Principles of Chemistry I
Principles of Chemistry I CHEM 1307
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This 41 page Class Notes was uploaded by Kara Dibbert on Thursday October 22, 2015. The Class Notes belongs to CHEM 1307 at Texas Tech University taught by Tamara Hanna in Fall. Since its upload, it has received 166 views. For similar materials see /class/226511/chem-1307-texas-tech-university in Chemistry at Texas Tech University.
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Date Created: 10/22/15
Energy amp Chemical Reactions Energy Most of the energy we use in our daily lives is obtained by carrying out chemical reactions ie burning fossil fuels Food is a source of chemical energy when we eat the energy stored in the compounds is released as the food is metabolized Thermodynamics is the science of heat and work Energy is defined as the capacity to do work You do work against the force of gravity when carrying yourself and hiking equipment up a mountain Energy Potential Energy Energy a motionless object has clue to its position Can be energy stored in chemical bonds chemical energy Kinetic Energy Energy associated with motion The motion of atoms macroscopic objects movement of e39 Potential and kinetic energy can be interconverted When energy is Potentialenergy Lg energy of position i Kinetic energy energy of motion wkk 5 is quot Heat a nd work thermal and mechanical energy transferred from one object to another it appears as workheat Thermal Energy The temperature of an object is a measure of its ability to transfer energy as heat When two objects at different temperatures are brought into contact energy will be transferred as heat from the one at the higher temperature to the one at the lower temperature Transfer of energy as heat will continue until both objects are at the same temperature and thermal equilibrium is reached Temperature determines the direction of thermal energy transfer The higher the temperature the greater the thermal energy System and Surroundings System An object or collection of objects being studied Surroundings 12 3974 SURROUNDINGS Everything outside of the system VSYSTiEiMi the surroundings LV the system SURROUNDINGS Direction of Energy Transfer When energy is transferred as heat between a system and its surroundings the direction is described as endo or exothermic Exothermic Energy is transferred from the system to its surroundings E of the system decreases E of the surroundings increases Endothermic energy transferred from surroundings to system Exothermic energy transferred from system to surroundings BrooksCole Cengage Learning Endothermic Energy is transferred from the surroundings to the system E of the system increases E of the surroundings decreases Energy Units Nearly all chemical changes involve an energy transfer typically in the form of heat The SI unit for energy is the joule 1 J 1 kg m2 52 If you drop a 6pack of soda on your foot the KE at the moment of impact is between 410 J 4184 1 ca A calorie is defined as the amount of energy required to raise the temperature of one gram of water by 1 C Joules and calories are small quantities so we use kJ and kcal Calories from food energy are actually kcal Specific Heat Capacity When an object is heated or cooled the amount of energy transferred depends on three things 1 The amount of material 2 The magnitude of temperature change 3 The identity of the material gaining or losing energy Specific Heat Capacity C The energy transferred as heat that is required to raise the temperature of 1 g of a substance by 1 kelvin The units of C are joules per gram Kelvin JgK q m x HAT q E gainedlost m mass of substance AT change in temp C is an intensive property physical property that does not depend on the amount of matter present Specific Heat Capacity Molar heat capacity is the amount of energy required to change the temperature of 1 mol of a substance by 1 Kelvin Substance Specific Heat Capacity Molar Heat Capacity lJgKl lJmolKl Aluminum 0897 242 Iron 0449 251 Copper 0385 245 Water 4184 754 Ethylene glycol 239 148 Wood 18 Glass 08 Specific Heat Capacity Implications When bread is wrapped in aluminum foil and heated in the oven you can remove the foil with your fingers The