The Enthalpies of Reactions

Transcription

1 The Enthalpies of Reactions Collect 2 Styrofoam cups & a cup lid Digital thermometer Stop watch (from TA) Prepare 400 ml beaker 50 ml graduated cylinder * Take the warm water heated in hood (2015/09/20 revised) 1

2 Objective & Skills Objective Determine the heat capacity of calorimeter and heat of neutralization heat of solution heat of redox reaction Use Hess law (the enthalpies of reactions are additive) to calculate the heat of formation ( H f ) for MgO Skills Use of digital thermometer Setup a simple calorimeter Operation of graduated cylinder 2

3 Introduction: Enthalpy of Reaction H: enthalpy change, or heat of reaction at constant pressure Exothermic reaction, H < 0 Endothermic reaction, H > 0 The expression of H +, - Numerical value Unit: kcal/mol, kj/mol Notice the significant figures Ex: heat of neutralization is -14 kcal/mol H H H + (aq) + OH - (aq) H < 0 H 2 O(l) NH + 4 (aq) + NO - 3 (aq) H > 0 NH 4 NO 3(s) + H 2 O (l) 3

4 Introduction: Calculating H Setup a simple calorimeter as an isothermal system (q system = 0) H rxn = - (q soln + q calorimeter ) Thermometer and probe q calorimeter : C T C: the heat capacity of calorimeter (cal/ o C) q soln = m s T q soln : change in heat of solution (cal) m: mass (g) s: specific heat of solution (1.0 cal/g- o C) T: change in water temperature ( o C) Two Styrofoam cups stacked together 400 ml beaker 4

5 Procedure 1: Determine the Heat Capacity of Calorimeter Measure 50 ml cold water into calorimeter Wait 3 min till equilibrium Record temperature Place graduated cylinder on the inner side of the lab bench to avoid breaking it 5

6 Procedure 1: Determine the Heat Capacity of Calorimeter Prepare warm water (10~15 o C warmer than cold water) Measure the volume Take temp. of warm water Mix hot and cold water in beaker thoroughly Obtain warm water with temp. ca. 35~40 o C Measure 50 ml warm water with graduated cylinder Place the thermoprobe in the center of soln and take the equilibrium temp. The probe should not touch the walls of cylinder 6

7 Procedure 1: Determine the Heat Capacity of Calorimeter Pour the prepared warm water into calorimeter Calculation: q 1 (heat released by warm water) = q 2 (heat absorbed by cold water) + q 3 (heat absorbed by calorimeter) Place the lid on Put the thermoprobe in cup Shake and mix the soln Record the equilibrium temp. (the highest temp. for exothermic rxn.) 7

8 Procedure 2: Molar Heat of Neutralization of HCl and NaOH Measure 50.0 ml of 1.0 M HCl into calorimeter; record the equilibrium temp. Measure another 50.0 ml of 1.0 M NaOH with graduated cylinder; record equilibrium temp. Pour NaOH(aq) into calorimeter Place the lid on Put the thermoprobe in cup Shake and mix the soln Record the equilibrium temp. (the highest temp. for exothermic rxn.) 8

9 Procedure 3: Neutralization of CH 3 COOH and NaOH Use 50.0 ml of 1.0 M CH 3 COOH to replace the HCl(aq) Repeat the above procedures Determine the molar heat of neutralization of acetic acid and sodium hydroxide 9

10 Procedure 4: Molar Heat of Solution of Ammonium Nitrate Measure 50.0 ml of H 2 O into calorimeter Take the equilibrium temp. Weigh ca. 4 g NH 4 NO 3 (s) and record the accurate wt. Add NH 4 NO 3 (s) into calorimeter Shake and mix the soln Take the equilibrium temp. (the lowest temp. for endothermic rxn.) * Solid reactants must be dissolved completely 10

11 Procedure 4: Molar Heat of Formation of Magnesium Oxide Heat of reaction of Mg and HCl(aq) Measure ca. 0.2 g of Mg React with ml of 1.0 M HCl in calorimeter Take the equilibrium temp. Heat of reaction of MgO and HCl(aq) Measure ca. 0.7 g MgO React with ml of 1.0 M HCl in calorimeter Take the equilibrium temp. * Solid reactants must be mixed and reacted completely Mg (s) + 2H + (aq) Mg 2+ (aq) + H 2(g) H 1 MgO (s) + 2H + (aq) Mg 2+ (aq) + H 2 O (l) H 2 H 2(g) + ½O 2(g) H 2 O (l) H o f (H2 O) = kcal/mol Mg (s) + ½O 2(g) MgO (s) H f o (MgO) H f o (MgO) = H 1 - H 2 + H f o (H2 O) 11

12 Notice Determine the heat capacity of calorimeter The temperature difference of warm and cold water should be about 10~15 o C Mix hot and cold water thoroughly in a beaker, then measure with graduated cylinder and take temperature The tip of the thermoprobe should be in the center of the solution to avoid touching the walls of the container (causing the temperature to be lower) After measuring the warm water temperature with thermoprobe, rinse probe with tap water to let it return to room temperature, then place back into calorimeter When mix warm water with cold water in calorimeter, equilibrium temperature is achieved with speed, temperature should be read immediately 12

13 Notice Wash and wipe dry the Styrofoam cups after each trial Determining the equilibrium temperature: Exothermic reaction: Stop recording temperature when the temperature rises to the highest point and starts to decrease Endothermic reaction: Stop recording when the temperature of solution decreases to the lowest point and starts to increase Solid reactants as NH 4 NO 3(s), Mg (s), MgO (s) need to be mixed thoroughly for complete reaction to avoid experimental errors 13

14 Notice Mg and MgO are not easily dissolved in acid. After the reaction, observe there are any remaining solids causing inaccuracy (include the observation in report) Recycle Waste liquids: the salt solution resulting from neutralization can be discarded in sink Styrofoam cups and lids should be handed back in for reuse Broken lids and cups can be discarded Bring calculators to class, scientific calculators are recommended Show all the calculations in your report: Notice about: +/-, significant figures, and units (kcal/mol or kj/mol) 14