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| when a polar molecule encounters an ion, the attractive forces depend on three factors | • The distance between the ion and dipole The closer the ion and dipole, the stronger the attraction • The charge of the ion The higher the ion charge, the stronger the attraction • The magnitude of the dipole The greater the magnitude of the dipole, the stronger the attraction |
| ion dipole forces | o forces of attraction between a +/- ion and polar molecules o are less than those for ion-ion attractions but greater than other types of forces between molecules whether polar or nonpolar o can be evaluated based on coulombs law |
| coulumbs law | the force of attraction between oppositely charged particles depends directly on the the product of their charges and inversely on the square of the distance between the ions. the energy of attraction is also proportional to the charge product, but is inversely proportional to the distance between them |
| enthalpy of hydration | the energy evolved when an ion is hydrated depends on the dipole moment of water, the ion charge, and the distance between centers of the ion and the polar water molecule. the distance increases as ion size increases |
| • kinetic molecular theory of gases | o assumes gas molecules or atoms are widely separated and that these particles can be considered to be independent of one another o thus we can relates properties of gases under most conditions by a simple mathematical equation PV=nRT. • Ideal gas law equation • However in real gases there are forces between molecules—intermolecular forces—and these require a more complex analysis of gas behavior • If intermolecular forces become strong enough, the substance can condense to a liquid and eventually to a solid. |
| • Intermolecular forces influence chemistry in many ways | o They are directly related to properties such as melting point, boiling point, and the energy needed to convert a solid to a liquid or a liquid to a vapor o They are important in determining the solubility of gases, liquids, and solids in various solvents o They are crucial in determining the structures of biologically important molecules such as DNA and proteins |
| • Bonding in ionic compounds depends on the ... | electrostatic forces of attraction between oppositely charged ions • The intermolecular forces attracting one molecule to another are electrostatic • As a general rule intermolecular forces are generally less than about 15% of the vales of bond energies |
| • Many molecules have dipoles, and their interactions occur by... | dipole-dipole attraction |
| dipole-dipole attraction | o influence among other things the evaporation of a liquid, and the condensation of a gas. An energy change occurs in both processes o evaporation requires the input of energy, specifically the enthalpy of vaporization. o The value for the enthalpy of vaporization has a positive signs, indicating that vaporization is an endothermic process. o The value for enthalpy change for condensation has a negative value and is the reverse of vaporization |
| The boiling point of a liquid also depends on... | intermolecular forces of attraction. o As temp of substance increase its molecules gain kinetic energy o When boiling point is reached the molecules have sufficient kinetic energy to escape the forces of attraction of their neighbors o for molecules of similar molar mass, the greater the polarity the higher the temp required for the liquid to boil |
| how intermolecular forces also influence solubility | o like dissolves like • polar molecules are likely to dissolve in a polar solvent, and non polar in nonpolar. Unlikely polar to dissolve in nonpolar and visa versa • e.g. ethanol and water polar molecules water-ethanol interactions are strong enough that the energy expended in pushing water molecules apart to make room for ethanol molecules is compensated for by the energy of attraction between the two kinds of polar molecules |
| In bonds between H and N,O, or F... | the more electronegative element takes on a significant negative charge, and the hydrogen atom acquires a significant positive charge |
| hydrogen bond | o There is an unusually strong attraction between an electronegative atom with a lone pair of electrons (most often, and N, O, or F atom in another molecule or even in the same molecule) and the hydrogen atom of the N—H, O—H, or F—H bonds. o are an extreme form of dipole-dipole interaction where one atom involved is always H and the other atoms is highly electronegative, most often O,N, or F. o hydrogen becomes a bridge between the two electronegative atoms. has important implications for any property of a compound that is influenced by intermolecular forces of attraction |
| • one reason fir ice’s unusual structure and water’s unusual properties is that... | each hydrogen atom of a water molecule can form a hydrogen bond to a lone pair of electrons on the oxygen atom of an adjacent water molecule. Bc the oxygen atom in water has two lone pairs of electrons, it can form two more hydrogen bonds with the hydrogen atoms from adjacent molecules o result seen particulars in ice is a tetrahedral arrangement for the hydrogen atoms around each oxygen, involving two covalently bonded hydrogen atoms and two hydrogen-bonded hydrogen atoms |
| o as a consequence of the regular arrangement of water molecules linked by hydrogen bonding... | ice has on open-cage structure with lots of empty space. • The result is that ice has a density about 10% less than that of liquid water • Can also see in this structure that the oxygen atoms are arranged at the corners of puckered hexagonal rings Snowflakes are always based on six-sided figures |
| • extensive hydrogen bonding is also the origin of the extraordinary high heat capacity of water | o although liquid water does not have the regular structure of ice, hydrogen bonding still occurs o with a rise in temp the extent of hydrogen bonding diminishes o disrupting hydrogen bonds requires energy |
