Lecture 3 & 4: Chapter 11 PDF

Lecture 3 and 4: Chapter 11 Surface Tension Viscosity Capillary Action Change of Physical States Vapor Pressure Enthalpy of Vaporization 1 2  Properties of Liquids: Surface Tension Surface Tension is the resistance of a liquid to spread out or...

Lecture 3 and 4: Chapter 11 Surface Tension Viscosity Capillary Action Change of Physical States Vapor Pressure Enthalpy of Vaporization 1 2  Properties of Liquids: Surface Tension Surface Tension is the resistance of a liquid to spread out or resistance to increase its surface area. “the energy required to increase the surface area by a unit amount” Surface tension is a physical property equal to the amount of force per unit area necessary to expand the surface of a liquid. Surface tension results from intermolecular force differences between molecules in the interior of a liquid and those on the surface. C6H6 = 28 mJ/m2 H2O = 72.8 mJ/m2 Mercury = 480 mJ/m2 3 4  Surface Tension Liquids tend to minimize their surface—a phenomenon we call surface tension. This tendency causes liquids to have a surface that resists penetration. The stronger the attractive force between the molecules, the larger the surface tension. 5  Reasons for Surface Tension Molecules in the interior => experience attractions to surrounding molecules in all directions. However, molecules on the surface => imbalance in attractions,=> effectively pulling them in. To minimize this imbalance and maximize attraction, liquids try to minimize the number of molecules on the exposed surface by minimizing their surface area. Stronger attractive forces between the molecules = larger surface tension. 6 Factors Affecting on Surface Tension The stronger the intermolecular attractive forces, the higher the surface tension will be Raising the temperature of a liquid reduces its surface tension raising the temperature of the liquid increases the average kinetic energy of the molecules the increased molecular motion makes it easier to stretch the surface 7 Effect of Temperature on Surface Tension 8  Viscosity Some liquids flow more easily than others. The resistance of a liquid’s flow is called viscosity. The stronger the attractive forces between the molecules, the more viscous the liquid is. Also, the less round the molecule’s shape, the larger the liquid’s viscosity. Some liquids are more viscous because their molecules are long and get tangled in each other, causing them to resist flowing. 9 Units of Viscosity Viscosity is the resistance of a liquid to flow 1 poise = 1 P = 1 g/cm∙s often given in centipoise, cP 1 centiPoise (cP) = I milliPascal second (mPa.s) H2O = 1 cP at room temperature 10 Effect of molar mass on Viscosity 11 Effect of Temperature on Viscosity 12 13 Capillary Action Capillary action is the ability of a liquid to flow up a thin tube against the influence of gravity the narrower the tube, the higher the liquid rises Cohesive and adhesive forces 14 Meniscus The curving of the liquid surface in a thin tube is due to the competition between adhesive and cohesive forces The meniscus of water is concave in a glass tube because its adhesion to the glass is stronger than its cohesion for itself The meniscus of mercury is convex in a glass tube because its cohesion for itself is stronger than its adhesion for the glass metallic bonds are stronger than intermolecular attractions 15 Phase Changes 16 Liquid/Vapor Phase Changes Evaporation phase change from liquid to gas always endothermic process Condensation phase change from gas to liquid always exothermic process 17  Evaporation The process of molecules of a liquid breaking free from the surface is called evaporation. Also known as vaporization. Evaporation is a physical change Liquid form => gaseous form. The gaseous form is called a vapor. 18 Evaporation Over time, liquids evaporate—the molecules of the liquid mix with and dissolve in the air. Molecules on the surface experience a smaller net attractive force than molecules in the interior. All the surface molecules do not escape at once, only the ones with sufficient kinetic energy to overcome the attractions will escape. 19  Escaping the Surface The average kinetic energy is directly proportional to the temperature in Kelvin. Those molecules on the surface that have enough kinetic energy will escape. 20 Factors Effecting the Rate of Evaporation Liquids that evaporate quickly are called volatile liquids, while those that do not are called nonvolatile. Increasing the surface area increases the rate of evaporation. More surface molecules. Increasing the temperature increases the rate of evaporation. Raises the average kinetic energy, resulting in more molecules that can escape. Weaker attractive forces between the molecules = faster rate of evaporation. 