Lecture Presentation Chapter 11 Liquids and Intermolecular Forces John D. Bookstaver St. Charles Community College Cottleville, MO
Properties of Gases, Liquids, and Solids State Volume Shape of State Density Compressibility gas assumes shape and volume of container low easily compressed liquid definite volume, assumes shape of container high nearly incompressible solid both definite shape and volume high nearly incompressible
Physical State and Energy of Attraction Physical State solid liquid gas Relation Between Energy of Attraction and Kinetic Energy of Molecules kinetic energy << energy of attraction kinetic energy energy of attraction kinetic energy >> energy of attraction
Because in the solid and liquid states particles are closer together, we refer to them as condensed phases. States of Matter
Differentiate between Intermolecular Forces and Intramolecular Forces Definition Intramolecular Bonds hold ATOMS together Intermolecular Attraction forces hold MOLECULES together Types Ionic, covalent, metallic van der waals, H-Bonding Strength Stronger Weaker Intermolecular forces are strong enough to control physical properties of a substances (eg: boiling poins, melting point, vapor pressure, viscosity).
Types of Intermolecular Forces London dispersion forces Dipole dipole forces Hydrogen bonding van der Waals forces
London Dispersion Forces Exists between non-polar molecules, and atoms. London dispersion forces, or dispersion forces, are attractions between an instantaneous dipole and an induced dipole. These forces are present in all molecules, whether they are polar or nonpolar.
Polarizability Polarizability: the tendency of an electron cloud to distort. Polarizability generally increases with the number of electrons in the molecule. More polarizable molecules have larger dispersion forces.
Factors Affecting London Forces 1. Molecular Weight The strength of dispersion forces tends to increase with increased molecular weight. Larger atoms have larger electron clouds that are easier to polarize. Substance Molar Mass Boiling Point ( C) F 2 38-188 Cl 2 71-35 Br 2 160 59 I 2 254 184 http://www.chem.purdue.edu/gchelp/liquids/boil.html
Factors Affecting London Forces 2. Shape of the molecule long, skinny molecules (like n-pentane) tend to have stronger dispersion forces than short, fat ones (like neopentane). This is due to the increased surface area in n-pentane.
Dipole Dipole Interactions Exists between polar molecules (as these have permanent dipole moments and therefore partial charges) When polar molecules are close to each other, they are attracted by dipoledipole forces Positive end of one molecule is attracted to the negative end of the other molecule
Factors to affecting dipole-dipole forces? For molecules of approximately equal mass and size, the strength of intermolecular attraction increase with increasing polarity The more polar the molecule, the higher the boiling point (bp)
Hydrogen Bonding Hydrogen bonding a special type of dipole-dipole force when H is bonded to N, O, or F Reason: N, O, and F are the most electronegative atoms producing a strong dipole with H Note: H-bonding is an attractive force not an actual bond!
Why does ice float? Hydrogen bonding causes ice to have a lower density than liquid water.
Ion Dipole Interactions Exists between an ion and a polar molecule. Positive ions (cations) are attracted to the negative end of a dipole. Negative ions (anions) are attracted to the positive end of a dipole. Explains solubility of ionic salts in polar solvents (eg: NaCl in water). What affects Ion-dipole attractions? the charges on the ion magnitude of the dipole of the polar molecule
Comparing Intermolecular Forces
Sample Exercise Predicting Types and Relative Strengths of Intermolecular Attractions List the substances BaCl 2, H 2, CO, HF, and Ne in order of increasing boiling point.
Visual Summary Kinetic energy vs energy of attraction Intermolecular forces Solids, Liquids, Gas Intramolecular forces Ion-dipole forces Hydrogen bonding Dipole-dipole forces Van der Waals forces London dispersion forces Ionic bond Covalent bond Metallic bond Polarizability
Sample Exercise Predicting Types and Relative Strengths of Intermolecular Attractions (a) Identify the intermolecular attractions present in the following substances, and (b) select the substance with the highest boiling point: CH 3 CH 3, CH 3 OH, and CH 3 CH 2 OH. Answers: (a) CH 3 CH 3 has only dispersion forces, whereas the other two substances have both dispersion forces and hydrogen bonds, (b) CH 3 CH 2 OH
Intermolecular Forces Affect Many Physical Properties The strength of the attractions between particles can greatly affect the properties of a substance or solution.
Viscosity Resistance of a liquid to flow is called viscosity. It is related to the ease with which molecules can move past each other. Viscosity increases with stronger intermolecular forces and decreases with higher temperature.
Surface Tension Surface tension results from the net inward force experienced by the molecules on the surface of a liquid.
Phase Changes The heat of vaporization is defined as the energy required to change a liquid at its boiling point to a gas. The heat of sublimation is defined as the energy required to change a solid directly to a gas. The heat of fusion is the energy required to change a solid at its melting point to a liquid.
Energy Changes Associated with Changes of State The heat added to the system at the melting and boiling points goes into pulling the molecules farther apart from each other. The temperature of the substance does not rise during a phase change.
Vapor Pressure At any temperature some molecules in a liquid have enough energy to break free. As the temperature rises, the fraction of molecules that have enough energy to break free increases.
Vapor Pressure As more molecules escape the liquid, the pressure they exert increases.
Vapor Pressure The liquid and vapor reach a state of dynamic equilibrium: liquid molecules evaporate and vapor molecules condense at the same rate.
The boiling point of a liquid is the temperature at which its vapor pressure equals atmospheric pressure. The normal boiling point is the temperature at which its vapor pressure is 760 torr. Vapor Pressure
Phase Diagrams Phase diagrams display the state of a substance at various pressures and temperatures, and the places where equilibria exist between phases.
Phase Diagrams The liquid vapor interface starts at the triple point (T), at which all three states are in equilibrium, and ends at the critical point (C), above which the liquid and vapor are indistinguishable from each other.
Phase Diagrams Each point along this line is the boiling point of the substance at that pressure.
Phase Diagrams The interface between liquid and solid marks the melting point of a substance at each pressure.
Phase Diagrams Below the triple point the substance cannot exist in the liquid state. Along the solid gas line those two phases are in equilibrium; the sublimation point at each pressure is along this line.
Phase Diagram of Water Note the high critical temperature and critical pressure. These are due to the strong van der Waals forces between water molecules.
Phase Diagram 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 5.11 atm; CO 2 sublimes at normal pressures.