Physical Properties of Liquids

• Surface tension is due to an increase in the attractive forces between molecules at the surface of a liquid compared to the forces between molecules in the center, or bulk of the liquid
• Bulk molecules hve attractive forces all around it
• Surface molecules don't have attraction at the surface, thus the attractive forces are stronger in the surrounding bonds
• Cohesive forces are attractions between identical molecules
• Adhesive forces are attractions between different molecules
• If cohesive forces are stronger than adhesive, the liquid will form a bead
• If adhesive forces are stronger than cohesive, than the liquid will spread out uniformly
• Surfactants decrease the cohesive forces and surface tensions of liquids
• Viscosity refers to a liquid's resistance to flow
• Viscosity depends on the attractive forces between a liquid
• Molecules are able to move more freely in solutions that have lower attractive forces
• Viscosity decreases as temperature of a liquid decreases
• Evaporation is the rate at which liquid turns into a gas
• Evaporation rate increases as temperature increases
• In order for a liquid to turn into a gas, it must have sufficient kinetic energy to overcome its attractive forces
• The escape energy is the minimum kinetic energy needed for a molecule of liquid to escape to gas phase
• At constant temperature, liquid will evaporate at a uniform rate
• Increased surface area of a liquid increases evaporation rate
• An increase in temperature increases the proportion of molecules that have kinetic energies above the escape energy
• Boltzman curves are explained because 1. when molecules escape, the average kinetic energy of the remaining molecules decreases 2. when average Ek decreases, the proportion of molecules above the escape energy decreases. These two reasons explain why rate of evaporation decreases
• Vapor pressure is the pressure that develops in the gas phase above a liquid when a liquid is placed in a closed container
• In a closed container, the pressure of a vaporizing liquid increases
• As more molecules enter the gas phase, the pressure increases, finally stopping at the vapor pressure
• The rate at which the liquid molecules evaporate is equal to the rate at which the gas molecules condense, called dynamic equilibrium
• At equilibrium, molecules leave and enter the liquid at the same rate
• Evaporation rate depends on temperature
• Condensation rate depends on the frequency at which the gas molecules collide with the wall of the container
• Vapor pressure depends only on the nature of the liquid (attractive forces) and the temperature (kinetic energy)
• As temperature increases, so does vapor pressure
• Boiling occurs when the vapor pressure of a liquid is equal to the prevailing atmospheric pressure around that liquid
• Normal boiling point is the boiling point measured when the atmospheric pressure is 1 atm
• Vacuum distillation is a technique used to purify heat sensative materials
• Vacuum distillation is decreasing the boiling point of a liquid by decreasing the pressure
• The heat of vaporization is 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 for heat of vaporization are J/g
• Unit for molar heat of vaporization are J/mol
•  ΔHvap is always positive
• ΔHvap = -ΔHcond
• Water has the highest ΔHvap and ammonia has the lowest amongst hydrogen bonded molecules
• Polar substances have higher heats of vaporization than similar-size nonpolar substances