Tro's Introductory Chemistry, Chapter 129  High densities compared to gases.  Fluid.  The material exhibits a smooth, continuous flow as it moves.  Take the shape of their container(s).  Keep their volume, do not expand to fill their container(s).  Cannot be compressed into a smaller volume.

Tro's Introductory Chemistry, Chapter 1210  The particles in a liquid are closely packed, but they have some ability to move around.  The close packing results in liquids being incompressible.  But the ability of the particles to move allows liquids to take the shape of their container and to flow. However, they don’t have enough freedom to escape and expand to fill the container(s).

Tro's Introductory Chemistry, Chapter 1211  High densities compared to gases and liquids.  Nonfluid.  They move as an entire “block” rather than a smooth, continuous flow.  Keep their own shape, do not take the shape of their container(s).  Keep their own volume, do not expand to fill their container(s).  Cannot be compressed into a smaller volume.

Tro's Introductory Chemistry, Chapter 1212  The particles in a solid are packed close together and are fixed in position.  Though they are vibrating.  The close packing of the particles results in solids being incompressible.  The inability of the particles to move around results in solids retaining their shape and volume when placed in a new container, and prevents the particles from flowing.

Tro's Introductory Chemistry, Chapter 1213  Some solids have their particles arranged in an orderly geometric pattern. We call these crystalline solids.  Salt and diamonds.  Other solids have particles that do not show a regular geometric pattern over a long range. We call these amorphous solids.  Plastic and glass.

Tro's Introductory Chemistry, Chapter 1214  The state a material exists in depends on the attraction between molecules and their ability to overcome the attraction.  The attractive forces between ions or molecules depends on their structure.  The attractions are electrostatic.  They depend on shape, polarity, etc.  The ability of the molecules to overcome the attraction depends on the amount of kinetic energy they possess.

Tro's Introductory Chemistry, Chapter 1215 PhaseDensityShapeVolume Relative strength of attractive forces GasLowIndefinite Weakest LiquidHighIndefiniteDefiniteModerate SolidHighDefinite Strongest

Tro's Introductory Chemistry, Chapter 1217  Generally, we convert a material in the solid state into a liquid by heating it.  Generally, we convert a material in the liquid state to a gas by heating it.  In both cases adding heat energy increases the amount of kinetic energy of the molecules in the solid.  Eventually, they acquire enough energy to partially overcome the attractive forces holding them in place.  This allows the molecules enough extra freedom to move around a little and rotate.

Tro's Introductory Chemistry, Chapter 1218  Liquids tend to minimize their surface—a phenomenon we call surface tension.  This tendency causes liquids to have a surface that resists penetration.  The stronger the attractive force between the molecules, the larger the surface tension.

Tro's Introductory Chemistry, Chapter 1219  Molecules in the interior of a liquid experience attractions to surrounding molecules in all directions.  However, molecules on the surface experience an imbalance in attractions, effectively pulling them in.  To minimize this imbalance and maximize attraction, liquids try to minimize the number of molecules on the exposed surface by minimizing their surface area.  Stronger attractive forces between the molecules = larger surface tension.

Tro's Introductory Chemistry, Chapter 1220  Some liquids flow more easily than others.  The resistance of a liquid’s flow is called viscosity.  The stronger the attractive forces between the molecules, the more viscous the liquid is.  Also, the less round the molecule’s shape, the larger the liquid’s viscosity.  Some liquids are more viscous because their molecules are long and get tangled in each other, causing them to resist flowing.

Tro's Introductory Chemistry, Chapter 1222  The process of molecules of a liquid breaking free from the surface is called evaporation.  Also known as vaporization.  Evaporation is a physical change in which a substance is converted from its liquid form to its gaseous form.  The gaseous form is called a vapor.

Tro's Introductory Chemistry, Chapter 1223  Over time, liquids evaporate—the molecules of the liquid mix with and dissolve in the air.  The evaporation happens at the surface.  Molecules on the surface experience a smaller net attractive force than molecules in the interior.  All the surface molecules do not escape at once, only the ones with sufficient kinetic energy to overcome the attractions will escape.

24  The average kinetic energy is directly proportional to the Kelvin temperature.  Not all molecules in the sample have the same amount of kinetic energy.  Those molecules on the surface that have enough kinetic energy will escape.  Raising the temperature increases the number of molecules with sufficient energy to escape.

Tro's Introductory Chemistry, Chapter 1225  Since the higher energy molecules from the liquid are leaving, the total kinetic energy of the liquid decreases, and the liquid cools.  The remaining molecules redistribute their energies, generating more high energy molecules.  The result is that the liquid continues to evaporate.

