General Chemistry CHEM 1310
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This 0 page Class Notes was uploaded by Tierra Ernser on Monday November 2, 2015. The Class Notes belongs to CHEM 1310 at Georgia Institute of Technology - Main Campus taught by Mostafa El-Sayed in Fall. Since its upload, it has received 8 views. For similar materials see /class/234319/chem-1310-georgia-institute-of-technology-main-campus in Chemistry at Georgia Institute of Technology - Main Campus.
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Date Created: 11/02/15
Chagter 10 Entropy and the Second Law of Thermodynamics Chapter 10 Brown and Holme 2e 101 INSIGHT INTO Recycling of Plastics 102 Spontaneity 103 Entropy 104 The Second Law of Thermodynamics 105 The Third Law of Thermodynamics 106 Gibbs Free Energy 107 Free Energy and Chemical Reactions 108 INSIGHT INTO The Economics of Recycling Chapter Objectives We need a state function for which the sign of its change can predict whether the change should occur spontaneously or not Spontaneously here does not mean occurring rapidly but naturally Is it the enthalpy change AH NO Explain the concept of entropy Deduce the sign of A5 for many chemical reactions by examining the physical state of the reactants and products Define the second law of thermodynamics and use it to predict spontaneity State the third law of thermodynamics Use tabulated data to calculate the entropy change in a chemical reaction Derive the relationship between the free energy change of a system and the entropy change of the universe Use tabulated data to calculate the free energy change in a chemical reaction Enthalpy and Spontaneity AH heat flow at constant pressure qp Exothermic reactions are generally preferred over endothermic reactionsMany reactions that go to completion have AH negative ie exothermic However Melting ice is an endothermic process but occurs spontaneously and Dissolving most salts in water sbsorbs energy ie endothermicThusEnthalpy is not the exclusive determinant of spontaneity Entropy is this additional factor Spontaneity Natures Arrow Some processes or reactions proceed in only one direction gt Gasoline reacts spontaneously with oxygen to form carbon dioxide and water gt but water and carbon dioxide never spontaneously react to reform gasoline A spontaneous process takes place without continuous intervention according to thermodynamics gt Spontaneous processes are not necessarily rapid processes gt The combustion of diamond is thermodynamically spontaneous but diamonds are considered to last forever Chemical Reactions Thermodynamics predicts direction Chemical Kinetics predicts speed rate and pathway from reactants to products Spontaneous Processes Occur without outside intervention Nature spontaneously proceeds toward the states with the highest probability of existing Energy Reactants Domain of thermodynamics the initial and final states Domain of kinetic S the reaction pathway l l l l l l l I I l I I l I I l I I I l l I I I I I I Products l I I Reaction progtess Probability and Spontaneous Change Probability has important uses in I Il ih chemistry nin I gtThe fundamentals of probability 4 x explained using the rolling of dice gtThe number of rollinga4for one die isl in 6 for two dice is 1 in 36 in g s I I 1 1 N 39 Probability 6 The probability Of h 39 39 f N IS the number of dice being thrown F i 39gg Thus as the number of dice increases the When rolling two probabilities of having an quotorganized set dice of fours decreases Probability and Spontaneous Change Why does an ideal gas expand into an evacuated bulb Spontaneously Vacuum Ideal gas Three possible arrangements micro states of four molecules in two bulbs TABLE 101 Arrangement Stale Arrangement The Microstates That Give a Particular Microsmrcs II III Arrangement III Probability of occurrence of a particular arrangement state depends on the number of ways microstates in which that arrangement can have 2 molecules Probability of nding 2 molecules on one say eftsideis 14 TABLE 102 Probability of Finding All the Molecules in the Left Bulb as a Function of the Total Number of Molecules Relative Probability of Finding All Number of Molecules Molecules in the Left BLle l l 2 l l l l 2 E X E I ixixiLi 2 2 2 23 8 i lyixixixil 2 2 2 2 2 2 32 1 1 10 W 1024 L i n n 2 2 7 1 6x103quot W B 6 X 10 1 mole 7 1071 40 l The chance of a highly specified arrangement in a collection of molecules with quot1023 particles is phenomenally small Domain of thermodynamics the initial and Chemical Reactions nal states Domain of kinetics the reaction pathway Spontaneous Processes Energy Entropy Is the driving force behind Reactam spontaneous processes 5 ls how energy is distributed among energy levels of quotparticlesquot Is a quantitative measure of the degree