Chemical Principles II
Chemical Principles II CHEM 112
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This 0 page Class Notes was uploaded by Herta Weber on Sunday November 1, 2015. The Class Notes belongs to CHEM 112 at Pennsylvania State University taught by Staff in Fall. Since its upload, it has received 38 views. For similar materials see /class/233157/chem-112-pennsylvania-state-university in Chemistry at Pennsylvania State University.
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Date Created: 11/01/15
Chemistry 112002 MO Theory25 Bonding in molecules 978 also review 946 Dr Raymond Schaak Penn State University SPRING 2009 OUTLINE 0 Experiment Lewis dot structure and condensation of O2 0 Hybrid orbitals 0 Molecular orbitals 0 Atomic vs molecular orbitals Generating molecular orbitals 0 MO diagram for H2 0 Bond order 0 Mixing of p orbitals 0 Rules for molecular orbitals 0 MO diagrams for N2 and O2 0 MO diagrams for 2nd row diatomics Bonding in molecules and solids How are atoms held together in molecules and solids How does this contribute to the properties of a molecule or solid Magnetism unpaired vs paired electrons Color beyond transition metals Electrical metals semiconductors insulators Mechanical hardness strength Thermal melting boiling decomposition Where we re heading The chemistry of modern materials solids after a brief look at some key aspects of bonding in molecules molecular orbitals First let s do an experiment Lewis dot structure for 02 Draw the Lewis dot structure for 02 What is the nature of the bonding in 02 Are the electrons paired or unpaired Would you predict O2 to be paramagnetic or diamagnetic based on the Lewis dot structure Experiment Let s do an experiment Condense 02 between the poles of a strong magnet using liquid N2 as a coolant Air contains by volume 7808 N2 BP 77 K 2095 02 BP 90 K 093 Ar 0038 CO2 What is happening in the experiment What does this tell us about the Lewis dot structure of 02 Modifying our theory How do bonds form Overla rwinn Aomsappmacheach other P b Ovcrinp region cbond 1tbond Hybrid orbitals Mix orbitals to get hybrid orbitals 1s A0 3p AO s 9 hybrid orbitals Hybrid orbitals are on the atom from which they originate l 2 g r Hyman m lorml39om sn Iwbm mbilals Molecular orbitals Hybrid orbitals form by mixing atomic orbitals from the same atom and they are centered on the atom from which they originate Molecular orbitals form by mixing atomic orbitals from different atoms and they are spread out beyond the atoms from which they originate spread out over the entire molecule Node I I 39 I quotIs I I I Energy I u o 15 ls H2 molecular orbitals Mix two atomic orbitals H 1s and H 1s and get two molecular orbitals H2 515 and H2 515 Atomic vs molecular orbitals Atomic orbitals Molecular orbitals Wave functions describe locations of electrons in molecules Wave functions describe locations of electrons in atoms Visualize as Visualize as contour plots contour plots Each molecular orbital can hold a maximum of 2 electrons Each atomic orbital can hold a maximum of 2 electrons Associated with an Associated with a entire molecule single atom H Y H 39 Overlap region N xdu r 39 l 39l l l l Generating molecular orbitals How do we generate molecular orbitals from atomic orbitals Combine their wavefunctions highlevel math but a simple picture works too gt J J W a 5 m 15 15 H atomic orbitals quot1 H1 molecular orbitals Constructive add vs destructive subtract interference generates two distinct molecular orbitals from the two atomic orbitals two atomic wavefunctions in two molecular wavefunctions out Note the symmetry about the internuclear axis What you learned for atomic orbitals still holds MO diagram for H2 Fill the orbitals from low to high energy halffilling each degenerate orbital first before pairing electrons in