Fundamentals of Chemistry
Fundamentals of Chemistry CHEM 116
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This 25 page Class Notes was uploaded by Rosendo Conn on Saturday September 12, 2015. The Class Notes belongs to CHEM 116 at West Virginia University taught by Staff in Fall. Since its upload, it has received 38 views. For similar materials see /class/202841/chem-116-west-virginia-university in Chemistry at West Virginia University.
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Date Created: 09/12/15
Chem 116 Mechanisms Reaction Mechanism Reaction Mechanism 39 A step by step sequence of elementary reaction by which the overall chemical change occurs 39 Describes at a molecular level how reactants react to form products step by step 39 Almost all reactions occur in a series of small steps called elementary reactions Elementary reactions 39 Involve the dissociation of one particle or the collision of two or three particles 39 Must add up to give the net or balanced equation n L I step i i i formed Molecul arity 39 The number of molecular species that are involved in a single reaction step 39 A theoretical concept which can only be applied to elementary reactions Uniniolecular a reaction involving one molecular species A Hproducts Bimolecular a reaction involving two molecular species A A a products or A B a products Termolecular a reaction involving three molecular species A A B Hproducts orAB c Hproducts Rate Determining Step RDS 39 The slowest step in a chemical reaction which also determines the rate of reaction Intermediate 39 A short lived molecular species that is formed in one elementary step and consumed in a subsequent elementary step Does not appear in the net reaction or rate aw 1 A A NO25 035 9 N045 025 510W N03g N045 9 N205g fast 39 39 a ti and writethe rate law for this process Identify intermediates and catalysts if any 2 A 39 39 L 39 below 033 9 023 03 fast reversible 023 05 9 2 023 510 39 39 quot 39 a ti and writethe rate law for this process Identify intermediates and catalysts if any 3 023 9 023 03 NO 033 9 N02g 025 N02g 05 9 N0 023 a ti and writethe rate law for this process Identify intermediates and catalysts if any Chem 116 Mechanisms Collision Theory Product formation can only take place when there are effective collision between reactants Not all collisions result in a chemical reaction Collisions that result in a chemical reaction are called effective collisions Two conditions for product formation must be met for molecular collisions to be effective Example Consider the elementary step shown below 0g HClg 9 0Hg Cl Activation Energy Ea the minimum energy necessary for a speci c reaction to occur Energy Level Diagram shows the reaction progress vs Potential Energy PE Indicates the relative energy of the following i reactants ii transition state TS and iii products 35195 Transition state Activation energy Energy Energy of Energy of reactants reaction 139 Energy of products Progress of reaction mum Thummn Harm mmquot Chem 116 Mechanisms Catalyst A substance that is added to increase the rate of a chemical reaction without being used up during the reaction 0 A catalyst is reacted in one elementary step and produced in the same form in a subsequent step Speeds up the chemical reaction by providing an alternative mechanism for the reaction that lowers the Activation Energy gt more collision have enough energy to overcome the activation energy barrier gt reaction rate increases Energy Activation energy for catalyzed reaction V Progress of reaction a 2mm mammn new Edummn Temperature As temperature increases Kinetic Energy increases