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