CHEM 103 Calorimetry and Hess s Law

Transcription

1 CHEM 103 Calrimetry and Hess s Law Lecture Ntes March 23, 2006 Prf. Sevian Annuncements Exam #2 is next Thursday, March 30 Study guide, practice exam, and practice exam answer key are already psted n the curse website Where t take yur exam: Last names starting with A-H, rm M Last names starting with I-Z, in ur regular lecture hall Study guide is available at the bkstre The bkstre is anxius t get the bks ff the shelf Grup prblem #5 frm Tuesday: every team gt a perfect scre! 2005 H. Sevian 1

2 Map f this chapter Energy in chemistry Kinetic and ptential energy changes as heat energy is added t a pure substance First law f thermdynamics Transfer f energy and the Law f Cnservatin f Energy Endthermic vs. exthermic changes Enthalpy Measuring heat energy (enthalpy) changes (ΔH) in the labratry Heat energy and heat capacity f a material Calrimetry techniue Using labratry measurements t calculate ΔH fr reactins we can t measure in the lab Reversing the directin f a reactin 1) Frmatin f water H 2 (g) + ½ O 2 (g) H 2 O (l) ΔH = kj 584_BKS_ _3446,00.html 2) Electrlysis f water H 2 O (l) H 2 (g) + ½ O 2 (g) ΔH = kj H. Sevian 2

3 Back t Liuid Water s Capacity t Absrb Heat Energy Measuring heat transferred frm a system Prblem Slving Strategy = m C ΔT w w w w where ΔTw = temp change f H 2 sys is ppsite f w O system system Knwn uantity (mass) f water in beaker insulatin Beaker image: cre.ecu.edu/chem/chemlab/ euipment/ebeaker.htm Calrimetry Measuring heat transferred frm a system system reactin takes place insulatin Prblem Slving Strategy = m C ΔT w w w w where ΔTw = temp change f H 2 sys is ppsite f g f H 2 O w Given infrmatin Mass f water = g Temperature f water befre = 23.3ºC Temperature f water after = 47.3ºC C water = J/g K The cnfusing part is that nce the change takes place, the system and the water are mixed tgether, and the heat energy gets distributed thrughut the mixture O Beaker image: cre.ecu.edu/chem/chemlab/ euipment/ebeaker.htm 2005 H. Sevian 3

4 Calrimetry Example Exercise n p. 186 When 50.0 ml f M AgNO 3 and 50.0 ml f M HCl are mixed in a cnstant-pressure calrimeter, the temperature f the mixture increases frm ºC t ºC. The temperature increase is caused by the fllwing reactin: Cmplete rxn: AgNO 3 (a) + HCl (a) AgCl (s) + HNO 3 (a) Net inic: Ag + (a) + Cl (a) AgCl (s) Calculate ΔH fr this reactin in kj/ml AgNO 3, assuming that the cmbined slutin has density and heat capacity apprximately eual t that f pure water since the slutin is dilute. (D = 1.00 g/ml and C = J/g ºC) Calrimetry prblems Water is smething we knw a lt f very accurate data abut Measure heat changes that get transferred t water by a (reactin) system Calculate amunt f heat that water received frm r gave t a system If the calrimeter is insulated, then all heat that enters (r leaves) the water must have cme frm (r gne t) the system being studied Figure ut things abut the system that yu didn t knw befre 2005 H. Sevian 4

5 Water: A Useful Substance Liuid water s capacity t absrb heat (C) is enrmus cmpared t mst materials Heat capacity f water is very accurately knwn: Jules per gram per degree Kelvin What this means: If yu have ne gram f liuid water T raise its temperature by 1 Kelvin (eual t 1ºC) Takes Jules f energy (eual t 1 calrie) Since = m C ΔT, if yu are wrking with water then yu knw C. Therefre, if yu have a specific uantity f water, and yu measure the temperature it changes by, yu can calculate the heat that transferred. Fr a 1.00-gram sample f liuid water 120 Temperature (Celsius) Energy added (Jules) 2005 H. Sevian 5

6 What happens if yu add mre heat energy t liuid water at 100ºC? It bils What is biling? There is a statistical range f kinetic energies (velcities) f particles in the liuid Sme particles will always have enugh energy t break away frm attractive frces that keep them in liuid evapratin As temperature rises, eventually it is high enugh that s many particles can break away that their gas pressure (vapr pressure) euals the pressure f the surrundings biling Biling cntinues with n change in temperature until all liuid particles have cnverted t gas phase Heating Curve f 1.00-gram Sample f Sme Substance (nt H 2 O) at Standard Pressure A = slid belw melting* B = slid has reached melting temperature, and is beginning t melt* C = liuid at melting temperature, has just cmpleted melting D = liuid has reached biling temperature, and is beginning t bil* E = gas at biling temperature, has just cmpleted biling* F = gas abve biling temperature Temperature (Celsius) A B C D Nte: Jules shwn are bgus The pint is t nte the frm f the graph Energy added (Jules) E F See H. Sevian 6

7 Heat Changes Mre Generally = mδ vap H vap gas = mc gas ΔT liuid = mc liuid ΔT = mδ melt H melt slid = mc slid ΔT this is Mre n Hess s Law Cncept is simple, mathematics seems mre cmplicated Hw is ΔH A related t ΔH B and ΔH C? Hw is ΔH X related t ther enthalpy changes in the diagram? Hess s law says that ΔH rxn fr a given reactin is eual t prducts minus reactants f the heats f frmatin fr the chemicals invlved in the reactin 2005 H. Sevian 7

8 What is a heat f frmatin? What is a frmatin reactin? Standard heat (r enthalpy) f frmatin, ΔH f, is the enthalpy f reactin assciated with a frmatin reactin A frmatin reactin fr a cmpund is a reactin that prduces ne mle f that cmpund frm the pure elements in their standard states (p=1 atm, T=25ºC) Examples: Frmatin f sdium bicarbnate, NaHCO 3 (s): 1 3 Na ( s) + H ( g) + C( s) + O2 ( s) NaHCO3 ( s) ΔH f = Frmatin f ethylene, C 2 H 4 (g): C( s) + 2H ( g) C H ( g) ΔH kj f = ml kj ml Using Heats f Frmatin t Calculate Enthalpies f Reactin Prblem: Use standard enthalpies f frmatin t calculate the enthalpy f reactin fr the cmbustin f ethanl, C 2 H 5 OH (l). Slutin: Start by writing the reactin C 2 2 H5OH ( l) + 3O2 ( g) 2 CO2 (g) + 3 H O( l) ΔH f =? Methd 1: Write all the frmatin reactins fr any nn-elements at standard state. Then figure ut hw t arrange thse reactins t sum t the verall reactin and d the same t the ΔH s. Methd 2: Use the euatin ΔH rxn = prducts ΔH f reactants ΔH f 2005 H. Sevian 8