leading to higher reaction rates The sizes of the shaded areas under T T2 gt T1 the curves are proportional to the total fractions of the collisions that involve the minimum activation energy or more Fraction of collisions with a given KE gt Kinetic energy gt Minimum KE needed for reaction to occur Chem 116 Mechanisms The Arrhenius Equation Temperature is included in the rate law via the rate constant k Arrhenius discovered that the temperature dependence of the rate constant could be described by an exponential equation k AeERT 1 Write the straight line form ofthe Arrhenius Equation 2 How could the activation energy and preexponential factor be graphically obtained 3 Derive the equation that relates the activation to k at two different temperatures 1 The rate constants for formation of H1 were measured at the two different temperatures shown below Calculate the activation energy preexponential factor and speci c rate constant for formation ofHI at 427 C Hzg ME 9 2 H1g is temperature 2710394 327 C 3510393 377 C 7 427 C 2 39quoth for the I 39 39 of 439 quot pentoxide are 37 x10395 s391 at 25 C and17 x10393 s391 at 55 C 2 NzOsg a 4 N02 g 02 g a What is the activation energy for this reaction in kJm0170uxmuml b What is the rate constant at 35 C rum 1 c What is the numerical value of the preexponential factor A ssxmw Chem 116 Eq in Soln of Weak AcidsBases Chapter 18 V4 amp Chapter 16 V5 Equilibria in Solutions of Weak Acids and Bases Weak Acids Only a small percentage of weak acid molecules ionize in water to give H3O r ions K A Acid Ionization Constant and speci es the equilibrium established by the acid in water Example For the weak acid HA Write the equilibrium of the weak acid HA in water ie the equilibrium speci ed by K A HA Eq The value of K A gives information on Q Which is the stronger acid acetic acid KA18X10395 hydro uoric acid KA68X10394 or hydrochloric acid KA 2X106 Write the eq speci ed by KA for these three acids Q Which is the stronger acid boric acid with pKa 914 or carbonic acid with pKa 635 Chem 116 Eq in Soln of Weak AcidsBases Weak Bases Only a small percentage ofWeak base molecules ionize in Water to give OH39 ions KB Base Ionization Constant and speci es the equilibrium established by the base in Water Example For the Weak base B Write the equilibrium ofthe Weak base B in Water ie the equilibrium speci ed by KB B Eq The value of KB gives information on 7 Q Which is the stronger base ammonia NH3 KB18 x 10395 methylarnine CHKNHZ KB44 x 1039 or dimethylarnine CH3ZNH KB96 x 1039 Write the equilibrium specified by KB for these bases pH of Weak Acids and Bases Sample Questions 1 Calculate the pH pOH and percent ionization of 25 M HF For HF KA 68x10394 2 Calculate the pH pOH and percent ionization of075 M ammonia For ammonia KB 18x10395 Chem 116 Eq in Soln of Weak AcidsBases 3 A solution ofthe weak acid formic acid HCOOH has a pH on 14 when the concentration offormic acid is 0 30 M What is the numerical value ofthe acid ionization constant for formic acid What is the percent ionization ofthe formic acid 4 M mhiu F W Nn 39 39 39 39 39 IfthepH ofa70X10394M solution of i i nioipnine is 9 5 what are the values obe pr and percent ionization a ix nun 527 w Polyprotic Acids r A 39 so H30 and OH39 in a 0 25 M st03 solution Kn 1 2x104 Kz o o x 10398 r A 39 nfall 39 r n HCOZ39 c0317 H30 and OH39 in a 0 020 M carbonic acid K 4 3 x 1077 KHZ 5 o x 104 on minced mummr p Hcoi 93xiuMcog scan an HxleM Chem 116 Eq in Soln of Weak AcidsBases ACId Base Properties of Salts Hydrolysis when a salt ionic compound is dissolved in water a neutral acidic or basic solution is obtained