Northwestern High School PODCAST 4.7 IB Chemistry Northwestern High School Dr. J. Venables

Some Properties of Liquids 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.

Viscosity

Surface Tension

Cohesive forces bind molecules to each other. 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.

Phase Changes Examples – Name each of the following phase changes: solid  gas: liquid  gas: solid  liquid: gas  solid: gas  liquid: liquid  solid:

Phase Changes solid  gas: sublimation liquid  gas: vaporization (boiling, evaporation) solid  liquid: melting gas  solid: deposition gas  liquid: condensation liquid  solid: freezing

Phase 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. cool gas  condense  cool liquid  freeze  cool solid is exothermic.

Plot of temperature change versus heat added is a heating curve. 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

Vapor Pressure 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.

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 Melting point (freezing) Boiling point (condensation) ΔHfus ΔHvap Vapor Pressure/volatility lower