Properties of Liquids PDF

PROPERTIES OF LIQUIDS Lesson objectives: 1. describe the properties of liquids: surface tension, viscosity, vapor pressure, boiling point, and molar heat of vaporization; 2. explain the effect of intermolecular forces on these properties; and 3. relate the properties of water to intermo...

PROPERTIES OF LIQUIDS Lesson objectives: 1. describe the properties of liquids: surface tension, viscosity, vapor pressure, boiling point, and molar heat of vaporization; 2. explain the effect of intermolecular forces on these properties; and 3. relate the properties of water to intermolecular forces that operate among its molecules. 2 Activity 2. Review Directions: List the type/s of intermolecular forces that exist between molecules (or basic units) in each of the following species: (a) benzene (C6H6) Dispersion Forces (b) CH3Cl Dispersion and dipole-dipole forces (c) PF3 Dispersion and dipole-dipole forces (d) NaCl Dispersion and Ion-dipole forces (e) CS2 Dispersion Forces 3 Activity 3: Infer the Topic Directions: Look at the pictures in a minute or less and record an inference about the upcoming subject of study. Based on the images seen, you should be able to make arguments to support your conclusion. 4 5  CAPILLARITY LIQUIDS DUE TO EVAPORATION INTERMOLECULAR VISCOSITY FORCES EXHIBIT VAPOR PRESSURE SURFACE TENSION AND BOILING POINT 6 Vocabulary words Fluid A liquid or gas; a substance that flows. Surface tension It is the elastic force in the surface of a liquid. It is the amount of energy required to increase or stretch the surface of a liquid by a unit area. Capillary action It is the tendency of a liquid to rise in narrow tubes or to be drawn into small openings. 7 Vocabulary words Viscosity A measure of a fluid’s resistance to flow. Vapor A gaseous substance that exists naturally as a solid or liquid at normal temperature. Vaporization The change of phase from liquid to gas/vapor. 8 Vocabulary words Vapor pressure of a liquid The pressure exerted by the vapor above the surface of the liquid in a closed container. It is the equilibrium pressure of a vapor above its liquid. Boiling point The temperature at which a liquid boils. Normal boiling point is boiling point of a liquid when the external pressure is 1 atm. Molar heat of vaporization (ΔHvap) The amount of energy (usually in kilojoules) required to vaporize 1 mole of a liquid at a specific temperature. 9 1. Surface Tension  The energy required to increase the surface area of a liquid.  Surface tension like viscosity, is higher in liquids that have higher intermolecular forces.  Both properties are temperature dependent because at higher temperatures, molecules have more kinetic energy to counteract the 10 Surface Tension Figure 1. Intermolecular forces acting on a molecule in the surface layer of a liquid and in the interior region of the liquid 11 Surface Tension Figure 2.Water beads on an apple, which has a waxy surface 12 Surface Tension Figure 3. Surface tension enables the water strider to “walk” on water 13 Capillary  The rising of the blood, or any liquid, in a tube is called capillarity or capillary action.  The results from competition between the intermolecular forces within the liquid molecules (cohesive forces) and those between the liquid molecules and the walls of the tube (adhesive forces.) 14 Capillary Cohesive forces bind molecules of the same type together Adhesive forces bind a substance to a surface 15 Figure 4. Capillary (a) When adhesion is greater than cohesion, the liquid (for example, water) rises in the capillary tube. (b) When cohesion is greater than adhesion, as it is for mercury, a depression of the liquid in the capillary tube results. Note that the meniscus in the tube of water is concave, or rounded downward, whereas that in the tube of mercury is convex, or rounded upward. 16  2. Viscosity  Viscosity is the ability of a fluid to resist flowing.  It is related to the movement of the molecules in the liquid and thus to the intermolecular forces present.  Viscosity of a liquid decreases with increasing temperature because at higher temperature, the average kinetic energy molecules that overcomes the 17 2. Viscosity  Viscosity can be measured using viscometer. It measures the time it takes for a known volume of a liquid to flow through the small neck. 18 2. Viscosity Table 1. Viscosity of Some Common Liquids at 20°C 3. Vapor Pressure  depends on the magnitude of intermolecular forces and on temperature. 20 3. Vapor Pressure Figure 6. Vapor pressure over a liquid 21 4. Boiling Point  Normal boiling point – the temperature at which boiling occurs at 1 a pressure of 1 atm.  This property changes as pressure changes. 22 4. Boiling Point  If the atmospheric pressure is below at temperatures lower than 100˚C.  At an altitude of 1600atm, atmospheric pressure is about 640 torr or 0.84atm, and water boils at 90 ˚C 23 4. Boiling Point Figure 7 24 5. Molar Heat of Vaporization  A measure of the strength of intermolecular forces in a liquid is the molar heat of vaporization (ΔHvap), defined as the energy (usually in kilojoules) required to vaporize 1 mole of a liquid.  The molar heat of vaporization has a direct relationship to the strength of intermolecular forces that exist in the liquid. 