dependent on the Identity 0 the cation and anion Keep in mind strong acids have conjugate bases that have i to react with water or hydrolyze while weak acids have conjugate bases that have 7 to react with water or hydrolyze How is a salt formed Example CASE 1 Basic Salts pH 50 gt7 Salts derived from Ex KNOZ NaF KcszZ Use equations to show how KNOZ forms abasic solution when dissolved in water CASE 2 Neutral Salts pH soh 7 Salts derived from Ex NaCl KBr CaNOKZ Kclo4 Use equations to show how KBr forms a neutral solution when dissolved in water CASE 3 Acidic Salts pH 50 lt 7 Salts derived from Ex N39ILCI CHKNHKBr Use equations to show how NHACI forms an acidic solution when dissolved in water Chem 116 Eq in Soln of Weak AcidsBases CASE 4 Acidic Salts pH soln lt 7 Salts derived from Ex AlClz FeBrz ZnClz FeIK CrBrK PbNOKZ Use equations to show how AlCl3 forms an acidic solution When dissolved in Water relative VMuex 0ng and Kafar the canugmex Sample Questions 1 Classify each ofthe following salt solutions as acidic basic or neutral KCIOA NaNo2 NHABr ZnN032 NHAF cho3 Kc2H302 FeClOA2 Naclo3 NaF 2 39V L ink A 39 A L r 39 39 39 basic will result P043 NIH I cr3 52 c1037 N37 CHjZNH Relatlonshlp Between K A and KB For a conjugate acidbase pair the two equilibrium constants KA and K B are related through K W Using the conjugate acidbase pair HF K155 8x10394F denve the relationship between KA for HF and KB for F39 HF Eq m Net Eq Calculation of pH of Salt Sample Questions 1 Calculatethe pH of045MNaF KAforIEF6 8x10 2 Calculatethe pH ofo75MNH clo KE forN39Hj18x10395 3 Calculate the pH for each ofthe followlng solutlons a 010MA1C13I A1H2053 1 4x 1052 93 b OZOMNaN02KAHNOZ46x10 832 Chem 116 AcidsBases A Second Look Chapter 17 V4 amp 15 V5 Acids and Bases A Second Look Acid Base Theories There are three acidbase theories used to define acids and bases 1 2 3 Arrhenius Theory Most Wecr39 c 0f the three acidbase theories Arrhenius Acid Arrhenius Base Only de nes acids and bases With Water as a solvent H must be present to de ne a substance as an acid OH39 must be present to de ne a substance as base Arrhenius theory does not explain the basicity of ammonia To define ammonia as a base 7 We need new de nitions of acids and bases Assesesese Chem 116 AcidsBases A Second Look Lew1s Theory Most general of the three acidbase theories Lewis Acid Lewis Base Examplesz Identify the Lewis acid and Lewis base in the following reactions What type of bond is formed in the product of a Lewis acidbase reaction 1 BCIK NH 9 ClKBN39HK 2 2Cl39 BeclZ 9 Beclf 3 6HZO Al 9 A1Hzog 4 H H20 9 Hp NOTE Lewis acidbase reactions are VERY important in organic chemistry BronstedLowry Theory BronstedLowry Acid BronstedLowry Base is In BronstedLowry reaction the proton is transferred from the to the Drawback to BronstedLowry Theory must be present to de ne a substance as an In the following acidbase reactions identify the acid and base 1 HPOf HSOA39 9 0427 HZPOA39 2 Hcogr H20 9 CG H30 3 Hsogr HS39 9 st 0ng 4 NH3 CH3COOH 9 NH CH3COO39 5 HZC03 co 9 2 HCO Chem 116 AcidsBases A Second Look l Conjugate AcidBase Pairs Two substances with ehemieal immlas that dt er om each other by ONE proton As an example consider the equilibrium of acetic acid established in aqueous solution Write the acetic acid equilibrium and identify the two conjugate acidbase pairs Acetic Acid Eq Examples Identify the conjugate acidbase pairs for the reactions below a Hsogr H20 9 st03 OH39 b Hsogr H20 9 H30 0327 Give the formulas for the conjugate acid and conjugate base of each of the following c032 H20 Hcogr NH3 H80439 Autoionzation or selfionization of water