the same orbital Energy Hatom Hatom 715 H2 molecule Bond order The bond order is defined as 12 bonding e39 antibonding e What is the bond order for H2 What does this physically mean What is the bond order for He2 What does this physically mean And how does this help us rationalize the chemistry of helium Mixing of porbitals The p atomic orbitals can mix either endon or sideways to create molecular orbitals a quotEndon overlap of p orbitals forms 039 and 9 M05 w 4 ZV 2 02 b Sideways overlap of p orbitals forms two sets of 1 and 7 M05 1 1 l l 1 1 l Rules for molecular orbitals Number of MO s formed equals the number of AO s combined AO s combine best with AO s of similar energy The effectiveness with which two atomic orbitals combine is proportional to their overlap size shape energy more overlap results in a larger splitting between the bonding lower energy and antibonding higher energy MO s Each MO holds a maximum of two spinpaired electrons For degenerate MO s same energy one electron same spin in each MO before pairing Hund s rule MO diagram for 2nd row diatomics quot3r 139 l l l I r l q 1 p I I 3 l 39 l l l m l I 1 J a f 2 l I i H It i r 772px 139 l l i m l 0 LE l l l Nud 1 I l I s q 2 Z I I There are both a and 1 bonding molecular orbitals and 6 and 1c antibonding molecular orbitals One last detail Increasing lsZy interaction Enm gy of 113 r molecularorbiialg A 371 o z a n n H M Smaller elements have large interaction between s and p orbitals ips the order u OfG and 75 035 BgCN3 Large ZsZp inlcmrtinn Small ZsZp intumclion B2 C1 NZ 02 El Ne2 quot51 7 it quot3w U ll zl l w W lnli l j 1 lll u lllt m lg Ul Elli ll lEl 1E n3 LT ll a V II M El E It FD Um er I l 1 t 2 l l Hm lllv tk mnl 0 an an lug n a llumllwl EH 1quotquot ll lllI 3l in ngmm twin nu l39nmnagnrllr limnmgnvlu39 ivmmgmur I39nmnmglwin DlMlmgnulh 7 MO diagrams for N2 and 02 Back to our experiment why does 02 behave the way it does What can MO theory tell us that Lewis dot structures can not N2 02 Total of e Total of e Bond order Bond order unpaired e unpaired e Chemistry 112002 Chem Structure and Acids1O Oxyacids and Lewis Acids 161011 Dr Raymond Schaak Penn State University SPRING 2009 OUTLINE Acids and bases WHY Bond strength and acid strength Trends in HX acid strength Oxyacids Electronegativity and oxyacids Oxyacid strength Lewis acids and bases Comparing acidity of Lewis acids Hydration of metal cations Why are metal cation solutions acidic Acids and Bases WHY Why are some OH groups acidic and others basic Why are some acids stronger than others Think about a molecule containing hydrogen Under what conditions will an Hcontaining molecule transfer a proton What about NaH CH4 Bond strength and acid strength What is the effect of bond strength on the ability of a H X molecule to donate a proton Strong bonds are moreless easily dissociated than weak bonds What about the effect of conjugate base strength Greater stability of conjugate base strongerweaker acid Acid strength is a combination of aH of these factors Trends in HX acid strength Same group column H X bond strength increasesdecreases as X increasesdecreases in size down column so acidity increasesdecreases Same period row Look at H X bond polarity As electronegativity of X increasesdecreases acidity increasesdecreases left to right or right to left Oxyacids H2804 HNOs H3PO4 etc Electronegativity and oxyacids What do eectronegative atoms do to the strength of oxyacids Shift of Electron density 7 F O H K 30 X10 H EN 30 11 H 19223 gtlt10quot39 EN25 anyligm n ma Pealsan Prenllce Hall Inc Oxyacid strength The strength of oxyacids increases with 1 Increasing of central atom 2 Increasing of central atom General rule for uncharged oxyacids HXEOy Ify x 2 2 then acid If y x lt 2 then acid Lewis acids amp bases Another way of viewing acidity Expands concept