21  Evaporation in a closed container When a liquid evaporates in a closed container, the vapor molecules are trapped. The vapor molecules may eventually bump into and stick to the surface of the container or get recaptured by the liquid. This process is called condensation. Condensation is a physical change gaseous form => liquid form. 22 Dynamic Equilibrium Evaporation and condensation are opposite processes. Eventually, the rate of evaporation and rate of condensation in the container will be the same. Opposite processes that occur at the same rate in the same system are said to be in dynamic equilibrium. 23 Evaporation and Condensation When water is just Shortly, the water added to the flask starts to evaporate. and it is capped, all Initially the rate the water molecules of evaporation is are in the liquid. much faster than rate of condensation Eventually, the condensation and evaporation reach the same speed. The air in the flask is now saturated with water vapor. 24 Which of the following liquids has the highest surface tension at room temperature? a) Ethanol b) Water c) Mercury d) Glycerol Answer: c) Mercury 25 What is the phase change called when water vapor cools down and turns into liquid water? a) Condensation b) Sublimation c) Melting d) Deposition Answer: a) Condensation 26 27  Vapor Pressure Once equilibrium is reached, the amount of vapor in the container will remain the same. As long as you don’t change the conditions. The partial pressure exerted by the vapor is called the vapor pressure. The vapor pressure of a liquid depends on the temperature and strength of intermolecular attractions. 28  Boiling In an open container, as you heat a liquid the average kinetic energy of the molecules increases, giving more molecules enough energy to escape the surface. So the rate of evaporation increases. Eventually, the temperature is high enough for molecules in the interior of the liquid to escape. A phenomenon we call boiling. 29  Boiling Point The temperature at which the vapor pressure of the liquid is the same as the atmospheric pressure is called the boiling point. The boiling point depends on what the atmospheric pressure is. The temperature of boiling water on the top of a mountain will be cooler than boiling water at sea level. 30 31 Temperature and Boiling As you heat a liquid, its temperature increases until it reaches its boiling point. Once the liquid starts to boil, the temperature remains the same until it all turns to a gas. All the energy from the heat source is being used to overcome all of the attractive forces in the liquid. 32  Practice – Which of the following is the most volatile? a) water b) TiCl4 c) ether d) ethanol e) acetone 33 Heat of Vaporization The amount of heat needed to vaporize one mole of a liquid is called the heat of vaporization or enthalpy of vaporization DHvap It requires 40.7 kJ of heat to vaporize one mole of water at 100 °C (40.7 kJ/mol) Always endothermic. Number is +. DHvap depends on the initial temperature. Since condensation is the opposite process to evaporation, the same amount of energy is transferred but in the opposite direction. 34 Heats of Vaporization of Liquids at Their Boiling Points and at 25 °C DHvap at Normal boiling DHvap at Chemical boiling point, 25 °C, Liquid formula point, °C (kJ/mol) (kJ/mol) Water H2O 100 +40.7 +44.0 Isopropyl C3H7OH 82.3 +39.9 +45.4 alcohol Acetone C3H6O 56.1 +29.1 +31.0 Diethyl C4H10O 34.5 +26.5 +27.1 ether 35 Calculate the mass of water (in g) that can be vaporized at its boiling point with 155 kJ of heat. 36 Practice: Calculate the amount of heat (in kJ) required to vaporize 2.58 kg of water at its boiling point. Hint: kg to g, g to moles, moles to kJ Practice—How Much Heat Energy, in kJ, is Required to Vaporize 87 g of Acetone, C3H6O, (MM 58.08) at 25 C? (DHvap = 31.0 kJ/mol) Ans. 46 kJ 38 Properties of Liquids Surface tension – the energy required to overcome the attractive forces between molecules at the surface Capillary action – the process whereby a liquid rises in a small–diameter tube due to noncovalent interactions between the liquid and the tube’s material Viscosity – resistance of a fluid to flow Vapor pressure/boiling point – the pressure of a vapor over a liquid at a given temperature enthalpy of vaporization – the energy required to convert a given amount of liquid into vapor at a constant pressure All related to IMF!! 39