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by: Frankie Wisozk


Frankie Wisozk
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Gerald Speitel

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Gerald Speitel
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This 24 page Class Notes was uploaded by Frankie Wisozk on Monday September 7, 2015. The Class Notes belongs to G E 206E at University of Texas at Austin taught by Gerald Speitel in Fall. Since its upload, it has received 40 views. For similar materials see /class/181898/g-e-206e-university-of-texas-at-austin in General Engineering at University of Texas at Austin.

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Date Created: 09/07/15
Chapter 9 Thermodynamics First Law References Chemical Principles Steven S Zumdahl Fifth Edition Houghton Mifflin Custom Publishing Boston MA Professor Paul MC Cord Notes CH 301 University of Texas at Austin 1 ENERGY Energy Capacity to do work or to produce heat Law of conservation of energy Energy can be converted from one form to another but can be neither created nor destroyed 39 Some of the energy of ball Awill be transferred as u heat frictional heating interaction between the rough surface if the hill and the ball M Some of the energy of ball Awill be transferred as work force acting over a distance There are two ways to transfer energy through work and through heat Total energy transferred will be constant but the amounts of heat and work will differ Energy change is independent of the pathway but work and heat are not State function The value of a state function does not depend on how the system arrived at the present state A change in this function property in going from one state to another is independent of the particular pathway sttem Part of the universe on which we wish to focus attention Surroundings Everything else in the universe R3 1 m Surrounding Exothermic Energy ows out of the 313131 l V system at CH4g 2029 0029 2H20g energy heat l nil In any exothermic reaction the potential energy stored in the chemical bonds is being converted to thermal energy via heat more energy is released in forming new bonds than consumed in breaking the bonds S Hum Surroundings Endothermic Energy flows into the 7 7 i all 3 u system l N2g 029 energy heat 2Nog at For an endothermic reaction the situation is reversed 39 First Law of thermodynamics The energy of the universe is constant Internal Energy Is the sum of the kinetic and potential energies of all of the particles in the system Q E AB change in system s q heat W work q W internal energy q gt 0 heat flowing into the system q lt 0 heat flowing out of the system w gt 0 surroundings do work on the system w lt 0 system does work on the surrounding Common type of work associated with chemical processes is work done by a gas expansion or work done to a gas compression quot Work Force applied over a given distance work 2 PAM Z PAV P external pressure 0082 L atm 831J A Expansion negative w work flows out of the system Compression positive w work flows into the system 2 ENTHALPY Enthalpy is a state function For a process carried out at constant pressure and only work allowed is that from a volume change H reactants For a chemical reaction AH H products At constant pressure Exothermic means AH negative Endothermic means AH positive 3 THERMODYNAMICS OF IDEAL GASES Molar Heat Capacity Energy required to raise the temperature of 1 mole of that substance by 1 K KE 3 avg Molar heat capacity of a monoatomic ideal gas 32R Ideal Gas Constant Volume No PV work all the energy is used to increase the translation energy of the gas molecules Cv R 2 heat required to change the temperature of 1 mol of gas by 1 K at constant volume Ideal Gas Constant Pressure Energy must be supplied both to change the translational energy and to provide the expansion work C RR R P 2 2 heat required to increase the temperature of 1 mol of gas by 1 K constant P szCvR Constant Volume AE qv PAV qv qv nCVAT 2 CV heat required to change the temperature of 1 AE QV nCVAT mol of gas by1 Kat constant volume Constant Pressure AE qp PAV AE nCVAT PAV nRAT nCVAT