Water is amphoteric 7 it can act both as an acid or base Water will act as an acid if a stronger base is present Water will act as a base if a stronger acid is present Water can selfionize ie undergo an acid base reaction with itself Write the reaction for autoionization of water where KW ionproduct constant or dissociation constant of water If KW 10 x 103914 at 25 0C what are H3O and OH39 in pure water at 25 C Chem 116 AcidsBases A Second Look BasicityAcidity of Solutions A NEUTRAL solution is one in which a b An ACIDIC solution is one in which a A BASIC solution is one in which 33 How are the concentrations ofOH39 and H 30 related Sample Questions At 25 C the HKO in a sample of lemon juice is 30 x10393 M Is the lemon juice acidic basic or neutral Calculate the concentration of OH39 ions 2 At 25 C the OH39 in a sample of seawater is 50 x 10396 M Is the seawater acidic basic or neutral Calculate 1130 in the seawater 3 Which is more acidic abeer with 1130 316 x10395 M or awinewith OH39 200 x 103911 M pH Scale Provides a more convenient nonexponential way ofexpressing H307 pH Examples A A so drink has H3O 20X10395 M What is the pH ofthis so drink Is the so drink acidic basic or neutral B A neutral solution has OH39 H3O l0XlO397 What is the pH of this solution An acidic solution is one in which a A basic solution is one in which a A neutral solution is one in which a Chem 116 AcidsBases A Second Look Relationship between pOH and pH pOH can be de ned in the same way as pH Use the ionproduct expression for water to derive the relationship between pH and pOH KW H3OOH391OX103914 The relationship between pH and pOH is neutral increasing H3O decreasing H30 increasing acidity decreasing acidity 01234567891011121374 pH I I J L I I l I L I I I acidic solution basic solution IIIIII llllll pOH14131211109876543210 4 L decreasing basicity l decreasing increasing basicity increasing neutral this solution a human blOOd pH 7 40 H3O 40X10398MOH39 25x 107 M iii H3O mm9 M iV OH39 mm9 M Sample questions 1 Calculate the pH of an aqueous ammonia solution that has OH 1 9 X 10393 M 2 Acid rain is a matter of serious concern because most species of sh die in waters having a pH lower than 4 5 5 0 Calculate the H3O in a lake that has a pH of 4 5 3 Calculate the concentrations of H3O and OH in each of the following solutions b a cola beverage pH 2 8H3O 2X103MOH 6X103912M 4 An aqueous solution has a pOH of 8 9 Is this solution acidic basic or neutral Calculate the H3O and OH in 5 Which of the solutions in iiV is MOST basic A solution with i pH 6 5 ii pOH 5 5 Chem 116 AcidsBases A Second Look pH Calculations of Strong Acids and Strong Bases Addition of strong acidstrong base to water suppresses the autoionization of water 1 Calculate the pH pOH Hzot and OH39 ofO 25 M HCIOA 2 Calculate the pH pOH H30 and OH39 ofO 0050 M Ba0H2 3 Calculate the pH ofa 1 0 x 1079 M HCl solution 4 Calculate the pH ofa 5 0 x 10m M Ca0H2 solution 5 39 ui a in 1 0 39 39 mL of solution A B NaZO C SrO Chem 116 Intro to Kinetics Chapter 15 V4 or 13 V5 Kinetics The Study of Rates of Reaction Reaction Rates Chemical Kinetics study of how fastslow rate a reaction occurs and the mechanism by which it occurs Some reactions happen VERY quickly While others happen VERY slowly We observe how fast the reaction occurs Balanced equation provides no information on rate of reaction Examples Rate of reaction Consider the theoretical reaction aA 9 bB Reaction A gt B Write the expressions for Rate of appearance of product Rate of disappearance of reactant Concentration of B NOTE Pure product may never be obtained 1 Time nnnnnnnnnnnnnnnnnnnnn an Chem 