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The concept of intermolecular forces and their impact on the properties of liquids. Topics include polar and non-polar intermolecular forces, polarizability, dispersion forces, dipole-dipole interactions, hydrogen bonding, and their effects on surface tension, viscosity, and phase changes. Learn about the differences between polar and non-polar molecules and how their intermolecular forces contribute to various liquid properties.
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โ The fundamental difference between states is the strength of the intermolecular force
phases
โ van der Waals constant for water (a = 5.28 L^2 atm/mol^2 ) vs O 2 (a = 1.36 L^2 atm/mol^2 ) โโ water is polar (draw diagram) and O 2 is nonโpolar โโโ recall the Electronegativity (EN) Trend โโ interaction between water molecules are electrostatic โ polar bonds and polar molecules โโ bond dipole โโโ change in EN between 2 atoms makes the bond connecting them polar โโโ this phenomenon leads to a bond dipole (arrow head points to the more EN atom) โโ permanent dipole moment (see figure ๏ฎ) โโโ a molecule has a permanent dipole moment when it possesses an asymmetric orientation of polar bonds
โโโ molecules that possess a permanent dipole: NH 3 , H 2 O, SO 2 , SF 4 , XeOF (^4) โโโ molecules that do not possess a permanent dipole: CBr 4 , BF 3 , BeCl 2 , PCl 5 , I 3 โ, SF 6 , XeF 4
โ Intramolecular =inside a single molecule versus Intermolecular = between two or more molecules โโ Intramolecular forces will impact bond energies (polar versus covalent) โโ Intermolecular forces will impact things like melting/freezing and boiling points โ Dispersion Forces Why do van der Waals constants have nonzero values for nonpolar species? โโ recall a = 1.36 L^2 atm/mol^2 for O (^2) โโ polarizability:^ refers^ to^ the^ distortion^ of^ the^ electron^ cloud^ around^ the^ atom's^ nucleus^ as^ another^ atom or molecule approaches โโ this distortion occurs as a result of electronโelectron repulsion btwn the atom and the approaching species โโ the larger a molecule is the less tightly the electrons are held to the nucleus โโโ this makes it easier to distort the electron cloud โโโ therefore larger molecules are more polarizable โโ comparison btwn He (a = 0.0341 L^2 atm/mol^2 ) versus Ar (a = 3.59 L^2 atm/mol^2 ) โโ London or dispersion forces: interactions btwn induced dipoles โโโ when an atom is polarized in the presence of another species this induced dipole occurs โโโ this is the type of interaction which happens btwn two nonโpolar species โ e.g. N 2 molecules โโโ factors that impact the strength of this force: โโโโ molecular weight (aka size) โ the larger the more polarizable and therefore the larger the force โโโโ molecular shape โ when two molecules have the samemolecular formula then the shape that maximizes surface area will have a greater induced dipole โโ Usually this intermolecular force is considered to be the weakest โโ It is also the only force that is present in ALL neutral molecules โ DipoleโDipole โโ we have attraction btwn the negative (pink) and positive โpolesโ in blue shown with solid red lines โโ we also have repulsion btwn the โpolesโ which are charged the sames shown with dashed blue lines โโ these dipoles happen because electron density is pulled from the less electronegative atoms toward the more electronegative ones
โ surface tension: energy needed to separate the molecules of unit area on the surface of a liquid โโ the reason a cold needle floats on the surface of water is because it is not dense enough to break the hydrogen bonds btwn the individual water molecules โโ a hot needle will sink because the added temperature is enough to break the Hโbonds โ meniscus: curved surface of a liquid as a result of cohesive (Hโbonds โ e.g. water with water) or adhesive (dipoleโdipole โ e.g. water with glass) forces btwn solvent and container molecules โ capillary action: rise of a liquid up a narrow tube โโ result of cohesive and adhesive forces โโโ cohesive forces occur btwn liquid molecules โโโ adhesive forces occur btwn liquid and solid molecules โโ the way in which water flows upwards into trees and plants from the soil โ viscosity: resistance of a fluid to flow โโ molasses is very viscous โโ water is not โโ heating a fluid causes the viscosity to lower
โโ All phase changes require energy input (endothermic) or release (exothermic)
required to vaporize 1 mole of liquid at a pressure of 1 atm โโ Other changes of state โโโ aside from going back and forth from liquid to gas we also have solid state transitions โโโ sublimation is another endothermic process in which solid goes to gas โโโ solidification is the opposite effect in which liquid/gas is solidified โโโ a solid may melt to form a liquid
โโโ one final note: when a change of state is performed the intermolecular forces which led to the initial state must be overcome (e.g. to boil something the intermolecular forces in the
liquid must be overwhelmed with our heat to the point molecules escape from the liquid to the gas phase) โ Heating Curves โโ AB we are heating up to the freezing point of water, Tf (recall Ch
change in temperature)
change from s to l โโ CD we are heating up the liquid from Tf to Tb (boiling point, use
phase change from l to g โโ EF we are heating up the gas from Tb to final temperature โ Critical Temperature & Pressure โโ All substances have a T & P in which the liquid and gas phases are completely indistinguishable this is called the critical point โโโ the density is the same for both states โโโ the liquid phase is less dense due to high T โโโ the gas phase is more dense due to high P โโ The name we give to this state is supercritical fluid โโ We will talk more about this in 11.
โ vapor pressure is a result of molecules escaping from the liquid phase as gas โ vaporization/evaporation is an endothermic process because energy/heat must be added to the system for a molecule to escape the liquid phase โ when the rate of the liquid escaping to the gas is equal to the rate of a gas returning to liquid we have an example of equilibrium โ Volatility, Vapor Pressure & Temperature โโ A volatile liquid is one that evaporates and does not readily return to liquid โโ hot water will evaporate more quickly than cold because there is more energy present in the form of heat to break the Hโbonds between water molecules โ Vapor Pressure & Boiling Point โโ As the temperature is increased so is the vapor pressure โโ when T increases so do the molecular motions and the ability for a molecule to escape from the liquid and go into the gas phase โโ since the pressure of the atmosphere is lower at higher elevations โ less temperature is required for water to boil โโ One of the way we can use vapor pressure is to calculate the heat of vaporization by plotting the ln of the vapor pressure versus the inverse of the corresponding T: โโโ This produces a linear equation in the slope