bread and foil are very hot but only a small mass of Al foil is used and it has a low specific heat capacity so only a small amount of energy is transferred to your fingers How much energy must be transferred to raise the T of a cup of coffee 250 mL from 205 C to 956 C Use the specific heat capacity for water q C X m X AT q 4184 JgK x 250 g x 3686 K 2935 K 78554 79 kJ Positive value means E transferred to the coffee as heat Specific Heat Capacity Practice We can use energy transfer as a way to determine the specific heat capacity of a given unknown substance A 550 g piece of metal is heated in boiling water to 998 C then dropped into a cold water bath containing 225 g of water with an initial temperature of 210 C The final temperature of both substances is 231 C What is the C of the metal Hot metal 550 9 iron Cool water 225 g immerse hot metal MetaL cools in m Water exothermic process 3139 of metal is negative qmetal is negative Water is warmed in endothermic process if of water is positive qwate is pesitive Specific Heat Capacity Practice Things to Think About 1 Metal and water are the system beaker and lab are the surroundings 2 The metal and water end up at the same temperature Assume energy is transferred only as heat 4 E transferred from the metal to the water qmetal is negative due to the change in temp qwater is positive because of the increase in temp 5 qwater qmetal O or qmetal qwater equal but opposite sign 9 4184 JgK225 g2963 K 2942 K C550 g2963 K 3730 K O cmetal 0469 JgK Specific Heat Capacity Practice An 885 9 piece of iron whose temp is 788 C is placed in a beaker containing 244 g of water at 188 C When thermal equilibrium is reached what is the final temp What we know 1 Energy transferred from metal to water 2 CImetal CIwater 0 CW X mW X AT CFe X mFe X AT O 4184 JgK x 244 g x Tf 292 K 0449 JgK x 885 g x Ff 352 K 0 10209Tf 298102 397Tf 13987 O 10606Tf 312089 Tf 294 K 21 C Energy and State Changes Changes of state result in energy transfer Heat of Fusion The E transferred as heat that is required to convert a solid at its melting point to a liquid Heat of Vaporization The E transferred as heat that is required to convert a liquid at its boiling point to a vapor Substance Melting Point Heat of fusion Boiling Point Heat of C Jg C Vaporization Jg H20 000 333 1000 2260 Fe 1535 267 2861 6088 C6H6 548 1274 800 393 CZHSOH 114 109 783 838 See Appendix D Table 12 Energy and State Changes Temperature is constant throughout a change of state During a change of state the added E is used to overcome molecular forces not to increase the temperature 1600 ION 200 C 1200 Evaporation LIQUID WATER O 100 C Energy kJ o 8 Energy 50 0 C Melting absorbed 50 O 50 100 150 200 Temperature C Energy and State Changes What is the energy required to convert 500 g of ice at 50 C to steam at 200 C Heat of Fusion 333 Jg Heat of Vaporization 2256 J g JLIE S iwii WW WWW quot jii W U q c x m x AT 206 JgK500 g273 K 223 K 515 x104J 33 J i mg m f u 392 1Ker q C x m x AT 4184 JgK500 g373 K 273 K 209 x 105 J mx H 5 g qtotal Q1 Q2 Q3 C14 C15 165X106J Energy and State Changes What is the minimum amount of ice at 0 C that must be added to a can of diet soda 340 mL to cool the soda from 205 C to 0 C Use the specific heat capacity and density of water qcola qice O Ccola x m x AT qice O 4184 JgK34O g273 K 2935 K 333 Jgmice o 333 x m 291625 Ice mice 876 g If more ice is used the final temperature will still be 0 C when thermal equilibrium is reached but some ice will remain If less ice is used the final temperature will be greater than 0 C and all the ice will be melted Energy and State Changes At a pressure of 1 atm what is the AH in kJ for the process of condensing vapor to liquid a 332 g sample of gaseous Hg at its normal bp of 357 C Heat of Vaporization 593 kJmol AHcond AHvapor 3323415 X 1 mol Hg 0166 mol Hg 2006gAlg AH 0166 mol x 593 kJmol 984 kJ Pay attention to the sign Energy and State Changes A 243 9 sample of liquid ether is initially at 603 C If the sample is heated at constant pressure how many k of energy are need to raise the temp of the sample to 504 C AHW 346 C 3575 Jg cliquid 2320 Jg C Temperature cgas 1460 Jg C 99099 OWL Problem A 3180 g sample of tin is initially at 221 C If 5014 J of