116 Intro to Kinetics Sample Questions 1 Consider the following equation ZNZOSg gt 4NOZg Ozg From the following experimental data calculate the rate of disappearance of N205 and the rates of appearances ofNOZ and 02 over the time intervals 0500 s5001000s10001500 s Time is 2951 0 500 Is the reaction rate constant 500 352 If not what does it depend on 1000 248 What is the difference between ANZOSAt and rdNZOSdt 1500 175 2 In aqueous solutions molecular bromine reacts with formic acid HCOOH as follows Brzaq HCOOH aq a 2Br39aq 2Haq COZg From the following ex erimental data calculate the rate of disappearance of Br and the rate of appearances ofBr39 an COZ over the time intervals 050 s 50100 s and 1007150 s Timegsl m Timegsl m Timegsl m 00 0013920 1500 000710 3000 000420 500 00101 2000 000596 3500 000353 1000 000846 2500 000500 4000 000296 Rate of Reaction and the Slope Instantaneous reaction rate rate at each individual point on the curve Graphically 1 2 A E Mathematically 1 lt2 LH 2 0 a o O dA dfO dt dt t2 t3 time s Chem 116 Intro to Kinetics Relating Rates oprpearance and Disappearance How related 1 Consider the following reaction CzHan t 502g a 3002 t 4HzOg If at a given moment CKH8 is reacting at rate of 0400 molL391 squot What are the rates offormation of CO2 and H20 What is the rate of disappearance of OZ 2 Hydrogen and nitrogen react to form ammonia according to the equation 3Hzg t Nzg a ZNHng Ifhydrogen is consumed at a rate of050 M squot What is the rate at Which nitrogen is consumed and What is the rate at Which ammonia is produced Rate Laws It turns out that the rate of a reaction is proportional to the concentrations of the different reactants raised to experimentally determined exponents A General Rate Law can be written for any reaction aA bB gt products Rate Where k speci c rate constant 1 2 3 m and n are order of reaction with respect to each reactant 1 Chem 116 Intro to Kinetics b c d e c in l The reaction N02g der in c halved What are the units ofk Rate Law Examples cog a 3025 NOg at 200 C is known to be second order in No2 and zeroth rite the rate law for this reaction What is the overall reaction order for this reaction What will happen to the reaction rate ifthe N02 is doubled What will happen to the reaction rate ifthe co is halved What are the units on k s nit bromide 2N0g Brig a 2NOBrg 1 2 39L 39 L 39 39 r Lofthereactantsandwhatis Whatwill 39 39 39 quot A L39 A ifBr CHZBraq OH39aq a CH30Haq Braq a Write the rate laW at leHEYHOIH b How 39 L an t I 0f5 Factors That Affect the Reaction Rate Example Consider the heterogeneous reaction studied during Exp 2 Chemical Reaction Rates General Rxn Ms 2Haq gt M2aq H2g Reaction Rate Chem 116 Intro to Kinetics Method of Initial Rates Experimental method used to determine orders of reaction mn and k Best way to learn this method is by example 1 Use the method of initial rates to nd the rate law for the following reaction ZNOClg a 2NOg Clzg at 27 c NOCll Initial Rate ofFormation ofNO 0 30 M 3 60X10399 Ms 0 60 M 1 44x108 Ms 0 90 M 3 24x108 Ms quotat L r 39 How 39 L39 39 change ifthe initial concentration ofNOCl is increased from 0 30 M to 1 2 M 2 Find the rate law for the following reaction given the data shown below BrOK39aq 5Br39aq 6Haq gt 3 HZO1 3 Brzaq 1mg 1 1H1 dBr0239J 010M 010M 010M 00012Ms 020M 010M 010M 00024Ms 010M 030M 010M 00035Ms 020M 010M 015M 00054Ms 0 30 M 0 30 M 0 30 M disappearance ofBrOZ39 for the last trial Chem 116 Intro to Kinetics 3 M500 cc 2 t n mu rnm CH2 given below EECOCHZJD Initial rate of decomposition of CHZCOCHZ Msj 601E1039 Mquot 52x10395 39 39 90xl03M 78xl05 a Determine the rate law