Chemistry Study Guide Test 1 CHM1045 Review Drawing Lewis Structures Lewis Structure A representation of covalent bonding in which shared electron pairs are shown either as lines or as pairs of dots between two atoms and lone pairs are shown as pairs of dots on individual atoms Lone Pairs Pairs of valence electrons that are not involved in covalent bond formation in the center o Draw the structure of the compound with the least electronegative element o Count the total number of valence electrons Add 1 valence electron for a negative charge Subtract 1 valence electron for a positive charge o Complete an octet for all of the atoms except hydrogen Octet Rule An atom other than hydrogen tends to form bonds until it is surrounded by eight valence electrons No2 Li o If the structure contains too many electrons form double and triple bonds on central atoms as needed Formal Charge The difference between the number of valence electrons in an isolated atom and the number of electrons assigned to that atom Assigning Formal Charge it resides in 1 Find the number of valence electrons of the atom by finding which group 2 Subtract that number from the number of bonds the ion is connected with 3 Subtract that number from the number of lone pairs around the ion 4 The sum of your formal charges must be equal to the charge on that molecule or ion o CH2O a If your ion is No2 then your final formal charge should equal 1 C 4 valence electrons and 4 bonds 4 4 0 Formal Charge O 6 valence electrons 2 bonds and 4 lone pairs 6 2 4 0 Formal Charge H 1 valence electron and 1 bond 1 1 0 Formal Charge o Since your final formal charges are 0 for C H and O and because CH2O is a neutral molecule 0 formal charge this would be your resulting balanced Lewis Structure Formal Charge and Lewis Structures o Lewis Structures with large formal charges are less likely than those with small formal charges If your final formal charge for a molecular compound is 0 Neutral Compound 1 Positive compounds like NH4 1 Negative compounds like No2 o In Lewis Structures the most likely structure is the one in which negative formal charges are placed on the more electronegative atoms Resonance Use of two or more Lewis structures to represent a particular molecule Resonance Structure When one of two or more Lewis structures for a single molecule cannot be represented accurately by only one Lewis structure Exceptions to the Octet Rule o BeH2 Odd Electron Molecules o NO 11 valence electrons Valence Shell Electron Pair Repulsion VSEPR Model Predicts the geometry of the molecule from the electrostatic repulsions between the electron bonding and nonbonding pairs BeCl2 has a linear molecular geometry with a 180 bond angle because it only has 2 atoms bonded to a central atom Bond Angles o Trigonal Planar 120 o Tetrahedral 109 5 o Trigonal Bipyramidal 90 and 120 o Octahedral 90 Predicting Molecular Geometry o Draw the Lewis structure for each molecule o Count the number of lone pairs on the central atom and the number of atoms bonded to the central atom o Use VSEPR to predict the molecules molecular geometry Hybridization Mixing of two or more atomic orbitals to form a new set of hybrid orbitals o Mix at least 2 nonequivalent atomic orbitals Ex S and P Hybrid orbitals have very different shapes from original atomic orbitals o The number of hybrid orbitals is equal to the number of pure atomic orbitals used in the hybridization process o Covalent bonds are formed by Overlap of hybrid orbitals with atomic orbitals Overlap of hybrid orbitals with other hybrid orbitals Chapter 11 Key Equations ln P H vap RT C ln P1 P2 H vap R H H fus H vap o Hess s Law Key Concepts o Clausius Clapeyron Equation for determining Hvap of a liquid o For calculating Hvap vapor pressure or boiling point of a liquid Polymer Huge molecule built up of thousands of the same unit o Proteins Polymers of amino acids A protein will fold into the exact same shape every time o Amino Acid Any of a class of organic compounds that contains at least one amino group NH2 and one carboxyl group COOH Intermolecular Forces The attractive forces between molecules or between molecules and ions Normally these attractive forces are weaker than bonding forces o Intermolecular Force Ion dipole interaction Example When you heat up a compound like sugar you are breaking intermolecular bonds o Evidence of Intermolecular Forces Boiling Point and Melting Point Stronger Attraction Higher Boiling Point Melting Point Hvap Liquid Gas Hfusion Solid Liquid Stronger Attraction Higher Hvap Hfusion Vapor Pressure Volatility Stronger Attraction Lower Vapor Pressure at specific temp H2O Solubility More Polar More H2O soluble o When there are more Polar Parts vs Nonpolar Parts of a molecule then Polar wins making the molecule H2O soluble o When there are more Nonpolar Parts vs Polar Parts of a molecule then Nonpolar wins making the molecule not H2O soluble Surface Tension Capillary Action H2o balls up on wax paper H2O climbs up thin glass tubes Viscosity Stronger Attraction More Viscous Intramolecular Forces The force that holds an atom together in a molecule Van Der Waals Forces Dipole dipole dipole induced dipole and dispersion o Ion dipole forces Ions and dipoles that are attracted to one another by electrostatic forces Not Van Der Waals forces Hydrogen Bonding a special type of dipole dipole interaction between the hydrogen atom in a polar bond and an electronegative atom o Hydrogen Bonding is like a string connecting the hydrogen atom in the polar bond such as N H O H or F H with another electronegative atom O N or F because they are so attracted to each other that they form a link together without having to combine and interact with one o Hydrogen Bonds between water molecules are very strong another Example DNA Dipole dipole forces Attractive forces between polar molecules Ion dipole forces Forces that attract an ion anion or cation and a polar molecule to each other molecule Induced Dipole Caused by the separation of positive and negative charges in the atom or nonpolar molecule due to the proximity of an ion or a polar o Ion induced dipole The attractive interaction between an ion and the induced dipole o Dipole induced dipole reaction The attractive interaction between a polar molecule and the induced dipole Ex H2O makes O2 Polar o Induced dipole Induced dipole A collection of nonpolar molecules gathered together I 2 create their own polarity Dispersion Forces The result of temporary dipole moments
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