of disorder randomness measured by the number of Rew10quot ngress Products quotmicrostatesquot Entropy Entropyis discovered to be a state function and O was first introduced in considering the For the CarnOt CyCIeI efficiency of steam engines gt Isothermal or constant temperature p gt Adiabatic or no heat exchange Isothermal expansion qTgt0 The sum of qTaround 4 Adiabatic closedpath is equal to Adlabaoc 2 expansion d f COIIlPIESSOH qT0 4 defines the state Isothermal function COEUOOD V The state function is The sum ofqTaround the cycle is zero entropy so there must be a state function that is given by this expression Definition of Entropy Entropy can be tentatively defined as a measurement of the degree randomness or disorder of a system Entropy is a measure of the number of ways of microstates S that can be written for a certain system of a certain energy 5 lgan b Boltzmann s constant the gas constant per molecule WM 0 the number of microstates of a given system at a certain P amp T Entropy of of different systems Number of MicroStates and thus the Entropy of the system increase as of moles quantity increases Volume increases Temperature increases Phase change Vapor gt Liquid gt solid Dissolution of solid increases Of Liquid depends As the size of the molecule increases or becomes more complex As the bonds in the molecule become weaker gt CHEAT SHEET Factors Influencing Entropy Temperature of Argon T K 273 295 298 S0 310 329 333 Phase Change Na H20 Cgraphite 5 5 or I 51 70 57 5 g 154 189 158 Factors Influencing Entropy Dissolution NOTEWhen you mix Nacl CZHSOH ethanol with water it becomes more organized resulting 5 5 or in less entropy S aq soln115 148 a Ethanol in NaCl 39 ethanol xim In 1 b Water In water c Ethanol in water C Solution 01 water andelhanol Factors Influencing Entropy Atomic SizeMolecular Complexity Li NaK Rsz Atomic Radiuspm152 186 227 248 265 Mass gmol 69 23 391 855 1329 5 29 516570 85 Factors Influencing Entropy 5 210 Complexity Vibrations and Rotations 240 304 Judging Entropy Changes in Processes 0 When a solid melts to form a liquid entropy increases In solids the particles are held in place rigidly limiting the number of ways a specific energy can be obtained the particles move past each other increasing the number of ways a specific energy can be obtained melting The numlg QOQTlicrostates ing aQe 1during melting Judging Entropy Changes in Processes A chemical reaction that generates two moles of gas when only one mole of gas was initially present will increase the entropy of a sample hea N204 g tgt2 N02 g colorless redbrown The number of possilbalversible increases as the number of particles increases When a sample is heated the temperature of the sample The number of ways to distribute the kinetic energy of the sample increases resulting in an increase in the number of microstates First Law of Thermodynamics The change in the internal energy of a system is equal to the work done g it plus the heat transferred M The Law of Conservation of Energy AE q w Second Law of Thermodynamics In a real spontaneous process the Entropy of the universe meaning the system plus its must increase ASunivelsegt 0 Third Law of Thermodynamics In any thermodynamic process involving only pure phases at equilibrium the entropy change S approaches zero at absolute zero temperature also the entropy of a crystalline substance approaches zero S0at0K LU The Second Law of Thermodynamics Whenever energy is from one form to another some energy is quotlostquot or quotwastedquot Not all of the energy available is directed into the desired process Entropy provides the key to understanding that quotlossquot of useful energy is inevitable The Second Law The second law of thermodynamics in any spontaneous process the total entropy of the universe is positive Augt 0 Asles AS sys surr A5 entropy of the universe ASSVS entropy of the system A5 entropy of the surroundings surr It is impossible to convert heat completely to work since vvork IS a process that Involves movrng random Into more ordered ones The Effect of Temperature on Spontaneity Consider Energy it is perhaps obvious that processes occur spontaneously to produce a state of lower energy Gas Liquid Solid E But a chunk of ice at Room Temperature spontaneously melts forming a state of higher Energy Apparently more than ener y is involved in determining the direction 0 spontaneous change This additional factor is the tendency of a system to assume the most random molecular arrangement possible Systems become disordered more random Disordered I Sl l l e State gt SOfdered SHZOa gt SHzoo Natural processes are favored which result in Decreased Energy favored Increased Entropy favored These two factors can oppose each other Which one wins out Disorder and Entropy Entropy is a quantitative