Lec_10 - Today 1. Report for Experiment 7 is due (40 pts)...

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Unformatted text preview: Today 1. Report for Experiment 7 is due (40 pts) 2. Unknown Summary Sheet for Experiments 6-7 is due (20 pts) 4. Pre-lab for Experiment 8 is due (5 pts) 5. Experiment 8. Thermochemistry. have a write-up in your notebook Next week 1. Report for Experiment 8 is due (40 pts) 2. Post-lab for Experiment 8 is due (10 pts) 3. Unknown Summary Sheet for Experiments 8 is due (20 pts) 4. Pre-lab for Experiment 9 is due (5 pts) 5. Experiment 9. Acid-Base Equilibria have a write-up in your notebook Spring 2007 Experiment 8. Thermochemistry PURPOSE AND LEARNING OBJECTIVES To construct and calibrate a simple "coffee cup" calorimeter To measure the specific heat capacity of an unknown metal To apply Hess's law to determine the change in enthalpy of a chemical reaction by indirect calorimetric measurements Spring 2007 Calorimetry Calorimetry experimental technique that is based on measuring the temperature change when a system absorbs or releases energy in a form of heat Chemical reactions Physical changes (melting, dissolution, etc.) Calorimeter thermally insulated container measures the temperature change of a known amount of substance (often water) of known specific heat capacity Specific heat capacity the amount of heat required to raise the temperature of 1 g of substance by 1 degree Kelvin under constant pressure (Jg1K1) Heat released by exothermic process = Heat absorbed by water + Heat absorbed by calorimeter Spring 2007 The heat capacity of the calorimeter The heat capacity of the calorimeter (calorimeter constant), Ccal, the amount of heat absorbed by the calorimeter per every degree of the solution temperature change. Has units of J/K. Cold water of known mass is placed in calorimeter and allowed to equilibrate A known mass of hot water is prepared and its temperature determined The hot water is quickly added to the cold water in the calorimeter Time of mixing 30.0 25.0 Temperature (C) TM 20.0 15.0 TC 10.0 The decrease in T of the hot water: TH = TM TH <0 The increase in T of the cold water: TC = TM TC >0 5.0 0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 Time (min) Spring 2007 The heat capacity of the calorimeter The hot water is cooled The amount of heat lost by the hot water: qH = mH cs TH The cold water is warmed The amount of heat gained by the cold water: qC = mC cs TC The calorimeter is warmed The amount of heat gained by the calorimeter: qcal = Ccal TC mH and mC masses of the hot and the cold water, respectively (g) cs = 4.184 JK1g1 specific heat capacity of water qC + qcal = qH qC + qcal + qH = 0 mC cs TC + Ccal TC = mH cs TH Ccal = - m H cs TH - m C cs TC TC Spring 2007 The heat capacity of an unknown metal The hot metal is cooled The amount of heat lost by the hot metal: qmetal = mmetal cmetal TH The cold water is warmed The amount of heat gained by the water: qC = mC cs TC The calorimeter is warmed The amount of heat gained by the calorimeter: qcal = Ccal TC qC + qcal + qmetal = 0 qC + qcal = qmetal mC cs TC + Ccal TC = mmetal cmetal TH c metal = - TC (m C cs + Ccal ) m metal TH Spring 2007 Hess' Hess's Law The enthalpy change for a reaction is the same regardless of whether it occurs in one step or by any series of steps to get from reactants to products For the general reaction of a A + b B c C + d D, the standard enthalpy change can be calculated as Hrn 0 = [c Hf0 (C) + d Hf0 (D)] [a Hf0 (A) + b Hf0 (B)] Spring 2007 (s Molar Enthalpy of formation of MgO (s) The standard molar enthalpy of formation, Hf0, of a substance is the enthalpy change for the reaction in which one mole of the substance in a specified state is formed from its elements in their standard states. Mg (s) + O2 (g) MgO (s) Mg (s) + 2 HCl (aq) MgCl2 (aq) + H2 (g) MgO (s) + 2 HCl (aq) MgCl2 (aq) + H2O (l) H2 (g) + O2 (g) H2O (l) Hf = ? H1 experimental H2 experimental H3 = 285.8 kJ/mole Spring 2007 Experimental details Work in the groups of two Reports and unknown summary sheets are still submitted individually Use the digital high precision thermometer for the calorimeter measurements; use a regular thermometer for all the other temperature measurements IMPORTANT: Swirl (mix) solutions in the calorimeter while taking measurements Heat capacity of the calorimeter, Ccal, is a small positive value: 0 100 J/K Measure "hot" temperatures right before mixing Relative percent error: H theo - H exp H theo 100% Waterproof/shockproof 8" stainless steel stem Updates readings every second Accuracy 0.2 C Resolution 0.1 C Spring 2007 ...
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This note was uploaded on 11/10/2008 for the course CH 204 taught by Professor Leytner during the Spring '08 term at University of Texas at Austin.

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