of acids and bases to molecules that do not contain H Lewis acid Electron pair Lewis base Electon pair Lewis acids amp bases Lewis acid Lewis base Comparing acidity of Lewis acids Major factor Next Metal Ion ChargeIonic radius Na 10 Li 15 Ca2 21 Mg2 31 Zn2 27 Cu2 28 N 67 Cr3 48 Fe3 47 Hydration of metal cations ciaoH 5 H What is happening here Hydration increasesdecreases with increasing chargesize ratio ofthe metal ions Hydrated metal ions have acidic character which increasesdecreases with increasing chargesize ratio of the metal ions Why are Mquot solutions acidic What does hydrolysis do to water MH20Z MH20n1OHz1 H FeH2063 FeH205OH2 H 39 39H20s3 Ka12x 105 FeH20 3 Ka67x 1o3 er H2042 Ka33x 1o1o AQH202 Ka12x 1o12 Chemistry 112002 Kl netics4 Catalysis and Enzymes Ch 147 Dr Raymond Schaak Penn State University SPRING 2009 OUTLINE Catalysts and catalysis Demonstrations H202 decomposition Catalysis and energy profiles Homogeneous vs heterogeneous catalysts Catalytic converters Spotlight on nanotechnology nanocatalysts Reaction rates lowering Ea Enzymes Control of enzymes Catalysis Demonstration H202 What isacatalyst 2 H202 9 2 H20 02 Reaction occurs slowly by itself faster wcatalyst How does a catalyst work How is this consistent with what we observe Potential energy Catalysis amp energy profiles What does this do to the energy profile Uncatalyzecl reachon 2 H202 2 Br 2Hquot H202 2 H20 132 ZIIZO03 zsrwzw Reaction pathway sapyngmmuna Pearson Prenlnx Haul m Demonstration H202 MnO2 cat 2H202 gt 2H20 02 What happens What species are present at the beginning What species are present at the end Homogeneouslheterogeneous Homogeneous catalyst Heterogeneous catalyst Examples of each Heterogeneous catalysts H2 1202 gt H20 Reaction requires breaking strong bonds and thus has a negligible rate without a catalyst Key point Hydrogen Ethylene C2H4 H2 onpyngnm 2005 Pearson Prentice Hall Inc What is happening on the surface of the catalyst NOTE netuning catalysts computer Catalytic converters Remove certain key pollutants from automobile exhaust NOX CO hydrocarbons 0 CO hydrocarbons 2 CO2 H20 N0N02 N2 Catalysts CuO Cr203 Pt Rh Catalytic converters metals Catalytic converters account for 35 Pt 65 Pd and 95 Rh used annually How much do reasonably high purity metals cost per gram Cgraphite Fe Al A9 Ti Au Pd Pt Rh Spotlight on nanotechnology Catalytic reactions heterogeneous happen on surfaces Bulk Pt vs Pt nanoparticles Question How much would it cost for1 m2 surface area of bulk Pt assume 1 m x1 m x 10 um vs 1 m2 surface area for Pt nanoparticles assume 3 nm diameter Reaction rates lowering Ea How does lowering Ea affect reactions rates Example from biology H202 9 H20 12 O2 Uncatalyzed reaction Ea1 72 kJ Catalyzed reaction catalase enzyme in liver Ea1 28 kJ What is k kLmoat at 37 C body temperature oat Reaction rates lowering Ea What does this tell us Another example peptidase enzymes break up proteins into amino acids Enzymes What are enzymes Biological catalysts produced by organisms to accelerate and control reactions and reaction rates Usually large proteins with molecular weights of 10000 to 1000000 gmol Very selective in the reactions they catalyze and absolutely specific work for only one substrate in only one reaction Enzyme active sites ideally suited for binding transition states enzymesubstrate complex stabilizes it How are vitamins related to enzymes Nonprotein parts of enzymes coenzymes Combine with the protein part to make enzymes Vitaminderived enzymes play citical roles in redox chemistry in the body source of heat and energy Control of enzymes Activating enzymes Some enzymes wait in an ofF state then activate chemically inorganic or organic when needed Activity depends on temperature pH and concentration of substrate Molecular shape depends on pH temperature and reactions