2013, 2011, 2009, 2008 AP

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1 Lecture 16 Fur Methds r Finding Thermdynamics II Calrimetry Hess s Law Enthalpy r Frmatin 1) Calculate it using average bnd enthalpies 2) Measure it using a calrimeter. 4) Calculate it using enthalpies rmatin Calrimetry Calrimetry The measurement heat transer Calrimeter A device used t determine the amunt heat ttranserred Heat lw cannt be measured directly. We measure the temperature bere and ater a reactin, and use that data t calculate heat transer. Calrimetry Speciic Heat Capacity (c) The amunt heat required t raise the temperature 1 g a substance by 1 K. A large value r water (4.184 J/g K) It takes a lt heat t increase the temperature water. A small value r irn (0.45 J/g K) It takes much less heat t raise the temperature irn. Heat lst r gained by a substance q = m c T Mass that substance Speciic heat capacity that substance Temperature change that substance T = T - T i Thermmeter Cee Cup Calrimeter Stir Stick Cee Cup Calrimetry When measuring the heat lst r gained in a chemical reactin that takes place in a cee cup calrimeter, we use the slutin as the basis r ur calculatins. Heat lst r gained by the slutin in the reactin q = m c T Mass the slutin Speciic heat the slutin J/g K is ten used i cncentratins are lw. Temperature change the slutin T = T - T i Cpyright 2013, 2011, 2009, 2008 AP Chem Slutins. All rights reserved. 1

2 Cee Cup Calrimetry The reactants and prducts are the system. The water which they are disslved in and calrimeter make up the surrundings. The heat lst r gained by the slutin is equal t the heat lst r gained by the reactin. e.g.) I yu calculated q slutin t be +568 J, the slutin gained heat. This means the reactin was exthermic, as the heat gained by the slutin is equal t the heat lst by the reactin (q = -568 J). Ex1) Calrimetry Ex1) A 97 g irn bar was heated t 143 C and placed in a cee cup calrimeter cntaining g water at C. The maximum temperature the water was measured t be C. a) Hw much heat was gained by the water? b) Hw much heat was lst by the irn bar? c) What is the speciic heat irn? Ex1) Calrimetry (cnt.) a) Hw much heat was gained by the water? Ex1) Calrimetry (cnt.) c) What is the speciic heat irn? b) Hw much heat was lst by the irn bar? Ex2) Calrimetry Ex2) When 20.0 ml 1.00 M NaCl at C is mixed with 20.0 ml 1.00 M AgNO 3 at C in a calrimeter, a white precipitate rms and the temperature the mixture reaches C. Assume that the speciic heat capacity the mixture is J/g K, its density is 1.00 g/ml, and the vlumes are additive. a) Write the net inic equatin r this reactin. b) Calculate ΔH r the reactin. Ex2) Calrimetry (cnt.) Step 1) Calculate q sl. Step 2) Calculate q. Cpyright 2013, 2011, 2009, 2008 AP Chem Slutins. All rights reserved. 2

3 Ex2) Calrimetry (cnt.) Step 3) Calculate Mles Ag + r Cl -. Step 4) Calculate ΔH. q vs. q = The heat lst r gained in the experiment that tk place in the calrimeter = The heat lst r gained in the balanced chemical equatin Fur Methds r Finding 1) Calculate it using average bnd enthalpies. 2) Measure it using a calrimeter. 4) Calculate it using enthalpies rmatin. Hess s Law I a reactin is carried ut in a series steps, the verall change in enthalpy will be equal t the sum the enthalpy changes r the individual steps. The verall enthalpy change will be the same i a reactin is carried ut in ne step r in several steps. Ex1) Hess s Law Ex1) Hess s Law (cnt.) Ex1) Calculate r the llwing reactin using the inrmatin in the table belw. 4 NO 2(g) + O 2(g) 2 N 2 O 5(s) Reactin Chemical Equatin Enthalpy Change 1 2 NO (g) O 2(g) N 2 O 5(s) = kj 2 2 NO (g) + O 2(g) 2 NO 2(g) = kj Cpyright 2013, 2011, 2009, 2008 AP Chem Slutins. All rights reserved. 3