Tro's Introductory Chemistry, Chapter 1226  Liquids that evaporate quickly are called volatile liquids, while those that do not are called nonvolatile.  Increasing the surface area increases the rate of evaporation.  More surface molecules.  Increasing the temperature increases the rate of evaporation.  Raises the average kinetic energy, resulting in more molecules that can escape.  Weaker attractive forces between the molecules = faster rate of evaporation.

Tro's Introductory Chemistry, Chapter 1227  When a liquid evaporates in a closed container, the vapor molecules are trapped.  The vapor molecules may eventually bump into and stick to the surface of the container or get recaptured by the liquid. This process is called condensation.  A physical change in which a gaseous form is converted to a liquid form.

Tro's Introductory Chemistry, Chapter 1228  Evaporation and condensation are opposite processes.  Eventually, the rate of evaporation and rate of condensation in the container will be the same.  Opposite processes that occur at the same rate in the same system are said to be in dynamic equilibrium.

Tro's Introductory Chemistry, Chapter 1229 Eventually, the condensation and evaporation reach the same speed. The air in the flask is now saturated with water vapor. We have reached the dynamic equilibrium Shortly, the water starts to evaporate. Initially the rate of evaporation is much faster than rate of condensation When water is just added to the flask and it is capped, all the water molecules are in the liquid.

Tro's Introductory Chemistry, Chapter 1230  Once equilibrium is reached, from that time forward, the amount of vapor in the container will remain the same.  As long as you don’t change the conditions.  The partial pressure exerted by the vapor is called the vapor pressure.  The vapor pressure of a liquid depends on the temperature and strength of intermolecular attractions.

Tro's Introductory Chemistry, Chapter 1231  In an open container, as you heat a liquid the average kinetic energy of the molecules increases, giving more molecules enough energy to escape the surface.  So the rate of evaporation increases.  Eventually, the temperature is high enough for molecules in the interior of the liquid to escape. A phenomenon we call boiling.

Tro's Introductory Chemistry, Chapter 1232  The temperature at which the vapor pressure of the liquid is the same as the atmospheric pressure is called the boiling point.  The normal boiling point is the temperature required for the vapor pressure of the liquid to be equal to 1 atm.  The boiling point depends on what the atmospheric pressure is.  The temperature of boiling water on the top of a mountain will be cooler than boiling water at sea level.

Tro's Introductory Chemistry, Chapter 1233  As you heat a liquid, its temperature increases until it reaches its boiling point.  Once the liquid starts to boil, the temperature remains the same until it all turns to a gas.  All the energy from the heat source is being used to overcome all of the attractive forces in the liquid.

Tro's Introductory Chemistry, Chapter 1234  As it loses its high energy molecules through evaporation, the liquid cools.  Then the liquid absorbs heat from its surroundings to raise its temperature back to the same as the surroundings.  Processes in which heat flows into a system from the surroundings are said to be endothermic.  As heat flows out of the surroundings, it causes the surroundings to cool.  As alcohol evaporates off your skin, it causes your skin to cool.

Tro's Introductory Chemistry, Chapter 1235  As it gains the high energy molecules through condensation, the liquid warms.  Then the liquid releases heat to its surroundings to reduce its temperature back to the same as the surroundings.  Processes in which heat flows out of a system into the surroundings are said to be exothermic.  As heat flows into the surroundings, it causes the surroundings to warm.

Tro's Introductory Chemistry, Chapter 1236  The amount of heat needed to vaporize one mole of a liquid is called the heat of vaporization.   H vap  It requires 40.7 kJ of heat to vaporize one mole of water at  100 °C.  Always endothermic. ▪ Number is +.   H vap depends on the initial temperature.  Since condensation is the opposite process to evaporation, the same amount of energy is transferred but in the opposite direction.   H condensation = −  H vaporization

Tro's Introductory Chemistry, Chapter 1237 Liquid Chemical formula Normal boiling point, °C  H vap at boiling point, (kJ/mol)  H vap at 25 °C, (kJ/mol) WaterH2OH2O Isopropyl alcohol C 3 H 7 OH AcetoneC3H6OC3H6O Diethyl ether C 4 H 10 O

Since the given amount of heat is almost 4x the  H vap, the amount of water makes sense. 1 mol H 2 O = 40.7 kJ, 1 mol = g 155 kJ g H 2 O Check: Solution: Solution Map: Relationships: Given: Find: kJmol H 2 Og H 2 O

Tro's Introductory Chemistry, Chapter 1239 Example 12.1: Calculate the amount of water in grams that can be vaporized at its boiling point with 176 kJ of heat.