qp nRAT qp nCVAT nRAT nCv RAT AH qp nCpAT CID heat required to change the temperature of 1 mol of gas by 1 K at constant pressure Polzatomic Gas Gas molecules absorb energy to increase rotational and vibrational motion as well as to translate at higher speeds C5312 Cv R C5912 C CVR 2 2 2 p Monoatomic Diatomic Polyatomic 4 CALORIMETRY heat absorbed Heat capacity C 5pm 1 Hm 1ncrease 1n temperature a HP at I rru II t 391 Specific heat capacity energy required to RHquot a raise the temperature of 1 g of a substance iLLiln 2 by 1 C x 2 139 winsquot hm nah Molar heat capacity energy required to 11139 raise the temperature of 1 mol of a substance by 1 C Constant Pressure Calorimetrv coffecum M V f Energy released by thereaction energy absorbed by thesolution specific heat cap acity rnass of solutionincrese in temperature K3132 m the water Energy released by thereaotion 2 AH cotfecup calorimeter Chapter 10 Thermodynamics The Second Law References Chemical Principles Steven S Zumdahl Fifth Edition Houghton Mifflin Custom Publishing Boston MA Professor Paul MC Cord Notes CH 301 University of Texas at Austin 1 ENTROPY What do we know Heat and Work are equivalents ways of changing the energy of a system The enthalpy change is equal to the heat released at constant pressure We can predict how much heat AH is produced or consumed by a chemical reaction First Law fa reaction takes place AE q w What are we going to learn todav Why one chemical reaction has a natural tendency to occur but another one does not second law of thermodynamics Spontaneous change A process is spontaneous if it has a tendency to occur without being driven by an external influence spontaneous need not be fast Energy and matter tend to become more disordered The entropy S is a measure of disorder Low entropy means little disorder high entropy means great disorder Second Law of Thermodynamics The entropy S of an isolated system increases in the course of any spontaneous change Change in the entropy at constant Temperature AS qrev T A8 change in the entropy of the system 1 Reversible process Process carried out so the system is always at equilibrium Heat is reversible transferred if the temperatures of the surroundings and the system are only infinitesimally different luml MM qrev wrev V1 2 A Molecular Interpretation of Entropy There is a natural zero of entropy state of perfect order Third law of thermodynamics The entropies of all perfect custals approach zero as the absolute temperature approaches zero Thus the entropy of any substance is greater than zero above T O K Boltzmann s Formula k Boltzmann s constant 1381X103923 JK391 W number of ways that atoms or S 2 C111 W molecules can be arranged at the same energy microstates Calculate the entropy of a tiny solid made up of 4 diatomic molecules of CO at TO when a the 4 molecules have formed a perfectly ordered crystal all the molecules are perfectly aligned b the 4 molecules lie in random orientations CC 53 DC DE CD CC DD COCO 0 CC DC DC DC DC CD CD CD can no 151 miva TI 4 CED at Entropy of CO at T 0K 8 46 J K In the CO crystal the molecules are indeed arranged nearly randomly not a perfect crystal The dipole of a CO molecule is very small molecules are not lying heat to tail For HCI the same experiment at T OK gives 8 O HCI molecule has a big dipole moment The Boltzmann formula relates the entropy of a substance to the number of arrangements of molecules that result in the same energy when many energy levels are accessible this number and the corresponding entropy are large From Boltzmann s equation V ASanln 2 V1 Combining land ll qrev 9 Q 61m nRT1n AS T Meaning 1 Entropy S is an state function depends on the current state and not on how that state was achieved If we want to calculate the entropy difference between a pair of states joined by an irreversible path we can look for a reversible path between the same two states and use the previous AS equation Entropy is a measure of disorder Entropy is a state function The entropy of an isolated system increases in any spontaneous process Changes in Entrogy Changes in Physical State Entropy S gt Heating increases