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Plan for Tues/Wed, 4/5 Nov 08

Turn in Exp 5 Report and Exp 6 Pre-lab Today : Exp 6, Calorimetry Purpose : PART A: To calibrate (determine the heat capacity of) a constant-pressure calorimeter by monitoring the heat transferred between two samples of water at different temperatures.

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Plan for Tues/Wed, 4/5 Nov 08

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  1. Turn in Exp 5 Report and Exp 6 Pre-lab Today: Exp 6, Calorimetry Purpose: PART A: To calibrate (determine the heat capacity of) a constant-pressure calorimeter by monitoring the heat transferred between two samples of water at different temperatures. PART B: To use your calibrated calorimeter to determine the heat of reaction of two acid-base neutralization reactions: HCl(aq) + NaOH(aq)  H2O(l) + NaCl(aq) HNO3(aq) + NaOH(aq)  H2O(l) + NaNO3(aq) THIS EXPERIMENT WILL BE WRITTEN UP AS A FORMAL LAB REPORT. NO LAB NEXT WEEK. Plan for Tues/Wed, 4/5 Nov 08

  2. Tips for success TECHNIQUE • The Vernier systems are EXPENSIVE. • BE CAREFULwhen placing the the LabQuest probes in your calorimeter…they are sharp and can pierce the bottom of your styrofoam cup. • MAKE SUREto keep your probe tip completely submerged in the water/solution, but DO NOT let it touch the bottom of the cup. • DON’T FORGET TO SAVE YOUR DATA. PROCEDURE / DATA ANALYSIS • In your calculations, BE VERY CAREFUL with your plus and minus signs!!! • Also, BE VERY CAREFUL with your significant figures!!! WASTE • All solutions go down the sink, accompanied by 10-20 seconds of tap water. • You may throw your internal cup in the trash. Return the external cup to the stockroom. SAFETY • HCl, HNO3, and NaOH are toxic and corrosive!! • Wipe down your entire work area before you leave!! • Wash your hands with soap and water!!

  3. Constant P Calorimetry • Most strong acid / strong base neutralizations are exothermic: NaOH + HX  H2O + NaX; DH < 0 • The heat evolved in these reaction increases the average Ek of the water molecules, leading to an increase in the temperature of the solution. • In a perfectly insulated calorimeter, the heat lost by the reaction is exactly equal to that gained by the solution. • However, some of the heat evolved in the reaction is absorbed by the calorimeter, so we have to account for that too:

  4. Monitor the heat transferred from a sample of hot water to a sample of cold water and to the calorimeter A. Calibrating the Calorimeter Cold water (Ti = ~25oC) 50 mL Hot water (Ti = ~60oC) 50 mL energy flow Lukewarm water Tf = ?? oC

  5. A. Calibrating the Calorimeter (cont) The heat lost by the hot water is equal to the heat gained by the cold water and the calorimeter: Finding qhot and qcold: dhot, dcold: From density of H2O calculator s = 4.184 J/(goC) Vhot, Vcold: the volumes you measured Tf: From regression analysis of temp data

  6. A. Calibrating the Calorimeter (cont) The mathematical formula for the heat gained by the calorimeter is: What you just calculated in the calibration trial. Same as the DT for the cold water in the calibration trial. Now that we have determined the heat capacity of the calorimeter (aka the “calorimeter constant”), we can experimentally determine DH for our acid-base reactions…

  7. B. DH for Acid-Base Rxns NaOH + HCl  H2O + NaCl DH < 0 NaOH + HNO3 H2O + NaNO3DH < 0 • The heat evolved in the reactions is absorbed by the solution and the calorimeter. ssoln = 4.184 J/(goC) msoln = msoln + cal - mcal Tf: from regression analysis Ti: avg of initial soln temps Ccal : From calibration trial

  8. B. DH for Acid-Base Rxns (cont) NaOH + HCl  H2O + NaCl DH < 0 NaOH + HNO3 H2O + NaNO3DH < 0 • The enthalpy of reaction in each case is reported as kJ per mole of water produced. • This means you will have to perform a limiting-reactant calculation based on the moles of NaOH and the acid. THIS means n = M*V calculations!

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