b Calculate the value ofthe speci c rate constant c r 18X1073M d mm How 39 39 60X10393Mt024X10392M sted below for the reaction 021an H N204 2H200 NH U 91ng Initial rate of consumption of Cl Ms 0 10 M 7 2 X 10396 4 Initial rate data at 25 DC are li NHAaq N 024M 012M 010M 35x106 012M 015M 54xl06 What is the rate law irate k rH mum What is the value ofthe rate constant 3 l in What 39s the initial rate when the initial concentrations are NH40 39 M and NOZ390 052 M 6 ix HIM15 M Chem 116 Equilibrium Introduction Chapter 16 V4 and 14v5 Chemical Equilibrium General Concepts Concentrations ReactionA B 2 C D 39 Equilibrium At Equilibrium the rate at which products are consumed equals the rate at which products are produced 0 concentration of reactants and products remain constant with time steady state Viewed at a submicroscopic level 0 both reactions forward and reverse reactions are still going on like crazy just occurring at the same rate Time Equilibrium is established Viewed at the human level 0 No changes in concentration with time ie a steady state situation Chem 116 Equilibrium Introduction Equilibrium For the general elementary step aA bB cC dD Write the rate laws for the forward and reverse reactions Rate for forward reaction Rate for reverse reaction At equilibrium rate of forward reaction rate of reverse reaction Rearrangement leads to the Equilibrium Constant and the Equilibrium Constant Expression On what does the value of the Equilibrium Constant depend Sample Questions reactants at equilibrium for the following reactions a N2g 3H2g ZNHj g Kc17x102at500K b N20 2N02g Kc464X10393at25 C 2 Ingeneralichgtgt103 L 39 n L L 39 quot 39 if Kcltlt10393if 10393ltKclt 103 3 Fm L L 1 H r a 4NH3g 5025 4N0g 6HZOg b 3Fes 4H20g F9104S 4Hzg c znzoa 2H2g 03 ol SiClAg 2H2g Sis 4HClg e 2H20g 2Hzg 2 f ngzaq 2Cl39aq HgZClzs g ZNaHC03s Na2C03s H200 C02g 4 An equilibrium mixture osz H2 and NH3 at 700 K contains 0 035 M N2 and 0 15 M H2 At this temperature Kc 0 29 for the reac 39on Nzg 3 Hzg NH 5 What is the concentration ofNHg SaxlmM Chem 116 Equilibrium Introduction Mathematical Manipulation of Kc Calculate the numeIical value of the equilibiium constant for each reaction below given the following Nzg 3 HZg 2 NH3g Kc 41 x 108 at 25 0c a 2NH3g Nzg t 3Hzg Kc What happens to the value of KC when the reaction is reversed b 3 Nzg 9 HZg 6 NH3g Kc 7 What happens to the value of KC when the coe icients are multiplied c 2NZ 32 HZg NH3g Kc d NH3g 2Nzg 32 Hzg Kc For Gas Phase Reactions One can write the equilibrium constant Kp in terms of partial pressure me me i ace dDg K PtomZlDiPaPbPcPd L 1 ppm 1 CHAS t 21255 3525 t 4125 2 2N0g 023 1023 u 4NH3g 5023 v 4N0g 6HZOg a 3Fzg t 3125 2C1Fzg u CHAS t 11205 C05 3125 Chem 116 Equilibrium Introduction Relationship between Kc and KP The pressure of concentration 1 Use the equilibrium shown below to derive an equation relating KC to Kg 2 SOZg 02g 9 2 sogg Kc 28x10Z at 727 c For this reaction What is the numerical value of Kp at 727 C 2 What is the general form ofthe equation relating Kp to Kc Sample Questions process 1s He ltggt CHAg C0 g 3 He g a mg 3 8x 103 at 1000 K what 1s thevalue 0pr at the same temperaturev b 1pr 61x10 at 1125 c what 1s the value orig at the same tanpemture7 u h A u The chemlcal equatlon for thls soecalled watengas Shlftreactlon 1s C0ltggt H20ggt COKE H2ltggt t V are131atmofco lOOatmofH206123tm ofCOz and 20 3 atm any 9 48 eld 2 NO 02g 2 N02g mg 6 9x105 at 227 c what 1s the value 0pr atthlstemperaturev 1pr 13 x102 at 1000 K what 1s the value orig at 1000K7 at 500K KP17XIU at1000K 1g11
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