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Figure: 11-T01
The molecular-level interactions that determine the states of matter - solid, liquid, and gas. Understand how dipole-dipole and London dispersion forces influence the order and properties of substances based on their boiling and melting points.
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Presentation Transcript
What is happening on the molecular level that causes a solid to be a solid ? What is happening on the molecular level that causes a liquid to be a liquid ? What is happening on the molecular level that causes a gas to be a gas ? Intermolecular forces the attractive forces between molecules
Types of Intermolecular forces 1. dipole - dipole the attractive forces between polar molecules + - Br-Cl is a polar molecule with a permanent dipole moment
dipole-dipole IM attractive force since unlike charges attract, polar molecules are attracted to each other on the molecular level
dipole-dipole interactions cause molecules to orient to maximize attractive forces between molecules molecules line up and assume a somewhat ordered state
The quantitative measure of the extent of Intermolecular forces is reflected in the molecules melting point or boiling point ! B A solid at 25 C liquid at 25 C A Which possesses a greater degree of order ? MP of A > 25 C while MP of B < 25 C Substance A exhibits more IM attractive forces vs. B
The higher the boiling point, the more order the substance exhibits ..(in other words) . The higher the boiling point, the greater the extent of the intermolecular forces between molecules
Types of Intermolecular forces 2. London dispersion forces the very short lived attractive forces caused by the instantaneous displacement of electrons Fritz London 1900 1954
electron clouds momentarily distorts to give rise to non-symmetric electron cloud producing an instantaneous dipole
instantaneous dipole distorted electrons give rise to a temporary dipole before instantaneous dipoles: He nonpolar instantaneous dipoles occurring: He slightly polar instantaneous dipoles provide some additional intermolecular ordering on the molecular level
Extent of London dispersion is determined by MW London dispersion increases as MW increases
Arrange the following in order of increasing boiling points; C3H8 , CH4 , C8H18
Arrange the following in order of increasing boiling point; F2 , H2S
Types of Intermolecular forces 3. Hydrogen bonding occurs between polar molecules that contain a hydrogen atom that is attached to a F, or O, or N H F or H O or H N H-bonding is an unusually strong dipole-dipole that provides more order on the molecular level than a routine dipole-dipole
H-bonding is responsible for holding the two strands of DNA together !
H-bonding gives rise to the open structure of ice, thus H2O (s) is less dense than H2O ( ) and thus, ice floats
London dispersion forces are present in all substances and increases with increasing MW H-bonding > dipole-dipole
Arrange the following in order of increasing boiling point; Kr, H2S, NaCl, Ne, NH3, F2
viscosity a liquids resistance to flow. The greater the viscosity, the slower the liquid flows viscosity increases with increasing intermolecular attractive forces viscosity decreases with increasing temperature
Which species has the greater viscosity ? C6H14 ( ) or CH3CH2CH2CH2CH2OH ( )
Which species is more viscous (the greater viscosity) ? CH3OH ( ) at 15 C or CH3OH ( ) at 45 C
surface tensionthe characteristic skin a liquids surface develops surface tension increases with increasing intermolecular attractive forces surface tension decreases with increasing temperature
Which species has the greater surface tension ? H2O ( ) at 15 C or H2O ( ) at 45 C
Which species has the greater surface tension 25 C ? CH3CH2CH2OH ( ) or HOCH2CH2OH ( )
vapor pressure the partial pressure vapor molecules exert at equilibrium h liquid h = height of the Hg = vapor pressure, VP VPwater = 17.5 mm Hg VPmethanol = 46.0 mm Hg at 20 C
VP H2O < VP CH3OH VP increases as intermolecular forces decrease VP increases as temperature increases Which species has the higher vapor pressure at 25 C ? CH3CH2CH2OH ( ) or HOCH2CH2OH ( ) Higher VP
boiling point temperature where the vapor pressure of the liquid is equal to the applied pressure normal BP temp of the boiling point at 1 atm
phase diagrams graphs of pressure and temperature phase changes
line AD represents melting or freezing point of substance line AB represents boiling point of substance line AC represents sublimation of substance
how does water exist at 100 C and 5 atm ? liquid how does water exist at 100 C and 0.5 atm ? gas yes can you boil water at 50 C ? can you boil water at -5 C ? no can ice exist at +2 C ? no
How do you make a snowball ? What is happening at point A ? triple point all three phases coexist together
How does CO2 exist at -40 C and 7 atm ? liquid What temp does CO2 boil ? depends Can CO2 melt at 4 atm ? No What is the normal BP ? doesn t have one -56.4 C, 5.11 atm What is the triple point ? What is happening at point Z ?
critical point no distinct difference between liquid and gas states at temperature and pressure above this point supercritical fluid a state that is inbetween a liquid and gas phase
green coffee beans are immersed in supercritical CO2 at 90 C and about 180 atm. The caffeine dissolves into the CO2 supercritical fluid and is extracted and removed