measure of the number of microstatesVD available to the molecules in a system s klnBOLTZMANN Entropy measures the degree of randomness or disorder in a system The Entropy of all substances is positive Ssolidlt S liquidltSgas AS is the Entropy Change in a system Entropy S which has the units JK39lmol39l E011 A CRYSTALLINE 80quot AT ZERO DEGREE W1 THEN S0 112f2011 25 Third Law of Thermodynamics AS 0 at 09K At absolute zero the Entropy term contributes nothing to the direction of spontaneous change The most stable state has the lowest energy A temperature increases molecular motion increases and the tendency to disorder becomes more important At sufficiently high temperatures the Entropy factor becomes large enough to overcome even an unfavorable energy change For H20 5 gt H20 I Above Tm the Entropy is dominant so spontaneous melting takes place Below Tm the energy decrease is dominant so spontaneous freezing takes place Temperature is a critical factor gtTm SHZO is dominant ltTm EHZO is dominant ASuniverse Assystem ASsurroundings The sign of AS SUIT depends on the 39 direction ofthe heat i flow T The magnitude of AS depends on the temperature SU r r AS AS system surroundings Asuniverse TABLE 104 Interplay of ASSYS and 155m in Determining the Sign of 35m Signs of Entropy Changes Assys Assun39 Asmliv Process Spontaneous Yes 7 7 No process will occur in opposite direction Yes if ASW has a larger magnitude lmn ASSurr 7 P Yes if ASW has a larger magnitude than AS ASumVgt 0 Spontaneous ASumV 0 Equilibrium ASH V lt 0 Nonspontaneous STUDENT NOTES Exp 2 Molecules Moles amp Chemical Reactions BottomleyampBottomley Chemistry 1310 2010 2011 Lab Manual Hayden McNeil POST LAB ASSIGWENT 39 Lab Report Due next lab Students Notes will be on T square Cover Sheet Title Name Lab Partners name Date Honor Pledge Data amp Results Data and calculated Results in Tables Sample CalculationsUse Equation Editor Discussion Written discussion that INCLUDES answers to any questions References Any references including the lab manual websites etc I Cover Sheet5 pts DataResults 50 pts PART 1 NASA NEEDS WATER Unknown B Before Heating Mass crucible lid g 2 22 3 0g Mass crucible lid sample 2 2 716 g Mass sample before healing 048 9 g Results Percent Water7ptsand Identity o Jnknown7pt Percent Water 483 Identity of Unknown Mg504 7H20 Epsomite 12 pts1pt each line After Heating MaSScrucible lid and sample g after 1st heating 2 239466g MaSScrucible lid and sample g after 2ml heating 2 239471g MaSScrucible lid and sample g after 3ml heating 2 239467g Massamyamassampie g 0236g Compounds Solubility Observations Solube S or Insolube 1 NaCl Murky but mostly clear some S particles still at bottom KNO 3 Totally clear S AgN03 Clear but some particles still oating S around CaC03 Cloudy murky white I Na2C03 Few particles mostly clear S PbN032 Clear some particles at bottom S CuSO45H20 Big clumps at bottom light blue S liquid CaS042H20 Cloudy particles oating around 1 white NaCH3C00 Clear no particles left S NaHC03 Clear no particles left S BaS04 Murky cloudy white 1 K1 Gives off gas bubbling clear S PART II SOLUBILITY CHEMICAL REACTIONS AND DISSOCIATION IN WATER 16 pts 2 pts per line 1 pts for balanced equation 8 1 pt for observations Reactions Observations include pH m5 and pH Fina if acidbase and the Balanced Reaction NaClaq AgN03aq 10PM pl15 NaClaq AgNO3aq gt NaN03 aq AgCls pH ml 4 White milky murky no solids formed NaClaq KN03aq NaClaq KN03aq gt NaClaq KN03aq prinaz 4 p H 4 p H 4 Clear liquid no precipitate noreaction CuSO4aq NaOHaq CuSO4aq 2Na0Haq gt Na504aq Cu0H2s prm 4 pH3 pH11 Turned light blue precipitate at bottom HClaq Na2C03aq 2HClaq Na2C03aq gt 2NaClaq H200 C02g prinal 6 pH1 pH10 Clear no visible reaction HClaq NaHC03aq pl11 pH6l HC1aq NaHC03aq gt NaCKaq H2 01 C02g prinal 58 Released gas bubbling some white precipitate AgNOg aq KIaq AgN03aq KIaq gt A91 W KN03aq prinal 5 pl15 pl15 Blue on top green middle yellow at bottom no precipitate NaOllaq llClaq Na0Haq HClaq gt NaClaq H200 pH ml 1 pH11 pH1 No precipitate clear no gas PbN032aql KHan 10PM pH5l PbN032ai 2K1a l 2KN03aq Pb12s prinal 54 Clear no visible reaction NaCH3COOaq AgN03aq pH8 pH5 NaCH3C00aq AgN03aq gt NaN03aq CH3C00Agaq prinaz 6 Clear no visible reaction CH3COOaq KOHaq CH3C00aq KOHaq gt CH3C00Kaq H20l0Hfml4 p H 3 p H 1 2 Clear no visible reaction Nll4OHaq H 2 SO4aq NH40Haq H2 504aq gt NH4H504aq H2 01 prinal 9 p H 1 1 p H 1 Clear no visible reaction 8 pts Describe the conductivity demonstration that your TA conducted and how it illustrated dissociation vs dissolved ions or molecules in aqueous solution The conductivity experiment done by the TA Allison Lord was to put a light bulb apparatus that had two conducting prods into the solution in order to test if the solution could conduct electricity and