thermal disorder Solid Entropy also increases when a given amount of matter spreads into a greater volume or is mixed with another substance Liquid Melting point Temperature T gt Boiling point Temperature Smquot Phase C J39K Lmol l Solid 273 0 K 34 0 432 liquid 0 652 ZU 396 50 753 IOU 868 vapor 100 1969 200 2041 Yquot r 71 Mmhui Intqu nir pn1i1Um xt lib lvaxnx Idling 39mnl numg i 7 891 anqu 7 7 7135 uK39fmgrj n w iZ N 324 w nl 1 9 397 quot quot 8 4 In 1 um H3 871 al39 ml 1 15 114 39H I m 412 2 w um 119 042 v x mule Hiquot 3971 l 39 mm 3 AH llli l r 7 NW 1 Iwnui Huhng ppm ts rhx39 imllmg rleprr 1mm 3 I wn Standard Molar Entrogies 1A8 73 Hm mwl lIlJr39 l rurnp m39 H I 39l I l39x 11ml Substancc Sm39 SUIJULL Sm39 Substation Sm39 Casts I Liquids Solids Ammunm 39H VGA I lullum quotl I I ll Lalcmnl tunic Ml W urban dumulr rrlmiml wk mm garhmmtr 0 ll 3 cl 14H I myquot Law J v1quot Mdmgcn ll H39h Itu LU CWquot dmmmld L Ill is l lmgut N l llm graphich quotMum U Mil I lcml l lw 143 NHnan n minn 3m urn m Append l 39 Jlurr 80 diamond C gt 80 lead Pb heavier atoms have more disorder than lighter atoms 80 nitrogen N2 gt 80 hydrogen H2 80 calcium carbonate CaCO3 gt 80 calcium oxide CaO large complex species have higher entropies than those of smaller simpler ones Examples Chapter 9 Thermodynamics Enthalpy and Hess s Law References Chemical Principles Steven S Zumdahl Fifth Edition Houghton Mifflin Custom Publishing Boston MA Professor Paul MC Cord Notes CH 301 University of Texas at Austin 1 HESS S LAW Enthalpy H is a state function Hess s Law In going from a particular set of reactants to a particular set of products the change in enthalpy is the same whether the reaction takes place in one step or in a series of steps NO2 example Characteristics of Enthalpy Changes 1 If a reaction is reversed the sign of AH is also reversed 2 The magnitude of AH is directly proportional to the quantities of reactants and products in a reaction If the coefficients in a balanced reaction are multiplied by an integer the value of AH is multiplied by the same integer XeF4 example 2 STANDARD ENTHALPIES OF FORMATION Obtain the Enthalpy change AH by using a calorimeter can be very difficult Standard enthalpy of formation AH The change in enthalpy that accompanies the formation of 1 mole of a compound from its elements with all substances in their standard states We can t measure H we can measure only changes in that property AH that is why we define a common reference state standard state Definitions of Standard States Coinpuairul AH llx nml Gas Pressure of 1 atm m iii Am Soution Concentration of 1 M at an applied Emilio7quot ff39 pressure of 1 atm 5 L 73937 1quot2 1 3quot Pure substance liquid or solid the pure liquid or igi39i m SOIid 11mg 771m H 4 ii i g 2 339 Eement the form in which the element eXIsts 33 va it under conditions of 1 atm and temperature of 250 The enthalpy change for a given reaction can be calculated by subtracting the enthalpies of formation of the reactants from the enthalpies of formation on the products 0 0 0 AH 2M th ZAHWMM Elements are not included in the calculation since elements require no change in form Kez concepts 1 When a reaction is reversed the magnitude of AH remains the same but the signs changes 2 When the balanced equation is multiplied by an integer the AH for that reaction must be multiplied by the same integer 3 Elements in their standard states are not included in the AH reaction calculations That is Ahof for an element in its standard state O 3 BOND ENERGY Average Bond Energies KJmol REFER To GASES Single Bonds Multiple Bonds H li 432 N H 391 I I H9 C 614 11 1 505 N N 160 1 H 308 L L 830 H Jl 427 N F 272 l Br IT S 04 495 HBr 363 NACI 100 CTW 7H Hl 295 NHBr 243 5 H 34 Leo Inquot 0 201 s F 32quot N O 60139 LTH 413 0 H 467 5 0 25 NN 418 1 C 347 0 0 146 S Br ZIS NtN 941 C N 305 O F 190 Sr 5 266 LrN 6 114 358 O Cl 203 J N 391 Cl 485 0 1 234 Sx Sl 340 CSl 339 Si H 393 CBr 27 1 F 154 Si 39 160 l 40 Eva 253 Si O 451 ms 159 PBr 33quot 9quot 339 Example HF Ll Br 118 Br B 193 C0 C011 AH 2 bonds broken Zbonds formed


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