4 Ex2) Hess s Law Ex2) Hess s Law (cnt.) Ex2) Calculate r the llwing reactin using the inrmatin in the table belw. 3 C (s) + 4 H 2(g) C 3 H 8(g) 2( 1) 4 H 2(g) + 2 O 2(g) 4 H 2 O (l) = 2( kj) Reactin Chemical Equatin Enthalpy Change 1 2 H 2(g) + O 2(g) 2 H 2 O (l) = kj 2 C 3 H 8(g) + 5O 2(g) 3CO 2(g) + 4H 2 O (l) = kj 3 C (s) + O 2(g) CO 2(g) = kj Fur Methds r Finding Standard Enthalpy Frmatin 1) Calculate it using average bnd enthalpies. 2) Measure it using a calrimeter. 4) Calculate it using enthalpies rmatin. A hypthetical value that indicates hw much heat wuld be lst r gained during the rmatin ne mle a cmpund rm the mst cmmn rm its elements in their standard states. standard state (25 C, 1 atm) Frmatin Standard Enthalpy Frmatin Fr Methanl C (s) +2H H 2(g) +½O 2(g) CH 3 OH (g) = -201 kj/ml Heat rmatin reactins are always written s that all reactants exist as they wuld under standard cnditins, and there is ne mle prduct. Enthalpy Frmatin The value r the mst stable rm any element in its standard state is zer. Element Element Ca(s) 0 Cl 2 (g) 0 Ag(s) 0 H 2 (g) 0 Na(s) 0 Fe(s) 0 O 2 (g) 0 N 2 (g) 0 Cpyright 2013, 2011, 2009, 2008 AP Chem Slutins. All rights reserved. 4

5 Frmula r Calculating the Enthalpy a Reactin under Standard Cnditins Sum = Σ n (prducts) Σ n (reactants) stichimetric ceicients Ex1) Enthalpy Frmatin Ex1) Find r the thermite reactin under standard cnditins. Fe 2 O 3(s) + 2 Al (s) 2 Fe (s) + Al 2 O 3(s) = Σ n (prducts) Σ n (reactants) = [2 ( (Fe (s) )) + 1 ( (Al 2 O 3(s) ))] [2 ( (Al (s) )) + 1 ( (Fe 2 O 3(s) ))] = [2(0) + 1( )] - [2(0) + 1(-821.4)] = kj/ml Ex2) Enthalpy Frmatin Ex2) Find r this reactin at standard cnditins. 2 H 2 S (g) + 3 O 2(g) 2 SO 2(g) + 2 H 2 O (g) = Σ n (prducts) Σ n (reactants) Enthalpy Frmatin Reactin H kj/ml Elemental metal 1 st used by humans 2 Au (s) + 3/2 O 2 (g) Au 2 O 3(s) +131 ~6000 BCE 2 Ag (s) + 1/2 O 2 (g) Ag 2 O (s) 31 ~4000 BCE Cu (s) + 1/2 O 2 (g) CuO (s) 156 ~4000 BCE Pb (s) + 1/2 O 2 (g) PbO (s) 217 ~3500 BCE Sn (s) + O 2 (g) SnO 2(s) 581 ~1800 BCE 2 Fe (s) + 3/2 O 2 (g) Fe 2 O 3(s) 822 ~1200 BCE Ex3) Enthalpy Frmatin Ex3) Use the enthalpy rmatin values belw t determine ΔH r the reactin between C (s) and Fe 2 O 3(s) t prduce Fe (s). 2 Fe (s) + 3/2 O 2 (g) Fe 2 O 3(s) C () (s) +O 2 () (g) CO 2() 2(s) H 822 kj/ml HH 394 kj/ml Ex) Enthalpy and Stichimetry 2 H 2 O 2(aq) O 2(g) + 2 H 2 O (l) = -186 kj Ex) Hw much heat is released when 3.56 g H 2 O 2(aq) decmpses int water and xygen gas? 1 ml H O 186 kj g H2O kj g H2O2 2 ml H2O2 There are 2 mles H 2 O 2 in the balanced equatin. Cpyright 2013, 2011, 2009, 2008 AP Chem Slutins. All rights reserved. 5