25 5. Molar Heat of Vaporization Table 2.Molar heats of vaporization and boiling points of some substances ΔHvap (kJ/ Boiling Point* Substance mol) (OC) Argon (Ar) 6.3 -186 Pentane(C5H12) 26.5 36.1 Acetone (CH3COCH3) 30.3 56.5 Ethanol (C2H5OH) 39.3 78.3 Water (H2O) 40.79 100 26 27  The Structure and Properties of Water  Water has a high surface tension  Water has high boiling point which is why it is liquid at room temperature.  The density of solid water or ice is less than the density of liquid water.  Water has a high heat of vaporization, due to strong intermolecular forces of H- bonding.  Water is a good solvent. 28 Table 3. The Specific Heats of Some Common Substances Substance Specific Heat (J/goC) Al 0.900 Au 0.129 C (graphite) 0.720 C (diamond) 0.502 Cu 0.385 Fe 0.444 Hg 0.139 H 2O 4.184 C2H5OH (ethanol) 2.460 29 Figure 8. Electrostatic potential map of water. 30  Figure 9. Left: Ice cubes float on water. Right: Solid benzene sinks to the bottom of liquid benzene. 31 Figure 10. The three-dimensional structure of ice. Each O atom is bonded to four H atoms. The covalent bonds are shown by short solid lines and the weaker hydrogen bonds by long dotted lines between O and H. The empty space in the structure accounts for the low density of ice. 32  Brown, 2015 Figure 11.Plot of density versus temperature for liquid water. The maximum density of water I reached at 4°C. The density of ice at 0°C is about 0.92 g/cm3. 33 ACTIVITY 3: Let’s check your understanding! Directions: Read and answer each question carefully. The boiling points, surface tensions, and viscosities of water and several alcohols are as follows: 34 a. For ethanol, propanol, and n-butanol the boiling points, surface tensions, and viscosities all increase. What is the reason for this increase? 35 b. How do you explain the fact that propanol and ethylene glycol have similar molecular weights (60 versus 62 amu), yet the viscosity of ethylene glycol is more than 10 times larger than propanol? 36 (c)How do you explain the fact that water has the highest surface tension but has the lowest viscosity? Water has the highest surface tension but lowest viscosity because it is the smallest molecule in the series. There is no hydrocarbon chain to inhibit their strong attraction to molecules in the interior of the drop, resulting in high surface tension. The absence of an alkyl chain also means that the molecules can move around each other easily, resulting in low viscosity. 37 ACTIVITY 4: Let’s think! Directions: Explain the following phenomena. a. A freely falling drop of water is spherical in shape. Surface tension is responsible for the shape of liquid droplets. Although easily deformed, droplets of water tend to be pulled into spherical shape by cohesive forces of the surface layer. 38 b. Your arm feels cool when alcohol evaporates from your skin. As the molecules evaporate, they remove heat from your body. This is heat transfer: thermal energy leaves your body as the liquid becomes a gas…. 39 c. On warm day, water droplets form on the outer side of the bottle of a carbonated beverage. Dew point is the temperature at which condensation happens. Condensation can produce water droplets on the outside of soda cans or glass of cold water. When warm air hits the cold surface, it reaches its dew point and condenses. This leaves droplets of water on the glass or can 40 d. The vapor pressure of water increases with increasing temperature. As the temperature of a liquid or solid increases its vapor pressure also increases. 41 e. Boiling point varies with location. Boiling point of a liquid varies depending upon the surrounding environmental pressure…A liquid at high pressure has a higher boiling point than with that liquid is at atmospheric pressure. 42 Let’s summarize what you learned: 1. Liquids tend to assume a geometry that minimizes surface area. Surface tension is the energy needed to expand a liquid surface area; strong intermolecular forces lead to greater surface tension. 2. Viscosity is a measure of the resistance of a liquid to flow; it decreases with increasing temperature. 3. Vapor - A gaseous substance that exists 43 4. Vaporization - The change of phase from liquid to vapor (gaseous phase). 5. A liquid in a closed vessel eventually establishes a dynamic equilibrium between evaporation and condensation. The vapor pressure over the liquid under these conditions is the equilibrium vapor pressure, which is often referred to simply as “vapor pressure”. 6. Boiling point - The temperature at which a liquid boils. The boiling point of a liquid when 44 7. Molar heat of vaporization (ΔHvap) - The energy (usually in kilojoules) required to vaporize 1 mole of a liquid at a given temperature. 8. Water molecules in the solid state form a three-dimensional network in which each oxygen atom is covalently bonded to two hydrogen atoms and is hydrogen bonded to two hydrogen atoms. This unique structure accounts for the fact that ice is less dense than liquid water, a property that enables life 45 9. Water is also ideally suited for its ecological role by its high specific heat, another property imparted by its strong hydrogen bonding. Large bodies of water are able to moderate Earth’s climate by giving off and absorbing substantial amounts of heat with only small changes in the water temperature. 46 Thank you!!! 47