Chapter 11: Liquids, Solids, and Intermolecular Forces Mrs. Brayfield
11.1: Intermolecular Forces Intermolecular forces are attractive forces that exist between all molecules and atoms The state of matter (solid, liquid, or gas) depends on the magnitude of these forces
11.2: Molecular Comparison of States Phase Density Shape Volume Intermolecular Forces Gas Low Indefinite Indefinite Low Liquid High Indefinite Definite Moderate Solid High Definite Definite High A change in state can be achieved by increasing/decreasing temperature and/or pressure *Note: solids can be either have well ordered (crystalline) or no order (amorphous)
11.3: Intermolecular Forces Intermolecular forces originate from the interactions between charges, partial charges, and temporary charges Remember Coulomb s Law: There are 3 types of intermolecular attractions
(London) Dispersion Forces This force is present in ALL molecules and atoms It is the result of fluctuations in the electron distribution within molecules or atoms In any one frame the electrons are not symmetrical around the atom which results in a temporary dipole
(London) Dispersion Forces An instantaneous dipole on any one atom will induce an instantaneous dipole on neighboring atoms, which will then attract each other The magnitude of the force depends on how easily the electrons can move As the molar mass increases the force also increases This is because of more electrons around a greater volume (easier to polarize) See page 392 for more info.
Dipole-Dipole Forces These forces exist in all polar molecules There is a permanent dipole from neighboring molecules that interact with each other To determine if a molecule has these forces, just determine if the molecule is polar! The polarity of molecules also determines miscibility (the ability to mix like dissolves like )
Dipole-Dipole Forces Example Which of the following molecules have dipole-dipole forces? CI 4 CH 3 Cl HCl Answer: CH 3 Cl and HCl
Hydrogen Bonding Is not actually bonding (no transfer or sharing of electrons) Polar molecules that contain a very electronegative atom (F, O, or N) exhibit this force This is a super dipole force The boiling point increases with molar mass
Hydrogen Bonding Example Which has the higher boiling point, HF or HCl? Why? HF because it can form hydrogen bonds which is much stronger than the dipole forces that HCl forms. The stronger the intermolecular force, the higher the boiling point.
Summary of Intermolecular Forces No ion-dipole interactions (see original table) Homework Problems: #1, 2, 4, 6, 8, 10
11.4: Intermolecular Forces in Action Surface tension the energy required to increase the surface area by a unit amount (liquids only) Molecules interact with their neighbors so the stronger the intermolecular attractions the higher the surface tension: This is what gives water droplets their circular shape
Viscosity Viscosity the resistance of a liquid to flow It is greater in molecules with stronger intermolecular attractions because if the molecules are strongly attracted to one another then they do not flow around each other as freely The viscosity of maple syrup is larger than that of water Viscosity is also temperature dependent (as temperature increases viscosity decreases)
Capillary action Capillary action the ability of a liquid to flow against gravity up a narrow tube This results from both the attraction between molecules (cohesive forces) and the attraction between the molecules and the surface of the tube (adhesive forces) The adhesive forces causes the liquid to spread out over the interior surface of the tube while the cohesive forces cause the liquid to stay together When adhesive > cohesive water When adhesive < cohesive mercury Homework Problems #12, 15
11.5: Vaporization and Vapor Pressure Vaporization the process by which thermal energy can overcome intermolecular forces and produce a phase change from a liquid to a gas Remember that molecules are in constant motion (see picture on page 401)
Vaporization Every liquid is vaporizing and condensing at the same time, but not always at the same rate (which results in liquids evaporating from a container) The rate can be affected by 3 things: The rate will increase with a temperature increase The rate will increase with increasing surface area The rate increases with decreasing strength of intermolecular forces Liquids that evaporate easily are volatile and those that do not are called nonvolatile
The Heat of Vaporization The heat of vaporization (ΔH vap ) is the amount of heat required to vaporize one mole of a liquid to a gas For example: The heat of vaporization is positive because the process requires energy
Heat of Vaporization Example Calculate the amount of heat (in kj) required to vaporize 2.58kg of water at its boiling point
Vapor Pressure Vapor pressure the pressure of a gas that is in dynamic equilibrium with its liquid Weak intermolecular forces results in a volatile substance which results in a high vapor pressure High intermolecular forces results in a nonvolatile substance which results in a low vapor pressure
Vapor Pressure When the system (in dynamic equilibrium) is disturbed the system responds to minimize the disturbance and return to a state of equilibrium The boiling point of a liquid is the temperature at which its vapor pressure equals the external pressure Boiling point DOES change with pressure Think of the boiling point of water here verses in Colorado Homework Problems: #18, 19, 22, 24
11.6: Sublimation and Fusion Sublimation the phase transition from solid to gas The opposite is called deposition This is why foods get freezer burned in the freezer the water from the food sublimated and is redeposited on the surface of the food The melting point is when the molecules have enough thermal energy to over the intermolecular forces to turn from a solid to a liquid (otherwise known as melting or fusion)
The Heat of Fusion The heat of fusion (ΔH fus ) is the amount of heat required to melt 1 mole of a solid For example: The heat of fusion is positive because the process requires energy
Heat of Fusion Example An 8.5g ice cube is placed into 255g of water. Calculate the temperature change in the water upon the complete melting of the ice. Assume that all of the energy required to melt the ice comes from the water. Homework Problems: #30, 32, 34
11.7: Heating Curve for Water
11.8: Phase Diagrams A phase diagram is a map of the phase of a substance as a function of pressure (y-axis) and temperature (x-axis) Homework Problems: #35, 36, 40
11.9: Water The high boiling point of water can be explained by its molecular geometry Which is Bent Water also has a high specific heat capacity which allows coastal cities to have small temperature fluctuations For example San Francisco's temp. fluctuation is ~10 F Water is also funky because it expands when it freezes Which is the opposite to most liquids This is why most living things do not survive freezing Homework Problems: #41, 42, 44
11.10 11.12 You are not responsible for knowing any material in sections 10 12 Review problems: #66, 71 http://www.youtube.com/watch?v=bqqjpcdmip8