Thermochemistry II: Hess\'s Law OBJECTIVES: Explore quantitative aspects of cons
ID: 507684 • Letter: T
Question
Thermochemistry II: Hess's Law OBJECTIVES: Explore quantitative aspects of constant-pressure calorimetry Check the validity of Hess's law DISCUSSION: In this part of the experiment, you will use your calorimeter to measure the beat released by three related Because two of the reactions sum up to give the third reaction, you can use the of measurements to check Hess's of summation. heat In the first reaction, solid sodium hydroxide dissolves in water: In the second reaction, solutions of sodium hydroxide and hydrochloric acid react to form water and aqueous sodium chloride: In the third reaction, solid sodium hydroxide reacts with aqueous hydrochloric to form water and aqueous sodium chloride: acid As in the first part of the lab, you will use an EPs cup supported in a beaker as the Although this assembly is not a perfect insulator, if we assume that the heat losses are not significantly different from one trial to the next, we can expect valid comparisons, PROCEDURE: Be sure to read the Safety and Disposal section at the end. Reaction 1 I. Measure 100.0 mL of DI water into the cup. Place the thermometer in the water. 2. Weigh the provided vial of NaoH pellets. Because sodium hydroxide readily picks up moisture from the air, you should weigh it quickly and go on to the next step without delay. CAUTION: Solid sodium hydroxide and NaoH solutions are quite caustic. Hvoid skin contact and wash thoroughly in the event an accidental exposure of 3. As it may take some time for the thermometer come into thermal equilibrium with the Na liquid, wait until you have at least three subsequent readings (at 15 second intervals) at the same temperature before adding the solid NaoH to the water in the calorimeter cup. Be sure 83 to stir the pellets gently but continuously with your th or a glass stiming-rod, to encourage complete dissolving. Collect data for a full em nometer least until the temperature readings begin 225 s, or at to drop after passing through a maximum value. 4. After collecting the temperature data, weigh the empty vial (with its cap) and record the mass of NaoH used in your experiment. Dispose of the solution into the proper waste container inse the thermometer, the cup, and the stiming rod with DI water, then dry them in preparation for the next reaction. 5. Read through the data to find the initial solution temperature, T, and the maximum solution equilibrium temperature, Tr Record Tr and Ta on your data sheet. Note: You may want to graph the data to determine the maximum eguilibrium temperature. REV 2014-11-03 PADE 6 opExplanation / Answer
For the given data
Calculations
Reaction 1 : H2O + NaOH
solution mass = mass of water + mass of NaOH
= 100 + 2.2
= 102.2 g
change in temperature = 31.9 - 24.8 = 7.1 oC
heat absorbed (q) = mCpdT
= 102.2 x 4.18 x 7.1 = 3.033 kJ
moles NaOH = 2.2/40 = 0.055 mol
Molar enthalpy = 3.033/0.055 = 55.145 kJ/mol
Reaction 2 : NaOH + HCl
solution mass = mass of HCl + mass of NaOH
= 50 + 50
= 100 g
change in temperature = 30.1 - 23.9 = 6.2 oC
heat absorbed (q) = mCpdT
= 100 x 4.18 x 6.2 = 2.592 kJ
moles NaOH = 1.0 M x 0.05 L = 0.05 mol
Molar enthalpy = 2.592/0.05 = 51.84 kJ/mol
Reaction 3 : NaOH + HCl
solution mass = mass of HCl + mass of NaOH
= 100 + 2.274
= 102.274 g
change in temperature = 36.5 - 24.7 = 11.8 oC
heat absorbed (q) = mCpdT
= 102.247 x 4.18 x 11.8 = 5.043 kJ
moles NaOH = 2.247/40 = 0.056 mol
Molar enthalpy = 5.043/0.056 = 89.77 kJ/mol
1. Net ionic equations,
Reaction 1 : NaOH(s) ---> Na+(aq) + OH-(aq)
Reaction 2 : H+ + OH- <==> H2O
Reaction 3 : HCl + H2O ---> H3O+ + Cl-
2. The values do no follow the hess's rule. The reason could be incorrect measurement of the final temperature of the solution. Some of the heat may have been lost due to mixing.
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