Property of liquid and Phase Diagram for EN 2017

Transcription

1 Property of Liquid and Phase Diagram by Assist.Prof.Dr.Choosak Poonsawat FB: เคม อ ช ศ กด You should know - Intermolecular forces - Properties of Liquids - Phase Diagram Properties of solids, liquids and gases Intermolecular Forces Type of Intermolecular Forces The term van der Waals forces is a general term including dipole-dipole and London forces. Van der Waals forces are the weak attractive forces in a large number of substances. London Dispersion Forces (แรงของการกระจาย) London forces are the weak attractive forces resulting from instantaneous dipoles that occur due to the distortion of the electron cloud surrounding a molecule. Dipole-Dipole Forces (แรงระหว างโมเลก ลม ข ว) Polar molecules can attract one another through dipole-dipole forces. d + H Cl d - d + H Cl d -

2 Hydrogen Bonding - A special case of dipole-dipole forces. - This intermolecular force is very strong. - Strongest of the three Van der Waal s forces (Hydrogen bonding, Dipole-dipole, London forces) - H-bonding requires H bonded to an electronegative element (most important for compounds of F, O, and N). Ion-Dipole Forces Interaction between an ion (+ or -) and a dipole molecule. Strongest of all intermolecular forces. ion Dipole molecule Properties of Liquids; Surface tension (แรงต งผ ว) The energy required to stretch or increase the surface of a liquid by a unit area. Resistance of a liquid to an increase in surface area. The net pull toward the interior of the liquid makes the surface tend to as small a surface area as possible and a substance does not penetrate it easily. Therefore strong intermolecular forces Falling raindrops are nearly spherical, minimizing surface area. High surface tension Temperature also affects Surface Tension. Super hydrophobic surfaces Does warm water speed up the cleansing process? 2

3 Capillary Action Spontaneous rise of a liquid in a narrow tube. Cohesive forces (การเช อมแน น) exist between liquid molecules. Adhesive forces (การย ดต ด) exist between liquid molecules and the walls of the container. water mercury Surface Tension is reduced by surfactant Adhesion > Cohesion Cohesion > Adhesion Concave เว า Convex น นออก Vapor Pressure (ความด นไอ) In a sealed container, some of a liquid evaporates to establish a pressure in the vapor phase. Vapor pressure: partial pressure of the vapor over the liquid measured at equilibrium and at some temperature. Dynamic equilibrium evaporation liquid condensation vapor Temperature Dependence of Vapor Pressures - The vapor pressure above the liquid varies exponentially with changes in the temperature. -The Clausius-Clapeyron equat n shows how the vapor pressure and temperature are related. It can be written as: ln P - + C RT T Clausius Clapeyron Equation A straight line plot results when ln P vs. /T is plotted and has a slope of H vap /R. At any two points, we get P2 ln - P RT T T2 lnp - + C RT T Carbon disulfide, CS 2, has a normal boiling point of 46 C (vapor pressure = 760 mmhg) and a heat of vaporization of 26.8 kj/mol. What is the vapor pressure of CS 2 at 35 C? Substituting into the Clausius-Clapeyron equation, we obtain: 3 P J/mol ln ( - ) (760mm Hg) 8.3 J/(mol K) 39 K 308 K P 2 (3225K) (-.2 0 K ) = 530 mmhg -4-3

4 Viscosity (ความหน ด) Measure of the resistance to deformation of a fluid under shear stress. Strong intermolecular forces High viscosity The viscosity of liquids decreases and the viscosity of gases increase with temperature. Boiling Point Temperature at which the vapor pressure of a liquid is equal to the external pressure above the liquid. At sea level. The atmospheric pressure is higher. Where is the boiling point of a liquid higher, at or above sea level? Which has lowest boiling point? (a) CS 2 (b) CH 3 OH (c) CH 3 CH 2 OH (d) H 2 O (e) C 6 H 5 NH 2 Which has lowest Intermolecular force? Energy of Heat and Phase Change Heat of vaporization: heat needed for the vaporization of a liquid. H 2 O(l) H 2 O(g) H = 40.7 kj Heat of fusion: heat needed for the melting of a solid. H 2 O(s) H 2 O(l) H = 6.0 kj Temperature does not change during the change from one phase to another. Start with a solution consisting of 50.0 g of H 2 O(s) and 50.0 g of H 2 O(l) at 0 C. Determine the heat required to heat this mixture to 00.0 C and evaporate half of the water. 4

5 Phase Diagrams Phase diagrams display the state of a substance at various pressures and temperatures and the places where equilibria exist between phases. The AB line is the liquid-vapor interface. It starts at the triple point (A), the point at which all three states are in equilibrium. It ends at the critical point (B); above this critical temperature and critical pressure the liquid and vapor are indistinguishable from each other. Each point along this line is the boiling point of the substance at that pressure. - Pressure = Force / area - Increasing pressure causes the molecules to be condensed making the molecular arrangement closer The AD line is the interface between liquid and solid. The melting point at each pressure can be found along this line. Below A the substance cannot exist in the liquid state. Along the AC line the solid and gas phases are in equilibrium; the sublimation point at each pressure is along this line. Phase Diagram of Water high critical temperature and critical pressure: - These are due to the strong van der Waals forces between water molecules. (London dispersion force, Dipole-dipole forces and Hydrogen bonding - Thus Increase the melting and boiling point of water The slope of the solid liquid line is negative. This means that as the pressure is increased at a temperature just below the melting point, water goes from a solid to a liquid. Phase Diagram of Carbon Dioxide Carbon dioxide cannot exist in the liquid state at pressures below 5. atm; CO 2 sublimes at normal pressures. The low critical temperature and critical pressure for CO 2 make supercritical CO 2 a good solvent for extracting nonpolar substances (such as caffeine). 5