Chapter 11 (11.1-11.5) Intermolecular Forces, Liquids and Solids CHEMISTRY The Central Science 9th Edition Chapter 11 (11.1-11.5) Intermolecular Forces, Liquids and Solids 1

11.1: A Molecular Comparison of Liquids and Solids

The fundamental difference between states of matter is the strength of the intermolecular forces of attraction. Stronger forces bring molecules closer together. Solids and liquids are referred to as the condensed phases.

Some place between liquids and crystalline solids are Amorphous Solids No regularly ordered pattern Close packed particles Motion limited to vibrations in place The forces holding solids and liquids together are called intermolecular forces

Something to think about: Using the particle nature of matter that was just presented, explain the following properties of solids, liquids and gases: Compressibility Volume and shape Diffusion Watch this: Is glass really a solid?

11.2: Intermolecular Forces The covalent bond holding a molecule together is an intramolecular force The attraction between molecules is an intermolecular force Much weaker than intramolecular forces Melting or boiling: the intermolecular forces are broken (not the covalent bonds) The stronger the attractive forces, the higher the boiling point of the liquid and the melting point of a solid

Ion-dipole forces exist between an ion and a polar molecule Partial negative end of the water molecule and the positive sodium ion are held together by an ion-dipole force of attraction Recall the process of solvation! Bond breaking: endothermic Bond making: exothermic

Dipole-dipole forces exist between neutral polar molecules in the liquid or solid state Oppositely charged ends of molecules attract Weaker than ion-dipole forces Dipole-dipole forces increase with increasing polarity Strength of attractive forces is inversely related to molecular volume

Text P. 449

London dispersion forces are the result of attractions between induced dipoles Weakest of all intermolecular forces Two adjacent neutral, nonpolar molecules The nucleus of one attracts the electrons of the other Electron clouds are distorted Instantaneous dipole Strength of forces is directly related to molecular weight London dispersion forces exist between all molecules

Instantaneous dipoles for two He atoms Text P. 447

London dispersion forces also depend on the shape of the molecule The greater the surface area available for contact, the greater the dispersion forces Text, P. 448

Something to think about: Order the halogens according to increasing boiling points. Explain your reasoning.

Something to think about: Order the halogens according to increasing boiling points. Explain your reasoning.

Watch this: Bozeman Science on Dipole Forces

Hydrogen bonding is an especially strong form of dipole-dipole force H-bonding requires H bonded to an electronegative element (most important for compounds of F, O, and N) The small size of electropositive hydrogen allows it to approach and form a strong force of attraction with a nonbonding electron pair on these very electronegative atoms

Hydrogen Bonding Text, P. 449 Boiling point increases with increasing molecular weight. The exception is water (H bonding)

Text, P. 451 - Solids are usually more closely packed than liquids (solids are more dense than liquids) - Ice is ordered with an open structure to optimize H-bonding (ice is less dense than water) Watch this: Don’t make this mistake with H bonding!

Dispersion forces<dipole-dipole<hydrogen bonding<ion-dipole For molecules of similar size and mass, intermolecular forces of attraction increase in the following way: Dispersion forces<dipole-dipole<hydrogen bonding<ion-dipole

Text, P. 453

11.3: Some Properties of Liquids Viscosity Viscosity is the resistance of a liquid to flow Molecules slide over each other The stronger the intermolecular forces, the higher the viscosity Viscosity increases with an increase in molecular weight

Surface Tension Surface molecules are only attracted inwards towards the bulk molecules Imbalance of IMFs at the surface of the liquid Molecules within the liquid are all equally attracted to each other

Surface tension is the amount of energy required to increase the surface area of a liquid Cohesive forces bind molecules to each other (Hg) Adhesive forces bind molecules to a surface (H2O) If adhesive forces > cohesive forces, the meniscus is U- shaped (water in a glass) If cohesive forces > adhesive forces, the meniscus is curved downwards (Hg in a barometer)

11.4: Phase Changes Text, P. 457 (Exothermic) (Endothermic)

Generally heat of fusion (melting 1 mole of a substance at constant T) is less than heat of vaporization (evaporation of 1 mole of a substance at constant T) It takes more energy to completely separate molecules than to partially separate them Text, P. 458

Plot of temperature change versus heat added is a heating curve Heating Curves Plot of temperature change versus heat added is a heating curve During a phase change, adding heat causes no temperature change (equilibrium is established) These points are used to calculate ΔHfus and ΔHvap Remember: Q = m·Cp·ΔT Watch this: Boiling and Evaporation

Added heat increases the temperature of a consistent state of matter Text, P. 459 Added heat increases the temperature of a consistent state of matter Energy used for changing molecular motion, no T change

Critical Temperature and Pressure Gases are liquefied by increasing pressure at some temperature Critical temperature: the maximum temperature for liquefaction of a gas using pressure A high C.T. means strong intermolecular forces At temperatures higher than the CT, KE is so great that the substance can only be in the gas phase Critical pressure: pressure required for liquefaction at the CT Brings the molecules sufficiently close together so that forces of attraction can take over

Nonpolar substances and those with low molar masses have low IMFs and low CT’s and CP’s Polar substances and those with higher molar masses have higher CT’s and CP’s because they are more polarizable and have higher IMFs

11.5: Vapor Pressure Explaining Vapor Pressure on the Molecular Level Some of the molecules on the surface of a liquid have enough energy to escape to the gas phase After some time the pressure of the gas will be constant at the vapor pressure (equilibrium is established)

Partial pressure of water vapor is constant Dynamic Equilibrium: the point when as many molecules escape the surface as strike the surface Partial pressure of water vapor is constant Vapor pressure is the pressure exerted when the liquid and vapor are in dynamic equilibrium in a closed system Text P. 462

Volatility, Vapor Pressure, and Temperature If equilibrium is never established then the liquid evaporates Nonpolar liquids having low molar masses usually have weak IMFs and (evaporate rapidly) The higher the T, the higher the average KE, the faster the liquid evaporates (hot water evaporates faster than cold water)

Something to think about: Place the following compounds in order of increasing vapor pressure: CCl4 , Cl4 , CBr4 . Justify your answer Watch This: Bozeman Science on Intermolecular Forces

Vapor pressure increases nonlinearly with increasing temperature Text, P. 463

Vapor Pressure and Boiling Point Liquids boil when the external pressure equals the vapor pressure Normal BP: BP of a liquid at 1 atmosphere Temperature of boiling point increases as pressure increases Watch this: Boiling, atmospheric pressure and vapor pressure

Something to think about: Predict whether glycerol, C3H5(OH)3 , or 1 propanol, C3H7OH, will have the higher: Viscosity Vapor pressure Boiling point Justify your response

Something to think about: Ice skaters demonstrate that ice melts under pressure. A skater glides over a liquid surface of water as her blades impart a force sufficient to temporarily melt the ice. What does this say about the relative densities of ice and liquid water? Describe the molecular structures of liquid and solid water that would explain this unusual property of ice.

Something to think about: Look at the picture on this page. List the four liquids in order of increasing vapor pressure at any given temperature and give their formulas. Justify the trend citing the intermolecular forces acting in each liquid.

End of Chapter 11 (11.1-11.5) Intermolecular Forces, Liquids and Solids