Intermolecular Forces and Physical Properties

Intermolecular Forces and Physical Properties - The Liquid...

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Unformatted text preview: The Liquid State Viscosity • Viscosity is the resistance to flow. – For example, compare how water pours out of a glass For compared to molasses, syrup or honey. compared • Oil for your car is bought based on this property. – 10W30 or 5W30 describes the viscosity of the oil at high 10W30 and low temperatures. and – Another way of looking at multi-viscosity oils is to think of viscosity a 10W-30 as a 10 weight oil that will not thin more than a 10W 30 30 weight would when hot. 30 Viscosities of several liquids. Liquids that are formed from molecules that H-bond are generally more viscous than those liquids that are not formed from H-bonding. Hg is an exception (very viscous). The atoms in liquid mercury are held together from type of metallic bonding. 2 1 1 Viscosity The long threadlike molecules of heavy hydrocarbon oils tend to tangle together resulting in molecules that do not move past one another very easily. These liquids are very viscous. • Viscosity is temperature dependent. Viscosity – Higher temperatures produce lower viscosity. Temp /°C 20 20 40 60 80 100 Viscosity Viscosity /cP 1.002 0.653 0.467 0.355 0.282 centipoise (cP). 1 cP = 0.001 N s m-2 Viscosity of Water as a Function of Temperature 3 3 4 Surface Tension The Liquid State Surface Tension o Surface tension is a measure of the unequal attractions that occur at the surface ur of a liquid. of o The molecules at the surface are attracted unevenly. Mosquitoes walk on water because of the surface tension. Surface tension is responsible for the shape of liquid droplets. Although easily deformed, droplets of water tend to be pulled into a spherical shape by the cohesive forces of the surface layer. Molecules within the liquid experience attractive forces from all directions Molecules at the surface experience a net inward force. The surface tension of water provides the necessary wall tension for the formation of bubbles with water. 5 6 6 1 Surface Tension The Liquid State Capillary Action • Capillary action is the ability of a liquid to rise (or fall) in a glass tube or glass other container. other • Cohesive forces are the forces that hold liquids together. • Adhesive forces are the forces between a liquid and another surface. – Capillary rise implies that the: • Adhesive forces > cohesive forces – Capillary fall implies that the: • Cohesive forces > adhesive forces Increasing temperature will lower the surface tension Soaps and detergents further lower the surface tension. Soaps 7 8 8 Capillary Action Capillary action is the result of adhesion and surface tension. surface Adhesion of water to the walls of a vessel will cause an upward force on the liquid at the edges and result in a meniscus which turns upward. and The surface tension acts to hold the surface intact, so instead of just the edges moving upward, the whole liquid surface is dragged upward. Capillary Action Meniscus curve turns up. Meniscus curve turns down. Water exhibits a capillary rise. Adhesive forces > cohesive forces Water exhibits a capillary fall. Cohesive forces > adhesive forces 10 10 9 The Liquid State Capillary action also affects the meniscus of liquids. Vapor Pressure • The pressure of a vapor in equilibrium with its non-vapor vapor phases. – All liquids and solids have a tendency to evaporate to a All gaseous form, and all gases have a tendency to condense back into their original form (either liquid or solid). – For a particular substance, at a specific temperature, there For is a pressure at which the gas of that substance is in dynamic equilibrium with its liquid or solid forms. • The equilibrium vapor pressure indicates a liquid's evaporation rate. The concave H2O Hg convex Measure the meniscus at eye level from the center of the meniscus. Water and most liquids have a concave meniscus. Mercury produces a convex meniscus. 11 11 – It expresses the tendency of molecules and atoms to escape It from a liquid or a solid. – A substance with a high vapor pressure at normal substance temperatures is referred to as volatile. volatile 12 2 Vapor Pressure The pressure exerted by an evaporated liquid above the surface of the liquid. Vapor Pressure The gas molecules will continue to accumulate above the surface of the liquid until it reaches atmospheric pressure. In a closed container, the number of gas molecules that evaporate will eventually establish equilibrium. At equilibrium, the number of gas molecules formed are 13 equal to the number of gas molecules that condense. Ordinary evaporation is a surface phenomenon - some molecules have enough Ordinary some kinetic energy to escape. If the container is closed, an equilibrium is reached where an equal number of If qual molecules return to the surface. The pressure of this equilibrium is called the saturation vapor pressure. vapor The vapor pressure is the same for any particular liquid, regardless of the is for volume of the liquid, if the temperature is the same. volume 14 Vapor pressure changes with temperature and atmospheric pressure • Vapor Pressures and Boiling Points A liquid boils at a temperature at which its vapor pressure is equal to the pressure of qual the gas above it. the The lower the pressure (atmospheric pressure) of a gas above a liquid, the lower the iquid, temperature at which the liquid will boil. Boiling points and vapor pressures of substances are determined by intermolecular Boiling forces and mass. forces Vapor pressure is determined by the intermolecular forces of the liquid substance. • • – Liquids held together with strong intermolecular forces have high boiling points and low vapor pressures boiling Ethyl ether dispersion Ethanol some H-bonds Water H-bonds 15 16 The boiling point of water is dependent on the atmospheric pressure. The atmospheric pressure is lower at higher altitudes. Vapor Pressures and Boiling Points • Liquids held together with weak intermolecular forces have Liquids high vapor pressures. high – As the mass of the molecules increase, the boiling point As increases and vapor pressure decreases. increases B.P. C2H6 < C3H8 < C4H10 < C5H12 V.P. C5H12 < C4H10 < C3H8 < C2H6 10 The atmospheric pressure is higher at lower altitudes. 17 18 3 ...
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