Tro's Introductory Chemistry, Chapter 1240

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Tro's Introductory Chemistry, Chapter 1243  As you heat a solid, its temperature increases until it reaches its melting point.  Once the solid starts to melt, the temperature remains the same until it all turns to a liquid.  All the energy from the heat source is being used to overcome some of the attractive forces in the solid that hold them in place.

Tro's Introductory Chemistry, Chapter 1244  When a solid melts, it absorbs heat from its surroundings, it is endothermic.  As heat flows out of the surroundings, it causes the surroundings to cool.  As heat flows out of your drink into the ice cubes (causing them to melt), the liquid gets cooler.  When a liquid freezes, it releases heat into its surroundings, it is exothermic.  As heat flows into the surroundings, it causes the surroundings to warm.  Orange growers often spray their oranges with water when a freeze is expected. Why?

Tro's Introductory Chemistry, Chapter 1245  The amount of heat needed to melt one mole of a solid is called the heat of fusion.   H fus  Fusion is an old term for heating a substance until it melts, it is not the same as nuclear fusion.  Since freezing (crystallization) is the opposite process of melting, the amount of energy transferred is the same, but in the opposite direction.   H crystal = -  H fus  In general,  H vap >  H fus because vaporization requires breaking all attractive forces.

Tro's Introductory Chemistry, Chapter 1246 Liquid Chemical formula Melting point, °C  H fusion, (kJ/mol) Water H2OH2O Isopropyl alcohol C 3 H 7 OH Acetone C3H6OC3H6O Diethyl ether C 4 H 10 O

47 Since the given amount of heat is almost 4x the  H vap, the amount of water makes sense. 1 mol H 2 O = 6.02 kJ, 1 mol = g 237 kJ g H 2 O Check: Solution: Solution Map: Relationships: Given: Find: kJmol H 2 Og H 2 O

Tro's Introductory Chemistry, Chapter 1248

49 Since the given mass is more than one mole, the answer being greater than  H vap makes sense. 1 mol C 3 H 6 O = 5.69 kJ at  C, 1 mol = g 87 g C 3 H 6 O kJ Check: Solution: Solution Map: Relationships: Given: Find: g C 3 H 6 Omol C 3 H 6 OkJ

Tro's Introductory Chemistry, Chapter 1250  Sublimation is a physical change in which the solid form changes directly to the gaseous form.  Without going through the liquid form.  Like melting, sublimation is endothermic.

Tro's Introductory Chemistry, Chapter 1252  Intermolecular attractions are a result of attractive forces between opposite charges.  + ion to – ion.  + end of one polar molecule to − end of another polar molecule.  H-bonding is especially strong.  Even nonpolar molecules will have temporary induced dipoles.  Larger charge = stronger attraction= higher melting and boiling ponts.

Tro's Introductory Chemistry, Chapter 1253  Also known as London forces or instantaneous dipoles.  Caused by distortions in the electron cloud of one molecule inducing distortion in the electron cloud on another.  Distortions in the electron cloud lead to a temporary dipole.  The temporary dipoles lead to attractions between molecules—dispersion forces.  All molecules have attractions caused by dispersion forces.

Tro's Introductory Chemistry, Chapter 1254

Tro's Introductory Chemistry, Chapter 1255  Depends on how easily the electrons can move, or be polarized.  The more electrons and the farther they are from the nuclei, the larger the dipole that can be induced.  Strength of the dispersion force gets larger with larger molecules.

Tro's Introductory Chemistry, Chapter 1256

Tro's Introductory Chemistry, Chapter 1257

Tro's Introductory Chemistry, Chapter 1258  CH 4 or C 3 H 8  BF 3 or BCl 3  CO 2 or CS 2 CH 4 or C 3 H 8. BF 3 or BCl 3. CO 2 or CS 2.

Tro's Introductory Chemistry, Chapter 1259  Because of the kinds of atoms that are bonded together and their relative positions in the molecule, some molecules have a permanent dipole.  Polar molecules.  The size of the molecule’s dipole is measured in debyes, D.

Tro's Introductory Chemistry, Chapter 1260  Polar molecules have a permanent dipole.  A + end and a – end.  The + end of one molecule will be attracted to the – end of another.

Tro's Introductory Chemistry, Chapter 1261

Tro's Introductory Chemistry, Chapter _ _ _ _ Dispersion forces—All molecules. Dipole-to-dipole forces—Polar molecules.

