Download - PODCAST 4.7
PODCAST 4.7PODCAST 4.7
IB ChemistryNorthwestern High School
Dr. J. Venables
Viscosity• Viscosity is the resistance of a liquid to flow.• A liquid flows by sliding molecules over each other.• The stronger the intermolecular forces, the higher the
viscosity.
Surface Tension• Bulk molecules (those in the liquid) are equally attracted
to their neighbors.
Some Properties of LiquidsSome Properties of Liquids
Viscosity
Surface Tension
Surface Tension• Surface molecules are only attracted inwards towards the
bulk molecules.– Therefore, surface molecules are packed more closely than bulk
molecules.
• Surface tension is the amount of energy required to increase the surface area of a liquid.
• Cohesive forces bind molecules to each other.• Adhesive forces bind molecules to a surface.
• Meniscus is the shape of the liquid surface. – If adhesive forces are greater than cohesive forces, the liquid
surface is attracted to its container more than the bulk molecules. Therefore, the meniscus is U-shaped (e.g. water in glass).
– If cohesive forces are greater than adhesive forces, the meniscus is curved downwards.
• Capillary Action: When a narrow glass tube is placed in water, the meniscus pulls the water up the tube.
• Surface molecules are only attracted inwards towards the bulk molecules.
Examples – Name each of the following phase changes:
• solid gas:• liquid gas:• solid liquid:• gas solid:• gas liquid:• liquid solid:
Phase ChangesPhase Changes
• solid gas: sublimation• liquid gas: vaporization (boiling, evaporation)• solid liquid: melting• gas solid: deposition• gas liquid: condensation• liquid solid: freezing
Phase ChangesPhase Changes
Phase ChangesPhase Changes
Energy Changes Accompanying Phase Changes
• Sublimation: Hsub
• Vaporization: Hvap
• Melting or Fusion: Hfus
• Deposition: Hdep
• Condensation: Hcon
• Freezing: Hfre
Energy Changes Accompanying Phase Changes
• Sublimation: Hsub > 0 (endothermic).
• Vaporization: Hvap > 0 (endothermic).
• Melting or Fusion: Hfus > 0 (endothermic).
• Deposition: Hdep < 0 (exothermic).
• Condensation: Hcon < 0 (exothermic).
• Freezing: Hfre < 0 (exothermic).
Energy Changes Accompanying Phase Changes• Generally heat of fusion (enthalpy of fusion) is less than
heat of vaporization:– it takes more energy to completely separate molecules, than
partially separate them.
Energy Changes Accompanying Phase Changes• All phase changes are possible under the right conditions.• The sequence
heat solid melt heat liquid boil heat gas
is endothermic.• The sequence
cool gas condense cool liquid freeze cool solid
is exothermic.
Heating Curves• Plot of temperature change versus heat added is a heating
curve.• During a phase change, adding heat causes no
temperature change.– These points are used to calculate Hfus and Hvap.
• Supercooling: When a liquid is cooled below its melting point and it still remains a liquid.
• Achieved by keeping the temperature low and increasing kinetic energy to break intermolecular forces.
From origin to “a” =
From a to b =
From b to c =
From c to d =
From d up =
From origin to a = solid
From a to b = melting/freezing point
From b to c = liquid
From c to d = boiling/condensation point
From d up = gas
Explaining Vapor Pressure on the Molecular Level• Some of the molecules on the surface of a liquid have
enough energy to escape the attraction of the bulk liquid.• These molecules move into the gas phase.• As the number of molecules in the gas phase increases,
some of the gas phase molecules strike the surface and return to the liquid.
• After some time the pressure of the gas will be constant at the vapor pressure.
Vapor PressureVapor Pressure
• Dynamic Equilibrium: the point when as many molecules escape the surface as strike the surface.
• Vapor pressure is the pressure exerted when the liquid and vapor are in dynamic equilibrium.
Volatility, Vapor Pressure, and Temperature• If equilibrium is never established then the liquid
evaporates.• Volatile substances evaporate rapidly.• The higher the temperature, the higher the average kinetic
energy, the faster the liquid evaporates.
Vapor Pressure and Boiling Point• Liquids boil when the external pressure equals the vapor
pressure.• Temperature of boiling point increases as pressure
increases.• Two ways to get a liquid to boil: increase temperature or
decrease pressure.• Pressure cookers operate at high pressure. At high pressure the
boiling point of water is higher than at 1 atm. Therefore, there is a higher temperature at which the food is cooked, reducing the cooking time required.
• Normal boiling point is the boiling point at 760 mmHg (1 atm).
Property Stronger forces mean…
Viscosity
Surface tension
Melting point (freezing)
Boiling point (condensation)
ΔHfus
ΔHvap
Vapor Pressure/volatility
Property Stronger forces mean…
Viscosity higher
Surface tension higher
Melting point (freezing) higher
Boiling point (condensation) higher
ΔHfus higher
ΔHvap higher
Vapor Pressure/volatility lower