Liquids and Solids Ch 11 Comparison of Liquids

Liquids and Solids Ch 11

Comparison of Liquids and Solids to Gases § Liquids & solids are much more dense than gases § Inorganic liquids and solids have densities ranging from 1 – 8 g/cm 3, some up to 20 g/cm 3 § Most organic liquids & solids have densities ranging from 0. 7 – 2. 0 g/cm 3 § Gas densities are usually between 10 -2 and 10 -4 g/c, 3

Comparison of Liquids and Solids to Gases § Gases expand to fill available space & must be kept in enclosed containers § Liquids fills any container from the bottom up to a level dictated on by the mass of the liquid present § Liquids conform to the shape of their container § Solids maintain shape without a container

Comparison of Liquids and Solids to Gases § Gases lack significant attractive forces § Liquids and Solids- significant attractive forces!

Intermolecular Forces § 3 Types to be aware of: § Dipole-Dipole Forces § London Dispersion Forces § Hydrogen Bonding

Dipole-Dipole Forces § Molecular compounds share electrons in a covalent bond, usually not equally! § e- congregate at 1 end of the molecule, giving it polarity, creating a dipole. § Polar molecules are attracted to each other. § Attractive forces are represented by the equation: § Force = (δ+)(δ-) r 2

Dipole-Dipole Forces § For gases to become a liquid – the attractive forces must overcome the KE of the moving gas molecule. § Decreasing the distance b/w molecules increases the attractive force. § Increasing P on a gas forces the molecules closer together § Cooling a gas reduces its avg KE § Force = (δ+)(δ-) r 2

Dipole-Dipole Forces § Boiling Pt (Condensation Pt) – indicator of the attractive forces b/w molecules § Measure of how much KE has to be increased so it overcomes the attractive forces in the liquid. § Low BP – low attractive forces § High BP – higher attractive forces § Highly polar molecules have higher BPs.

London Forces of Attraction § Explain how nonpolar gases develop the forces necessary to condense into liquids. § Nonpolar atoms & molecules may become momentarily polar when an unsymmetrical distribution of their e- results in instantaneous dipoles. § Sometimes called dispersion forces, instantaneous dipole forces, or induced dipole forces.

London Forces of Attraction § Very weak attractive forces, leading to very low BPs. The halogens, like the noble gases, don’t have permanent dipoles, but… Iodine is a solid and bromine is a liquid at room T.

London Forces of Attraction § What’s up with I 2 and Br 2? § Polarizability of e- clouds! § The ease with which the e- cloud around an atom or molecule can be deformed into a dipole. § Small atoms/molecules have e- clouds held tightly to nucleus- low polarizability. § Large atoms/molecules, w/ loosely held e- have high polarizability

London Forces of Attraction § These forces can explain the behavior or many molecules. § The more e- in a molecule, the more opportunity to form instantaneous dipolesso… increase in attractive forces means higher BPs. Aklanes – Cn. H 2 n+2 Called normal alkanes, nalkanes, because the vary in a regular way. (homologous series)

Hydrogen Bonding § Extraordinarily large dipole-dipole forces attributed to the large electronegativity difference between H and the other atom on the next molecule (F, N, or O).

Physical Properties of Liquids § Surface tension § Viscosity § Evaporation § Vapor Pressure § Boiling Pt. § Heat of Vaporizaiton

Surface Tension § Caused by an increase in the attractive forces b/w molecules at the surface of a liquid compared to the forces b/w molecules in the center (bulk) of the liquid. § Causes fluids to minimize their surface area… § Small droplets form spheres

Surface Tension § Look at a molecule on the interior… § The solvent molecule is surrounded by other solvent molecules on all sides. § Look at a molecule on the surface… § Some of the molecules surrounding the solvent molecules have been removed so the surface molecules will compensate by attracting neighboring molecules more strongly to reduce added potential energy. § Causes surface molecules to be closer to each other.

Surface Tension § Cohesive forces – attractions b/w identical molecules in the liquid § Adhesive forces – attractions b/w different molecules, like a liquid and a flat surface If cohesive forces are stronger than adhesive forces… If adhesive forces are stronger than cohesive forces…

Viscosity Low viscosity High viscosity § A liquid’s resistance to flow. § Attractive forces are responsible for viscosity. § Molecules move more freely in solutions with low attractive forces § Liquid alkanes have lower viscosities because they only have London forces § Water is more viscous because it has hydrogen bonding § Syrup is very viscous because all the bulky sugar molecules have lots of –OH groups, which hydrogen bond to the water in the mixture.

Viscosity § Decreases as the liquid’s T is increased. § Molecules have higher KE, weakens intermolecular forces (IMFs).

Evaporation § The process in which a liquid in an open container is slowly converted into a gas at the surface of the liquid. § Some liquids evaporate more rapidly than others. § Reverse of condensation, must have enough sufficient KE to escape the attractive forces

Evaporation § Factors that affect evaporation § Surface area of the liquid – the greater the surface area, the greater the evaporation

Evaporation § Factors that affect evaporation § Temperature – Increasing the T increases the # molecules with enough KE to escape as a gas

Evaporation § Boiling – when T is increased enough, boiling occurs. § Molecules do not have to reach the surface to enter the gas phase.

Vapor Pressure § Pressure that develops in the gas phase above a liquid when the liquid is placed in a closed container. Dynamic equilibrium – occurs when the rate the liquid evaporates equals the rate the gas condenses

Vapor Pressure § Rate a liquid evaporates – dependent on T § Rate a gas condenses – dependent on the frequency the gas molecules collide with the liquid “wall” of the container. § Therefore – vapor pressure depends only one the nature of the liquid (attractive forces) & the temperature (KE) § If T increases, Vapor Pressure increases.

Boiling Point § Boiling occurs when the vapor pressure of the liquid is equal to atmospheric pressure § Normal boiling point- refers to the boiling point when atmospheric pressure is 760 m. Hg

Heat of Vaporization § ΔHvap - the energy needed to convert 1 gram of liquid into 1 gram of gas at a temperature equal to the normal boiling point of the liquid. § Units are J/g or J/mol (if using molar heat of vaporization) § ΔHvap = -ΔHcond

Heat of Vaporization § There are differences in the heats of vaporization that can be related to the IMFs § For similar-size molecules, hydrogenbonded substances have largest ΔHvap. § Polar substances have higher ΔHvap than similar shape nonpolar substances § Increasing London forces increases ΔHvap

The amount of heat needed to vaporize a liquid is very large. This explains why water can be quickly raised to its boiling point, but a long time is needed to boil away all the water. Heat of Vaporization Compound Formula ΔHvap (k. J/mol) Attractive force Water H 2 O +43. 9 H bonding Ammonia NH 3 +21. 7 H bonding Hydrogen fluoride HF +30. 2 H bonding Hydrogen chloride HCl +15. 6 Dipole-dipole Hydrogen sulfide H 2 S +18. 8 Dipole-dipole Fluorine F 2 +5. 9 London Chlorine Cl 2 +10. 0 London Bromine Br 2 +15. 0 London Methane CH 4 +8. 2 London Ethane C 2 H 6 +15. 1 London Propane C 3 H 8 +16. 9 London