could therefore be called an electrolyte or nonelectrolyte In chemistry an electrolyte is any substance containing free ions that make the L 39 iLdlly J 39 quot quotquot 39r J39 Allison had three beakers one filled with acetic acid one with HCl and one with Sucrose sugar When inserted into the solution of HCl the bulb glowed brightly meaning that the of HCl 139 39 J p making HCl a strong electrolyte When inserted into the acetic acid the molecules only partially dissociate where H is the only part that dissociates and the rest of the molecule stays together The bulb glowed dimly while the bulb did not glow at all when placed into the solution of sucrose which proves it is a nonelectrolyte Since these molecules do not ionize they furnish no charge carriers in solutionquotBottomley 17 Ill Discussion 30 pts 20 pts General Discussion 10 ptsquestions 5 pt each Part 1 NASA Needs Water The goal of the rst experiment conducted on September 9 h was to find out the percentage of water in an unknown substance and from the ratio of hydrate discern which of four different substances it could beStudents first had to get all the equipment needed for the experiment including the Bunsen burner crucible tongs wire gauze and scale After obtaining a crucible and lid and cleaning them students lit the Bunsen burner and adjusted the air intake and gas ow until there were almost no yellow ames It was then required that students heat the crucible until it was red hot After heating the crucible for about fteen minutes to rid it of any water or impurities a sample of approximately 5g is taken and the letter of the unknown sample B recorded for later in the experiment The crucible was then weighed with just the lid and crucible itself which came to be 22230g The unknown sample was transferred to crucible and then the lid crucible and unknown sample were weighed together resulting in 22716g From the two measurements one can infer that the unknown sample weighed 0489g Following all the measurements the crucible was heated slowly at rst then heated by a strong ame for fteen minutes After cooling on the wire gauze the crucible lid and unknown sample were weighed accurately to 001g The first time the objects were weighed the weight was 22466g which is a significant difference from the weight it was before heating The heating was meant to help evaporate any water in the sample so it makes sense that the total weight got lighter However the second time the mass of everything was weighed the weight was 22471 g which means it somehow got heavier This error in measurement may have been due to small unknown objects on the scale that the students had not seen The third time the mass of the objects was 22467g which was similar to the rst measurement The weight seemed to be rather consistent so it was safe to infer that the anhydrous solid had been dried to constant weight After dispensing of all the materials the mass of the anhydrous sample was calculated and resulted in being 0236g This kind of system could be used when trying to discover how much water is in a certain compound and how much gets removed when heated This could be helpful if perhaps you wanted to find how much water could be stored for space travellers in the future Part 2 Dissociation Solubility and Chemical Reactions in Water After obtaining all of the different test tubes and various solutes a toothpick tip was used to deliver a small amount of solid to each 36 mL of deionized water was then added to each test tube and mixed Each reaction was then recorded to see if the solid was soluble or not After doing the solubility tests students had to do acidbase reaction tests in a 24well plate The acids were available in bottles and each reaction was recorded and observed in the lab book A few of the reactions however do not seem as accurate as they should be which is attributed to the way students put in the acidsbases without measuring very accurately and it skewed some of the results For example when mixing a strong acid and strong base the resulting solution should be of a neutral pH In the experiment one of the strong acidbase reactions the reaction between NaOH aq HClaq came out having a pH of 1 instead of having the ideal pH of 7 This experiment shows how in real life scientists can use how certain acids and bases react to one another before using them in a product or mixture Discussion Questions Question Epsomite MgSO47HZO has been found on Mars The minimum amount of water it takes to sustain a human s need is 15 kgday How much epsomite would you have to dehydrate to sustain four astronauts for one week The amount of Epsomite that one would need to dehydrate in order to sustain four astronauts for one weekis 82 X 1049 M9504 7H20tra a
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