heat are added to the sample at constant pressure which of the following are true AHqu 232 C 596 Jg AHvap 2270 C 1939 Jg csolid 0226 Jg C cliquid 0243 Jg C The sample is at a temp greater than 232 C The sample is at exactly 232 C The sample is a solid in equilibrium with a liquid The sample is a gas The sample is a liquid q1 79 J q2 1895 J q3 15748 J a amp e First Law of Thermodynamics The energy change for a system AU is the sum of the energy transferred as heat q and the energy transferred as work w Work Energy transfer that occurs as a mass is moved against an opposing force If a system does work on its surroundings Esystem will decrease If work is done by the surroundings Esystem will increase heat energy transferred 1 work energy A q w transferred internal energy change Internal Energy Changes Change Sign Convention Effect of Usystem E transferred as heat to q gt 0 U increases system endothermic E transferred as heat from q lt O H U decreases the system exothermic E transferred as work done w gt O U increases on system E transferred as work done w lt O H U decreases by system Surroundings PV Work Work associated with a change in volume AV that occurs against a resisting external pressure P When P is constant wPxAV Initial state wePAV Internal Energy Summary energy transfer in energy transfer out endothermic q exothermic q W transfer in w transfer out W w Internal Energy Problems A chemical reaction is run in which 125 J of heat are generated and the internal energy changes by 637 J Calculate the amount of work done AU q w w AU q Heat is generated so q w 637 J 125 J 762 J w gt 0 so work is done on the system Internal Energy Problems An automobile engine provides 620 J of work to push the pistons In the process the internal energy changes by 2960 J Calculate the amount of heat carried away by the cooling system AU q w q AU w Work is done by the system so w q 2960 J 620 J 2340 J q lt 0 so heat is given off by the system Enthalpy Heat energy transferred at constant pressure AU qp wp AU qp PAV qp AU PAV AH qp AH AU PAV AH E transfer from the system AH E transfer to the system AH and AU differ by PAV AV is typically very small therefore work is small AV is large when gases are formed or consumed State Functions A state function depends on the initial and final state of the system but is independent of the path taken Doesn t matter how you got to the end product No matter how you go from reactants to products in a reaction the values of AH and AU are always the same P V and T are also state functions q and w are not state functions A O G Standard Reaction Enthalpy Enthalpy changes accompany all chemical reactions The standard reaction enthalpy ArH is used when all reactants and products are in their standard state ie 02 g graphite s The standard state typically implies P 1 bar T 25 C H20 9 gt H2 9 12 02 9 AH 2418 kJmol Positive value for enthalpy indicates an endothermic reaction 2 H2 9 02 g gt 2 H20 9 ArH 4836 kJmol Reverse reaction has an enthalpy with an opposite sign If two moles are used in the reaction the enthalpy is doubled Standard Reaction Enthalpy The state of matter effects the magnitude of the enthalpy H2 9 12 02 g gt H20 9 AH 2418 kJmol H2 9 1 02 g gt H20 I ArH 2858 kJmol The difference in the value is equal to the enthalpy change for the condensation of 1 mol of water vapor to 1 mol of liquid water Keep in Mind 1 AH is specific to each reaction it depends on the reactants products and their states of matter 2 AH depends on the number of moles of reactant 3 AH exothermic reaction AH endothermic reaction Enthalpy How many grams of H202 I would have to react to produce 245 kJ of energy 2 H202 I gt 2 H20 I 02 9 AH 196 kJmol 245k3 x M x mg 85 9 H202 196k ImeI HEO Z39 Ethene and hydrogen react to form ethane How much heat is released when 156 g of ethane form CZH4 g H2 9 gt CZH6 9 AH 137 kJmol 156meMg EXLMg ixl37kJ 71kJ 3092 inlet 63H lmet EEFI Constant Pressure Calorimetry The experimental determination of the enthalpy change of a reaction Exothermic Twater increases Endothermic Twater decreases qreaction qwater 0 Styrofoam cups Assume no E transfer insulation beyond solution Water surroundings Coffee Cup Calorimetry A chunk of lead C 0159 Jg C weighing 188 grams