Tro's Introductory Chemistry, Chapter 1263  All molecules are attracted by dispersion forces.  Polar molecules are also attracted by dipole- dipole attractions.  Therefore, the strength of attraction is stronger between polar molecules than between nonpolar molecules of the same size.

Tro's Introductory Chemistry, Chapter 1264 CS 2 Nonpolar bonds = nonpolar molecule. CH 2 F 2 Polar bonds and asymmetrical = polar molecule. CF 4 Polar bonds and symmetrical shape = nonpolar molecule.  CS 2  CH 2 F 2  CF 4 

Tro's Introductory Chemistry, Chapter 1265  Like dissolves like.  Miscible = Liquids that do not separate, no matter what the proportions.  Polar molecules dissolve in polar solvents.  Water, alcohol, CH 2 Cl 2.  Molecules with O or N higher solubility in H 2 O due to H- bonding with H 2 O.  Nonpolar molecules dissolve in nonpolar solvents.  Ligroin (hexane), toluene, CCl 4.  If molecule has both polar and nonpolar parts, then hydrophilic-hydrophobic competition.

Tro's Introductory Chemistry, Chapter 1266 When liquid pentane, a nonpolar substance, is mixed with water, a polar substance, the two liquids separate because they are more attracted to their own kind of molecule than to the other.

Tro's Introductory Chemistry, Chapter 1267  HF, or molecules that have OH or NH groups have particularly strong intermolecular attractions.  Unusually high melting and boiling points.  Unusually high solubility in water.  This kind of attraction is called a hydrogen bond.

Tro's Introductory Chemistry, Chapter 1268 NameFormula Molar mass (g/mol) Structure Boiling point, °C Melting point, °C Solubility in water EthaneC2H6C2H Immiscible EthanolCH 4 O Miscible

Tro's Introductory Chemistry, Chapter 1269

Tro's Introductory Chemistry, Chapter 1270  When a very electronegative atom is bonded to hydrogen, it strongly pulls the bonding electrons toward it.  Since hydrogen has no other electrons, when it loses the electrons, the nucleus becomes deshielded.  Exposing the proton.  The exposed proton acts as a very strong center of positive charge, attracting all the electron clouds from neighboring molecules.

Tro's Introductory Chemistry, Chapter 1271  Hydrogen bonds are not chemical bonds.  Hydrogen bonds are attractive forces between molecules.  Chemical bonds are attractive forces that make molecules.

Tro's Introductory Chemistry, Chapter 1272

Tro's Introductory Chemistry, Chapter 1273

Tro's Introductory Chemistry, Chapter 1274  formaldehyde, CH 2 O  g/mol  polar molecule  dipole – dipole attractions present ▪ polar C=O bond & asymmetric  fluoromethane, CH 3 F  g/mol  polar molecule  dipole – dipole attractions present ▪ polar C−F bond & asymmetric  hydrogen peroxide, H 2 O 2  g/mol  polar molecule  dipole – dipole attractions present ▪ polar H−O bonds & asymmetric  H−O bonds  Hydrogen bonding present formaldehyde, CH 2 O g/mol. polar molecule  dipole–dipole attractions present.  Polar C=O bond and asymmetric. fluoromethane, CH 3 F g/mol. polar molecule  dipole–dipole attractions present.  Polar C−F bond and asymmetric. hydrogen peroxide, H 2 O g/mol. polar molecule  dipole–dipole attractions present.  Polar H−O bonds and asymmetric. H−O bonds  hydrogen-bonding present.

Tro's Introductory Chemistry, Chapter 1275  CH 3 CH 2 OCH 2 CH 3 or CH 3 CH 2 CH 2 CH 2 CH 3.  CH 3 CH 2 NHCH 3 or CH 3 CH 2 CH 2 CH 3.  CH 3 CH 2 OH or CH 3 CH 2 CH 2 CH 2 CH 2 OH.

Tro's Introductory Chemistry, Chapter 1276  CH 3 CH 2 OCH 2 CH 3 or CH 3 CH 2 CH 2 CH 2 CH 3 contains polar O.  CH 3 CH 2 NHCH 3 or CH 3 CH 2 CH 2 CH 3 contains polar N.  CH 3 CH 2 OH or CH 3 CH 2 CH 2 CH 2 CH 2 OH contains less nonpolar parts.

Type of force Relative strength Present inExample Dispersion force Weak, but increases with molar mass All atoms and molecules H2H2 Dipole– Dipole force Moderate Only polar molecules HCl Hydrogen Bond Strong Molecules having H bonded to F, O, or N HF