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Intermolecular Forces Important differences between gases, solids, &liquids: – Gases - expand to fill their container. – Liquids - retain volume, but not shape. – Solids – retain volume and shape. Question :Why ?? Properties can be understood in terms of how tightly the molecules are packed together and the strength of attractions between them. State the differences between Intermolecular & Intramolecular forces. The differences between Intermolecular & Intramolecular forces Intramolecular forces Intermolecular forces The attractions within The attractions between molecules molecules Always stronger than Always weaker than intermolecular forces Intramolecular forces Control the chemical Control the physical properties properties of the substance. of the substance. Which of these are intermolecular forces ??? 1.Metallic bond 2.Ionic bonds 3.Dipole-dipole attractions. – 1 % - 5% of strength of covalent bond. 4.London Dispersion forces.(Van der waals Forces) 5.Ion-dipole 6.ion-induced dipole attractions 7. Hydrogen bonding . – 5- 10 % of strength of covalent bond 1- Metallic Bond 2-Ionic Bond 3-Dipole-dipole attractions – Polar molecules tend to align their partial charges – The attractive force is about 1% of a covalent bond and drops off as 1/d 3 (d=distance between dipoles). 4-London forces – The (very) weak attractions between nonpolar molecules – Arise from the interactions of instantaneous dipoles on neighboring molecules An instantaneous dipole on one molecule can produce and induced dipole on another. The net interaction of these over time is attractive. – These instantaneous dipole-induced dipole attractions are called London dispersion forces, London forces, or dispersion forces – London forces decrease as 1/d 6 (d=distance between molecules) Strength of London forces depends on three factors: a. Polarizability is a measure of the ease with which the electron cloud on a particle is distorted. It tends to increase as the electron cloud volume increases. b. Number of atoms in the molecule c. Molecular shape a- Polarizability :Large electron clouds are more easily deformed than small ones. The magnitude of the resulting partial charge is larger. The larger molecules experience larger London forces than small molecules. Boiling point of halogens and noble gases demonstrate this: b-London forces depend on the number of atoms in the molecule. The larger the molecular formula the larger the boiling point of hydrocarbons. Hexane, C6H14, (right) has a BP of 68.7oC while the BP propane, C3H8, (left) is –42.1oC because hexane has more sites (marked with *) along its chain where attraction to other molecules can occur. c-Molecular shape affects the strength of London forces – More compact molecules tend to have lower London forces than longer chain-like molecules. For example the more compact neopentane molecule (CH3)4C has a lower boiling point than n-pentane, CH3CH2CH2CH2CH3 . Both have formula C5H12. The H atoms in the more compact neopentane cannot interact as well with neighboring molecules as the H atoms in the more chain-like n-pentane. 5-Ion-dipole and 6-ion-induced dipole attractions are the attractions between an ion and the dipole or induced dipole of neighboring polar molecules. (a) The negative ends of water dipoles surround a cation. (b) The positive ends of water dipoles surround an anion. The attractions can be quite strong because the ions have full charges. 7-Hydrogen bonding Very strong dipole-dipole attraction that occur when H is covalently bonded to to a small, highly electronegative atom (usually F, O, or N) – Typically about 10 times stronger than other dipole-dipole attractions . – Are responsible for making water a liquid , for high boiling point of water & for the expansion of water as it freezes. (a) Polar water molecule. (b) Hydrogen bonding produces strong attractions in the liquid. (c) Hydrogen bonding (dotted lines) between water molecules in ice form a tetrahedral configuration. Electronegativity describe the relative attraction of an atom for the electrons in a bond. The element with the larger electronegativity will carry the partial negative charge. In general, electronegativity increases bottom to top in a group and left to right in a period. Order of electronegativity F > O >N 1. Dipole-dipole: occur between molecules with permanent dipoles; about 1% - 5% of a covalent bond. 2. London dispersion: present in all substances; are weak, but can lead to large net attractions. 3. Ion-dipole: occur when ions interact with polar molecules; can lead to large net attractions. 4. Ion-induced dipole: occur when an ion induces a dipole on neighboring molecule; depend on ion charge and the polarizability of its neighbor. 5. Hydrogen bonding: occur when molecules contain N-H , O-H & F-H bonds; about 5% to 10% of a covalent bond. Heat of vaporization (∆Hvap ) The liquid begins to evaporate in the closed container. Avg. kinetic energy of remaining molecules is less, so the temperature is lower. Dynamic Equilibrium . The molar heat of vaporization or enthalpy (∆Hvap ) heat absorbed when 1 mole of liquid is changed to 1mole of vapor at constant T&P. It is measured at the normal boiling point of a substance • The rate of evaporation depends on: Thank you Wikipedia encyclopedia Thank you Wikipedia encyclopedia It is directly proportional to strength of intermolecular attractions between molecules. It increases with molecular mass. Example: The molar heat of vaporization of water is 40.6 kJ/mol. How many kJ of heat energy are required to convert 1.0L of water to steam? Solution: 1.0L=1000 mL x1.0 g/mL = 1000 g 1000 g x 1mol x 40.6 kJ = 2260 kJ of heat . 18g mol Factors that affect Vapor pressures : 1-Increasing temperature. It increases the amount of vapor and decreases the amount of liquid. At higher temperature, the total fraction of molecules with kinetic energy large enough to escape to vapor phase is larger so the rate of evaporation is larger. 2- Vapor pressure increases with decreasing intermolecular forces. Factors that do not affect Vapor pressures: Volume changes can effect vapor pressure for a short time then equilibrium is re-established and the vapor pressure returns to its initial value Vapor pressures of Solids: • Solids also have vapor pressures. The pressure of the vapor that in equilibrium with the solid is called the equilibrium vapor pressure of the solid. • At a given temperature, some of the solid particles have enough kinetic energy and escape into the vapor phase • The molar heat of fusion(∆Hfus) is the heat absorbed by one mole of solid when it melts to give a liquid at the same temperature and pressure. • The molar heat of sublimation (∆Hsub) is the heat absorbed by one mole of a solid when it sublimes to give one mole of vapor at constant T & P. • All of these quantities tend to increase with increasing intermolecular forces.
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