of the enzyme can turn the enzyme on and off Inhibiting enzyme activity Denature Competitive inhibition bind another molecule to the active site drugs and poisons cutting it off Noncompetitive inhibition molecule binding one place modifies the active site Irreversible inhibitors strong covalent bonds with enzyme ENZYMES Enzymes are biological catalysts Enzymes are produced by organisms to accelerate and to control reaction rates Enzymes are typically large protein molecules or combinations of proteins with other molecules Lock and Key Model Substrate active binding site 4Pmduas Enzyme Enzyneubstrale Enzyme mm lex canyngmozuuspgarsan N in Reactant called substrate The region where the substratels bind is called the active site ENZYMES Nature s catalysts how do they increase rates by gt 1010 Enzymes are superior to manmade catalysts 1 2 3 More efficient Speed up rates by factors of 106 or more Absolute specificity Regulated Rate can be controlled by changing enzyme activity ENZYME CATALYSIS lysozyme b Oapyngm mans Pearson Prentice Hall Inc N Enzyme active sites are ideally suited for transition state binding lowers IEi by lowering the energy of the transition state Enzymesubstrate complex stabilizes the transition state Juxtaposition of reactants high effective concentration increases A Each factor enhances rate by 2 105 ENZYME CATALYSIS Enzyme Activity depends on empera u e pH Concentration of substrate Enzyme specificity The active site depends on the enzyme conformation shape gtMolecular shape depends on pH temperature and reactions of the enzyme If this shape is altered the active site no longer functions and the enzyme is turned off denatured ENZYME ACTIVE SITES Left Representation of an active site in an enzyme Right Denatured enzyme parts of the active site are no longer in close proximity Enzyme Example The enzyme carbonic anhydrase uses a Zn2 ion at its active site to accelerate the reaction co2 H20 H2CO3 Zn2 at active site Eapyriglil a mm P eeee on r rrrr inc Nan Inc Carbonic anhydrase Carbonic Anhydrase co2 H20 Hzco3 Ridding the body of C02 Lung Blood vessel Tissue 02 in i 1 a l Hemoglobin Red blood cell i Jquot i i i o2 co2 H20 i C02 iHO L J Carbonic 2 F anhydrase HC03 Ouvvnuhi n ma Pearson Prentice Halt inc CONTROL OF ENZYMES Some enzymes wait in the off state such as bloodclotting and digestive proteins They are activated reacted to make the active orm when needed Activator inorganic substances that increases the activity of an enzyme Example Zn2 is activator for carbonic an hyd rase Metal ions play a role in redox chemistry of the body CONTROL OF ENZYMES Enzyme activity can be inhibited 1 Denature change shape via Temp or pH change 1 Competitive inhibition bind a molecule to its active site blocking any catalytic activity Many drugs and poisons work by this mechanism Competitive inhibitors can be overcome by increasing the concentration of substrate 3 Noncompetitive inhibition Bind a molecule to another site on enzyme this results in a modification of the active site 4 lrreversiblelnhibitors Form strong covalent bonds to enzyme Increasing concentration of substrate has no effect CO FACTO RS Small non protein molecule that plays a role in enzyme activity Cofactor bind strongly Coenzyme bind reversibly Vitamins are an example of coenzymes Enzymes derived from vitamins play critical roles in redox chemistry in the body which is the source of heat and energy ATP and ADP use coenzymes made from nucleotides to regulate the energy use in the body DRUGS Penicillin antibiotic blocks an enzyme that bacteria use to build cell walls People do not have this enzyme Bacterial cells only are poisoned HIVprotease inhibitors bind to the active site of an enzyme that releases the viral coat proteins preventing the production of the HIV virus 1 H o l l NAN wxquot GAIL3 39 