and originally at 9764 C is dropped into an insulated cup containing 786 grams of water at 2286 C What is the final temperature of the water qwater qlead 0 CW X mW X AT CPb X mPb X AT O 4184 JgC x 786 g x Tf 2286 0159 JgC X 188 g X F 9764 0 3289Tf 75178 299Tf 2919 0 3319Tf 78097 T 235 C Constant Volume Calorimetry Used for reactions that involve gases or high temperatures Can allow calculation of AU Wagequot WW 7 thermumeter Wmquot was 4 gt my a WWW Wm musing mm mmi mm mm Wm WWW mm mm 997 Encyclavzedu WWW m Qreaction qwater qbomb 0 Constant Volume Calorimetry 100 g of octane C8H18 is burned in a bomb calorimeter insulated with 120 kg of water The temp of the water and bomb rises from 29815 K to 30635 K What is the heat of combustion per gram of octane Per mole Cbomb 837 JK C8H18 I 125 02 g gt 8 C02 9 9 H20 I CIreaction CIwater CIbomb 0 Calculate qwater qwater c x m x AT 4184 JgK1200 g82 K 411713 Calculate qbomb qbomb C X AT 837 JK82 K 6863 J Calculate qreaction qreaction 41171 J 6863 O CIreaction 39480341 AHcomb AHcomb 48034 Jgrx 1142 gmol 549x106 Jmol Hess s Law AH for an overall process is the sum of the AH values of all the individual steps Some reactions are complicated and AH cannot be measured Cgraphite 12 02 g gt co 9 We can combine multiple reactions to calculate AH Cgraphite 12 02 g gtce16 AH1 M V2 02 g gt CD2 9 AH2 283 kJmol Cgraphite 02 g gt C02 9 AH3 393 kJmol AH3 AH1 AH2 AHl 39 AH1 393 kJmol 283 kJmol 110 kJmol Hess s Law Calculate the AH for the formation of methane from graphite and hydrogen gas Desired reaction Cs 2 H2 9 gt CH4 9 Eqn 1 C s 02 g gt C02 9 AH1 393 kJmol Eqn 2 H2 9 12 02 g gt H20 I AH2 286 kJmol Eqn 3 CH4 9 2 02 g gt co2 g 2 H20 I AH3 890 kJmol 393 kJmol c s M gteez 6 AH1 2 H2 9 W gt m 2 xAH2 572 kJmol M2 H50 m gt CH4 9 2976 AH3 890 kJmol Cs 2 H2 9 gt CH4 9 Aern AH1 AH2AH3 Aern 393 kJmol 572 kJmol 890 kJmol 75 kJmol Standard Molar Enthalpies of Formation The enthalpy change for the formation of 1 mol of a compound directly from its elements in their standard state AHf does not always describe a reaction that can be done in the lab For an element AHf 0 If AHf is negative formation of the compound is exothermic is positive formation of the compound is endothermic The more negative the AHf the more stable the compound Which of the hydrogen halides is most stable Compound AHf kJmol HF g 2733 HCI g 9231 HBr g 3529 HI g 2536 Standard Molar Enthalpies of Formation Which of the following chemical equations does not correspond to a standard molar enthalpy of formation a Ca s c s 32 02 g gt Caco3 s b C s 02 g gt C02 9 d N2 9 2 029 gt N204 g 9 Iquot2 9 12 02 9 gt Iquot20 I Have to form ONLY 1 mol of product Elements must be in standard states No compounds as reactants Check Out Appendix L Formula AHfkJmol Formula AHfkJmol Formula AHfkJmol calcium silver Ca 5 0 A 0 hydrogen 9 S CaO s 6351 AgCl s 1270 H g 218 Caco3 s 12076 H2g 0 sodium carbon nitro en Cgraphite 0 N g 0 Na 5 0 Cdiamond 18 2 9 Na g 1073 NH3g 459 NaCl s 4111 CO 9 1105 NO 9 90 3 C02 9 393935 I sulfur CH4g 749 oxygen 58rhombic 0 CH3OH I 2384 029 0 5029 2968 039 1427 so g 3958 chlorine H20 9 2413 3 CI 9 1213 H20 I 2858 Clz 9 0 Ha g 923 Enthalpy Change for a Reaction Aern 2 AHf products Z AHf reactants Add up all the AHf for the products multiplied by their stoichiometric coef cients and subtract from the sum of the products How much heat is required to decompose 1 mol of CaCO3 CaCO3s gt CaO s 02 9 Compound AHf kJmol CaCO3 5 12076 CaO s 635 1 C02 9 3935 Aern 6351 kJmol 3935 kJmol 12076 kJmol Aern 179 kJmol decomposition of CaCO3 endothermic Enthalpy Change for a Reaction Calculate the enthalpy change that occurs when 100 g of nitroglycerin are detonated AHfnitroglycerin 364 kJmol 2 C3H5N033 l gt 3 N2 9 V2 029 6 C02 9 5 H20 9 From Appendix L AHfCOZg 393 kJmol AHfN2g 0 kJmol AHfHZOg 242 kJmol AHfOZg 0 kJmol Solve for Aern Aern 2 AHf products Z AHf reactants Aern 3 mol N2 x 0 kJmol 12 mol 02 x 0 kJmol 6 mol CO2 x 393 kJmol 5 mol H20 x 242 kJmol 2 mol C2H5NO33 x 364 kJmol 2840 kJmol rxn Calculate moles of nitroglycerin 100 x 1 molC NO 00440 moIC3H5NO33 2271 5 3 3 Calculate enthalpy change for 00440 mol C3H5NO33 00440meh x 1Mx 2840kJ 625 kJ 2M 1
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