H H o 0H N 8 Fr Ritonavir inhibitor Chemistry 112002 Hyd rides22 Chemistry and applications of metal hydrides Dr Raymond Schaak Penn State University SPRING 2009 Hydrogen Diatomic gas colorless odorless tasteless Most abundant element in the universe 70 but rare in its elemental state H2 Reactivity of hydrogen Strong HH bond 436 kJmol Comparison ClCl bond in Cl2 is 242 kJmol Not reactive at room temperature Once activated heat radiation catalysis H2 reacts rapidly and exothermically with many substances Common oxidation states 0 molecular hydrogen 1 proton 1 hydride H vs H Let s look at a technologically relevant class of hydrogenbased materials metal hydrides Binary hydrogen compounds Formed by reacting elements with H2 Ionic hydrides Form when react with H2 2 Li H2 2LiH Ca H2 CaH2 Ionic hydrides can be used as a source of H2 CaHz Molecular hydrides Form when react with H2 Acids HCI HBr Recall periodic trends concerning acid strength PH3 H28 HCI H20 H28 H28e H2Te Bases NH3 No acidbase properties CH4 Binary hydrogen compounds Metallic hydrides Form when react with H2 Retain Amount of hydrogen in metal hydrides nonstoichiometric Hydrogen is typically absorbed by the metal fitting between the metal atoms in the crystal interstitial sites Example palladium hydride PdHX Metallic Hydrides a rerungen 39 she ll Jquot I I I I I I I A i I I I I I In 39 5 39l I I I I I I I a A Palladium hydride 5491viy1mv39ylt dad LaNi5 and LaNi5H6 Technology challenge Efficient and effective hydrogen storage materials will be a key enabling technology for fuel cell technologies Transportation challenge Storing releasing and refueling enough hydrogen for gt300 mile driving ranges within the practical constraints of weight volume efficiency safety cost References httpwww1eereenergygovhydrogenandfuelcellsstorage httpwwwfuelfromthewatercomstorage US Department of Energy Objectives for hydrogen storage materials 2010 6 wt hydrogen 2015 9 wt hydrogen Hydrogen storage systems Pressurized storage tanks Compressed hydrogen Engineering challenges to contain the pressure Usually heavy inconvenient sizevolume expensive Liquid hydrogen tanks Ultracompressed hydrogen Higher energy H2 density than compressed gas Boiloff of hydrogen Liqui cation is energyintensive and expensive Insulation Materialsbased hydrogen storage Absorption metal hydrides Hydrogen inserts between all atoms in a crystal Adsorption carbon and zeolites Hydrogen sticks to surfaces Chemical reaction molecular hydrides Chemical reactions trigger hydrogen release Hydrogen storage systems Hydrogen an be Stored in Different Forms In tanks make npussne m suns Vargzlquammes ofhydmgen m smaHelvo andrempevamresdnse mourn temps3mm Fma y hydrogen ral mmpmmd mmmng hydvngen armquot 1 Wmquot mnlenhurmrmms 3 rhem I bummam cComplnx Hydride Hydride d Chemiul Hydride o Hydmgennmml IncreasingDonsity M M 1 H1 rogen new hNn39IIme1 eere hrml Absorption metal hydrides Solidstate onboard hydrogen storage and release at low temperatures and pressures LaNi5H6 Pros Cons Alanates AIH4 Pros Cons NaAH4 13 NaaAle 23 Al H2 NaaAle 3NaH Al 32 H2 Chemical reaction molecular hydrides Hydrolysis Hydrogen is generated via reaction with water NaBH4 2H20 e NaBo2 4H2 Hydrogenation Dehydrogenation Decalin 9 naphthalene 210 C yields 73 vvt H2 C10H18 9 C10H8 5H2 00 Ammoniaborane Lewis acidbase adducts NHsBHs e NHzBHz H2 9 NHBH H2 Adsorption surfaces Hydrogen can adsorb on surfaces both external and internal Carbon nanotubes Under appropriate conditions H2 can be stored on activated carbon and in carbon nanotubes Metalorganic frameworks H2 can be stored in highly open porous solids that contain connected metal complexes bridging ligands define metalmetal distances and chemical functionality for H2 to adsorb in the interior spaces
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